Ashley River bed level investigation

Transcription

Ashley River bed level
investigation
Report No. R09/71
ISBN 978-1-877542-12-1
A J Boyle
M R Surman
September 2009
Report R09/71
ISBN 978-1-877542-12-1
58 Kilmore Street
PO Box 345
Christchurch 8140
Phone (03) 365 3828
Fax (03) 365 3194
75 Church Street
PO Box 550
Timaru 7940
Phone (03) 687 7800
Fax (03) 687 7808
Website: www.ecan.govt.nz
Customer Services Phone 0800 324 636
Executive summary
This report explores the history of flooding and human responses to that flooding with particular
emphasis on sedimentation in the Ashley River from the Okuku confluence to the coast.
The Ashley River has a long history of flooding combined with substantial sediment movement and
erosion. The Ashley River stopbanks have not been breached since 1953 although break-outs almost
occurred in 1986 and 2002.
The report is part of the implementation of the Waimakariri District Floodplain Management Strategy.
The stopbanks were last upgraded in 1976 to contain floods up to 2400 m3.s-1 with a freeboard of
600mm. Recent survey and modelling suggest the river has more than that capacity (3000 to 3500
m3.s-1, or more, throughout most of the river).
The revised 100 year flood estimate recommended for design purposes is 3250 m3.s-1.
The changes in mean berm, mean channel (fairway) and channel invert (lowest) levels between
1960/62, 1976, 1997 and 2008/09 are recorded and analysed.
A trend of degradation of the fairway which has accelerated markedly since the 1997 survey is evident
from the survey record. In the period since 1997 degradation far exceeds the reported gravel
extraction.
A range of indicators have been tabulated in Section 21.1 which together provide a record of how the
capacity and erosion risk have changed over time.
Recommended target mean bed levels are set out based on 4 criteria related to the indicators. The
criteria are intended to:

3 -1
Effect a minimum flood capacity to safely pass a flow of 3250 m .s to the sea.

Limit berm edge heights to enable erosion risk to be managed and to protect existing
infrastructure such as bridges and intakes.

Reduce the variability of mean bed slope without introducing large changes in slope to assist
with river bed stability and consistency of bedload transport capacity.

Maintain a consistency of flood capacity over the length of the scheme.
The report concludes with the following recommendations:


Adopt the levels set out in Section 22 as minimum bed levels with the exception that bed
levels be allowed to drop up to 200mm further in the reach downstream of SH1. Gravel
extraction should be limited to the few locations where drops in bed level have been
recommended. The volumes involved are small in relation to historic extraction rates and
current applications in process.
3. -1
The Ashley River Rating District consider adopting 3250 m s (plus 600mm freeboard) as
the design flow for the river.
Environment Canterbury Technical Report
i

It is apparent that significant changes can occur in a relatively short time, therefore, where
large scale excavation is occurring the cross sections upstream of, through and downstream
of, the excavation areas should be surveyed annually. In the absence of extraction the reach
downstream of SH1 be surveyed at least 5-yearly so that the capacity of the reach can be
managed. Elsewhere, a maximum of 10 years between surveys (or more frequent if large
floods appear to have altered the bed substantially) is considered adequate. This is longer
than the period recommended in the Waimakariri District Floodplain Management Strategy
but when considered along with the annual surveys recommended above is a more strategic
approach.
Allowing extraction of gravel and sediment down to the proposed minimum bed levels would yield
3
3
about 440,000 m (as at the most recent survey of 2008/09) of which 240,000 m is available for
current applications.
ii
Table of contents
Executive summary....................................................................................................i
1
Introduction .....................................................................................................1
2
Context.............................................................................................................1
3
Methodology....................................................................................................1
4
Physical setting of the Ashley catchment ....................................................1
5
Structural setting of the catchment...............................................................4
5.1
Geomorphology ..............................................................................................................6
6
History of flooding ..........................................................................................9
7
Previous schemes ........................................................................................60
8
7.1
Population, land-use, and future development .............................................................60
7.2
Perception of risk and public participation ....................................................................61
7.3
Structural integrity of the existing system .....................................................................61
Hydrology ......................................................................................................63
8.1
July 2009 review ...........................................................................................................63
9
Sedimentation ...............................................................................................64
10
Climate change .............................................................................................69
11
Survey comparisons.....................................................................................70
12
Hydraulics......................................................................................................72
13
Mean fairway levels ......................................................................................75
14
Invert levels ...................................................................................................77
15
Channel widths .............................................................................................79
16
Mean berm level comparisons.....................................................................81
17
Fairway capacity relative to mean berm levels ..........................................83
18
Mean berm level minus mean fairway level comparisons.........................84
19
Mean berm level minus channel invert comparisons ................................85
20
Slopes ............................................................................................................87
21
20.1
Fairway slopes ..............................................................................................................87
20.2
Channel invert slopes ...................................................................................................90
Criteria ...........................................................................................................90
iii
21.1
Indicators ......................................................................................................................91
22
Recommended bed levels ............................................................................92
23
Sediment budget ...........................................................................................93
24
Implications for gravel extraction consents ...............................................95
25
Conclusion ....................................................................................................96
26
Recommendations ........................................................................................96
27
Peer review ....................................................................................................97
28
Acknowledgements ......................................................................................97
29
References.....................................................................................................97
Appendix 1
iv
Bed level decision matrix........................................................99
List of photographs
Photo 6-1
Photo 6-2
Photo 6-4
Photo 6-5
Photo 6-6
Photo 6-7
Photo 6-8
Photo 6-9
Photo 6-10
Photo 6-11
Photo 6-12
Photo 6-13
Photo 6-14
Photo 6-15
Photo 6-16
Photo 6-17
Photo 6-18
Photo 6-19
Photo 6-20
Photo 6-21
Photo 6-22
Photo 6-23
Photo 6-24
Photo 6-25
Photo 6-26
Photo 6-27
Photo 6-28
Photo 6-29
Photo 6-30
Photo 6-31
Photo 6-32
Photo 6-33
Photo 9-1
Photo 9-2
Photo 9-3
Photo 9-4
March 1902 Flood............................................................................................................14
March 1902 Flood............................................................................................................14
March 1902 Flood............................................................................................................16
March 1902 Flood............................................................................................................17
May 1923 Flood...............................................................................................................19
The long deep lagoon, 9/09/09........................................................................................22
The Café Bank, 9/09/09 ..................................................................................................22
February 1945 Flood .......................................................................................................33
February 1945 Flood .......................................................................................................33
‘Emergency Bank’ post 1951 (note the undesirable alignment of the main channel) .....34
Reinstatement of the ‘Break-Bank’ post 1953 (Reinforced Earth Technique) ................35
Reinstatement of the ‘Break-Bank’ post 1953 (Reinforced Earth Technique) ................35
Protection Plantings on the riverside of the re-instated ‘Break-Bank’ .............................36
Protection Plantings on the riverside of the re-instated ‘Break-Bank’ .............................36
April 1951 Flood ..............................................................................................................38
April 1951 Flood ..............................................................................................................38
January 1953 Flood.........................................................................................................41
January 1953 Flood.........................................................................................................42
January 1953 Flood.........................................................................................................42
January 1953 Flood.........................................................................................................43
January 1953 Flood.........................................................................................................44
January 1953 Flood.........................................................................................................46
January 1953 Flood.........................................................................................................46
January 1953 Flood.........................................................................................................47
January 1953 Flood.........................................................................................................48
January 1953 Flood.........................................................................................................50
January 1953 Flood.........................................................................................................52
January 1953 Flood.........................................................................................................52
October 1956 Flood.........................................................................................................54
August 1986 Flood ..........................................................................................................57
August 1986 Flood ..........................................................................................................57
August 1986 Flood ..........................................................................................................58
Ashley River mouth, low tide 9/09/09..............................................................................65
Ashley River near the mouth, (low tide) 9/09/09 .............................................................66
Ashley River and Okuku River confluence, 1977............................................................67
Ashley River and Okuku River confluence, 2001............................................................68
List of maps
Map 4-1
Map 5-1
Map 6-1
Map 11-1
Physical setting..................................................................................................................3
Overview of landforms.......................................................................................................5
Historic outflows ..............................................................................................................59
Location of cross sections ...............................................................................................71
v
List of figures
Figure 5-1
Figure 7-1
Figure 13-1
Figure 14-1
Figure 15-1
Figure 16-1
Figure 18-1
Figure 19-1
Figure 20-1
Figure 22-1
Figure 23-1
Schematic representation of the Ashley floodplain ...........................................................8
Elements and functions of protection works....................................................................62
Mean fairway level changes since 1960/62 ....................................................................76
Invert level changes since 1960/62 .................................................................................78
Fairway and berm widths ................................................................................................80
Mean berm level changes ...............................................................................................82
Mean berm minus mean fairway level changes ..............................................................85
Mean berm level minus channel inverts ..........................................................................87
Slopes between cross-sections.......................................................................................89
2008/09 bed level relative to recommended bed level....................................................93
Location of changes in volume and reported extraction volumes 1997 to 2008/09........94
List of tables
Table 8.1
Table 12.1
Table 12.2
Table 13.1
Table 14.1
Table 15.1
Table 16.1
Table 17.1
Table 18.1
Table 19.1
Table 20.1
Table 20.2
Table 22.1
Table 23.1
Table 23.2
Table 24.1
vi
Flood frequency estimates (Griffith, 2009) ......................................................................64
Modelled flood levels and freeboards (1997 cross-sections)..........................................73
Modelled flood levels and freeboards (2008/09 cross-sections).....................................74
Mean fairway levels .........................................................................................................75
Invert levels .....................................................................................................................77
Fairway, berm and total channel widths ..........................................................................79
Mean berm level comparisons ........................................................................................81
Relative fairway capacity.................................................................................................83
Mean berm level minus mean fairway level comparisons...............................................84
Channel depths ...............................................................................................................86
Fairway slopes and change in slope ...............................................................................88
Slope of the channel invert..............................................................................................90
Recommended bed levels...............................................................................................92
Volume changes and extraction returns 1960/62 to 2008/09 .........................................93
Annualised volume changes and extraction returns 1960/62 to 2008/09 .......................93
Potential gravel extraction volumes.................................................................................95
1
Introduction
This report explores the context and history of flood control works on the Ashley River. It examines the
response of the river to protection works and gravel extraction, examines options for adopting bed
levels and recommends fairway bed levels for the short to medium term (10 years).
2
Context
Flood hazard is the potential damage to people and/or property caused by floodwaters escaping from
a river during a period of higher flow.
Responses involve action plans consisting of a number of measures to minimise the impacts of
flooding and deposition of sediment. Examples of measures are: stopbanks, protection plantings, rock
protection, gravel extraction, raising of floor levels, flood warning and land use controls.
To be effective, the action plans must have the support of the communities where they are to operate
and from where they are funded and must be based on sound appreciations of the physical and social
aspects of the flood hazard.
Particularly because of the success of current and historical structural works, the Ashley floodplain
residents are less aware of flood hazard than many other floodplain residents around the region.
The issues were re-visited in the early 1990’s culminating in the report Ashley River Floodplain
Management Regional Plan, Technical Investigation, May, 1995 and subsequently the adoption of the
Waimakariri District Flood Hazard Management Strategy by the Canterbury Regional and Waimakariri
District Councils.
There is a Rating Area Liaison committee which oversees the maintenance of the existing structural
works and capital upgrades.
The influence of gravel extraction on bed levels of the Ashley River has recently been explored in the
broader regional context and is described in the Regional Gravel Management Report (CRC, 2006).
Demand for gravel has increased to the extent that in many areas in the Ashley the flow capacity is
substantially more than that needed to pass the existing design flood and an appropriate bed level
limit needs to be set.
The calculated net supply rate of gravel is very small in comparison to the apparent scale of gravel
extraction in recent years.
3
Methodology
The methodology for this investigation was developed on the Pareora and Opihi Rivers, refined on the
Waimakariri River and further developed herein.
4
Physical setting of the Ashley catchment
(Refer Map 4.1)
(Ashley River Floodplain Management Regional Plan, Technical Investigation – Physical setting – the
late Mr. R. I. McPherson.)
The Ashley River is a relatively steep braided river which together with its major tributary the Okuku
drains a catchment of 1,340 square kilometres containing seven distinct physical landforms.
Commencing with headwaters in the Puketeraki Range in the northwest of the catchment the
succession of landforms continues downstream to the east through Lees Valley, the Mt. Thomas
Range, Loburn Basin, hill country at Ashley Forest and into the beginning of an unconfined floodplain
below the junction of the Ashley and Okuku Rivers. The floodplain then spreads out from both river
banks below Ashley Township to merge seaward into a broad belt of coastal lagoon, beach and dune
deposits.
The highest points in the catchment are along the crest of the Puketeraki Range with at least nine of
the peaks in excess of 1600 metres in elevation, for example, Black Rock at 1708m, Mt Pember at
1
1692m and Chest Peak – the highest, at 1935m. This range together with the Lees Valley Range and
its extension north over Okuku Saddle into Pig Flat on the upper reaches of the Okuku River and
intervening hill country of the Pancake Range combine to form an upper catchment of 671 square
kilometres – half the total area of the catchment.
The Ashley River drains the Puketeraki Range by way of the Lillburn, Whistler and Townshend Rivers.
A further tributary, Duck Creek, drains the hill country in the Pancake Range around to the Okuku
Saddle. Lees Valley wedged between these ranges and the frontal range to the southeast (Mt.
Richardson and Mt. Thomas) has been infilled by extensive fans of gravel brought down by the
tributary rivers to form a broad valley plain.
Both rivers, Ashley and Okuku, leave the upper catchment by deep narrow gorges through the frontal
hill range. The main sector of this range lying between the two gorges is unnamed on Survey and
Land Information Maps but is referred to as the ‘Mt. Thomas Range’ in this report. Major peaks in this
sector include Okuku Hill, 1143m, Ashley Hill, 1018m, Mt. Richardson, 1048m, and Mt. Thomas,
1023m. The continuation of the hills northeast of Okuku River is formally recognised on maps as the
Okuku Range (heights of 800-900m). The sector of hills to the southwest of Ashley Gorge is locally
known as the ‘Oxford Hills’ with Mt. Oxford the highest at 1356m.
The downland and plains section of the catchment referred to here as the Loburn Basin commences
downstream of the Ashley and Okuku Gorges. It forms a partially enclosed basin ringed by a crescent
of hill country to the north and east but is mostly open to the south. To the west, the foothills at Mt.
Thomas abruptly terminate on a north east trending front of steep slopes 600 to 800 metres in height.
To the north, the Okuku Range continues east in an arc of hills as far as Mt. Grey (934m) then curves
south through Ashley Forest almost to Ashley Township.
The plains of the Loburn Basin are built of a succession of alluvial fan aggradation gravels and silts of
varying ages. The youngest named as the Okuku Surface in this report is extensive and forms a large
featureless plain with an average seaward gradient of 6 metres per kilometre. Most of the gentler
downland standing above this plain is underlain by older fan gravels. The steeper downland and
neighbouring hill terrain, for example, at Mairaki Downs and Ashley Forest is underlain by compacted
silts and gravels more resistant to erosion known as the Kowai Formation.
From Ashley Gorge to the Okuku River confluence where the Ashley River floodplain commences on
2
the south side the strongly braided river is joined by the Glentui River (a sub-catchment of 20.6 km ),
2
2
2
Garry River (55.8 km ), Mt Thomas Stream (24.0 km ) and Bullock Creek (24.0 km ). Downstream of
its Gorge the Okuku is joined by the Karetu and Grey Rivers both of which drain the hills of Mt Grey
Forest. Below Loburn the Ashley is joined by the Makerikeri River (34.3 km2) which drains the western
slopes of Ashley Forest.
All the rivers have now cut below the Okuku Surface each leaving behind a succession of river
terraces on their flanks during the course of downcutting. These are generally referred to as
degradation terraces (or ‘surfaces’) to distinguish them from the fan aggradation surface above (in this
case the Okuku) into which the river has initially cut.
The unconfined floodplain commences on the south bank of the Ashley downstream from the
confluence with the Okuku River. It spreads a large fan of alluvium southeast around the edge of
Mairaki Downs and out towards Rangiora and Southbrook. The situation is different on the north side
of the Ashley River where the floodplain is confined by a prominent 5 to 10 metre-high boundary bank
which extends downstream from the Okuku River to as far as Ashley Township. The top of the bank
marks the south edge of the Okuku surface referred to earlier. Beyond the township younger fan
sediments of the floodplain have been able to spread out to the north-east around the flank of Ashley
Forest.
Nor’ easters tend to dominate the weather pattern in the lower part of the catchment from the Mt.
Thomas Range down to the coast. On occasion cyclonic rain associated with southward-moving
tropical weather systems also enters the catchment from this direction. Southerlies, south easterlies
and to a lesser extent nor’ westers, also contribute to increased river flows.
This account demonstrates the importance of the alluvial fan-building process in creating the
landforms seen in Lees Valley and Loburn Basin and in creating the classic fan forms of the modern
alluvial floodplain on which Fernside, Pegasus Township, Rangiora, Southbrook, Waikuku and
Woodend are built.
2
Physical setting
3
Map 4-1
5
Structural setting of the catchment
(Refer Map 5.1)
(Ashley River Floodplain Management Regional Plan, Technical Investigation – Structural setting – the
late Mr. R. I. McPherson)
As with other large Canterbury rivers the geomorphic history has been one of response to long-term
variations in climate.
But in the Ashley the changes have also taken place in an active tectonic setting and in this respect it
differs markedly from the large plains area immediately to the south.
The catchment broadly divides into two.
The western half extends from the headwaters in the Puketeraki Range east across Mt. Thomas
Range to Mt. Grey. This area is made up of hard greywacke, sandstone and mudstones.
The eastern half, extending seaward from the Loburn Basin east across the arch of low hills at Ashley
Forest contains younger and softer sedimentary rocks (covering strata). In turn these rocks are largely
covered by a succession of extensive fan aggradation gravels and silts.
The history of fan development is of interest to this study in so far as it relates to the disposition of
floodable ground – the present-day riverbeds and coastal floodplain.
Most of the catchment is subject to the same levels of differential uplift, seismic faulting and folding
which characterise North Canterbury.
Vertical uplifts of several kilometres have occurred along range-bounding faults.
Most of the catchment experiences a frequency of earthquakes and shallow faulting not seen on the
plains to the south.
4
Overview of landforms
Map 5-1
5
5.1
Geomorphology
(Refer Map 5.2. Because of its size this map is not included with the report. It can be viewed on
request.)
The Ashley River like other Canterbury rivers from the Clarence to the Waitaki flows from the
mountains and foothills and discharges floodwaters and sediment onto its floodplain.
The Ashley floodplain is built of alluvial sediments.
The frequency of discharges onto the floodplain has been reduced substantially by physical controls.
The Ashley fan starting from the coast as a wide expanse of shingle which narrows in a triangular
fashion to a point at Rangiora. It overlays an older fan which protrudes through in some places which
also narrows to a point at the Okuku confluence.
Obviously the elevations of the fans increase in the inland direction and are inlaid with previous flow
channels which have formed the fans.
In its natural state prior to any containment works the shingle deposition would cause the channel to
switch direction and thus over time the channel would roam backwards and forwards across the fan
building it up.
The process is described very succinctly in documents referred to in the References section of this
report such as the Ashley River Floodplain Management Regional Plan, Technical Investigation
Report.
The alluvial fan that covers the upper part of the floodplain is the oldest. According to Brown (1973) it
is composed of stony silts distinguished from the younger lower fan by a ‘stony surface’ developed at
a higher level.
The upper fan commences at Banner Downs (close to Hillcrest) and slopes gently away from the river
south-east around Mairaki Downs. The fan maintains a relatively uniform gradient as far east as
Rangiora then abruptly increases for about 2 kilometres across the built-up area of the town. East of
the town the gradient flattens markedly and the fan becomes constricted to residual arms of high
ground which maintain height and extend coastward beneath the younger sediments.
Immediately to the south of the river the fan is separated from the riverbed by both the stopbank and
the long eastward-draining floodway corridor occupied by Northern Creek. The floodway is corrugated
by numerous shallow channels probably cut into riverbed which fed downstream to the younger fan.
Further to the south the fan carries imprints of shallow channels and remnants of braiding directed
south-east towards the Cust catchment. Here the older fan meets the Springbank fan sloping in from
the Cust River. Both fans merge, the junction being the drainage channel of Dockeys Stream.
East of Dockeys Stream the boundary between the older fan and the younger fan alluvium is very
difficult to define.
The younger fan spreads symmetrically out from both banks of the Ashley River over a wide area
downstream from Ashley township to meet the coastal succession of beach sands, interdune flats and
dunes. Subject to extensive surface flooding and containing the majority of floodways most of the land
in these areas was found by Torlesse and Boys in 1848-1850 to be covered by swamp or under ‘flax
grass and tutu’ with some areas in native bush. Most of the swamp areas are now drained.
Two entirely separate and very shallow drainage depressions lie to the south and south-east. The
larger depression lies on the south and forms a sub-catchment occupied by the Cam River (formerly
the Korotuaheka) and its tributaries – the Waikoruru, Northbrook and Southbrook. A smaller
depression lies to the southeast enclosed between arms of higher ground (the older fan) which
extends down to Tuahiwi to the west and Woodend to the east. Formerly occupied in part by Tuahiwi
Bush and in the lower reaches by Maori gardens this depression drains into the lower part of the Cam.
Brown (1973) described the deposits in these areas as consisting of three depositional units:
6



The majority of fan surface, a relatively stone-free alluvial silt lower than the stony silts on the
older fan perhaps as a result of differential compaction following post-european settlement of
the land.
Shallow peat deposits.
Flood sediment deposits (gravel, sand or silt) in the floodways and adjacent areas episodically
flooded prior to the construction of the stopbanks.
Characterising the fan surface and its periphery has proved difficult on the north side of the Ashley.
The surface is marked by sinuous flood channel-ways and the imprints of river loops and point bars
which indicate a southward deflection renewed after each incursion by the river.
The fan surface slopes fairly steeply away to the north from the stopbanks. The effect of this has been
to divert local streams draining off higher ground at Ashley Forest in the same direction and onto the
relatively featureless lower slopes of the fan (Saltwater Creek, Stony Creek and The Boyne). The
drainage is then diverted south again where it meets the inner edge of coastal deposits – the bars of
former beaches, dunes and interdune flats first mapped by Shulmeister and Kirk (1993).
The southern boundary to the younger fan mapping has been drawn on changes in the direction of
drainage and contours across the lower Cust River depression. The area south of this boundary has
been named here as the Flaxton Fan. Formerly swampland the Flaxton Swamp is now drained for
farming. However, it does have limitations as it floods on a regular basis.
Figure 5-1 illustrates the Ashley floodplain.
7
Figure 5-1
8
Schematic representation of the Ashley floodplain
6
History of flooding
“If you ignore history, you are compelled to doom, because history is bound to repeat itself.”
George Bernard Shaw.
A full history of flooding is set out in the report Ashley River Floodplain, Technical Investigation.
Excerpts from that history are reproduced here, with particular reference to the larger damaging floods
of 1868, 1924, 1936, 1945, 1951, 1953, 1979 and 1986. Historical break-outs are shown on Map 6.1.
(Ashley River Floodplain Management Regional Plan, Technical Investigation – History of Flooding the late Mr. R. I. McPherson.)
Introduction
The record of flooding since European settlement insofar as the research has gone is presented here
as a date file in which each event is covered as far as possible by quotations from the original source
supplemented, if need be, by additional comments. Considerable use is made of the Press and other
newspaper accounts namely the Canterbury Times, Lyttelton Times, Star Sun, North Canterbury
Gazette and the Christchurch Star. Other sources include Acheson 1968, the N.Z. Rivers Commission
(1921), an important flood history compiled for the North Canterbury Catchment Board from newsprint
records (MacDonald 1956), Soil Conservation and Rivers Control Council (SCRCC 1957), the
Torlesse Papers, 1848-51 (Maling 1958) and accounts of flooding written within an extensive history of
Rangiora (Hawkins 1983).
Floods have been a part of life in the Ashley region since records commenced. The problem was
pinpointed as early as 1921 when the N.Z. Rivers Commission reporting on the Ashley stated:
“evidence showed that in times of very high flood, water has escaped the Ashley River and run into
the low country lying to the north and west of Kaiapoi. It is, therefore, essential for the safety of this
country that the Ashley be kept within its banks. The flooding of the adjacent lands is caused by the
fact that along some portions of the river the natural banks are lower than the grade of a high flood
and, furthermore, by the fact that the river in common with most other Canterbury rivers is running on
a ‘fan’ and once the floodwaters get over the immediate river bank they tend to follow old channels
that lead away from the main river and do not return lower down as is the case with valley rivers.”
Further on in discussing the flood hazard to which Kaiapoi has been subjected the Rivers Commission
also make important observations with regard to the Ashley vis a vis the Waimakariri when they state:
“in addition to these floods of 1868 and 1887 emanating from the Waimakariri, Kaiapoi has been
inundated by local floods from the Eyre and Cust Rivers and also by flood overflows from the
Ashley River. As a rule, floods in the Eyre and Cust Rivers do not synchronise with those of the
Waimakariri but this happened in 1868 and 1905” (1905 notable also for overflow from the Ashley).
The Years of Settlement – 1850 to 1899
>>>May, 1862
Lyttelton Times, 24 May, 1862: “The unusual severity of the weather during the past week deserves a
passing notice. On Sunday morning a sou-wester accompanied with snow and rain set in which lasted
with scarcely any intermission till Thursday night…..During the late heavy fresh the Ashley overflowed
its banks and submerged to a considerable depth the whole township of Sefton and surrounding
district .” This account suggests a break-out of the river at some point near to and below Ashley
Township. Sefton, built at the head of a local fan and thus above flood levels emanating out of
the Ashley was probably inundated by backup followed by overflow from the local creek
(Pembertons).
>>>February, 1868
9
The Ashley area, in general
Rivers Commission, 1921: “It appears that in 1868 an extraordinary heavy flood occurred. If such a
flood were to occur again the damage would be enormous owing to the fact that the surrounding
country is now densely settled, whereas in 1868 it was sparsely populated.” Note: This observation
is even more pertinent today.
SCRCC: “Rangiora suffered a similar fate when the Ashley broke its banks and it was here that two
children were drowned. Many bridges were swept away and stock losses were heavy.”
North of the Ashley River
Hawkins (1983): Page 157: “Northport on the north bank of Saltwater Creek was promoted by C.E.
Dampier and was first mentioned in newspaper reports late in 1859. The creek was navigable to small
vessels but Dampier also hoped that it would become the market centre for that part of Canterbury
north of the Ashley. It had two wharves, two hotels, two stores and a handful of tradesmen during its
brief career as a coastal port.” Its role as a potential port and settlement was effectively
destroyed by the 1868 flood.
Lyttelton Times, 6 February, 1868: “In the Rangiora and Kaiapoi districts the accounts of loss and
suffering are heart-rending in the extreme. Hitherto, the former district has been able to look on at the
periodic flooding of its unfortunate neighbour in tranquil safety but this last fresh has most disastrously
encroached upon the previously unflooded grounds. This done, the overflow – emanating from the
Ashley and Cust swept irresistibly on towards Kaiapoi where, joining the Cam and flowing into the
Waimakariri a little above the township so increased the quantity of water in the north branch as to
immeasurably exceed all former floods and entirely neutralise all efforts of the residents during the
past two or three years to protect themselves.” And “the fact of the Ashley and Cust joining the
Waimakariri below the girder bridge also accounts for the giving way of the Kaiapoi Swing Bridge
which was much the stronger of the two.”
Press, 8 February, 1868: “River Ashley having burst its banks about two miles from the Creek”
referring to Saltwater Creek township. “Cameron’s store washed away and water 6 feet deep in
Turner’s Hotel and 3 feet deep in the Road Board’s office.” Of Thomas Topping’s property situated just
where the river burst its bank: “whole of farm levelled and transferred into riverbed.”
Lyttelton Times, 8 February, 1868: The flood “causing on Monday afternoon the Ashley to commence
to overflow and by five the same evening the whole neighbourhood was a complete waste of waters.”
Wycombe’s house was the only one except Dampier’s to escape inundation. The latter was probably
near the old bluegums on Main North Road between Saltwater Creek and Mt. Grey Road (MacDonald,
1956).
South of the Ashley River
Press, 6 February, 1868: The “Ashley overflowing at the bridge and in the neighbourhood of
Rangiora.” “A prediction that the Ashley might at no remote date flow through Kaiapoi may
come true as it was evidently from that river, the Cust, the Cam and the drains from the swamp
that flooded that town on the memorable 4th February, 1868.”
“The inhabitants (Rangiora) had been on the alert all night and at midnight several had to clear out of
their houses before the rush of water coming in from the Ashley, the banks of which had given way
near the Lion Hotel as well as at a point near Baugh’s assembly rooms and public-house.”
Again referring to Kaiapoi and the district north of it: “On this occasion, unlike all former ones,
the overflow was not caused by the Waimakariri alone, but by the Ashley, having burst its
banks above Rangiora, flooding that town and the plains beyond it and doing incredible
damage to property. Almost the whole of the district between the Ashley and Waimakariri is
submerged.”
10
In Rangiora “in the part which was flooded a great proportion of the crops are destroyed and the
fences either swept away or damaged.”
“On the farms behind the Church Bush and on the other side of the Cam opposite the Maori Pa the
water was very deep.” “On nearing Rangiora the scene of wreck was awful; fences, cut corn and all
sorts of articles strewing the road and crops flattened to the ground in every direction. All the road that
is formed up from Rangiora in the direction of Oxford was more or less damaged, the shingle being
washed and the road so full of holes as to be dangerous. Two children (Wilkenson’s) drowned at Drain
Road a mile from the Masonic Hotel.”
Lyttelton Times, 7 February, 1868: “A mighty rush of water such as no living inhabitant has seen has
visited Rangiora and the adjacent parts. Between eight and nine on Monday evening an extraordinary
and in every way unprecedented overflow from the Ashley River ran through Rangiora down the
Ashley Road taking the town about Thompson’s corner and then pursued its course along the Drain
Road to Southbrook.”
“On leaving Rangiora and going towards Fernside the road shows evident marks of the damage done
by the flood. At Capt. Parson’s place the garden is destroyed, fences levelled and the gate torn up
with the posts. The water reached here about six o’clock, coming from the direction of Mr. Dalziel’s
farm and making its way towards Rangiora. Beyond Mr. Dalziel’s place we hear of some houses
having been destroyed. One, belonging to Mr. Land and another, the water up to the thatch before it
gave way.”
“Mr. Blackett and some friends on Wednesday inspected the banks of the Ashley and found that the
water had made three large breaches; between the trig-pole below Mr. Mannering’s and that
gentleman’s residence; there is another not so large as either of these in the rear of Mr. F. Rickman’s
place. The river also overflowed its banks at John McFarlane’s. Returning to Rangiora the scene of
wreck near the Masonic Hotel is dreadful. The bridge which used to cross the creek near the hotel is
washed downstream and now lies bodily against the corner of Steggall’s mill.”
“Mannering’s (now Carpenter’s) lost 2000 sheep. The other severe sufferers in the Fernside area were
A. and W. Milne, Tooney, Guiney, McIntyre, Land, Dalziel, Dunlop, Woodley, Arnett, J. Williams,
Johnstone, O’Rourke, Capt. Parsons and Mrs. Watt.”
Press 7 February, 1868: “Opposite Mr. Blacketts’s store in Rangiora the water was flowing over the
crown of the road.”
MacDonald (1956) extracted from the writings of Mannering in 80 years in New Zealand, that the
“1868 flood came right up to the small terrace at the road (Mt.Thomas Road at Carpenter’s drive).”
This suggests flood flows were contained within the Northern Creek floodway.
Hawkins: Pages 149-50: “in 1868 the Mandeville district was battered by one of the great floods of its
time. It happened early in February about eight or nine o’clock on a Monday evening and the whole
basin between Rangiora and Kaiapoi was submerged from Fernside to the sand hills. And it being
February, the fields were in the flush of harvest, full of potatoes and cocked hay, wheat, oats and
barley, in full head, in stook, or in stack. Prolonged heavy rain brought the Ashley down in full spate
unfettered by any artificial banks and it surged out across country from three places; near T.S.
Mannerings farm at Fernside, behind Rickman’s at Rangiora and at John Macfarlane’s, Coldstream.
The water came down the Ashley Road into Rangiora took the Direct Road at Thompson’s store and
flowed on down to Southbrook arriving there about eleven. Between the Ashley and Southbrook,
residents had to turn out with little warning to rescue their property. Sod and cob cottages collapsed
and one family, the Hepworths, spent the night on their stable roof. When it was over the recentlyformed road to Fernside looked like a riverbed.”
“Out across the fields lay a new harvest of pots and pans, carts, dead stock, clothing, dairy utensils
and the scattered remains of crops and farmyards, while miles of close-clipped young hedges of gorse
and quicks lay levelled and broken. For days afterwards people drove out over the fields in their drays
searching the debris for possessions. Southbrook suffered badly; the area around the Masonic Hotel
was in a dreadful mess. The new bridge was gone; Steggall’s flour mill behind the hotel was badly
11
damaged and dead cows and three or four stacks, quite intact, littered the road. Two girls from a
family named Wilkinson, who lived further along the Drain Road, were drowned.”
“The flood of 1868 left a large area of newly-settled farmland without fences or recoverable crops. The
losses were inestimable and so much of the little that many families owned was destroyed that they
had to rebuild from the beginning again. It was also a timely warning to the Mandeville and Rangiora
Road Board, just four years old, that a lot of extensive and expensive river protection work lay ahead
of it if a repetition was to be prevented.”
February 1873, to April 1875
Page 210: “Many of the Road Board’s troubles in the late seventies were caused by the Ashley.” “In
December, 1874, the river threatened the Ashley School so that it had to be removed and the
Tipping’s house near Waikuku was washed away. In the years that followed the small plain on the
Kowai side below Sefton was inundated several times and the approaches to the North Road Bridge
were often washed out. Built low over the shingle the first Waikuku Cart Bridge was replaced by
another with the same fatal fault and by 1877 the road was being constantly blocked there, farmland
was being washed away and the course of the river was changing.”
>>>October, 1875
Hawkins: Page 193: “The Association’s next show (Northern Agricultural and Pastoral) in October
drew about 386 entries even though the flooded Ashley prevented many exhibitors from getting their
stock across.”
October, 1877
>>>July, 1887
SCRCC, 1957: “The ‘jubilee flood of 6 July, 1887, did not get into Christchurch but reached a greater
height by 2ft 4 in. than the 1868 flood. There was extensive flooding between the Waimakariri and
Ashley Rivers, but this time the Ashley did not overflow into the Kaiapoi.”
North of the Ashley River
Press, 7 July, 1887: “The Ashley has broken out somewhere about the same place as it did in 1868
near Mr Topping’s and all the places near that are nearly as bad as in that memorable flood. Mr
Wyllie’s old homestead had a depth of 3ft. of water in it and Messrs. Hopkins, McKay, McIntosh and
several others are in the same position.”
South of the Ashley River
Press, 7 July, 1887: “The Ashley River is higher than it has been for years and from the large quantity
of water that is coming across the country between Rangiora and Fernside it appears there must be a
break in the banks and the water is coming over. Near Coldstream there is also a large quantity of
water.”
“Towards the Ashley River a large gully is full of water which is coming from the Mairaki Downs and
running towards Rangiora. A small portion of it finding its way on to the Oxford Road and thence on to
the West Belt of the Borough and eventually into Southbrook Creek.”
“The Ashley River is running from bank to bank and opposite Coldstream has overflowed and has
inundated a considerable quantity of land.” “So far as we have heard no damage has been done in
this direction but at Waikuku serious floods have occurred and the lands of Messrs. Leggett and
McDonald are submerged.”
Lyttelton Times, 7 July, 1887: “the river broke its south bank opposite Mr. M. McFarlane’s property
Coldstream flooding out two families and spreading over a wide track of country.”
12
“There was very little inconvenience experienced in the town from water, the only sufferers being, as
usual, those people living in the southwestern portion of the Borough. A large volume of water was
finding its way down the Oxford Road from a gully known as ‘Racecourse Gully’ which drains a large
area of land between the road and the Ashley. The stream took its course down Lehmans Road to the
Oxford Road which it transformed for a time into a small river and thence into the West Town Belt.
From the Belt the water flowed partly into the new drain in course of construction by the Borough
Council and the balance greatly augmented at the end of the Belt by a strong stream from Bruin’s
corner found its way either to Mr. Buckham’s corner or into the Southbrook where it increased to such
an extent as to cause it to flood several properties along its course.” Note: ‘Racecourse Gully’ is
obviously Northern Creek.
Lyttelton Times, 8 July, 1887: The Ashley “broke over its bank to the north west of Fernside and threw
a large volume of water into the ‘Big Creek’ which in turn overflowed in the direction of Fernside
flooding a large area of land. The Ashley also broke at Waikuku doing an immense amount of
damage. Leggitt, McDonald and Bishop suffered damage.” Note: This description is vague enough
for ‘Big Creek’ to be ‘Northern Creek’ or even ‘Dockeys Stream’. In either case flood water
could have overflowed into the two normally dry minor stream channel courses shown on the
geomorphic map to bracket Fernside.
Press 8 July 1887; “A good deal of interest is manifested in Rangiora as to whether the Ashley really
did overflow its banks, that being a matter of serious moment to the Rangiora people and after very
careful enquiry and personal examination the Mayor Mr. Boyd has been able to satisfy himself that a
portion of the water coming down a large gully by Fernside and running parallel to the river came from
the Ashley having entered the gully close to Fernside sheep station about seven miles from Rangiora.
This was augmented by a quantity of water from the Mairaki Downs which caused the gully to overflow
the lands on the north side of the Oxford (railway) line and wash away a portion of the line itself.”
Note, yet again, a confusing description of flood paths which might include not only Northern
Creek but also Dockeys Stream and the minor stream channels bracketing Fernside.
“The Ashley also broke loose opposite the Coldstream farm and inundated the country there but with
these exceptions the river kept its course in the Rangiora District but on the North Road near
Saltwater Creek a quantity of water from the river overflowed the land of Messrs. Tipping, Leggett and
others, doing some damage to the fences and crops.”
“At Flaxton, the land between the Flaxton Railway Station and the Eyreton Line was one sheet of
water several feet deep and a strong current was running towards the Cam. The line was damaged in
several places, one length of about three chains being the worst.”
1900 to 1924
>>>24 March, 1902
Press, 25 March, 1902: “Following the flood of March 24th “a visit to the Ashley River this morning
early disclosed the sight of about eighty to one hundred yards of the railway embankment and the
twisting up of the rails and sleepers from the piles that supported them. But the more serious part of
the damage is the south part of the bridge structure itself which has been considerably twisted and it
looks as if a fortnight will hardly be long enough to repair damages.”
13
Photo 6-1
March 1902 Flood
Damage to the Railway Bridge built in 1875 over the Ashley River Rangiora and Districts Early
Records Society.
Photo 6-2
March 1902 Flood
The extent of damage to the Railway Bridge seen from the riverbed after the flood of 24th
March. Source:-Weekly Press (2/4/1902) in “Canterbury Times” collection. Canterbury Museum.
14
“The embankment can soon be made up by trains conveying spoil but the driving of piles at the bridge
will occupy some time since two or three bays at the south end are affected and the upper structure is
hanging over about seven feet.” “Mr. Gray went to the Ashley Traffic Bridge but found there was no
chance of getting over the seething torrent several hundred yards in width between the south bank
and the approaches to the bridge.”
“At the north end of the bridge one bay of the bridge has been carried away and this will require some
pile-driving. The riverbed was full of water from bank to bank and without question the work of
embankments etc. has driven the main stream into the centre course with the result that the bed is
scoured out.”
“From the Ashley (traffic) Bridge towards Rangiora it is seen how the mighty torrent of Sunday in one
place tore up some chains of roadway and in another carried away ballast from the railway in its
sudden escape from the Ashley towards Maria Andrews Park and adjoining lands. At the mills bridge
there was a washout and some damage was done to the Woodend Rangiora Road.” Note: that part
of Maria Andrews Park is now occupied by Park Lawn Cemetry, and, presumably, ‘mills bridge’
is the bridge at Cam Corner.
“This morning the escape of water from the Ashley to the Cam had ceased. For a time there was very
rapid subsidence but towards noon the river began to rise again and scores of persons were out to
watch developments. It seems fortunate that the river found an escape for had the flood risen and
overflowed in the direction of Rangiora Town there is a fall of 14 ft. from one end (the north) of Ashley
street to the Bank of New Zealand (at the south).”
“News from the north bank of the Ashley is to the effect that the river extended well out towards the
lower part of Sefton and Saltwater Creek which presented a veritable sea and in instances was
covering crops and surrounding stacks. The great North Road was blocked and some large mobs of
sheep were held up.”
Elsewhere in an account describing arrangements made for transporting railway passengers between
Sefton and Rangiora passengers’ descriptions “of the havoc wrought by floods in that portion of the
country they saw from the Ashley to about half way to Rangiora showed that they had been greatly
impressed by the damage done.”
“The paddocks were covered with river mud or silt whilst on the gorse hedges and in some cases on
the wire fences were seen to be deposited mangels which had been uprooted by the flood. For a mile
or a mile and a half south of the Ashley River the damage was cruel. On the way one portion of the
road was under water which reached as high as the horses’ bodies. There are rumours of large
numbers of sheep having been destroyed one statement being that Mr. R. Evans of Flaxton had lost
about one thousand sheep.”
“Near Saltwater Creek and Sefton Messrs. Wyllie Bros. are heavy losers of grain and crops of beans
and damage to fences from the rush of Ashley overflow. The flood also reached Mr. Thompson’s land
and at Mr. E. d’Auvergne’s carried away a crop of newly-cut beans. Parnham Bros., Messrs. R.
Wilson, Connell and Laing received damage to their crops. The river Ashley broke in Mr. D. Dick’s
farm and after covering a wide area returned to the river between Saltwater Creek and Ashley Bridge.
A very heavy sediment was left which while enriching the land will preclude the prospect of grass
crops this season. Mr Ensor of Mt. Grey kindly offered some of the Sefton settlers a run for stock until
the effects of the flood had passed.”
“At Coldstream, on the opposite side of the river Ashley, Mr. M. McFarlane had 150 acres under
water.”
Weekly Press, 2 April, 1902: “As a result of the heavy rains in the Canterbury district last week most of
the rivers were in heavy flood.”
15
“The effect of the rain was most severely felt in the river Ashley where the water rose to a greater
height than has been the case at any time during the past nine years. The Railway Bridge at Ashley
which is situated twenty-two miles from Christchurch suffered most severely and the large
embankments were quickly scoured away. On Sunday night two spans at the north end of the bridge
were washed away, the Railway Bridge being left suspended between the embankment at the
northern end and the broken bridge.”
Weekly Press 9 April, 1902: “The Ashley Gorge Bridge was strongly built and the fact that two spans
were completely washed away shows the force of the flood.”
Photo 6-3
March 1902 Flood
The road traffic bridge at Ashley Gorge. The Weekly Press commented “the fact that two
spans were completely washed away shows the force of the flood.”
Source:- Weekly Press (2/4/1902) In “Canterbury Times” collection, Canterbury Museum.
Hawkins 1983: “The deluge which produced this flood in late March, 1902, filled the Flaxton Basin to
the fence-tops and the torrent which came down the river seemed to be deflected by the new
embankment straight into one section of the Railway Bridge. It was a long time before the damage
could be repaired and the route was kept open with twelve hired drags which plied between the Ashley
and Rangiora Stations by way of the ford.”
16
Photo 6-4
March 1902 Flood
After the collapse of the rail bridge in the March 24th flood rail passengers and mail were
conveyed across the bed of the Ashley in “drags” (seen here) supplied by the Christchurch
Tramway Company. Rangiora and Districts Early Records Society.
>>>23-24 June, 1905
SCRCC “In 1905 the flood waters of the Ashley River dammed up by the railway embankment
overflowed and came through into the Cam and found their way down to Woodend. A considerable
amount of damage was done to the Railway Bridge. The Railway Bridge had been reduced in length
from 3,000 ft to 1,700 ft in 1902 but after the flood of 1905 considerable work was done to protect the
approaches.”
>>>11-12 September, 1917
“the heavy rain which set in on Monday from the southwest continues. It has been the heaviest fall
experienced in the North Canterbury district for several years. Much of the low-lying country is under
water.” “The Ashley is in heavy flood and is still rising.” “The rain ceased at about 9.45 p.m. yesterday
after having fallen continuously for 37 hours.”
17
8 April, 1919
Rivers Commission established
Acheson 1968: “From the time of the consolidating river and drainage legislation in 1908 up to 1919
the position in regard to deterioration of river channels, flooding and erosion damage and inadequate
drainage was cause for concern. This was no doubt partly due to inadequate maintenance and
increased urban and rural land development requiring a higher degree of protection but there was also
a growing awareness of the relation of catchment condition to the problems. Many of the River and
Drainage Boards had found their financial and other resources were too limited to tackle their
problems and looked to Government for assistance.”
“In 1919 a Royal Commission was appointed to report on a number of rivers (the Ashley included)
mostly in the South Island. The Commission consisted of F. W. Furkert, Chairman, with Ashley John
Hunter and F. C. Hay as Commissioners.”
5 March, 1921
Rivers Commission Report on the Ashley River
Extracts: “A considerable amount of levee-building, protective planting and groyne work has been
done by the Sefton-Ashley Drainage Board, and, to a lesser extent, by other interested parties. After
the flood of 1905, the Railway Department did a considerable amount of work to protect the
approaches to the Railway Bridge and to lead the floodwaters safely under the bridge. While the river
in the past has overflowed its banks it appears now to be fairly well controlled. A good deal of
evidence was given by witnesses as to the rising of the bed. That some such rising has occurred
appears to be confirmed by the fact that it was found necessary to raise the Lower Ashley Bridge 4 ft.
and also by the fact that the piles cut off at the time of bridge renewals are now completely covered
with shingle. The absence, however, of definitive levels makes it impossible to decide definitely the
extent of this filling-up or the length of the river over which it extends.”
“The fact that in 1902 the Railway Bridge was reduced in length from 3,000 ft. to 1,700 ft. would tend
to cause silting up of the bed for some distance above the bridge and cause scouring under the
bridge. Nothing very decided, however, appears to have been noticed since that date.”
“The riverbed is very much overgrown with gorse, willows and broom, while evidence showed that
before the progress of settlement it was a clean river except for tussocky islands. The presence of
exotic vegetation cannot but tend to silt up the river and it is essential that a sufficient width of riverbed
to accommodate the river should be kept permanently clear.” [The fairway]
It was stated in evidence “that recent floods though in volume less than those of 1868 and 1905 have,
nevertheless, risen higher in the Lower Ashley. This will no doubt be due to the choking-up of the
riverbed and also to the rise in the level of shingle due to the first cause.”
“The local bodies interested in the control of the Ashley River are; the Sefton-Ashley Drainage Board,
the Rangiora County Council, the Rangiora Borough Council and the new special local authority which
your Commissioners have recommended in connection with the control of the Waimakariri River.”
“Your Commissioners consider that this new body will be interested in the control of the Ashley River
because evidence showed that in times of very high flood the water has escaped from the Ashley
River and run into the low country to the north and west of Kaiapoi. It is, therefore, essential for the
safety of this country that the Ashley be kept within its banks.”
“The Railway Department is also interested in controlling the river so that its floodwaters are passed
safely under the Railway Bridge just north of Rangiora and also in its control below the Railway-Bridge
as if it flowed on the north side it would probably injure the railway line to Culverden which for a long
distance is now built at a level considerably below that of the river with which it runs roughly parallel.”
18
>>> 6-8 May, 1923
SCRCC, 1957: “The most disastrous floods since 1868 swept through the province causing colossal
damage to public and private property. Rainfall was torrential in some areas particularly in North
Canterbury - 12.41 inches in 5 days (4-8 May) at Ashley Gorge. A large amount of damage was done
to Kaiapoi and Rangiora was similarly affected and at Waikuku the wool works were flooded to a depth
of 8 ft. The Ashley River peaked at 45,000 cusecs [1275 m3.s-1] at Rangiora.”
Photo 6-5
May 1923 Flood
“The flood attacking the Ashley Railway Bridge on May 8th. The water is up to the decking of
the bridge but the structure stood all strains.” This bridge replaced the structure destroyed in
the 1902 flood.” Source:- Weekly Press and New Zealand Referee (17/5/1923) in the New
Zealand Room, Canterbury Public Library
Lyttelton Times, 8 May, 1923: “Between Kaiapoi and Southbrook the country is a sea of water in some
places up to the tops of the fences…the main road from Flaxton to Southbrook was transformed into a
river of water from fence to fence…” “In the Southbrook Township heavy streams of water were
flowing down both sides of the road and in some cases over the roadway. Victoria Street leading up to
the centre of Rangiora was also carrying a large body of water on each side….”
“In Rangiora there were running streams and sheets of water in the main streets making it impossible
to get about without wading or being ferried over the worst parts in motorcars. In some places the
footpaths were covered to the front doors but no damage was done to stock by water. Two big
streams were flowing down Ashley Street and two down King Street giving cause for fear that the
Ashley River had overflowed between the embankment and Fernside. Fortunately this was not the
case the water having come mostly from overflowing water races. In the southwestern portion of the
Borough where the land is low-lying many acres were submerged and John Street was a miniature
river from end to end carrying a large volume of water from the intersecting streets and the water
races and discharging it into Northbrook causing it to overflow into adjacent paddocks.” “The most
serious damage done in the Borough, however, was the flooding of the gasworks.”
“The Ashley River was in heavy flood; in fact it has not been so high since the fresh that badly
damaged the Railway Bridge many years ago (1902). The Traffic Bridge embankment held but a large
stream of water percolated through and rendered the road leading up to the bridge impassable to
motor traffic.”
19
“The river has been responsible for a large amount of damage. It overflowed its south bank near
Coldstream and after running through the farms of Messrs. H. Mehrtens and O. Barber inundated the
Coldstream Estate and neighbouring farms.”
“Some of the families were in a bad plight. On Mr George Lowe’s farm the water was 3 ft. deep in the
house and it was not until the afternoon that Mr. and Mrs. Lowe were rescued from the house.”
“At Waikuku, conditions were even worse the water being out over the whole country from the Beach
Road to the river. The water invaded some of the houses near the twine mills and also submerged
part of Macdonald’s wool works, the engine being several feet under water.”
“The Misses Leggitt and their household were compelled to take to the upper storey of their house
from which they were rescued in the afternoon in a boat procured from Kaiapoi.”
“Thirty pigs drowned, belonging to M. Stokes.”
“On the north side of the river the protective works failed to keep the water within bounds. In three
separate places it broke over the bank and inundated hundreds of acres of the rich pastoral and
agricultural land between Sefton and Saltwater Creek doing a large amount of damage. At Saltwater
Creek Mr. Parkinson had 4 ft. of water in his house and in the early morning he rescued a neighbour
Mr. E. McMillan and family from their house where they were waist deep in water. Mr. Parkinson had
100 head of cattle washed out to sea. Mr. J. D. Wyllie had 300 sheep drowned. It is reported that there
are severe losses of stock. In the Sefton township there was a depth of 8 ft. of water in Mr. J. Barclay’s
store caused by the overflow of Pemberton’s Creek and a family in a house near by were forced to
leave home during the night. The railway line in the station yard is considerably damaged by the rush
of water from the same creek (Pemberton’s) and further along the line there is a washout extending for
half a mile.”
“In Ashley County, the Gray, Okuku and Makerikeri Rivers were in heavy fresh and there was a large
quantity of water on the roads and paddocks. Damage was done to the approach to the Makerikeri
bridge and it is now closed to traffic.”
“Ashley in high flood and has overflowed its banks. Cars (with passengers and mail for Kaikoura)
could not get past Waikuku being stopped by a sheet of water which completely covered the road and
paddocks. The cars then went to Rangiora to try to get across the ferro-concrete bridge over the
Ashley there. But here, a similar experience was met with the bridge being unapproachable on
account of a huge sheet of water.”
“Mr. J. Robb of Cust who travelled by train to Christchurch yesterday morning informed a reporter that
the flood waters were about a foot above the railway line in places. ‘Between Cust and Rangiora we
travelled at a snail’s pace and from Fernside to Rangiora a ganger and a platelayer in a ‘jigger’
proceeded in front of the engine to make sure that the line was safe for the train’ stated Mr. Robb. At a
number of places the ballast was washed away from under the rails. When the train pulled into the
Rangiora station the water was from 18 inches to 2 feet deep almost to the level of the platform.
Between Rangiora and Flaxton the train had to stop several times owing to the water which was
rushing across the line in a torrent in a number of places.”
The Press, 8 May, 1923: “It is many years since North Canterbury was visited with such a long spell of
heavy rain commencing on Thursday night (May 3rd) about 7 o’clock with a slight drizzle which
continued all night and all next day. The downfall increased to heavy rain on Saturday afternoon (May
5th) which has continued with more or less severity ever since.” “On Sunday (May 6th) the creeks
began to rise and the stormwater was soon overflowing the banks and all the low-lying lands were in a
short time under water and still the rain fell without a break.”
“Ashley causes trouble: Early yesterday morning (May 7th) water began to flow into Rangiora from
Fernside where the Ashley river had overflowed its banks and soon the drains and culverts were not
large enough to cope with the volume of water which came rushing down and some of the residents in
the western part of the town had a very anxious time.”
20
Note: The belief that the Ashley had overflowed its banks close to Fernside is refuted in the
Lyttelton Times of May 8th and again on May 11th. Flood flow into Rangiora, therefore, appears
to have come through the network of drains and races upstream as far as Mairaki Downs. The
Cust overflowed its banks and Dockeys Stream was almost certainly in spate.
Lyttelton Times, 9 May, 1923: “At Waikuku township the flood had receded slightly since Monday. The
lower Ashley River a few hundred yards further along the road was in heavy flood and the road was
entirely blocked to motor traffic (submerged). The bridge across the river was completely out of sight.”
“Traffic possible over the Ashley: The Ashley River had fallen from two to three feet yesterday and it
had ceased to overflow its banks. When at its height on Monday afternoon it broke through the traffic
bridge embankment near the end of Bell’s Road and the water that found its way past the bridge
added considerably to the stream which cut off motor traffic on the road leading to the bridge. The
stream had so far decreased yesterday that motorcars were again able to get through.”
Note: The “stream” referred to above (stream of water?) probably occupied the original bed of
Northern Creek which at that time crossed the upper end of Ashley Street.
“Buildings carried out to sea: flood water from the river found its way in a heavy stream through the
Waikuku beach domain and washed to sea Mr. H. M. Hawkin’s building used as a grocery store during
the picnic season and a ‘bach’ belonging to Messrs. Coates Rangiora. The bathing sheds on the
beach belonging to the Domain Board are also said to have been carried out to sea.”
“Bridges damaged: The flood was severe on the bridges in Ashley County. Both approaches to the
new Garry bridge were cut through also the south approach to the Makerikeri Bridge and one of the
approaches to the Karetu bridge. The Grey River bridge has also suffered some damage.”
Lyttelton Times, 10 May, 1923: Flooding in Kaiapoi “results from overflowing of Ashley, Eyre and
Main Drain.” “On the north side of the north branch (the Waimakariri) the flood water came
from the Ashley and Cam Rivers and the main drain there being a wide sheet of water from
Fernside where the Ashley broke through right up to Ranfurly St. in Kaiapoi.”
Lyttelton Times, 11 May, 1923: Statement that the above report is quite erroneous as “there was no
overflow of the river above the Railway Bridge, consequently, no water from the river could have
reached Kaiapoi. The Ashley is responsible for a large amount of damage but it was all caused by
overflows between the Railway Bridge and the sea.”
Hawkins (1983): Page 303: “Steady southeast rain caused the Cust and the Eyre to pour high
floods into the low-lying farmland of the old Mandeville and Rangiora swamp. The Ashley,
running high, then broke out above Rangiora and added its waters to the cascade which
swirled its way across country to the coast. Rangiora suffered some local (surface) flooding
but Kaiapoi was inundated. Two hundred families were evacuated from their homes and the
Revell family marked a new flood level at their home fifteen inches above the 1868 mark. The
main break, however, was at Coldstream through which much of the Ashley found its way
overland to Waikuku.” Note: This report has a break-out above Rangiora.
“Waikuku Beach in 1923 was becoming an increasingly popular picnic spot and holiday resort. A
Domain Board had been set up in 1905 to develop the place and it had planted pines and macrocarpa
and had provided water, a shelter and a grassed playing area.” “In 1923 water from the Coldstream
break went straight through the gate of the domain washed Hawkin’s store and a bach out onto the
beach and scoured out the long deep lagoon (photo 6-6) which has been a feature of Waikuku Beach
to this day.”
21
Photo 6-6
The long deep lagoon, 9/09/09
Photo 6-7
The Café Bank, 9/09/09
22
>>> 20-21 October, 1924
The Press, 21 October, 1924: “On Sunday night (19th) a strong wind got up from the south with much
colder conditions and heavy rain fell at night.”
“The Ashley river was in very high flood yesterday morning causing some anxiety to settlers on the
lower lands from the bridge down to Coldstream. The river was practically normal on Sunday evening
but during the night very heavy rain fell and yesterday morning the river rose very rapidly. Stock
grazing on the riverbed was removed with difficulty. At 10 a.m. the water was within a few feet of the
bridge at the Ashley and the river began to trickle over the embankment on the south side. Opposite
the golf links by Mrs. Mehrten’s farm the river overflowed and all the low-lying land was under water
and the Coldstream road was flooded for about a mile. By 9 a.m. (the 20th) the rain ceased and about
11 a.m. the water began to subside. At the Lower Ashley Bridge the water was just up to the planking.
Lower down by Leggat’s it had broken through and was running down the Waikuku Beach road but
instead of making its way to the sea it ran into the lagoon towards Kaiapoi. Some residents at Waikuku
beach had to be brought away yesterday in drays. Fortunately the water subsided before any damage
was done.”
>>> 1-2 November 1924
SCRCC, 1957: “Rivers in North Canterbury were in high flood (not specifically mentioning the Ashley)
but no damage was caused by them apart from inundation in the Flaxton-Tuahiwi area when the Cam
River overflowed.”
The Press, 1 November, 1924: “After several days of fine north-west weather the conditions changed
to a southwest wind on Wednesday night (October 29th) accompanied by heavy rain. Very heavy rain
fell over North Canterbury on Thursday night and yesterday (October 31st) particularly from the
Amberley district northwards. The fall in the locality mentioned exceeded 2 in. Judging by the
conditions of the Waimakariri and Ashley rivers the fall in the back country has been heavy and both
branches of the Kowai are also showing an unwanted vigour.”
The Press, 3 November, 1924: Leithfield and Sefton were flooded by local streams (Kowai River, and
Pemberton Creek). “Exceptionally heavy rain fell at Rangiora during the weekend (4.62 inches in two
days). The Ashley River was flowing bank to bank but did not overflow. A good deal of water came
through the Makerikeri stream which runs into the Ashley but no serious damage was done.”
1925 to 1949
>>> 1-2 July, 1925
The Press, 3 July, 1925: “Thaw sets in; snow rivers flooded; Eyre and Ashley break out. The Ashley
River is very high and is overflowing its banks at Coldstream. A great deal of anxiety is felt by farmers
as all the low-lying land is under water. All settlers have been warned to move their stock.”
The Press, 4 July, 1925: Rivers subsiding. “Traffic interrupted: The weather shows no sign of clearing
at Ashley Bank. Creeks were all running high on Thursday (2nd) but as little rain fell during the night
they were much lower yesterday morning. The roaring of the Ashley River can be heard at a great
distance. It is in high flood and the low-lying ground in the neighbourhood of the bridge is under water.
At the bridge, the water yesterday afternoon was within two feet of the supports below the mark
reached by the previous flood of two years ago. Cars and vehicular traffic were unable to get through
to Rangiora owing to the river breaking out in the vicinity of the Racecourse and forcing its way to the
north end of Ashley street and filling the hollow there. The water at this point was up to the shafts of
one vehicle that made its way through.”
The Press, 8 July, 1925: “During the recent heavy flood in the Ashley River a large volume of water
made a set for the south bank above the end of the embankment constructed to train the river to flow
under the Traffic Bridge. Some acres of flat in front of the bank were carried away including the portion
of the rifle range from the 700 yds. peg to the 1000 yds. (one).”
23
“After making a deep bight into the land the stream carried away about three-quarters of a chain of the
bridge embankment at the south end and flowed through blocked traffic on the road approaching the
bridge. Yesterday afternoon members of the Ashley, Rangiora and Kowai County Councils and the
Rangiora Borough Council paid a visit to the locality. After inspecting the damage done by the river
they decided that measures would have to be taken at once to prevent further erosion and to protect
the bridge embankment, there being danger of another flood forming a strong permanent stream in
front of the south approach to the bridge. The Ashley County Council, the controlling body of the
bridge and embankments, was instructed to obtain estimates for the construction of three or four
groynes to throw the stream into its old bed to join the main stream which flows under the south end of
the bridge. It is hoped that later on there will be a River Trust formed to undertake the conservation of
the river.”
The Press, 16 July, 1925: “Wellington, July 15: In the House of Representatives, this afternoon, the
Hon. D. Buddo asked a question in regard to the damage by the Ashley River floods, by which, he
said, the Traffic and Railway bridges were endangered. He suggested that the Government should
expedite the passage of the River Trust Bill and make some provision to contribute to the cost of
protecting the banks. He read a letter he had received from the Rangiora Borough Council, in
reference to the damage done. Both private and Government property, he said, were at stake.”
1925 – Ashley River Improvement Bill
Hawkins 1983: Pages 326-7: “In the mid-twenties something was at last done about flood protection.
By far the worst for many years the 1923 flood had exposed the vulnerability of the whole district from
the Ashley to Kaiapoi to sudden devastation and the river now received that special attention which
had been given to the Waimakariri some sixty years before. An Ashley River Improvement Bill set up a
River Trust District in 1925, the Trust to be responsible for the building of flood banks and other
protection work. Always reluctant to become involved in the Ashley, Rangiora now found itself again
rated to help supply the necessary money for the work; not only rated but in the highest category of
district rating and it filed an appeal. After evidence was given that the Rangiora Borough had never
been actually affected directly by floodwater from the Ashley – which was not strictly true if the
newspapers of 1868 are to be believed – the Borough was reduced to a Class B classification.”
>>> 15-16 July, 1925
SCRCC, 1957: “Heavy rain again brought floods” and “the Ashley River broke out on the south side of
the Traffic Bridge at Rangiora and blocked traffic.”
The Press, 16 July, 1925: “The weather; still more rain: The fine weather of Tuesday (14th) was of
short duration as rain began to fall in Rangiora during the night and continued during the whole of
yesterday. Owing to so much rain during the last few weeks the ground is thoroughly soaked and
water is lying everywhere. The Ashley River is again reported to be bank high and has broken through
on the south side of the Ashley Traffic Bridge and motor traffic is unable to get through. The low-lying
lands are again becoming a sheet of water.”
>>>23-24 September, 1925
SCRCC, 1957: “A high northwesterly gale brought melting snow and rivers throughout the province
rose in moderate flood. The Ashley River was also in high flood but no damage was reported.”
>>> 15-16 January, 1930
SCRRC, 1957: “A remarkably unseasonal rainfall amounting in some districts to over 4 in. in twentyfour hours resulted in heavy floods in many parts of the Canterbury province. The Ashley River was
also at a high level and overflowed its bank in several places. At Rangiora, 2.76 in. of rain was
recorded in twenty-four hours and 3.37 in. in forty-eight hours” with “6.16 in. at Ashley Gorge (in the
forty-eight hour period).”
24
The Press, 16 January, 1930: “Rangiora experienced mild flood conditions yesterday and much
surface water lay about. The Ashley River was in high flood yesterday morning (15th) and on the lowlying lands overflowed its banks.”
“Near Waikuku, a big area of country was under water. Campers at Waikuku Beach had a rather
anxious time as the flood waters overflowed and the road approach to the beach was submerged. The
Ashley cut a new channel to the sea and now runs straight out. No serious damage has yet been
reported to any protective works on the Ashley.”
“Slip at Ashley Gorge; hillside falls away. A landslide at Ashley Gorge on Tuesday afternoon when a
large area of hillside crashed into the Ashley River gave cause for concern to residents nearby. The
slip took place on the side of the hill above the Ashley Domain opposite the tearooms conducted by
Mrs. Reynolds where a small stream about a yard wide has run for the last thirty or forty years. A gully
several yards wide has been scoured out. For some time it seemed that an embankment would be
formed in the already flooded river but fortunately the debris was soon carried down by the current.”
>>> 5-7 August, 1930
SCRCC, 1957: “At Rangiora, 4 in. of rain was recorded in a twenty-four hour period. All rivers were in
moderate flood but caused no damage.”
The Press, 7 August, 1930: “Not since 1923 has there been such a heavy fall of rain as was
experienced on Monday night (the 4th) at Rangiora when four inches of rain fell for the 24 hours ended
on Tuesday morning at 9 o’clock. From then until about 10 a.m. yesterday a further inch of rain fell.”
“Although the fall was so heavy there was not nearly as much water in the Ashley as was expected.
The river rose but not enough to overflow its banks. No damage was done to the protective works or to
the new groynes which were recently put in. Several of the smaller streams near Southbrook and also
the Cam were unable to carry off the surface water and they overflowed.” There was widespread
temporary surface flooding over the low-lying area in and around Flaxton.”
>>> 11 April, 1931
SCRCC, 1957: “Heavy rain accompanied by a southwest wind caused a general rise in North
Canterbury rivers.” Rainfall over 24 hours amounted to “1.96 in. at Rangiora” and “the Ashley River
overflowed and covered the Main Road.”
The Press, 13 April, 1931: “In the higher back-country there was generally more rain and the hills in
some parts received a slight coating of snow.” “The Ashley was in high flood and had overflowed and
covered the Main North Road just above the Lower Ashley Bridge.”
>>> 20-21 February, 1932
SCRCC, 1957: “Heavy rain throughout the province caused high floods. Rivers in North Canterbury
were in high flood, the Ashley River running bank high.”
>>> 24-25 October, 1932
SCRCC, 1957: “Following three days of heavy rain some flooding occurred in parts of North
Canterbury. Coastal rivers such as the Selwyn, Ashley and Eyre were in high flood but the
Waimakariri, Rakaia and Rangitata were not affected.”
The Press, 26 October, 1932: “At the main North Road Traffic Bridge on the Ashley, “the water was a
foot above the planking”.
The Press, 27 October, 1932: “The Main North Road was clear of water up to Rangiora.” “The Ashley
was particularly high being up to within three feet of the planking of the Main North Road Bridge
though it was not overflowing its banks.” Article accompanied by photograph showing the flow “then
estimated to be 12 feet above its normal level.”
25
1933 – The Depression
Hawkins 1983: Pages 333-4: “The Rangiora Unemployment Committee co-ordinated the various
projects which were started under the Government’s work schemes.” “The unemployed planted trees
down by the river, trimmed hedges etc.”, “but there were too many men and too few jobs. Work was
rationed so that they would earn at least a few shillings for three or four days employment.”
May-June 1933 – The Flood Protection Scheme
Hawkins: Page 334: “The situation was alleviated by the commencement of a flood protection scheme
at the Ashley by the Public Works Department of Government. Classed as developmental work the
Ashley project required the construction of high flood banks and groynes along the river from the
junction with the Okuku down to the sea, especially on the lower south side where the river had
regularly broken out over the years from Fernside to Waikuku. Because of its scope the scheme was
used to help mop up unemployment elsewhere in Canterbury and the Borough Council had to fight to
have Rangiora men included in it. But it succeeded and work began in May-June, 1933. Two camps
were built, one at Fernside for Christchurch men and the other at Rangiora on the west side of the
southern approach to the Traffic Bridge at Waikuku. The men built the stopbanks with picks, shovels
and wheelbarrows, filling their barrows in the riverbed, wheeling them up long narrow planks to the
bank face, then tipping them over the end. They were paid by the day at first but a roster system
operated here also and the best a man could hope for was a fortnight on and a fortnight off.”
January, 1936 – The Ashley Relief Workers Strike
Hawkins: Page 336: “After that brighter Christmas of 1935 and the New Year holiday the relief workers
returned to the Ashley to find that the basis of their rates of pay had changed. Their daily rate of twelve
shillings had been exchanged for a piecemeal per barrow-load of sand and shingle and claiming that
they would now earn less than before the whole Ashley work force went on strike. The Ashley relief
workers strike was seen as the first test of strength of the new Labour Government by its supporters
and its new and untried Minister of Public Works, Robert Semple, hastened to the river on 9 January,
1936. With the first of many Semplean homilies which the Minister was to deliver during his years in
office he told the Ashley workers – ‘I want to tell you straight that no one is going to play pitch and toss
with the Labour Government. Anyone who gets in our way is our enemy. If any man thinks he is going
to dump his lazy frame on the Public Works Department he is making a hell of a mistake.’ Semple
offered the men another penny a barrow and they went back to work to discover that their earnings
had increased to over fifteen shillings a day.”
>>> 20-22 February, 1936
SCRCC 1957: “A southwest rainstorm culminated in heavy floods throughout the province resulting in
widespread damage. Rainfalls in some districts were heavy. Rangiora experienced 5.50 inches in 17
hours (and East Oxford 9.40 inches from 17 to 22 February inclusive).”
“The floods were most serious in North Canterbury and three rivers, the Ashley, Kowai and the Waiauuha broke their banks with the result that thousands of acres of farmlands were flooded. Farmers had
no opportunity to move their stock to safety and hundreds of cattle and sheep drowned. The Ashley
rose by 8 inches per hour to a record peak flow at Rangiora and water entered the Borough but
caused no damage. On the Main North Road the northern end of the bridge and part of the
approaches on either side were washed away. The most serious damage was done at Waikuku where
water flowed through Coldstream over towards Woodend and then across to Waikuku, the flood
devastating thousands of acres of farmlands and crops.” Discharge on the Ashley was “60,000 cusecs
3 -1
[1700 m .s ] at Rangiora off 443 square miles.”
MacDonald, 1956: With respect to the vulnerability of the stopbanking system to break-outs (in 1956)
MacDonald wrote that “as the stopbanks were only partially completed in 1936, the flooding in that
year would give a better indication of what can be expected if the stopbanks are left to deteriorate than
would the floods of 1951 or 1953. Unfortunately, however, the records of flooded areas in 1936 are
incomplete being limited to a comparatively narrow strip along the river.”
26
Hawkins, 1983: Page 337: “About six weeks after the strike the relief workers swiftly abandoned their
camp by the bridge (Rangiora Traffic) escaping along the railway embankment as the Ashley again
rose to a very dangerous height. Their new stopbank was breached in three places and the overflow
left much the same mess and destruction down the south side to Waikuku as it had in 1923. Strong
criticism of the protection works was heard but Robert McIntyre the engineer in charge pointed out that
the new banks although much higher than they had been in 1923 had actually been topped not broken
through and that this confirmed his view that this flood of about 83,000 cusecs [2350 m3.s-1] was much
greater than that of 1923. Rangiora had, in fact, been saved from disaster, for the riverbed above the
town was higher than High Street. The flood of February 1936, was a good test of the bank and
groyne system and while the damage ensured more work for the unemployed it also proved that the
Rangiora area could never really be absolutely protected from those rare and unpredictable high
floods which surged down the Ashley every twenty years or so.”
The Press, 21 February, 1936: “Heavy flooding in the Ashley and Kowai rivers late yesterday
afternoon developed to alarming proportions before midnight and caused extensive damage washing
out a considerable section of the Main North Road, smashing several of the bridges and by
widespread flooding of farm properties. Damage in Rangiora County alone was roughly estimated last
night at 50,000 pounds and the flood was the worst since 1923. On the south side of the Ashley the
Main North Road was covered to a depth of up to nine feet for a considerable distance and half a mile
north of Woodend a section of the concrete highway disappeared leaving a yawning gap 15 ft. deep
and increasing in width every minute as the fast swirling flood waters forced their way across country.”
“The main Ashley Bridge decking disappeared under the flood early in the afternoon. The Upper
Ashley Bridge close to the Rangiora Showground was also awash and the overflow swept over the
relief campsite and was checked only by the sharp rise into High Street (probably the northern end of
Ashley Street) half a mile from the main street (High Street) of Rangiora. In the meantime the Ashley
had broken its banks at Fernside causing extensive flooding on farms where houses had to be
abandoned. Much stock was marooned and heavy losses are reported.”
“Close to Rangiora the most serious break was across farm properties between Rangiora and the
Main North Road slightly north of the town” (probably means Coldstream). Note: the properties of
Crotty, Stalker and Pethig (all on Smarts Road) suffered inundation and damage.
“The flood swept with the fall of the land across the Main North Road a few hundred yards past the
Woodend church. The raging torrent carried away the Main North Road, scouring away the concrete
highway and culverts as if they were paper. Within half an hour the road was eaten away for several
chains.”
“At midnight although the river itself raged unabated there were indications that the breakaway south
to Woodend was checking. Trouble on the Main North Road, however, was increasing as the
floodwaters turned in search of a sea outlet and there was very heavy scouring all along the Main
North Road from Woodend to the Ashley River Bridge with many complete wash-outs.”
“In three places at least the Ashley broke through protective works yesterday afternoon. Through two
gaps separated by about two miles the flood swept across farms near Fernside surrounding houses
and causing heavy losses of sheep. A mail-carrier driving on the Mount Thomas Road alongside the
river was caught when a stopbank collapsed under the terrific pressure and had to abandon his van
and trudge through two feet of water for nearly four chains before he was safe. With the flood waters
running through their home ten members of one Fernside family (Mr. and Mrs. Jack Miles) had to seek
shelter with a neighbour on higher ground. The river broke its banks with startling suddenness and
farmers had little time to rescue stock. Horses and cattle were able to swim to safety but in spite of the
efforts of the farmers many sheep had to be left and were swept away and drowned. A break in the
banks higher up brought the flood through Rangiora and water was lapping the doors of shops in the
main street.”
“The effect of the flood in the Ashley became more noticeable after dark. The river also broke its bank
about 30 chains north of the Upper Ashley Bridge from Ashley Street and the flood swept down
through Coldstream towards the coast.” “Floodwaters spreading a mile and a half on either side.”
27
A further statement that “a break in the Ashley riverbank higher up brought three streams of floodwater
through Rangiora” is refuted elsewhere in this account – see MacDonald’s comments within the N.C.
Gazette entry for February 21st, below.
“Fifty relief workers in a camp on the north side of the Ashley river on the Main North Road are
isolated. Attempts were made last evening to send boats to their rescue from points both north and
south of the river but the strong flood had washed out the north road on both sides and the trucks
carrying the boats were unable to get through.”
North Canterbury Gazette, 21 February, 1936: Referring to the Borough of Rangiora: “houses beside
Bell’s Station were completely surrounded by water.” “The Waterworks had to be protected by
sandbags to prevent silt in the water flowing down Lover’s Lane from getting at the pumps and wells.”
“At 6 o’clock last night Borough workmen had to borrow the Southbrook Fire Brigade’s manual pump
for use at the Gas Works and the steady flow of surface water from Edward St. kept them busy until
midnight. The water reached within inches of the fire but sandbagged doorways kept it out.”
MacDonald (1956) had these comments to make concerning reports of water flooding into Rangiora
from breaks in the Ashley stopbanks: “I have been unable to definitely establish that the Ashley
contributed to flooding in the Rangiora Borough in 1936. The water was very dirty but the Cust had
overflowed at Stokes and there was a considerable flow down the Oxford Road. Cust water, therefore,
could account for the silty state of the water in Rangiora. On the other hand, although Cust water
could flow down High Street it would not be expected to spread as far north as Lover’s Lane. In
addition, there would be a considerable quantity of downs water and local water as a rainfall of 7.82
inches in 18 hours was recorded at the High School. At one stage over an inch fell in one hour.” Note:
If the Cust overflowed it is also likely that Dockeys Stream was in spate under prolonged heavy
rain. Any back-up of water in Dockeys Stream would eventually flow over towards Fernside and
the Oxford-Rangiora Road finding its way through normally dry channels which groove the
surface of the Rangiora Fan in the vicinity of Fernside.
The Press, 22 February, 1936: “The most serious damage has been done at Waikuku. Smashing
down through Coldstream over towards Woodend and then across to Waikuku the flood devastated
thousands of acres of farmlands hurling with it wheat stooks, stock, tree trunks and similar debris. At
Waikuku it went into houses that no other floods have penetrated. On the beach the residents were in
extreme peril until the water which had been banking up in the lagoon forced a channel to the sea.”
“Towards Coldstream there was widespread havoc. On each side of Gressen’s Road in the path of the
detour from Rangiora to Waikuku (this taken to mean both Waikuku Stream and Coldstream-Woodend
floodways) stooks of wheat have been caught by the fences and hedges. Debris was strewn across
the road. Some of the roads nearby were impassable. The road was under water to a depth of three
feet.”
“The flow in the Ashley River which broke its banks and spread through Woodend and Waikuku was
estimated by engineers of the Public Works Department, to have been at a rate of more than 60,000
3 -1
3 -1
cusecs [1700 m .s , see Robert McIntyre’s estimate of 2400 m .s ]. This is the highest recorded
figure for this river. At the peak of the disastrous flood of 1923 the flow of the Ashley was 45,000
cusecs [1275 m3.s-1] and at no other time since has it been more than 30,000 cusecs [850 m3.s-1].”
“Properties at the junction of Beach Road with the Main North Road, Waikuku, received the full force
of the flood for the flow from the Ashley was augmented by that from Waikuku Creek which crosses
the Main North Road to run along the side of the road to the beach. The flood was considerably higher
than that of 1923 and at the Waikuku Wool Works a few hundred yards down Beach Road there was
a depth of nine feet in one shed. That was three feet higher than the 1923 flood.”
“Residents at Waikuku Beach had a terrifying experience as the flood rushed into the lagoon and piled
up the water. The pressure of water forced an outlet from the lagoon to the sea. Homes had water up
to the mantelpiece and residents broke into unoccupied baches on higher ground for refuge.”
28
The Press, 24 February, 1936: “Damage to river protection works: Three groynes washed away and
several breaks in the stopbank were found when the engineer in charge of the Ashley river protection
works (Mr. R. McIntyre) and the District Public Works engineer (Mr. F. Langbein) carried out an
inspection of the flood damage on Saturday. ‘In every case of the failure of the bank it was clear that
the water had got over the top Mr. McIntyre said after the inspection. No failures were due to the bank
failing to withstand the pressure of the water. The loss in the groynes would be about 32,000 yards
with another 20,000 yards in stopbanks.”
“The flood was the biggest in the Ashley. An accurate estimate of the flow of water would be nearer
80,000 cubic feet a second [2,300 m3.s-1] than the 60,000 at first mentioned. As the scheme had been
built to withstand a flood of only 44,000 cusecs, or 70,000 cusecs with the water flowing level with the
top of the banks, it was remarkable that the damage was not greater. Groynes which were a complete
loss were near Mr. J.W. Brosnan’s farm at Fernside, at the mouth of the Makerikeri river and near Mr
H. I. Mehrten’s farm at Coldstream. The gabion head of the groyne at the Rifle Range was damaged
but this groyne probably saved Rangiora. There are gaps in other groynes but most of them are intact.
Breaks in the stopbank were seen at Fernside near the Mount Thomas Road and at the old Fernside
camp.”
“Gangs are ready to begin work today and repairs will be made shortly.”
“A survey to collect data and compute the volume of water which was carried by the river will be
carried out immediately. All relief workers who were employed at the beginning of last week will
receive full pay for the week and from today those who are not working will receive sustenance until
they are employed again.”
>>>8-9 March, 1936
SCRCC, 1957: “Incessant rain for two days again caused serious flooding in Christchurch City (3
inches in 24 hours) and in many parts of the province.” The Waimakariri was “in moderate flood
peaking at 14,000 cusecs” but “the Ashley River rose to a high level and at the peak was discharging
40,000 cusecs [1130 m3.s-1] at Rangiora. Some flooding also occurred at the northern approach to the
Traffic Bridge on the Main North Road.”
20-21 April, 1938
SCRCC, 1957: “North Canterbury rivers and creeks in flood but no damage reported. At Rangiora,
1.43 in. of rain fell in twenty-four hours but no mention of the Ashley River.”
1938 – Protection Works Completed.
Hawkins, 1983: Page 337: “The Ashley protection works were completed as a standard pay job
towards the end of 1938 and handed back to the Ashley River Trust. The scheme, first devised by an
engineer named F.C. Hay in 1926 to cope with a flood of 45,000 cusecs [1275 m3.s-1] had cost over
200,000 pounds, had taken five years to complete and at one time had provided employment for up to
five hundred men. Thirteen miles of stopbanks had been built as well as twenty-two angled groynes on
each side of the river while nine traction engines had been used to haul out willow thickets choking the
lower reaches. After the 1936 flood the height of the banks was increased and belts of willows were
planted as an additional defence.”
29
>>> 25-26 August, 1939
SCRCC, 1957: “Heavy rain in North Canterbury brought a rapid rise in creeks and rivers. The Ashley
was running bank high and properties adjoining the Eyre had large areas under water.”
>>> 7-9 May, 1940
SCRCC, 1957: “Heavy rain over a period of about four days brought most of the rivers of Canterbury
into high flood.” “In North Canterbury the Ashley River rose some 3 feet above normal at the Rangiora
Traffic Bridge and was discharging 25,000 cusecs [710 m3.s-1].”
The Press, 9 May, 1940: “Brief flood in Ashley River: There was a sharp rise in the level of the Ashley
River yesterday morning as a result of heavy rain. The river rose all morning and was at its peak about
midday when the level was three feet above normal at the Rangiora Bridge.”
“The Resident Engineer of the Public Works Department at Rangiora (Mr. R. McIntyre) stated that no
damage had been reported and that the flood would clear the channels in the river by cutting away
silted-up shingle and small islands. At one stage the river was carrying 25,000 cusecs, the average
being less than 1000 cusecs. The flood dropped quickly in the afternoon.”
1941 – The Soil Conservation and Rivers Control Act, 1941
Acheson, 1968: Page 19: The Act passed in 1941 “provided for the establishment of the Soil
Conservation and Rivers Control Council as the central authority on Soil Conservation and Rivers
Control work responsible to the Minister of Works and the establishment of Catchment Districts with
Catchment Boards as the district executive authorities to plan and carry out works under the Council’s
general supervision and direction.” “This Act was considered to be one of the most advanced Acts of
the type anywhere in the world bringing together Soil Conservation, River Control and Drainage
problems under unified control both at National and District level.”
>>> 18-19 March, 1941
SCRCC, 1957: “All rivers in Canterbury were in high flood after three days of continuous easterly rain,
the rainfall at Ashley Gorge totalling 13.89 inches in 6 days (March 14th to 19th inclusive).”
“The most disastrous flooding was caused by an unusually rapid rise in the Ashley River which broke
its banks in four or five places. Many acres of land were flooded, several hundred sheep were lost and
many farm houses were invaded, several occupants being rescued by boat. The river rose in twentyfour hours from a mere trickle to a flow of 50,000 cusecs [1416 m3.s-1] at the Rangiora Bridge. The
Main North Road was under water at the State Highway Bridge and traffic was stopped.”
The Press 19 March, 1941: “All rivers north from Christchurch in high flood after three days easterly
rain – the Ashley river broke its stopbanks in four or five places three of them close to the Main Road
Bridge.”
“Rapid rise of Ashley”: On the 18th March the river “was a torrent flowing at 50,000 cusecs [1416 m3.s1
] making the abnormally rapid rise of more than five feet within 24 hours. A further rise of three feet
brought it near the danger level at 2 p.m. at the Main Highway Bridge and not long before 3 p.m. the
water came over the top of the inland northern bank. Almost at the same time a flood gate on the
southern side about a quarter of a mile below the bridge gave way.”
“The river cut great gaps in the banks at these two points most of the water taking to the northern side
where a wide area of country was flooded so quickly that there was no time to remove stock. Here Mr.
James Wyllie lost 300 ewes and lambs. On the southern side Mr. John Wesley lost 80 sheep.” Mr.
Wyllie had to be rescued by boat from a haystack and “on the south bank at the same time an elderly
woman Miss M. Leggatt was rescued by boat from the upper storey of her home.” “Both had been
isolated by the flood for two and a half hours.”
30
Events were spectacular on the north side with the outflow in spate across the lower end of the bridge
approach. “Great stumps of trees swept down – the surface of the bridge approach tearing off in
sheets – a surging mass of yellow water with a few haystacks and trees above it.”
The flood was less spectacular on the southern side but “by 4 p.m. the water had backed up across
the road to Waikuku for a distance of about a mile. The lagoon at Waikuku began to rise flooding
some houses. A party of 20 men set to work cutting a channel through the sand spit between the
lagoon and the sea hoping that this would carry off some of the water when the tide fell.”
“With the main road impassable it was fortunate that the alternative route over the Rangiora Traffic
Bridge remained open. Here the Ashley rose to its highest level since the flood of 1936 but the banks
held. Slight flooding of the road on the Rangiora side of the bridge did not interrupt traffic.”
“At Fernside the Ashley came over the bank at 4 p.m. and Mr. Adam Smith reported that although he
had moved a large number of sheep to safety there were still some hundreds not accounted for. They
might, however, have reached higher ground on the far side of the flooded area.” This report
suggests the break-out was contained within the Northern Creek floodway, the flood thus
running between the stopbank and the low terrace cut along the upper edge of the Rangiora
Fan. Page 10 notes the effectiveness of this edge in the flood of 1868.
“Numerous wash-outs occurred on roads in Ashley County, all rivers and streams running practically
bank to bank and in some cases overflowing. At Loburn the Makerikeri river ran over its banks and
flooded the road to a depth of six inches. Rainfall figures at Loburn showed more than nine inches of
th
rain had fallen from Sunday night to 4 p.m. yesterday (Tuesday, March 18 ).”
“The Grey river broke its banks at Whiterock and traffic to Birch Hill was held up. The Rangiora-Oxford
Road via Loburn was closed when the Okuku River burst its banks and no traffic could get through to
Whiterock because of extensive damage to the Karetu Bridge. A small bridge on Beard’s Road Ashley
was washed away.”
The Press, 20 March, 1941: “The Ashley which caused most spectacular damage on Tuesday (18th)
subsided steadily during the day (19th) and by 8 p.m. was six feet below its peak level. A large volume
of water was still flowing through the break on the north side of the Main North Road Bridge but much
of the flood water on farmlands on the south side had got away.” “A report from Lees Valley in the
morning (19th) stated that after a short break in the weather torrential rain had again set in and another
rise was noticed in the Ashley.”
Waikuku Beach reported that “the channel, which was cut through the sand spit between the Lagoon
and the sea on Tuesday night (18th) worked successfully at the turn of the tide. The river scoured a
wide opening and the water which had banked up on Tuesday was drained away in an hour.”
“Few bridges in Ashley County escaped damage. Glentui Creek washed over its banks carrying away
part of the bridge. The approach to the Garry was swept away, the river scouring away 20 feet of soil
and shingle. The breakwaters of the Okuku river stood up well to a terrific pressure of water and
yesterday afternoon the river had dropped slightly. One pile of the Grey River Bridge collapsed. The
Karetu Bridge was wrecked and the Rydal Downs Bridge was washed away.”
>>> 16-18 August, 1941
SCRCC, 1957: “Southwesterly storm accompanied by rain, hail and snow swept the province and
caused some damage.” “The Ashley River was also in high flood discharging some 30,000 cusecs
[934 m3.s-1] at the Rangiora Bridge from 443 square miles. Some areas were inundated (the province
in general) but stock losses were mainly due to snowfall.”
>>> 9-10 September, 1943
SCRCC, 1957: “Heavy rain in North Canterbury caused slips on both roads and railways.” (3.05 in. in
24 hours at Waipara). “The Waimakariri and Ashley Rivers were in moderate flood but dropped fast.”
31
>>>20-22 February, 1945
SCRCC,1957: “A complex succession of weather systems commenced during the period 9 to 13
February culminating (for Canterbury) in a strong easterly flow of cold air undercutting the warmer and
moister northerly air mass to the north of the front. Heavy rainfalls were experienced in the centre of
the South Island during the night of the 20th as the depression moved across the Alps reaching the
Pegasus Bay area by the next morning. The rains continued and the winds veered to strong
southerlies during the afternoon of the 21st as the low-pressure centre re-curved east-north-eastwards
out of Pegasus Bay. By the morning of the 22nd weather conditions ameliorated and the continuous
heavy rain of the previous day was replaced by increasing fair periods. Ashley Gorge logged 7.99
inches of rain in the period 19th to 21st February.”
“East of the Southern Alps the zone of maximum rainfall extended from Glentunnel to Waimate with
the axis roughly parallel to the coast.” “In North Canterbury, the Ashley River broke its banks and
flooded an extensive area of country as well as blocking the Main North Road.”
The Press, 23 February, 1945: “One serious washout on the Main North Road where the Ashley River
broke its bank near Waikuku”. On the north side of the river “the Main North Road was covered
yesterday with flood water for a distance of several hundred yards reaching in places a depth of three
feet.” The flooding was caused by a break in the bank about 5 chains upstream of the Traffic Bridge.
“The road was impassable and a detour was necessary over the Rangiora Traffic Bridge.”
“The deputy-chairman of the Ashley River Trust (Mr. C. S. Ayers) said the flood had demonstrated that
an overflow was necessary where the river entered the bottleneck just before it reached the bridge.
Such an overflow would have prevented the break on the north side by acting as a safety valve. Work
with a bulldozer which had been carried out below the bridge had helped the water to keep its course
so that no breaks had occurred between the bridge and the sea.” Note: Mr. Ayers has identified
here the need for the works that are in the Waimakariri District Floodplain Management
Strategy related to secondary bank protection to the south of the river and to the north of
Rangiora.
“The Makerikeri bridge on the east side has been washed out for about 20 feet and a span of the
bridge has been carried away.” “At the Loburn Homestead 4.52 inches of rain was recorded from noon
th
nd
on Tuesday (20 ) until 9 a.m. yesterday (22 ). Around Rangiora the fall has been lighter.”
1946 – Administration of Protection Works passes to North Canterbury Catchment
Board
North Canterbury Catchment and Regional Water Board, 1982:
“The old banks (completed in 1938-9) generally had a small cross-sectional area built of ‘poorly
compacted silt material.’ As was only to be expected in a scheme of this type damage occurred during
floods in the years following completion. Bank failures occurred at several points on both sides of the
river and severe damage was sustained by several of the more exposed major groynes. A number of
the gabion groyne heads were completely lost. A proper appreciation of the finance required to
maintain the work had not been made and the result was that the limited money available was
completely used up in repairing flood damage.”
“The administration was taken over by the North Canterbury Catchment Board in 1946 and it was
soon realised that the original asset would continue to deteriorate unless a realistic approach was
made to finance the continuing work necessary to restore and maintain the scheme.”
32
Photo 6-8
February 1945 Flood
Surface flooding in High Street Rangiora caused by the heavy rain. A depression known as
Norfolk’s Gully originally entered the town from west of the Plough Inn and ran close to or
along parts of High Street down to and beyond the Lion Hotel. It was filled in during the last
century. Donated by R. Hassall to Rangiora and Districts Early Records Society.
Photo 6-9
February 1945 Flood
Surface flooding from heavy rain at the lower end of High Street
Rangiora looking east to the Main North Railway crossing.
The greater depth of floodwater seen here gives a better idea of
how the town would look if the river broke through down Ashley Street.
Donated by Mrs C. Tyler to Rangiora and Districts Early Records Society.
33
SCRCC, 1957: “Heavy rain which fell throughout the province did no damage apart from road
blockages in North Canterbury. At Ashley Gorge 2.6 in. of rain was recorded in twenty-four hours; 0.82
in. at Rangiora.”
1950 to 1974
The file of flood events beyond 1960 remains incomplete partly because the records of some floods
known to (or thought to) have taken place in the 1960’s and early 1970’s are either incomplete or
missing. No easily accessible documentation has been found for major flood protection works which
were commenced after the large floods of 1951 and 1953 other than photographic – this being
considerable.
One example of this is the re-instatement of the southern primary stopbank after the flooding of 1953.
The bank had been breached during the flood of 1951. An emergency bank was constructed south of
the primary bank parallel with it until close to the bridge where it angled sharply towards the bridge.
Prior to the 1953 breach a sharp north to south flow alignment had developed directly into the change
in angle.
Not surprisingly, the emergency bank breached at the change of angle in 1953.
The primary bank was re-instated with reinforced earth techniques and then heavily planted out on the
river side. It is proposed to strengthen and upgrade the emergency bank as far as Milton Avenue and
then construct an overlapping echelon bank from the eastern side of Milton Avenue to the southern
edge of the Railway Gap. The primary bank upstream of the Gap will be made sacrificial for a period.
Note: In this regard note should be taken of the North Canterbury Gazette observation in the
third paragraph of page 39.
Photo 6-10
34
‘Emergency Bank’ post 1951 (note the undesirable alignment of the main channel)
Photo 6-11
Reinstatement of the ‘Break-Bank’ post 1953 (Reinforced Earth Technique)
Photo 6-12
Reinstatement of the ‘Break-Bank’ post 1953 (Reinforced Earth Technique)
35
Photo 6-13
Protection Plantings on the riverside of the re-instated ‘Break-Bank’
Photo 6-14
Protection Plantings on the riverside of the re-instated ‘Break-Bank’
>>> 21 August, 1950
SCRCC, 1957: “Heavy rain in the hills caused a thaw in the snow and resulted in flooded rivers
particularly in South Canterbury. The Eyre River rose 6 ft. at Oxford and the Ashley by the same
amount at the Gorge.”
23-24 January, 1951
SCRCC, 1957: “Heavy rain in North Canterbury caused rivers to rise appreciably but no damage was
reported apart from crop losses which were serious. During the twenty-four hours 1.76 in. was
36
recorded in the Ashley State Forest and the Ashley River was in high flood running from bank to
bank.”
>>> 10 February, 1951
SCRCC, 1957: “Heavy rain caused serious flooding in parts of Canterbury. The Ashley River was
In…flood discharging 20,000 cusecs [570 m3.s-1] but the Cam and Cust Rivers were in high flood the
Cust peaking at 1,511 cusecs off 45 square miles of catchment.”
>>> 1 March, 1951
SCRCC, 1957: “Many rivers in the province rose appreciably following heavy rain which fell over two
days. At Rangiora, 2.28 in. fell in sixteen hours of which 1.32 in. fell in eight hours. No road or rail
blockages were reported. The Ashley River was in slight flood but was well contained within its banks.”
>>> 1-3 April, 1951
SCRCC, 1957: “Heavy rain along coastal areas of the province brought flooding to many areas. At
Rangiora, 1.77 in. fell in eight hours. There were, however, only small floods in the Cust, Ashley, Cam
and Halswell Rivers.”
>>> 17-19 April, 1951
SCRCC, 1957: “Following heavy falls of rain in both North and South Canterbury rivers rose in very
high flood and extensive and serious damage resulted.” Rainfall at Lees Valley amounted to 11.71
inches in forty-eight hours.” “Practically all the rivers in the province except the snow-fed Waimakariri
and Rakaia Rivers reached the top of their banks and there were few which did not overflow.” The
Ashley attained a peak discharge of “100,000 cusecs from 443 square miles” [2,830 m3.s-1 a record
high!]. Note: The SCRCC give the dates of this flood as 17-18 April but the Press account
(below) suggests the 18th to 19th are closer to the actual dates.
The Press, 18 April, 1951: “The rivers are rising quickly in many parts of Canterbury after heavy rain
yesterday and last evening. Serious flooding was not expected…but…with the heavy rain continuing
last evening it is likely there will be flooding today.” “More than two inches were recorded in the hills
about Loburn.” “From Woodend to Waipara the rain was exceptionally heavy from early yesterday
morning to late last evening when it was continuing.” “A strong easterly and later southeasterly wind
blew throughout the day and temperatures were cool.”
Star Sun, 19 April, 1951: Copied from MacDonald (1956): “the banks broke above the Main North
Road and water crossed the road in a wide arc towards Woodend where it banked up in a swamp
area.” “Water through the Latex Rubber Mill. Break at Rangiora Traffic Bridge 10 chains long. “Most of
the water is going back into the river through a breach near the Railway Bridge but the remainder is
flowing across country towards the Cam River.” “The danger from the swollen Cam was increased by
water from the Ashley.”
The Press, 19 April, 1951: “Wide areas of North Canterbury were flooded yesterday.” “From 9 a.m.
Tuesday to 6 p.m. yesterday nine inches of rain fell at Glentui on the Loburn-Ashley Gorge Highway
and two spans of the Glentui Bridge were washed away. The road at The Poplars has been badly
scoured and the cutting at Ashley Gorge is similarly affected. The settlers in Lees Valley are isolated,
five inches of rain having fallen there yesterday. One and a half chains of the approaches to
Gillespie’s Bridge in the valley itself have been washed away.”
“The road between Rangiora and the Ashley Traffic Bridge was covered by water in several places.
The Okuku, Karetu and Station Road Bridges were all badly damaged and were closed to traffic
yesterday afternoon. The damage would be already greater than in the 1945 flood.” “From 8 a.m. on
Tuesday until 4.30 p.m. yesterday 3.04 inches of rain fell in Rangiora. This brought the total up to
19.78 inches for the year.”
37
Photo 6-15
April 1951 Flood
This photo, of the peak flood of April 18th is part of a larger frame printed with the caption: “The
swollen Ashley River looking upstream from the Main North Road Bridge yesterday afternoon.
The trees prominent toward the left of the picture indicate the normal riverbank.” Christchurch
Star.
Photo 6-16
38
April 1951 Flood
“The Ashley River near the height of the flood.” This flood peak of approximately 2,800 m3.s-1
on April 18th. Christchurch Star.
“The Main North Road at the Ashley Traffic Bridge was made impassable by the river breaking its
banks on both sides last evening. The first break was on the north side about 6 p.m. and later in the
evening there was a further break on the south side. The homestead of Mr. J.H.L. Wyllie was
surrounded by water to a depth of three or four feet.”
“Members of the Soil Conservation and Rivers Control Council who were to have arrived in
Christchurch yesterday to inspect parts of the North Canterbury Catchment Board’s district in the next
three days have been stranded by the floods in South Canterbury.”
North Canterbury Gazette, 20 April, 1951: (Recopied from MacDonald (1956)): “Reports of overflows
above Fernside were advised but most of this water was re-entering the river further down.” “From
Bruere’s corner near the hospital to the Traffic Bridge was one sheet of water. A big lake quickly
formed in the parking area of the Showgrounds. The overflow poured south between the
Showgrounds and the railway eventually finding its way into the creek through Mr. J. Tutton’s property
on East Belt above the High School. From there it made its way into the Cam. The main body of water,
however, found its way safely back into the river at the Railway Embankment.” [Note: the designers
of the proposed Secondary Protection in this vicinity will need to carefully consider this
report.]
The Press, 20 April, 1951:
“Waikuku Beach isolated – Bread taken in by boat – One man walks out: Practically every river in
Canterbury fell yesterday but the Ashley River and Taranaki Creek rose yesterday morning and about
150 permanent residents of Waikuku Beach were in danger. Supplies were taken to them by rowing
boats.”
“On Wednesday night and yesterday morning the Ashley River breached its banks at three or four
places. On the south side of the river water flowed across the Main Road down to Waikuku Beach and
on to Woodend Beach. After high tide yesterday it was obvious that the floodwaters would not rise and
all residents remained at Woodend Beach. However, at Waikuku Beach the flood waters rose so high
that all exits from the beach were blocked and the only way out to the Main Road was by rowing boat.
After high tide the flood waters subsided gradually.”
“The only man to get out of Waikuku Beach yesterday morning was Mr. W. Muir. Mr. Muir’s home is on
a rise and when he went to bed on Wednesday evening there was no sign of flooding. Mr. Muir went
to his garage in King’s Avenue early yesterday morning and found that the water was well over the
floorboards of his car. Although his garage was flooded he thought he could get through to the Main
Road and he set out to walk. He headed for what little high ground lies between the beach and the
Main Road…and…found that all the high ground was also flooded and eventually went back to the
road. He had to wade about 220 yards through (fast flowing) water.”
“Across the Main South Road of the Ashley Bridge water was about 3 feet 6 inches deep for most of
the day and 3 houses on the Waikuku side of the road were flooded. ‘It was not as bad as the 1936
flood when the water was around the mantlepiece’ said Mrs. J. Sincock just before she waded through
several feet of water to the cottage.” Note: The three houses are most likely situated at the
intersection of Beach and Main North Roads where the same family (Sincock) is also
mentioned in The Press of 22 February, 1936.
“The Railway Bridge over the Ashley was intact yesterday but several piles were strained and the rails
twisted. Near the Rangiora Hospital there was a strong ‘tributary’ of the Ashley flowing across the road
and water covered the Rangiora Showgrounds.”
The Press, 5 May, 1951: Contains a report by H. W. Harris to North Canterbury Catchment Board
stating: “For a few hours on the morning of April 19th a small overflow occurred at the shingle pit near
the railway embankment and at the peak almost 150 cusecs got into the bend of the Cam and came
down through Kaiapoi.” “The water near the hospital had only one foot to rise to get into Rangiora via
Ashley Street.” (text, the same as copied by MacDonald, 1956).
39
>>> 29 August, 1952
SCRCC, 1957: “Heavy rain in North Canterbury brought flooding in some areas but no damage was
reported. The Ashley River was at a high level, the rainfall at Lees Valley being 1.6 in. in thirty hours
including 0.91 in. in seven hours.”
>>> 21 October, 1952
SCRCC, 1957: “Heavy rain throughout the province caused some minor flooding and stock losses
were reported. The Ashley and Cust were running high but they caused no damage.”
6-8 November, 1952
SCRCC, 1957: “Heavy rain over the plains brought serious floods to all parts of the province.”
“Rangiora experienced 1.82 inches in 24 hours.”
>>> 25-26 January, 1953
SCRCC, 1957: “As a result of heavy rain throughout the province widespread and serious flooding
occurred and all road and rail communications north and west of Christchurch were severed. In North
Canterbury the river most affected was the Ashley due to the abnormally heavy rain in the upper parts
of its catchment.”
“Rainfalls recorded at Ashley Gorge were as follows: 22 January, 2.00 in., 23 January, 0.13 in., 24
January, 2.65 in., 25 January, 1.85 in., 26 January, 0.10 in.; total 7.38 in. At Lees Valley, 5.36 in. of
rain was recorded on the 24th, 5.31 in. on the 25th and 1.93 in. on the 26th. At noon on the 25th the river
broke through the ‘emergency stopbank’ above the Rangiora traffic bridge and flowed towards
Rangiora, Kaiapoi, Woodend and Waikuku. The railway embankment also collapsed and allowed the
greater part of the river to flow through Coldstream towards Waikuku. Much water found its way into
the Waimakariri by way of the Cam. Many families in these areas were forced to abandon their homes
and considerable damage was done. Road traffic over the Ashley River was stopped for three days,
railway traffic for a week and telephones were out of action for several days. The peak discharge at
the Gorge was 33,000 cusecs [935 m3.s-1] from 201 square miles. Two culverts in Rangiora County
were washed away and many bridges were damaged.” Note: The flow was about 1,860 m3.s-1 at the
Rangiora Traffic Bridge.”
The Press, 26 January, 1953:
“Canterbury Rivers in High Flood – Ashley menaces Rangiora and Waikuku: North Canterbury rivers
and creeks in high flood after exceptional rain in the foothills cut railways and roads at several points
yesterday (Sunday 25th). The worst flooding was in the Ashley River which broke its banks near
Rangiora and at one stage threatened to invade the town. Late last evening flood waters were pouring
across country towards the Main North Road and Waikuku. In 24 hours 7.4 in. of rain fell in Lees
Valley in the Ashley River catchment.”
“Volunteers turn flood away: The Borough of Rangiora was threatened by one of the heaviest floods in
its history when the Ashley broke its banks shortly before midday. The Ashley overflowed an
‘emergency bank’ southwest of the traffic bridge and then carried the bank away. The RangioraLoburn highway was impassable at the Rangiora Traffic Bridge.”
“The flood was heavier than that of April, 1951, when floodwaters from the river submerged the
highway under three feet of water. Yesterday, half a mile of the highway on the Rangiora side was
under five feet of water and when the flood began to cover Ashley Street and move down towards the
Borough a call was made for volunteers to dam the street with sandbags. Trucks belonging to the
Rangiora Borough Council and North Canterbury Transport were used to cart sand from the Council’s
yard. The water was turned from the Borough towards the Coldstream area. The Ashley River which is
normally about 150 yards wide and carrying only a small amount of water was yesterday a roaring
torrent about half a mile wide.”
40
Note: Although the discharge was some 1000 m3.s-1 less than 1951, the effect was much more
marked in the vicinity of the ‘Break-Bank’, most probably because of the bad flow alignment
against the emergency bank.
“The home of Mr. G. Miles at Coldstream a mile from Rangiora was threatened by the flood about 2.30
p.m. Mr. Miles called the Rangiora police and a heavy high deck lorry was obtained from G. W.
Pearson and Sons to rescue the family and tow their car out to the Main North Road. Mr. Pearson also
rescued an elderly man a short distance from the Miles family. His house had been invaded by
floodwaters and he was on his bed when rescuers arrived. There are several houses in the
Coldstream area which are in the path of the flood and some concern is felt for the safety of the
occupants.”
Photo 6-17
January 1953 Flood
th
Rangiora-Loburn traffic bridge at the height of the flood on the 25 January, 1953. Donated by
Mrs C. Tyler to Rangiora and Districts Early Records Society.
41
Photo 6-18
January 1953 Flood
“Looking across Ashley at break in Railway embankment; Railway bridge at right centre.
Rangiora Borough at left. View looking southwest taken 26 January, ’53 between 1240 and 1340
hours.” Photograph by L. Ernie Clark for North Canterbury Catchment Board.
Photo 6-19
January 1953 Flood
An aerial view taken after the height of the flood showing extensive breaching of an area of
stopbank known euphemistically as the ‘Break-Bank’ upstream of Rangiora traffic bridge.
Ashley Bank Township is visible on the north bank; Ashley Street and the Recreational
Reserve remain inundated on the south. Rangiora And Districts Early Records Society.
42
Photo 6-20
January 1953 Flood
The flooded approach to Rangiora Traffic bridge looking northwest from Bruere’s Corner on
Ashley Street to where the road crosses the Recreational Reserve Rangiora and Districts Early
Records Society.
43
Photo 6-21
January 1953 Flood
Emergency sandbagging on Ashley Street Rangiora to divert flood water from the river
eastward into Coldstream Road. The photograph faces south into Rangiora – There is a fall of
6.35 metres (20.83 feet) from Bruere’s Corner down to High Street. Rangiora and Districts Early
Records Society.
“Large numbers of sheep were moved from areas adjacent to the Ashley yesterday afternoon.
Farmers in the Coldstream and lower areas have had newly-sown paddocks drowned, outstanding
crops flattened and hundreds of bales of hay still in the paddocks ruined.”
“Break in Embankment: Last evening (Sunday) the flood waters from a break in the railway
embankment five to seven chains long just south of the Ashley Railway Bridge were pouring out over
the Coldstream district and moving in the direction of Waikuku. By 10 p.m. there were three sheets of
water – one 15 chains long and a foot deep extending over the Main North Road at Waikuku
Township. There were also two stretches of water five and three chains long and about a foot deep at
Woodend near the church. By midnight the depth of the water below the Main North Road Bridge had
increased to three feet. An officer said that the main body of the overflow would pass by the twine
works at Little Ashley and on the Main North Road at Little Ashley Creek and Waikuku Creek which is
about half a mile south of the Main North Road Traffic Bridge.”
“The Ashley Gorge Bridge was unsafe yesterday morning. The river was lapping onto the deck of the
bridge and in the estimation of contractors of the North Canterbury Catchment Board it only needed a
heavy tree to come down the river to wash a span of the bridge away. On the Okuku River, a big
tributary of the Ashley a pier of the Okuku Bridge has been damaged. The level of the river was falling
last evening at the Gorge Bridge. At 6 p.m. it had dropped 18 inches.”
“Precautions at Beach: Broadcast warnings to farmers and residents of districts east of the Ashley
Railway Bridge where the washout occurred were heeded by residents at Waikuku Beach who
remembering the devastation caused by the 1951 flood lost no time in putting furniture and other
44
valuable equipment in a place where they would not be accessible to flood waters. Campers at the
beach were warned by the caretaker – and the majority of them packed and left.”
“The North Canterbury Catchment Board sent a bulldozer to the beach in the afternoon with
instructions to stand by. To facilitate the swifter flow of water should the flood waters reach the beach
the bulldozer was employed during the afternoon in cutting a wider outlet from the lagoon to the sea
and it is thought that this will aid considerably to drain the area should flooding occur.”
“At 9.30 p.m. the Ashley River was running very high and in one Waikuku beach resident’s opinion
was much higher than it was during the 1951 flood but up to that time the main flood waters were still
confined to the river near the beach. High tide at the beach was expected at 12.30 a.m. and it was
considered that from this time until the tide began to ebb would be the critical period. Few occupants
of baches left the beach.”
“Rail Services Disorganised – Washout at Annat and Ashley: Heavy flooding in the Ashley River has
pushed the Railway Bridge out of alignment by at least 4 in. and this together with a washout on the
approach to the bridge will prevent the running of normal railway services north of this point for several
days. The major washout at the approaches to the Ashley Bridge combined with the misalignment of
the bridge will render the stretch of line between Rangiora and Waipara unusable. The full extent of
the damage was not known at a late hour last evening.”
The Press, 27 January, 1953:
“Ashley Flows Over Farmlands – Fall of five feet from peak level – railways and roads still blocked:
Floodwaters from the Ashley river still flowed over North Canterbury farmlands yesterday but at the
Ashley Gorge Bridge the river had dropped 5 ft. 6 in. at 4 p.m. The river reached its peak level about
midday on Sunday (25th).”
“Water from the Ashley flowing east over the Coldstream district reached Waikuku Beach settlement
early yesterday morning but a cut to the sea made by a North Canterbury Catchment Board bulldozer
let the water away quickly to the sea and minimised flooding in the settlement. About four houses in
low areas had to be evacuated and the caretaker’s house close to the lagoon through which water
was pouring to the sea was also evacuated lest it should be undermined.”
“There has been flooding at Woodend Beach and some people have been evacuated it is reported but
the settlement could not be reached by telephone last evening. There was a foot of water in the store
at the beach yesterday. The road from the Main North Road to the beach is covered with water for
about a quarter of a mile and is impassable.”
“Floodwaters from the Ashley also moved toward Kairaki and The Pines yesterday afternoon but the
water had not crossed the road to the settlements last evening and unless a big volume has still to
come from Woodend the settlements are expected to be safe.”
“Floodwater from the Ashley caused the Cam to overflow its banks and with high tide in the afternoon
it was feared that the river might break its banks and flood the low-lying Camside area of Kaiapoi
Borough but the banks held and the river level later dropped.”
“Rail Services - Damage at Ashley Bridge – repairs likely to take some time: The repairs to the
Railway Bridge over the Ashley river at Rangiora are likely to take nearly a week according to the
District Civil Engineer of the Railways Department (Mr. O. J. Doidge). Investigations had been made
yesterday to see if there was any damage but no estimate had been made because of the high level of
the flood waters. If the water subsided it was possible that a line might be put through by the end of
the week Mr. Doidge said.”
“Road Damage – Both Coast Routes Impassable – highway under water at Waikuku: The Ministry of
Works advised yesterday afternoon that there was four feet of water over the Main North Road
opposite the flax mills at Waikuku. Taylor’s culvert near the Woodend church had collapsed and an
attempt to erect a temporary bridge was made last evening.”
45
“Telephone Lines Affected – difficulties at Rangiora: When 10 poles beside the railway line at Ashley
were swept away communications with districts north of the Ashley were cut off. The four Sefton lines
and three Amberley and Ashley Bank lines from the Rangiora telephone exchange were affected as
well as between 50 and 60 local subscribers.”
Photo 6-22
January 1953 Flood
“Flooding at Waikuku Beach and Waikuku emergency opening to sea centre foreground. View
looking west upstream (across Kings Avenue to SH1), taken on 26 January ’53 between 1240
and 1340 hours.” Photograph by L. Ernie Clark for North Canterbury Catchment Board.
Photo 6-23
46
January 1953 Flood
“Flooding at Woodend Beach and east of Woodend. Sea beach in foreground. View looking
west along Woodend Beach Road taken on 26 January ’53 between 1240 and 1340 hours.” Note
the course of the prominent floodway crossing Woodend Beach Road in the middle distance
by which water is fed from Coldstream. Photograph by L. Ernie Clark for North Canterbury
Catchment Board.
Photo 6-24
January 1953 Flood
“Looking across Ashley River at flooding between Waikuku Beach and Woodend Beach.
Emergency outlet to sea on left centre. View looking south taken on 26 January ’53 between
1240 and 1340 hours.” The coastal flood extends south beyond Woodend Beach towards The
Pines and Kairaki in the far distance. Note also the extent of floodwater still remaining in the
vicinity of Kings Avenue and the future Queens Avenue. Photograph by L. Ernie Clark for North
Canterbury Catchment Board.
“Assistance For Farmers – Requests Made To Ministers: Mr. W.H. Gillespie, M.P. for Hurunui, sent
telegrams to the Minister of Works (Mr. W.S. Goosman) and the Minister of Agriculture (Mr. K.J.
Holyoake) asking for the assistance of their departments to County Councils and farmers in the
affected areas – particularly in the Ashley area so that access could be given to people who had been
completely cut off.”
“Mr. Goosman said he was distressed to learn of the seriousness of the flooding. He had instructed
engineers of the Ministry of Works and the Main Highways Board to give every possible help. The
Ministry was doing its best to have a good supply of Bailey Bridges made available so that roads could
be opened and officers of the Soil Conservation Council were leaving Wellington last evening to make
an inspection of the damage.”
“Spent Night In Tree – Woodend Farmer’s Plight – rescue by rocket line: A Woodend man spent about
12 hours perched 14 feet up in a pine tree last night. He was Mr. C. Lynskey who lives about half a
mile on the Woodend side of the Rangiora-Woodend road.”
“Mr. Lynskey went out about 5.10 p.m. on Sunday to milk his cows. He said that normally when floods
came through the area there was a small stream of water between his house and the milking sheds
but after he had been milking for about 20 minutes on Sunday a wave of water five feet high bore
down on the shed. The force of the water was so great that it thrust open the doors of the shed and
47
swept out the eight cows that were inside at the time. Then in six feet of water Mr. Lynskey made his
way half swimming to a fence line near the shed and along the fence line to a pine tree about three
chains from the shed. He climbed up about 14 feet into the tree and there he perched (overnight) until
he was rescued about 12 hours later around 6 a.m. yesterday.”
“Captain J. Duncan the Lyttelton Harbour Board’s second pilot played a key role in Mr. Lynskey’s
rescue. Armed with the Harbour Board’s line-throwing Schumley rocket gun Captain Duncan was
rushed by truck to Woodend. Then from a point within 150 yards of the tree to which Mr. Lynskey was
clinging Captain Duncan set up the gun and fired a shot. The first shot carried the line into the top of
the tree. Mr. Lynskey climbed up and fastened the rope to the trunk. A boat was then hauled up along
to the tree by means of the rope and Mr Lynskey was taken aboard. During his long vigil Mr. Lynskey
saw a number of dead animals and a neighbour’s cocksfoot crop float by. It was moonlight at the
time.”
Note: Enquiries made to the Lynskey family indicate their former relative’s dairy farm was located
close to or within the floodway running between Coldstream and Woodend.”
“Ashley River Flooding – Rangiora Saved by Break at Bridge – water enters Waikuku homes:
Rangiora was saved from serious flooding when the Ashley river broke through the approach to the
Railway Bridge and spilled out over the country towards Coldstream and Waikuku on Sunday. The
approach to the bridge is built across a former bed of the river. It is five chains long and yesterday the
railway line was suspended across this area. ”Note: This is where the additional secondary
protection is to be constructed.
Photo 6-25
January 1953 Flood
The collapsed section of Main North Line embankment which leads to the Railway Bridge over
the Ashley. The view is from the south (Maria Andrews Park) end of the break taken at the
height of overflow coming across from the lower end of the Showgrounds (compare this with
the aerial view, Photo 6-18).
48
“A number of residents in the Waikuku district have had their homes badly flooded. Water six inches
deep ran through the home of Mr. G. Goodwin and in the yard it was waist deep. The home of Mr. R.
E. Gillman was flooded by six or seven inches of water and out in the paddocks it was waist deep. As
Waikuku is almost wholly given over to dairying few losses of sheep were reported. Motor vehicles
have been severely damaged by the flood. Garages on properties in the Coldstream-Waikuku areas
were invaded by deep water and the insides of the vehicles have been covered by mud and silt.”
“On the Woodend road a mile east of Rangiora the Rangiora County Council was yesterday repairing
the culvert over the Cam Creek which has been torn apart by the force of the flood waters. The
concrete sides of the culvert have been split apart and a huge cavity gouged in the road. Intermittent
rain was still falling in the Ashley catchment last evening.”
“Flood at Waikuku Not Serious - water runs into sea: Though isolated, Waikuku Beach settlement did
not suffer serious flooding. The North Canterbury Catchment Board sent a bulldozer to the beach on
Sunday (25th) and this was used to cut a channel to the sea. This channel which was opened early
yesterday morning allowed a big part of the floodwaters to get away quickly and greatly lessened
flooding in the settlement.”
“Some water got into the oval at the settlement but the camping ground was high and dry said Mrs. N.
M. Harper (storekeeper at Waikuku Beach). Four homes had to be evacuated in the lower areas of the
beach settlement and the caretaker (of the camping ground) – Mr. C. Hurse had to leave his home at
the side of the lagoon in case it should be undermined by waters making their way to the sea. These
waters are taking some land with them. The settlement bridge has been badly damaged and is
impassable. The settlement has accordingly been cut in two.”
“The Press, 28 January, 1953:
“Rail Service: Too much water was flowing in the Ashley to make a concerted start on the breach in
the approach to the bridge said the District Civil Engineer to the department (Mr. O. J. Doidge)
yesterday afternoon. Although a thorough inspection had not been made he did not think the bridge
had been seriously damaged.”
“Bailey Bridge at Woodend: The Ashley overflow has crossed the Main North Road at three main
culverts which have been inundated Mr. Grant (Assistant District Engineer to Ministry of Works) said
last evening. The first over the Waihora creek near the Woodend church which had been washed out
was spanned yesterday with 60 feet of Bailey bridging.”
“The Taranaki creek culvert nearer to Waikuku has survived the flood with minor damage. Little Ashley
the third main culvert had 3 ft. 7 in. of water over it last evening and has been reported as being badly
damaged and only a small section remains safe. The culvert must be considered unsafe for traffic Mr.
Grant said. The water was reported as falling steadily last evening.”
“Aerial Survey of Floods – Engineer of Rivers Control Council – Ashley by far the worst: The Ashley
River was by far the worst that I saw said Mr T.H. Nevins engineer to the Soil Conservation and Rivers
Control Council in Christchurch last evening after he had returned from a flight over the flooded areas
in an aircraft of the Royal New Zealand Air Force. There was still a steady flow coming through the
break above the Rangiora Road Bridge. That was where most of the water was coming out of the
river.”
49
Photo 6-26
January 1953 Flood
Scouring by the flood through Cam Corner on Highway 72 Woodend to Rangiora (vicinity of L.
H. Leech’s property). Donated by Mrs C. Tyler to Rangiora and Districts Early Records Society.
50
“The North Canterbury Catchment Board reported last evening that all rivers were falling. Some of the
floodwaters which are still flowing from above the Rangiora Traffic Bridge across the Ashley through
the gap in the railway embankment towards Waikuku were reported to be returning to the river through
a 20-yard gap which has been eroded by the floodwaters about three miles downstream near L. Kerr’s
property.” (Tulls Road)
“Shingle Pit Destroyed – Caught by Ashley River: The shingle crushing plant of North Canterbury
Transport Ltd. which was situated beside the Ashley River a mile from Rangiora has been almost
completely destroyed by the floods. At present there is more water flowing across country than there is
in the Ashley riverbed and this enabled two employees of the company stationed at Sefton to walk
from the Main North Road to the bridge on the Loburn-Rangiora Highway to see what had become of
the plant. Only the bins have been left standing. A sizeable shed was washed away and there is no
sign of a petrol pump which was installed by the shed.”
“When the river was rising on Sunday morning employees of the company moved a bulldozer and a
dragline to safety and while four trucks spent an hour shifting gear from the site of the plant two men
stood on the bank above the bridge to give warning of any danger as the bank above the bridge was
expected to break at any time. When the river burst its banks the area half a mile wide was covered in
10 minutes.”
“Army To Help Farmers – Clearing up after Floods – ‘an enormous task’: ‘A tremendous number of
farms are almost devastated’ said Mr. W. H. Gillespie M.P. after a tour yesterday by horse and lorry of
the flooded Ashley area. He flew over the area later.”
“The cleaning-up would be an enormous task said Mr. Gillespie. Through the Minister of Defence (Mr.
T. L. Macdonald) he had obtained an assurance from Southern Military District that groups of soldiers
would be available to help farmers clear their properties. I am grateful to the army officials for their cooperation and look forward to their doing a good job of work.”
“Mr. Gillespie said he had been endeavouring to arrange within the Ministry of Works for a Bailey
Bridge to be sent to the Ashley Gorge to bridge the 30-foot gap on the Glentui side of the Gorge
Bridge. The end of the bridge was resting on some rotten rock and if this subsided the gap would be
increased to 80 feet.” A temporary footbridge was to be erected over the gap. “Still raining in the Lees
Valley area at midday yesterday and up to that time 12.81 inches of rain had been recorded in the
valley.”
“Air Lift Across Ashley – Newspapers, Bread, and Mail: Three Tiger Moth aircraft belonging to Airwork
N.Z., Ltd. were chartered yesterday to fly bread, ‘The Press’ and mail to residents on the north side of
the Ashley river. A total of 2000 loaves were handled as well as mail. It was estimated that the three
aircraft in use had made nearly 60 flights.”
51
Photo 6-27
January 1953 Flood
Scoured approach to the southern end of the Rangiora Traffic Bridge after the flood had
subsided. Donated by Mrs C. Tyler to Rangiora and Districts Early Records Society.
Photo 6-28
January 1953 Flood
Fine weather returns and the sandbagged diversion from Ashley Street into Coldstream Road
is left high and dry. Donated by Mrs C. Tyler to Rangiora and Districts Early Records Society.
52
“Bread for Waikuku: residents received their first supplies of bread yesterday since last Friday.
Supplies were taken from Rangiora to the Waikuku Beach Road where boats were used to take them
to the settlement. The caretaker’s house at the beach was reported to be in a precarious position.
Flood waters are receding although there is heavy flooding in the lagoon and the bridge is not being
used. The steel pipeline across the lagoon has been cracked and is threatening the supplies of fresh
water to residents on the south side. Yesterday morning North Cantebury Transport Ltd. took wire
down to the Waikuku Beach Road. The wire was used to tie boughs to the side of the badly-scoured
road to the beach.”
>>> 28 March, 1953
SCRCC, 1957: “As a result of southeasterly rain in North Canterbury, rivers and streams rose in flood.
The heaviest falls of rain reported were in the foothills the highest figure being at Ashley Gorge where
5.20 in. fell in twenty-four hours. For the same period 4.50 in. fell at Lees Valley and 4.30 in. at
Glentui. The Ashley River rose more than 10 ft. at the Gorge Bridge but although high it caused no
serious flooding.”
The Press, 30 March, 1953:
“Ashley rises more than 10 feet – erosion at Waikuku Beach: Rivers were in fresh after widespread
southeasterly rain during the weekend. No major damage has been reported but the swift-flowing
waters of the Ashley caused some erosion at the northern end of the Waikuku Domain near the river
mouth. Employees of the North Canterbury Catchment Board and voluntary workers from the beach
settlement yesterday carried out emergency protection near the end of the Café road.”
“The flood was at its peak at the Rangiora Highway Bridge about 8 a.m. yesterday. By 9 a.m. the rain
had eased to a light drizzle in Lees Valley and the Ashley River had fallen 2 ft. In the lower reaches
the level had dropped 4 ft. by 5 p.m. yesterday. In Lees Valley the river was still running fairly high but
the level was falling. All three bridges in the valley were impassable because of scoured approaches
but the hill road was intact.”
>>>3 October, 1956
The Press, 4 October, 1956:
“Heavy Rain Causes Flooding – Ashley Railway Bridge Damaged:
Heavy rain which continued to fall over most of Canterbury yesterday caused rivers to rise. Flood
waters closed the Oxford-Loburn Road and Lees Valley has been isolated by slips since Monday. All
rivers in Canterbury which are now snow-fed were affected by the rain yesterday. The Ashley, Eyre,
Cust, Selwyn, Waiau, Kowai and Hawkins rivers were all in moderate flood. Floods reached their
peaks in the rivers early yesterday morning between 4 p.m. and 6 p.m.”
“A pile driver working from the tilting Ashley Railway Bridge was expected to start late last night on
strengthening piers which subsided early in the morning through the flood in the river. Although the
river was dropping in the afternoon it was not possible to assess the damage but it is hoped to have
railway traffic restored late on Friday or at the weekend.”
“The Oxford-Loburn Road which runs through the Ashley Gorge was also impassable all day. A creek
on the Loburn side of the Gorge Bridge and about a mile from it rose quickly in the morning and more
than 2 ft. of water covered the road by 9 a.m. Big slips and shingle slides have isolated Lees Valley.”
53
Photo 6-29
October 1956 Flood
Newsprint caption reads: “Subsidence in the Main North Line Railway Bridge over the Ashley
River. Floodwaters carrying with them tree stumps and other debris caused heavy scouring
around one of the bridge piles resulting in a pier collapsing. The bridge has been closed
indefinitely. Christchurch Star.
54
The Press, 5 October, 1956: “The Picton-Christchurch railway line which was blocked at the Ashley
River when a pier subsided on Wednesday is expected to be open to traffic at 10 a.m. today.” “At the
Rail Bridge the most difficult part of the work - driving the first of three new piles – was completed early
yesterday afternoon. Railway workers finished driving the third pile about 10 p.m. after working under
floodlights.”
“Two piles had been completely washed away and a third was broken. A large amount of debris which
had collected around the bridge caused one of the major difficulties in the work. From the top of the
bridge and the riverbanks workmen used power winches to clear away tree trunks and logs from
around the piers.”
“The road to Lees Valley is now clear but because the approaches to two bridges over the Ashley
River in the valley itself have been damaged all the farms are still isolated. The river has broken out in
so many places that the road is not expected to be reopened before Monday or Tuesday.”
>>>12-13 May, 1961
The Press, 13 May, 1961:
“Rivers rise swiftly: Main West Coast Road closed: Two days of heavy rain (in the high country)
caused rivers in South and North Canterbury to rise quickly yesterday (Friday 12th). In most areas
there were reports of minor flooding as tributaries of main rivers overflowed on to roads. The Ashley,
Selwyn and Eyre rivers were all in light flood yesterday afternoon. The Ashley river rose 9 ft. above
normal at the Ashley Gorge Bridge. When the last major flood occurred in Canterbury the Ashley rose
14 ft. above normal. In the catchment area of these rivers the rainfall was about 2 in. yesterday and it
was still raining heavily last evening.” Note: The 1961 flood on the Ashley was computed at 1,100
m3.s-1.
The Press, 15 May, 1961: “Heavy rain in North Canterbury and Marlborough since Friday had caused
surface flooding and disrupted rail services between Christchurch and Kaikoura a Railways
Department train control official said yesterday afternoon. Railway gangs worked all yesterday to
repair flood damage to the Ashley River Bridge just north of Rangiora but it is not expected to be open
before noon today. Repair work on the bridge will continue from daylight today.”
The Press 16 May 1961: “The Ashley River Railway Bridge was opened at 6.30 p.m. yesterday after
men had worked long hours repairing the approaches.”
“Floodwaters of the Ashley River severely damaged two groynes, numbers 5 and 21, the Chief
Engineer of the North Canterbury Catchment Board (Mr. H.M. Reid) said yesterday. He said the
Ashley had been in the highest flood for some years and when the river was low enough to be
inspected more damage could be found.”
>>>15-16 July, 1963
(Partly researched)
The Press, 18 July, 1963:
“Repairing Flood Damage – Gangs Busy on Roads and Railways: The peaks past Canterbury’s
flooded rivers dropped rapidly yesterday and repair gangs started work on damaged railway lines
roads and bridge approaches.”
“The Cust river had held a high level but dropped rapidly yesterday. The level of the Ashley River also
dropped quickly. At Ashley Gorge 8.76 in. of rain fell in four days. Water drained rapidly off the Sefton
Swamp yesterday leaving a few hundred acres still under water. A lot of water still lies on Stevenson’s
Swamp and the Flaxton Swamp just west of Kaiapoi, but the Catchment Board says this is usual after
heavy rain.”
55
“N. Canterbury took ‘hammering: No major damage was reported from the Kowai, Rangiora and Eyre
Counties and the flatter sections of Oxford. Lees Valley road is still impassable but it is hoped to have
the road open by Saturday. In Ashley County the washed-out approach to the Makerikeri was in use
again yesterday. Much of the surface flooding in Eyre and Rangiora Counties receded quickly
yesterday morning with drains and creeks discharging through floodgates when the water levels in the
main drains dropped after the turn of the tide.”
>>> 1965
Not researched.
>>> 1972
Not researched.
>>> 3-5 September, 1974
(Partly researched)
The Press, 5 September, 1974: “Road and rail traffic has been disrupted and hundreds of acres of
farmland submerged as a result of the heavy rain that has fallen over Canterbury during the last two
days.” Photographs in The Press show River Road submerged and the electricity substation at
Southbrook surrounded by floodwater.
“Some of the most extensive flooding was between Leithfield and Saltwater Creek where water 3 ft. to
4 ft. deep covered hundreds of acres. In the Rangiora, Flaxton and Swannanoa districts farmland was
covered with sheets of water. Water flowed across roads and paddocks and one farmer used bales of
hay to protect his house from the water.”
“The temporary bridge over Stoney Creek between Ashley and Sefton was washed away. The road
from Rangiora to the Ashley-Loburn bridge was impassable when a depression in the road filled with
water. Heavy traffic was able to use a shingle road to the bridge.”
The Press, 6 September, 1974: “After only four days Rangiora’s rainfall has already set a record for
September since records were started 57 years ago. By yesterday morning 166.4 mm (6.55 in.) had
fallen.”
“The 6 in. main pipeline in the Ashley County’s rural reticulated stock and domestic water supply
scheme has been washed out at the Stoney Creek and South Kowai River crossings cutting the
supply to the north of the County.”
“Several bridges in Ashley County have been washed out or undermined by the now receding
floodwaters. In the Loburn district the main road to Loburn and White Rock was closed yesterday
when the Makerikeri River Bridge had its approach undermined. Motorists are able to detour via Carrs
and Station Roads.”
3.3.6 1975 to 2009
Since 1974, there have been numerous discharges of the order of 500 to 600 m3.s-1 and four
significant floods. The first, on 11 August, 1986, tested the stopbanks with a flow of 1407 m3.s-1 and
the system almost failed on the right bank of the river downstream from State Highway 1 Traffic
Bridge. A Civil Defence emergency was declared and Waikuku Beach evacuated. More recently,
stopbanks were again severely tested when there was a serious flood of 1890 m3.s-1 on 23 December
1993, a further flood of 1540 m3.s-1 on 26 July 1994, 1740 m3.s-1 on 19 August, 2000 and, finally, 1530
m3.s-1 on the 13th January, 2002. This flood stripped away the berm (a precursor to bank failure)
immediately to the west of the ‘Break-Bank’.
56
Photo 6-30
August 1986 Flood
“Fisherman’s hut near the mouth of the Ashley River.”
Christchurch Star.
Photo 6-31
August 1986 Flood
“Gumboot brigade at Waikuku school.” Christchurch Star.
57
Photo 6-32
August 1986 Flood
Horses rescued at Saltwater Creek. Christchurch Star.”
58
Map 6-1
Historic outflows
59
7
Previous schemes
(Ashley River Floodplain Management Regional Plan - Technical Investigation)
(Note: The following is embedded throughout the previous Section.)
During the latter part of the nineteenth and the first quarter of the twentieth century protection works
throughout Canterbury were ‘piecemeal’ and aimed at protecting some assets perhaps at the expense
of others.
Serious flooding problems persisted and eventually the Ashley River Improvement Act was passed in
1925. This Act and subsequent amendments concerned the safe passage of Ashley flows without
flooding of the surrounding land. The Act constituted the Ashley River Trust which was empowered to
carry out works to protect riverbanks from erosion, prevent or mitigate flood damage and extend river
works at sites known to be susceptible to overflows.
3 -1
Stopbanks for a scheme to carry 2000 m .s bank-full were designed and constructed by the Public
Works Department during the 1930’s. The construction was by hand labour and wheelbarrows. These
banks were ‘mauled’ over-topped and seriously damaged by floods in 1936, 1951 and 1953.
The Soil Conservation and River Control Act 1941 resulted in the Ashley River Trust being dissolved
in 1947 and its powers, functions, assets and liabilities transferred to the North Canterbury Catchment
Board. That Board strengthened and upgraded the protection system so that the system was aimed at
passing 2400 m3.s-1 safely to the sea with 600mm freeboard.
The groynes were strengthened between 1955 and 1966 and the stopbanks after that along with
protection planting infill.
7.1
Population, land-use, and future development
(Ashley River Floodplain Management Regional Plan, Technical Investigation)
(Note: This is of interest because the floodable areas are vastly more populated now than
when some of the historical break-outs occurred. Also because it is possible to site new
development away from the historic flood paths.)
The existing population of the Waimakariri District is of the order of 46,000 and it is expected that
this will increase to around 64,000 by the year 2031. A significant portion of this existing
population and most of the projected increase in population will live on the floodplain.
A significant factor on the past and projected growth of population on the floodplain is its proximity
to Christchurch.
The development of the Northern Motorway from Christchurch to Kaiapoi (and beyond) opened in
1968 provides rapid access to the city for commuters. This road link has been a major factor in the
rapid population growth of Kaiapoi and Rangiora over the past 40 years and will contribute to the
projected increase over the next decades.
The lower floodplain is densely populated with a significant urban population while the upper
floodplain is sparsely populated and rural.
Kaiapoi and Rangiora are important service centres for the surrounding rural community and are
expanding apace.
Woodend is being further developed, Waikuku Village is experiencing Rural-Residential
development, Tuahiwi’s already subdivided lots are being built on, Pegasus Bay Township is
being developed and the beach settlements of Kairaki, The Pines, Waikuku Beach and Woodend
are popular recreational and holiday settlements with increasing permanent population in recent
years.
60
The rural activities include concentrated dairying, horticultural and market gardening activities in
the east and in the west, dry-land sheep farming and forestry.
In 2002 grassland (mostly dairying) represented the dominant land use seaward of Rangiora.
Arable crop land and horticulture made up about 8% of the land use with forestry at 10% and
extensive grazing at 23%.
More concentrated subdivision is occurring on the floodplain.
The projected future development pattern for the floodplain is for further urban growth and more
subdivision of the rural land.
7.2
Perception of risk and public participation
As a general rule the floodplain residents perceive themselves to be relatively safe from flooding.
This is because floodwaters from the Ashley have been contained since 1953.
There was extensive consultation culminating in the adoption of the Waimakariri District Floodplain
Management Strategy. This investigation is part of the implementation of that strategy.
7.3
Structural integrity of the existing system
The objective of the current protection system is to pass 2400 m3.s-1 safely to the sea without
overflow or outflow.
This is achieved by minimising channel bank erosion which could threaten the structural integrity
of the stopbanks and contribute to the supply of sediment which could lead to aggradation and
consequent reduction in flood capacity in the lower reaches of the river. Minimising bank and berm
erosion also contributes to improved water quality.
Figure 7-1 below illustrates the shape and form of the protection works.
61
Figure 7-1
Elements and functions of protection works
The prime protection mechanisms are the stopbanks which are the outer limits of the suite of
measures which make up the structural system. Inside the stopbanks there is a central gravel bed
channel or fairway defined by control lines. This central channel is kept clear of planting and
vegetation. It is here that the bulk of the high velocity floodwaters are carried.
Flanking the central channel are berms. Two significant protection mechanisms are employed on
the berms - protection plantings and groynes. Their purpose is to deflect river flows into the central
channel and to keep them away from the primary stopbanks. The protection plantings which are
between the groynes and also where there are no groynes are aimed at slowing water flows and
62
developing a cushion of slow moving water which along with the plantings protects the stopbanks
from the raging main channel flows. The concept is akin to the method of ‘fighting fire with fire’.
However, braided river systems such as the Ashley are by nature dynamic and with continuing
attacks from floodwaters and continuous changes of the braided patterns areas of weakness
develop and the protection system becomes vulnerable at various places and at various times.
The maintenance programme monitors and remedies any undesirable effects of these processes.
The structural strength of the existing works was the subject of a sub-investigation of the 1995
project and as with the other component parts of the overall study there was a separate report
describing the investigation and the results.
Four main conclusions were drawn:




8
The stopbanks are designed and built to a uniform standard but they are not designed to
withstand overtopping by floodwaters.
The greatest risk of failure of the stopbank system is from erosion and such a failure could
3 -1
occur at a flood significantly less than the design flood (2400 m .s ).
Stopbank erosion is dependent not only on discharge but also on other variables whose
influence may be paramount.
It is not possible to predict the exact point location of erosion failure although constant
vigilance and a pro-active maintenance programme can lessen the probabilities of erosion
failure.
Hydrology
During the early 1990’s there was much analysis of the hydrology of the Ashley River.
A value of 4030 m3.s-1 was the estimated 100 year return period flood which was considerably in
excess of the previously estimated 2400 m3.s-1.
The estimate was updated to 3500 m3.s-1 in 2001. Because of continuing uncertainties, as part of
this investigation, a further review was commissioned which has once more lowered the 100 year
assessment slightly to 3250 m3.s-1.
8.1
July 2009 review
The review report concluded:
“Comparison of the flood frequency estimates in Table 5 shows that for return periods greater than
five years the largest values are given by regional analysis whereas the at site and composite
values agree within the 95% confidence limits. (The at-site and regional values would probably
also agree within these limits if they were calculable in the regional case). The probable reason for
this is that the group data demonstrate EV2 rather than EV1 or Gumbel behaviour – a feature
noted previously by Connell and Pearson (2001) in their analysis of the flood behaviour of
Canterbury Basins.
For a conservative estimate of say the 100 year flood at the Rangiora Traffic Bridge it is
3 -1
3 -1
recommended that the regional estimate of 3250 m .s (the actual figure was 3228 m .s . It has
3 -1
been rounded to 3250 m .s which is the recommended new design flow) be employed:
otherwise the at site value of 2500 ± 830 m3.s-1 should be used.
63
Table 8.1
Flood frequency estimates (Griffith, 2009)
Return
period
(T) (yrs)
Annual
exceedance
probability
AEP
Regional Flood
Estimate
(m3/s)
2
0.5
538
5
0.2
1083
10
0.1
1526
20
0.05
1997
50
0.02
2668
100
0.01
3228
200
0.005
3825
500
0.002
4685
1000
0.001
5380
Discussion
Over the 3 revisits the Regional Estimate has lowered consistently. Because the length of record is
still short the recommended figure for use at this time is 3250 m3.s-1. It is of interest to note that 1500
m3.s-1 has been exceeded 4 times in the 18 years of record and that the 10 year at site analysis
estimate is even slightly lower than that.
9
Sedimentation
The rise and fall of the shingle bed of an alluvial fan over the years is termed aggradation and
degradation.
Where there is a sufficient natural supply of gravel alluvial fan rivers have a tendency to aggrade
which can be exacerbated where the fan building process is confined to a narrow strip of the fan by
the protection works.
However, degradation can occur if the river sluices more material through the system than is being
supplied or, importantly, if the gravel extraction rate exceeds the supply rate.
If the river is aggrading then the design standard of the scheme is being eroded as shingle replaces
available waterway area. If the river is degrading then the structural stability of the banks and groynes
are endangered due to undermining. A common misconception is that degradation is a good thing in
that in entrenching itself the river does away with the need for banks. What then happens is that the
system reverts to a natural alluvial fan behaviour with natural avulsion from the degraded channel and
then begins to roam back and forth across the fan at random.
Sections 12-20 analyse the available survey data for trends in aggradation and degradation. In
summary although there has been some aggradation on the berms the Ashley River has generally
degraded in recent years. The management of gravel extraction will need to balance flood risk
objectives against an increased risk of undermining of the banks, berms and stopbanks.
Sediment budgets are calculated between surveys in Section 23. Even accounting for reported gravel
extraction there has been a net deficit in gravel supply in recent years, a reversal of earlier trends
where a net surplus of sediment was offset by gravel extraction. It is not clear where the degraded
material has gone. The sandy nature of the mouth and the beaches near the mouth are inconsistent
with large volumes being passed to the sea. It is possible that there has been somewhat of an
interruption in the gravel supply due to congestion by willows and other vegetation upstream of the
64
Okuku confluence (in the Ashley River between the Okuku confluence and the Ashley Gorge and in
the tributaries such as the Okuku), however, this does not explain the large difference between
reported shingle extraction and the volumes that have disappeared from the system since the mid
1990’s.
Photos 9-1 and 9-2 show the Ashley River near the mouth, showing the normal situation where the
river mouth is dominated by sand. However, there is evidence of gravel reaching the coast
occasionally, for example, during the major floods in the early 1950’s (Little, 1991, p49). Other reports
(Single, 2005) and anecdotal evidence suggest it has occasionally reached the coast. Cockle beds
near the coast were reported as being covered by gravel in the early 2000’s (G Scholes, pers.comm).
The position of the river mouth influences the slope and therefore the sediment transport and
depositional characteristics of the lower river. There have been periods (including 1980-1993) when, in
the absence of major floods to breach the spit, the mouth has migrated several hundreds of metres
northwards. It is likely that there is greater deposition in the lower reaches of the river (particularly
downstream of the monitored cross-sections) during these periods, although the extent of this
deposition has not been explored.
Photo 9-1
Ashley River mouth, low tide 9/09/09
65
Photo 9-2
Ashley River near the mouth, (low tide) 9/09/09
Photos 9-3 and 9-4 show the same reaches of the Ashley River and the Okuku River around their
confluence in 1977 and 2001 respectively. There is a marked increase in the density of vegetation
within the active fairway of both rivers upstream of the confluence – the active fairway has effectively
been narrowed and more substantial islands have formed. This is not the case downstream of the
confluence, where the Rating District maintains the fairway clear of woody vegetation.
66
Photo 9-3
Ashley River and Okuku River confluence, 1977
67
Photo 9-4
68
Ashley River and Okuku River confluence, 2001
10 Climate change
“Preparing for climate change: A guide for local government in New Zealand (MfE, 2008) summarises
the main elements of relevant technical reports including “Climate Change Effects and Impacts
Assessment” produced by MfE following the release of the Fourth Assessment Report of the
Intergovernmental Panel on Climate Change (IPCC) in 2007.
It includes summaries of climate change predictions, for example, that there is moderate confidence
the frequency of an extreme rainfall event of a given intensity will lie within the range of 1 to 2 times as
frequent as current rainfall intensities by 2040 implying that floods of a given size could become about
twice as frequent as they are now. However, there are significant regional and seasonal differences
predicted within the range of estimates.
On the Ashley River, floods are largest when the catchment is affected by easterly tending weather
patterns.
3 -1
The Ashley River stopbanks were built to contain up to a certain size flood (2400 m .s ). The report
indicates the frequency of the design flood will probably increase over time. The mid-range estimates
for temperature change in Canterbury is for an increase of about 1 degree by 2040 and 2 degrees by
2090 although the spring estimates are slightly less. Low frequency extreme rainfall events are
expected to increase by about 8% per degree warming (“preliminary assessment scenario”). Assuming
that an increase in extreme rainfall intensities corresponds to a similarly proportional increase in flood
flow the mid-range estimates are for Ashley flood flows to increase by about 8% by 2040 and 16% by
2090. (The model range corresponds to increases of about 2% to 16% by 2040 and 7% to 40% by
2090).
More frequent events are not predicted to increase by as much as extreme events. For example, the
preliminary assessment scenario indicates mean annual rainfall intensity will increase by about half as
much as the 100-year rainfall intensity or about 4% per degree warming.
Most bedload sediment is transported by smaller, frequent events which are not predicted to increase
much so that the pattern of supply of gravel from upstream in the next 10 to 20 years should be much
the same as in the past.
The issues being considered here are for the medium term (0-10 years) management of the bed level
of the river. The proposed bed levels will need to be reviewed in the future. The appropriate flood
capacity of the stopbank system will also need reviews from time to time, although this can be a
separate exercise. The natural variation in sedimentation rates, gravel extraction rates and the
acceptable variation in bed levels is high enough that management of the river bed level is not likely to
be greatly affected by climate change over the 10-20 years being considered here.
In the long term the gravel extraction rate will need to approximate the sedimentation rate or
other solutions to aggradation and degradation found.
69
11 Survey comparisons
Sections 12 to 20 compare the surveyed levels at a series of cross-sections which have been
repeated several times since 1960/62. In most cases comparisons have been made with 1960/62
surveyed levels because this gives the longest view but, where relevant, comparisons have been
made with the 1976 survey because the stopbanks were last redesigned around that time. Some
comparisons have also been between the 2009 survey and the next most recent full cross-section
survey in 1997.
Other surveys not included here include a full survey in 1986/88, partial surveys in 2001 and 2005 and
a LIDAR survey also in 2005. The LIDAR survey is an aerial survey that covers the whole bed (not just
the cross-sections) but does not include the levels below the water surface. While the data gives
excellent coverage and includes areas between the standard cross-sections there is no simple and
accurate way to adjust for levels below the water surface and compare to previous surveys.
The cross-section locations are shown in Map 11.1.
70
Map 11-1
Location of cross sections
71
12 Hydraulics
As noted previously, the original scheme designed in the 1930’s was designed to provide stopbanks
that would carry 2000 m3.s-1 with no freeboard. These banks were overtopped and seriously damaged
by floods in 1936, 1951 and 1953.The stopbanks were strengthened in 1968 and 1973 to a level
approximately two feet (600mm) above the levels that occurred in the floods of 1951 and 1953.
The present scheme was designed in 1976 to carry 2400 m3.s-1 which was estimated at that time to be
the 100 year discharge. The design stopbank heights provided a minimum of 600 mm freeboard
above this design flood level. In the event the existing bank heights were generally higher than the
design bank height so were retained. However, more recent estimates (Griffiths, 2009) suggest the
frequency of this event is likely to be greater than was thought at the time perhaps with an average
recurrence interval of around 40-50 years.
Because there is only limited site data (18 years of record at the Rangiora Traffic Bridge) there is still
considerable uncertainty about the likelihood of occurrence of flood events of a certain size particularly
of larger floods. Griffiths (2009): “For a conservative estimate of say the 100 year flood at the Rangiora
Traffic Bridge, it is recommended that the regional estimate of 3250 m3/s (rounded up) be employed:
otherwise the at site value of 2500 ± 830 (m3/s) should be used”.
In 2007, a computer simulation was carried out of the river system from the mouth up-river some 21
kilometres. The model was calibrated using the 1997 cross-sections against the observed cross
section flood levels and the discharges of the December, 1993, flood (1950 m3.s-1) and the 1986 flood
(1406 m3.s-1). The model was run for 2400 m3.s-1, 3000 m3.s-1, and 3500 m3.s-1. The results are shown
in Table 12.1 below, with freeboards less than 600mm in bold.
Worthy of note is the fact that for much of its length the river system designed at the last re-visit for
2400 m3.s-1 had a standard of 3000 m3.s-1 plus, because of the decision to adopt the level of the
existing banks as the design bank level where it was higher than the 2400 m3.s-1 flood level and also
because of targeted gravel extraction.
72
Table 12.1
Modelled flood levels and freeboards (1997 cross-sections)
Ashley River Bank Levels & Flood Levels (1997 sections)
No Breakout Scenario (bed resistance 0.03, Cone Rd & SH1 bridges modelled)
(Modelled Tony Oliver, August 2007)
X/Section
Mike11
Chainage
0.30
0.80
1.61
2.41
2.96
3.22
4.02
4.83
5.63
6.44
7.24
8.05
8.85
9.66
10.46
11.27
12.07
12.87
13.68
14.48
15.29
16.09
16.90
17.70
19.31
20.92
99700
99200
98390
97590
97040
96780
95980
95170
94370
93560
92760
91950
91150
90340
89540
88730
87930
87130
86320
85520
84710
83910
83100
82300
80690
79080
Left Bank
Right
Bank
2400
3 -1
m .s
2400
3 -1
m .s
Minimum
Freeboard
3000
3 -1
m .s
3000
3 -1
m .s
Minimum
Freeboard
3470
3 -1
m .s
3470
3 -1
m .s
Minimum
Freeboard
n/a
6.10
7.91
9.32
10.00
12.32
14.65
17.50
19.37
22.32
25.23
28.48
31.95
35.11
40.61
44.71
49.41
56.44
54.78
68.77
73.91
75.94
78.49
n/a
89.13
4.09
6.04
7.58
9.53
10.25
12.20
14.87
17.41
19.32
22.49
25.52
28.56
32.17
43.35
38.92
43.19
45.95
50.58
54.03
58.10
62.69
67.13
72.41
79.71
88.51
3.04
5.40
7.24
8.78
9.39
11.02
13.28
16.16
18.61
21.20
23.94
27.23
30.51
34.04
37.64
40.93
44.92
48.90
52.99
57.21
61.55
65.70
70.05
79.00
87.26
1.05
0.64
0.34
0.54
0.61
1.18
1.37
1.25
0.71
1.12
1.29
1.25
1.44
1.07
1.28
2.26
1.03
1.68
1.04
0.89
1.14
1.43
2.36
0.71
1.25
3.22
5.73
7.63
9.15
9.78
11.39
13.60
16.48
18.97
21.48
24.17
27.44
30.72
34.27
37.92
41.25
45.11
49.08
53.16
57.39
61.73
65.85
70.21
79.18
87.45
0.87
0.31
-0.05
0.17
0.22
0.81
1.05
0.93
0.35
0.84
1.06
1.04
1.23
0.84
1.00
1.94
0.84
1.50
0.87
0.71
0.96
1.28
2.20
0.53
1.06
3.35
5.96
7.90
9.43
10.07
11.66
13.84
16.72
19.23
21.68
24.33
27.59
30.87
34.44
38.11
41.47
45.24
49.21
53.28
57.52
61.85
65.97
70.32
79.30
87.58
0.74
0.08
-0.32
-0.11
-0.07
0.54
0.81
0.69
0.09
0.64
0.90
0.89
1.08
0.67
0.81
1.72
0.71
1.37
0.75
0.58
0.84
1.16
2.09
0.41
0.93
In 1997, the freeboard was less than design at two cross-sections in the 2400 m3.s-1 event, 6 crosssections for the 3000 m3.s-1 event and 8 cross-sections for the 3470 m3.s-1 event. Overtopping of
stopbanks was almost certain in events over about 2900 m3.s-1.
For this re-visit the model was re-run with the 2008/09 cross-sections. Results are tabulated in Table
12.2 below.
73
Table 12.2
Modelled flood levels and freeboards (2008/09 cross-sections)
Ashley River Bank Levels & Flood Levels (2008/09 Sections)
No Breakout Scenario (bed resistance 0.03, Cone Rd & SH1 bridges modelled)
(Modelled by Tony Boyle, August 2009)
X/Section
Mike 11
Chainage
0.30
0.80
1.61
2.41
dssh1
2.96
3.22
4.02
4.83
5.63
6.44
7.24
8.05
8.85
9.66
10.46
11.27
dsrdbr
rdbr
rdbr
usrdbr
12.07
12.87
13.68
14.48
15.29
16.09
16.90
17.70
19.31
20.92
99700
99200
98390
97590
97050
97040
96780
95980
95170
94370
93560
92760
91950
91150
90340
89540
88730
88510
88502
84498
88490
87930
87130
86320
85520
84710
83910
83100
82300
80690
79080
Left Bank
Right
Bank
2400
3 -1
m .s
2400
3 -1
m .s
Minimum
Freeboard
3000
3 -1
m .s
3000
3 -1
m .s
Minimum
Freeboard
3500
3 -1
m .s
3500
3 -1
m .s
Minimum
Freeboard
n/a
6.10
7.91
4.09
6.04
7.58
0.89
0.99
0.54
44.71
49.41
56.44
54.78
68.77
73.91
75.94
78.49
n/a
89.13
43.19
45.95
50.58
54.03
58.10
62.69
67.13
72.41
79.71
88.51
3.55
5.65
7.71
8.84
8.97
9.62
11.63
13.81
16.66
19.28
21.69
24.03
27.44
30.76
34.41
38.02
38.84
38.86
39.41
39.47
41.24
44.79
48.83
52.99
57.43
61.84
65.93
70.29
79.07
87.87
0.54
0.39
-0.13
9.53
10.25
12.20
14.87
17.41
19.32
22.49
25.52
28.56
32.17
43.35
38.92
3.40
5.38
7.42
8.52
8.66
9.31
11.34
13.55
16.41
19.01
21.46
23.84
27.28
30.60
34.22
37.80
38.60
38.62
39.00
39.06
40.99
44.60
48.65
52.85
57.30
61.71
65.81
70.17
78.93
87.71
0.69
0.66
0.16
9.32
10.00
12.32
14.65
17.50
19.37
22.32
25.23
28.48
31.95
35.11
40.61
3.20
5.05
7.04
8.13
8.24
8.91
10.99
13.23
16.10
18.68
21.18
23.60
27.08
30.38
33.98
37.53
38.36
38.37
38.52
38.58
40.67
44.35
48.43
52.68
57.13
61.54
65.64
70.00
78.76
87.49
1.08
1.09
1.21
1.42
1.31
0.64
1.14
1.63
1.40
1.57
1.13
1.39
2.52
1.60
2.15
1.35
0.97
1.15
1.49
2.41
0.95
1.02
0.66
0.69
0.86
1.10
1.00
0.31
0.86
1.39
1.20
1.35
0.89
1.12
2.20
1.35
1.93
1.18
0.80
0.98
1.32
2.24
0.78
0.80
0.35
0.38
0.57
0.84
0.75
0.04
0.63
1.20
1.04
1.19
0.70
0.90
1.95
1.16
1.75
1.04
0.67
0.85
1.20
2.12
0.64
0.64
In the latest model, the freeboard is marginally less than design at one cross-section in the 2400 m3.s-1
event and less than design at 2 cross-sections in the 3000 m3.s-1 event and 7 cross-sections in the
3500 m3.s-1 event. Overtopping of stopbanks is almost certain in events over about 3200 m3.s-1.
The most dramatic differences between the two models include:




74
A reduction in flood levels of about 0.2m in the critical area around 2.4km where the flood
capacity was most constrained in 1997.
A reduction in flood levels of about 0.4m at the SH1 bridge (2.96-3.22km) where mean bed
levels have dropped by about 1.0m (see Section 13).
A reduction in flood levels of 0.2-0.4m upstream of the Rangiora Traffic Bridge (12.07 to
14.48km) where bed levels have dropped by around 0.3-0.8.m.
A rise in flood levels of about 0.3m at the Okuku confluence (20.92km) where bed levels rose
by a similar amount.
13 Mean fairway levels
From Reid, H.M.: Dick, R.D. 1960: Waimakariri River Improvement Scheme 1960:
“In the first place it should be made clear what is meant by mean bed level. It is obvious that at
any one cross-section the bed of the Waimakariri [Ashley] is not uniform in level across the
width of the river. The difference between the highest and lowest points can be as much as 18
feet. What then can be taken as bed level at any particular mileage on the river? The mean bed
level is a horizontal straight line at such a level that there is as much bed above it as there is
below it. It is as though a bulldozer worked on a straight line across the river pushing the high
spots into the hollows until it graded a perfectly horizontal surface. This surface would be
equivalent to mean or average bed level. If the bulldozer repeated the process after there had
been aggradation another surface would be obtained at a higher level. The difference in level of
these two surfaces would be the increase in bed level.”
A distinction is also made between the fairway and the berms which together make up the full width of
the river. The mean fairway is typically lower than the mean berm levels and, therefore, also lower
than the mean level of the whole river. The edge of the fairway is generally clearly defined by a bank
or vegetation change.
Table 13.1
Location
Waikuku
SH1
Toppings
Road
Tulls Rd
Smart Rd
Marchmont
Rd
Rail Br
Rangiora T
Br
Mean fairway levels
Distance
(km)
0.80
1.61
2.41
2.96
3.22
4.02
4.83
Mean Fairway Level (MFL) (m)
MFL Change (m) since
1960/62
MFL Change (m)
since 1976
MFL
Change
(m) since
1997
1960/62
1976
1997
2008/
09
1976
1997
2008/
09
1997
2008/
09
2008/09
1.08
1.97
4.33
5.41
5.69
8.66
1.04
2.36
4.35
5.47
5.79
8.49
1.26
2.64
4.28
5.88
5.70
8.41
1.11
2.20
4.16
4.89
5.67
8.51
-0.04
0.39
0.02
0.06
0.11
-0.17
0.18
0.67
-0.05
0.47
0.01
-0.25
0.03
0.23
-0.18
-0.52
-0.01
-0.15
0.22
0.28
-0.08
0.41
-0.09
-0.07
0.07
-0.16
-0.20
-0.59
-0.12
0.02
-0.15
-0.44
-0.12
-1.00
-0.02
0.10
10.68
10.70
10.75
10.67
0.02
0.07
-0.01
0.05
-0.03
-0.08
5.63
6.44
7.24
8.05
13.93
16.27
19.21
13.68
16.27
19.40
13.68
15.93
19.26
13.54
16.11
19.21
-0.25
0.00
0.20
-0.25
-0.34
0.05
-0.40
-0.16
0.00
0.00
-0.34
-0.15
-0.14
-0.16
-0.19
-0.15
0.18
-0.05
22.30
22.38
22.32
21.84
0.08
0.01
-0.47
-0.06
-0.54
-0.48
8.85
9.66
10.46
11.27
25.89
29.41
32.73
25.88
29.11
32.37
25.55
28.77
32.19
25.49
28.52
32.12
0.00
-0.30
-0.35
-0.33
-0.64
-0.53
-0.39
-0.89
-0.61
-0.33
-0.34
-0.18
-0.39
-0.59
-0.26
-0.06
-0.25
-0.07
36.31
36.03
35.50
35.29
-0.28
-0.81
-1.01
-0.53
-0.74
-0.20
39.92
43.76
39.54
43.51
39.31
43.10
39.05
42.27
-0.38
-0.26
-0.61
-0.66
-0.86
-1.49
-0.23
-0.40
-0.49
-1.24
-0.25
-0.83
48.15
47.97
47.53
47.01
-0.17
-0.62
-1.13
-0.45
-0.96
-0.52
51.76
55.93
59.98
51.48
55.98
60.18
51.81
55.95
60.34
51.33
55.85
60.34
-0.28
0.05
0.21
0.05
0.02
0.36
-0.43
-0.08
0.36
0.33
-0.03
0.15
-0.15
-0.13
0.15
-0.48
-0.10
0.00
64.58
64.62
64.57
64.38
0.05
0.00
-0.20
-0.05
-0.25
-0.20
68.82
77.54
68.75
77.48
68.83
77.73
68.77
77.35
-0.07
-0.06
0.01
0.19
-0.05
-0.19
0.08
0.25
0.02
-0.14
-0.06
-0.38
86.33
86.25
85.84
86.09
-0.08
-0.50
-0.25
-0.42
-0.16
0.25
-0.06
0.04
-0.25
-0.03
-0.14
0.05
-0.60
0.02
-0.35
-0.15
-0.91
-0.12
-0.08
0.02
-0.35
0.05
-0.29
-0.18
-0.67
-0.09
-0.21
-0.20
-0.31
-0.14
12.07
Groyne 1
12.87
Lehmans
13.68
Rd
Groyne 2
14.48
Airstrip
15.29
Groyne 3
16.09
Dalzeills
16.90
Rd
Groyne 6
17.70
Groyne 8
19.31
Okuku
20.92
Confluence
Average
Average 0 to 8.05
Average 8.85 to 13.68
75
Mean fairway level changes since 1960/62
1.0
Change in level (m)
0.5
0.0
1976
1997
2008/09
-0.5
-1.0
-1.5
-2.0
0
5
10
15
20
River distance (km)
Figure 13-1




76
Mean fairway level changes since 1960/62
There was a modest average drop in mean fairway levels over the period 1960/62 to 1997 at a
fairly uniform rate. Degradation has accelerated in the 1997 to 2009 period.
The drop has been more pronounced from Marchmont Road to Lehmans Road (8 to 14 km)
with an average drop of 0.9 m.
Only 3 of 25 cross-sections have recorded a rise in mean fairway level between 1960/62 and
2008/09.
The stopbanks were redesigned following the 1976 survey. As at the 2008/09 survey only 3 of
the 25 cross-sections had recorded a rise in mean fairway level since 1976.
14 Invert levels
The channel invert level is the lowest part of the channel across a given cross section.
Table 14.1
Location
Waikuku
SH1
Invert levels
Distance
(km)
Lowest Channel Level (m)
Lowest Channel Level Change (m)
since 1960/62
1960/62
1976
1997
2008/09
1976
0.80
-0.54
-0.48
-0.96
-0.44
0.06
-0.42
0.10
0.53
1.61
0.84
1.22
0.63
0.29
0.38
-0.21
-0.56
-0.35
1.95
3.07
2.62
-0.75
-0.45
4.10
2.71
2.41
2.70
2.96
3.80
3.22
3.97
3.76
4.48
3.57
4.02
6.69
6.78
6.84
9.55
8.97
9.75
1997
2008/09
Lowest
Channel
Level
Change
(m) since
1997
2008/09
0.38
-0.07
0.30
-1.09
-1.39
-0.21
0.51
-0.40
-0.91
6.69
0.09
0.15
0.00
-0.16
9.03
-0.58
0.20
-0.52
-0.72
Toppings
Road
4.83
5.63
11.60
12.02
12.54
11.10
0.42
0.94
-0.50
-1.44
Tulls Rd
6.44
14.29
15.48
14.57
14.06
1.19
0.28
-0.22
-0.51
Smart Rd
7.24
17.78
18.14
17.91
16.08
0.36
0.13
-1.70
-1.82
Marchmont
Rd
8.05
20.81
20.71
20.74
19.98
-0.09
-0.07
-0.83
-0.76
8.85
24.69
24.87
24.24
24.06
0.18
-0.45
-0.63
-0.18
9.66
28.14
27.83
27.53
26.50
-0.31
-0.61
-1.64
-1.03
Rail Br
10.46
31.40
31.12
30.77
29.82
-0.28
-0.63
-1.58
-0.95
RTB
11.27
34.58
34.42
33.91
33.86
-0.16
-0.67
-0.72
-0.05
12.07
38.60
38.16
37.45
37.34
-0.44
-1.15
-1.26
-0.11
Groyne 1
12.87
42.38
42.19
41.97
41.31
-0.18
-0.41
-1.07
-0.66
Lehmans
Rd
13.68
46.30
46.09
45.80
44.99
-0.20
-0.50
-1.30
-0.81
-0.80
Groyne 2
14.48
49.88
50.46
50.14
49.34
0.58
0.26
-0.54
Airstrip
15.29
54.30
54.18
54.32
54.47
-0.12
0.02
0.16
0.15
Groyne 3
16.09
57.40
58.92
58.79
57.94
1.52
1.39
0.54
-0.85
Dalziells
Rd
16.90
62.55
63.19
62.81
62.90
0.65
0.27
0.35
0.09
Groyne 6
17.70
67.24
67.36
67.08
66.38
0.13
-0.16
-0.86
-0.70
Groyne 8
19.31
76.26
75.65
76.40
75.41
-0.61
0.14
-0.85
-0.99
Okuku
Confl.
20.92
83.23
83.86
84.06
83.41
0.63
0.83
0.18
-0.65
Average
0.09
0.02
-0.60
-0.62
Average 0 to 8.05km
0.09
0.20
-0.53
-0.73
Average 8.85 to 13.68km
-0.20
-0.63
-1.17
-0.54
0.40
0.39
-0.14
-0.54
77
Invert level changes since 1960/62
2.0
1.5
Change in level (m)
1.0
0.5
1976
1997
2008/09
0.0
-0.5
-1.0
-1.5
-2.0
0
5
10
15
20
River distance (km)
Figure 14-1




78
Invert level changes since 1960/62
To 1997 the invert rose an average of 0.2-0.4m between the coast and Marchmont Rd (08km) and Groyne 2 to the Okuku confluence (14-21km). This trend has reversed in the 19972009 period with average falls of about 0.6m in these areas.
The invert has fallen an average of about 1.2m between Marchmont Rd and Lehmans Rd
(8.5-14km).
The trends are consistent with the changes in mean fairway levels.
The largest drop was 1.7m at 7.24km near Smarts Rd. Similar incision has occurred at 9.66
and 10.46km (Rail bridge) with noticeable incision at several other locations including at the
bridges.
15 Channel widths
From time to time fairway widths are redefined to account for changes in the position of the fairway
and/or banks. The following widths have been used in the calculation of the fairway and berm levels.
Table 15.1
Fairway, berm and total channel widths
Location
Distance (km)
Waikuku
Fairway Width
(metres)
Berm Width (metres)
Left
Right
Total Width
(metres)
0.80
398.8
21.5
276.6
696.9
1.61
195.1
143.0
126.4
464.5
2.41
231.8
70.3
74.1
376.2
2.96
231.6
11.1
97.9
340.6
3.22
136.3
96.4
131.3
364.0
4.02
309.1
85.9
63.1
458.1
4.83
211.9
187.6
201.0
600.5
5.63
202.4
141.3
97.0
440.6
Tulls Rd
6.44
231.6
139.6
109.4
480.6
Smart Rd
7.24
413.7
47.0
101.2
561.9
Marchmont Rd
8.05
476.0
285.2
163.3
924.5
8.85
422.9
337.8
181.4
942.1
9.66
361.0
353.2
190.4
904.6
Rail Br
10.46
359.3
200.9
54.9
615.0
RTB
11.27
275.1
164.1
35.9
475.1
12.07
519.6
26.0
69.6
615.2
Groyne 1
12.87
287.6
351.5
86.5
725.6
Lehmans Rd
13.68
468.3
501.8
262.2
1232.3
Groyne 2
14.48
459.4
432.7
354.8
1246.9
Airstrip
15.29
577.3
257.7
67.6
902.6
Groyne 3
16.09
585.6
243.8
213.9
1043.3
Dalziells Rd
16.90
500.5
577.9
150.0
1228.4
Groyne 6
17.70
467.0
549.3
144.5
1160.8
Groyne 8
19.31
485.5
366.7
245.2
1097.4
Okuku Confl
20.92
454.9
279.3
106.9
841.1
Average
370
235
144
750
Average 0 to 8.05km
276
112
131
519
385
276
126
787
504
387
183
1074
SH1
Toppings Rd
79
Fairway and berm widths
1400
1200
Width (m)
1000
Fairway width
Left berm width
Right berm width
Total width
800
600
400
200
0
0
5
10
15
20
River distance (km)
Figure 15-1
80
Fairway and berm widths

The channel narrows towards the coast in terms of fairway width, berm width and total width.

The narrowest berms are near the bridges where the fairway widths are also less than
average (bold in Table 15.1).
16 Mean berm level comparisons
Table 16.1
Location
Mean berm level comparisons
Distance
(km)
Mean Berm Level (m) 1960/62
Mean Berm Level (m) 2008/09
Change in Berm Level (m)
1960/62 to 2008/09
Left
Berm
Right
Berm
Difference
Left
Berm
Right Berm
Difference
Left
Right
Weighted
average
0.80
1.74
0.83
-0.91
1.87
1.66
-0.20
0.13
0.84
0.79
1.61
2.92
2.88
-0.05
3.30
3.21
-0.09
0.38
0.33
0.36
2.41
4.84
5.09
0.25
5.1
5.36
0.18
0.33
0.27
0.30
2.96
6.48
6.84
0.36
6.67
6.83
0.16
0.19
-0.01
0.01
3.22
7.39
7.72
0.33
7.45
7.67
0.21
0.07
-0.05
0.00
4.02
8.27
10.08
1.80
8.98
10.08
1.10
0.71
0.00
0.41
Toppings
Rd
4.83
11.75
11.85
0.10
11.84
11.93
0.09
0.09
0.08
0.09
5.63
13.12
14.37
1.25
14.00
14.68
0.68
0.88
0.31
0.65
Tulls Rd
6.44
16.55
16.00
-0.55
16.75
16.97
0.22
0.20
0.97
0.54
Smart Rd
7.24
19.30
19.66
0.36
19.44
20.02
0.59
0.14
0.36
0.29
Marchmo
nt Rd
8.05
22.52
22.89
0.37
22.53
23.14
0.61
0.00
0.24
0.09
8.85
26.18
26.40
0.22
26.34
26.38
0.03
0.16
-0.02
0.10
9.66
29.45
29.89
0.44
29.58
30.04
0.45
0.13
0.15
0.14
Rail Br
10.46
33.61
33.59
-0.02
33.61
33.60
-0.01
0.00
0.02
0.01
RTB
11.27
36.94
37.76
0.82
36.76
37.60
0.84
-0.18
-0.16
-0.18
Waikuku
SH1
12.07
39.69
40.43
0.74
39.91
40.50
0.59
0.23
0.07
0.11
Groyne 1
12.87
43.95
44.39
0.45
43.91
44.47
0.55
-0.03
0.07
-0.01
Lehmans
Rd
13.68
48.38
47.88
-0.50
48.71
47.93
-0.77
0.33
0.05
0.23
Groyne 2
14.48
52.01
52.53
0.52
51.98
52.16
0.18
-0.03
-0.37
-0.18
Airstrip
15.29
55.78
56.71
0.93
56.15
56.59
0.45
0.37
-0.12
0.27
Groyne 3
16.09
60.28
60.66
0.37
60.21
60.72
0.51
-0.07
0.06
-0.01
Dalziells
Rd
16.90
64.71
65.13
0.42
64.64
65.13
0.48
-0.06
0.00
-0.05
Groyne 6
17.70
69.14
68.62
-0.52
69.21
69.02
-0.19
0.07
0.40
0.14
Groyne 8
19.31
78.00
77.71
-0.29
77.92
78.03
0.12
-0.08
0.32
0.08
Okuku
Confl
20.92
86.92
85.97
-0.95
86.98
85.96
-1.02
0.07
-0.01
0.05
Average (absolute)
0.54
0.41
0.17
Average 0 to 8.05km (abs)
0.57
0.38
0.32
Average 8.85 to 13.68km (abs)
0.46
0.46
0.06
Average 14.48 to 20.92km (abs)
0.57
0.42
0.04
81
Mean berm level changes 1960/62 to 2008/09
1.2
1.0
Change in level (m)
0.8
0.6
0.4
Left berm
Right berm
0.2
0.0
-0.2
-0.4
-0.6
0
5
10
15
20
River distance (km)
Figure 16-1





Mean berm level changes
On average, mean right berm levels are higher than mean left berm levels, even in the 1 to
7 km reach where berm widths are similar.
The berms have risen on average 0.17 m between 1960/62 and 2008/09.
Not shown in the table, the berm levels in 1997 were very similar to the 2008/09 levels – there
was no significant berm aggradation in the period, perhaps partly due to the increased fairway
capacity resulting from degradation of the fairway.
The rises are more pronounced in the lower reaches (downstream of Marchmont Rd, 8 km)
particularly at 5.63, 6.44 and 0.80 km, where the average rises are up to 0.79 m. Aggradation
of the right berm at 0.80 km has been encouraged since the floods of the 1950’s threatened
the stopbanks in this area.
Although left and right berm levels are quite different at some locations the average difference
in mean berm levels is small and has slowly reduced.
Aggradation on the berms is generally likely to be silt and sand sized sediment rather than gravel
because of the slower velocities within the vegetated berms, particularly in the lower reaches of
the river.
82
17 Fairway capacity relative to mean berm levels
The 2 year flood flow is estimated to be approximately 600 m3.s-1 (although the recorded median
annual flood flow is somewhat less than that). A model run was completed so that comparisons of
relative fairway capacity can be made by comparing the water level to the mean berm level on each
side of the river. A negative “berm level minus water level” means that that berm is likely to be flooded
in modest events.
Table 17.1
Relative fairway capacity
Location
Distance
(km)
Flow level (m)
3 -1
at 600 m .s
Min mean berm
level (m)
Berm level
minus
water level
(m)
Ratio of min bermfairway level to 600
3 -1
m .s flow depth
Waikuku
0.80
2.35
1.66
-0.69
0.45
low
1.61
3.52
3.21
-0.31
0.76
medium
2.41
5.46
5.1
-0.29
0.78
medium
DsSH1
Relative fairway
capacity
6.46
SH1
2.96
6.47
6.67
0.20
1.13
medium-high
3.22
7.14
7.45
0.31
1.21
medium-high
4.02
9.53
8.98
-0.55
0.46
low
11.84
0.00
1.00
medium
11.84
Toppings Rd
4.83
5.63
14.73
14.00
-0.73
0.39
low
Tulls Rd
6.44
17.29
16.75
-0.54
0.54
low
Smart Rd
7.24
19.99
19.44
-0.55
0.29
low
Marchmont Rd
8.05
22.12
22.53
0.41
2.42
very high
8.85
26.03
26.34
0.31
1.58
high
9.66
29.29
29.58
0.29
1.38
high
Rail Br
10.46
32.87
33.60
0.73
1.97
very high
RTB
11.27
36.29
36.76
0.47
1.47
high
very high
Dsrdbr
36.97
Rdbr
36.98
Rdbr
37.06
Usrdbr
37.09
12.07
39.43
39.91
0.48
2.28
Groyne 1
12.87
43.17
43.91
0.73
1.80
very high
Lehmans Rd
13.68
47.39
47.93
0.54
2.44
very high
high
Groyne 2
14.48
51.80
51.98
0.18
1.39
Airstrip
15.29
56.32
56.15
-0.17
0.63
low
Groyne 3
16.09
60.79
60.21
-0.58
-0.28
very low
Dalziells Rd
16.90
64.94
64.64
-0.30
0.47
low
Groyne 6
17.70
69.24
69.02
-0.22
0.54
low
Groyne 8
19.31
77.82
77.92
0.10
1.20
medium-high
Okuku Confl
20.92
86.51
85.96
-0.55
-0.31
very low
Average
-0.03
Average 0 to 8.05km
-0.25
0.51
-0.22


On average, the fairway capacity before overflow onto the berms is of the order of 600 m3.s-1.
The capacity is significantly higher in the 8 to 14km reach, indicating that erosive forces may
be higher in that reach in larger events because the fairway will contain a high proportion of
the flow.
83

Overflow onto the berms is most likely to occur in some isolated parts of the upper reaches
where berms are lower than the mean fairway level.
18 Mean berm level minus mean fairway level
comparisons
Table 18.1
Location
Mean berm level minus mean fairway level comparisons
Distance
(km)
Waikuku
SH1
Difference between
mean berm and mean
fairway level 1960/62
(m)
Difference between
mean berm and mean
fairway level 1997 (m)
Difference between
mean berm and mean
fairway level 2008/09 (m)
Average change
1960/62 to
2008/09
Left
Right
Left
Right
Left
Right
0.80
0.66
-0.25
0.59
0.37
0.76
0.55
0.45
1.61
0.95
0.91
0.59
0.84
1.10
1.01
0.12
0.48
2.41
0.51
0.76
0.89
1.02
1.02
1.20
2.96
1.07
1.43
0.85
1.05
1.78
1.94
0.61
3.22
1.70
2.03
1.70
1.99
1.78
1.99
0.02
4.02
-0.39
1.41
0.45
1.71
0.47
1.57
0.51
Toppings
Rd
4.83
1.07
1.17
1.10
1.28
1.17
1.26
0.10
5.63
-0.82
0.43
0.36
0.91
0.46
1.14
0.99
Tulls Rd
6.44
0.28
-0.26
0.88
1.00
0.64
0.86
0.74
Smart Rd
7.24
0.09
0.46
0.26
0.68
0.22
0.81
0.24
Marchmont
Rd
8.05
0.22
0.59
0.21
0.80
0.69
1.30
0.59
8.85
0.29
0.51
0.79
0.89
0.85
0.88
0.46
9.66
0.04
0.48
0.82
1.23
1.06
1.52
1.03
Rail Br
10.46
0.88
0.86
1.41
1.54
1.49
1.49
0.62
RTB
11.27
0.64
1.46
1.29
2.20
1.47
2.31
0.84
12.07
-0.23
0.51
0.63
1.30
0.86
1.45
1.01
Groyne 1
12.87
0.18
0.63
0.87
1.43
1.64
2.20
1.51
Lehmans
Rd
13.68
0.23
-0.27
1.15
0.40
1.69
0.92
1.32
Groyne 2
14.48
0.25
0.77
0.17
0.41
0.66
0.84
0.24
Airstrip
15.29
-0.15
0.79
0.17
0.71
0.30
0.75
0.20
Groyne 3
16.09
0.31
0.68
-0.10
0.44
-0.13
0.39
-0.36
Dalziells
Rd
16.90
0.13
0.55
0.06
0.46
0.27
0.75
0.17
Groyne 6
17.70
0.32
-0.20
0.37
0.19
0.44
0.25
0.29
Groyne 8
19.31
0.46
0.17
0.19
0.29
0.57
0.68
0.31
Okuku
Confl
20.92
0.58
-0.37
1.11
0.32
0.90
-0.13
0.28
Average
0.37
0.61
0.67
0.94
0.89
1.12
0.51
0.44
Average 0 to 8.05
0.49
0.79
0.72
1.06
0.92
1.24
0.29
0.60
0.99
1.28
1.30
1.54
0.97
0.27
0.34
0.28
0.40
0.43
0.50
0.16
84
Berm level minus mean fairway level
2.5
2.0
Difference (m)
1.5
Left berm 1960/62
Right berm 1960/62
Left berm 2008/09
Right berm 2008/09
1.0
0.5
0.0
-0.5
-1.0
0
5
10
15
20
River distance (km)
Figure 18-1



Mean berm minus mean fairway level changes
The differences have increased from an average 0.5 m in 1960 to 1.0 m in 2009.
The largest increases are between Marchmont Rd and Lehmans Rd (8.5-14 km); there is a
modest increase from the coast to Marchmont Rd (0 to 8 km); and little increase upstream of
Groyne 2 (14 to 21 km).
The largest average difference in 1960/62 was 1.9 m at 3.22 km. The differences at 2.96 km,
11.27 and 12.87 km have now risen to similar differences (from typically 1.1 m or less in
1960/62 to 1.9 m in 2009). The right berm at 11.27 km is now the highest above mean bed
level at 2.3 m (berm relatively narrow at 36 m).
The percentage of berms less than 0.5 m above the fairway has reduced from 50% in 1960/62 to 20%
in 2008/09. The percentage of berms greater than 1.5 m above the fairway has increased from 4% in
1960/62 to 20% in 2008/09. The percentage of berm/fairway differences between 0.5 and 1.5 m has
increased from 46% in 1960/62 to 60% in 2008/09. The trend is one of increasing berm levels above
mean fairway levels.
19 Mean berm level minus channel invert
comparisons
The difference between mean berm level and invert level is indicative of berm edge heights. The
sediment sizes transported by the river (as bedload) gradually reduce in the downstream direction as
the river slope decreases. Where there is a greater proportion of silt and clay sized particles
transported by the river there is a greater degree of cohesion of deposited sediment so one would
expect stable river-edge bank heights (and slopes) to increase in the downstream direction.
85
Table 19.1
Location
Channel depths
Distance
(km)
Mean Berm Level - Channel Invert Level (m)
1960/62
1997
2009
Change
1960/62 to
19997
Change
1960/62 to
2009
Change
1997 to 2009
0.80
1.82
2.34
2.20
0.52
0.38
-0.14
1.61
2.06
2.52
2.97
0.46
0.91
0.45
2.41
2.27
1.89
2.64
-0.38
0.37
0.75
2.96
2.86
2.22
4.04
-0.64
1.18
1.81
3.22
3.58
3.07
3.99
-0.51
0.41
0.92
4.02
2.48
2.50
2.84
0.02
0.36
0.34
4.83
2.25
2.02
2.85
-0.23
0.61
0.84
5.63
2.14
1.40
3.24
-0.74
1.10
1.83
Tulls Rd
6.44
1.99
2.03
2.79
0.05
0.81
0.76
Smart Rd
7.24
1.70
1.65
3.65
-0.06
1.94
2.00
Marchmont
Rd
8.05
1.90
2.12
2.85
0.22
0.95
0.73
8.85
1.60
2.21
2.30
0.61
0.70
0.09
9.66
1.53
2.22
3.31
0.70
1.78
1.08
Rail Br
10.46
2.20
2.95
3.79
0.76
1.59
0.84
RTB
11.27
2.77
3.32
3.32
0.55
0.55
0.00
12.07
1.46
2.59
2.87
1.13
1.41
0.27
Waikuku
SH1
Toppings Rd
Groyne 1
12.87
1.79
2.29
2.88
0.50
1.09
0.59
Lehmans Rd
13.68
1.83
2.37
3.33
0.53
1.49
0.96
Groyne 2
14.48
2.39
2.07
2.73
-0.32
0.34
0.66
-0.17
Airstrip
15.29
1.94
2.07
1.90
0.13
-0.04
Groyne 3
16.09
3.07
1.66
2.53
-1.41
-0.55
0.86
Dalziells Rd
16.90
2.37
2.03
1.99
-0.34
-0.39
-0.05
Groyne 6
17.70
1.64
1.86
2.73
0.22
1.09
0.87
Groyne 8
19.31
1.60
1.34
2.57
-0.26
0.97
1.22
Okuku Confl
20.92
3.21
2.37
3.06
-0.85
-0.15
0.70
Average
2.18
2.20
2.93
0.03
0.76
0.73
Average 0 to 8.05km
2.28
2.16
3.10
-0.12
0.82
0.94
1.88
2.56
3.11
0.68
1.23
0.55
2.32
1.91
2.50
-0.40
0.18
0.59
86
Mean berm level minus channel invert
4.5
4.0
Difference (m)
3.5
3.0
1960/62
1997
2008/09
Trendline (1960/62)
Trendline (1997)
Trendline (2008/09)
2.5
2.0
1.5
1.0
0.5
0.0
0
5
10
15
20
River distance (km)
Figure 19-1





Mean berm level minus channel inverts
There has been an average increase in mean berm levels minus channel inverts of 0.76 m
between 1960/62 and 2008/09. The major part of this increase is evident since 1997.
Before 1997 the main increases (average 0.7 m) occurred between Marchmont Rd and
Lehmans Rd (8.5 and 14 km) with average decreases of about 0.3 m elsewhere.
Increases have since occurred throughout with channel incision and berm aggradation leading
to increases of an average of 0.9 m in the 0-8 km reach and continued incision in the 8.514 km reach (average 0.6 m). Incision (average 0.6 m) has also started to occur in the 1421 km reach.
In the 2008/09 survey 4 of the cross-sections had a mean berm level minus channel invert of
greater than 3.6m while in the 1960/62 and 1997 surveys there were no differences of this
magnitude.
The trends are consistent with incision of the channel in the middle reaches in the period to
1997 and incision throughout the river in the period 1997 to 2008/09. In the lower reaches this
is exacerbated by the trends of berm aggradation.
20 Slopes
20.1 Fairway slopes
Table 20.1 presents the slope measured between the mean fairway levels recorded at each crosssection for 1960/62, the average of 4 surveys (1960/62, 1976, 1997 and 2008/09) and the most recent
survey (2008/09). The changes in slope between one pair of cross-sections and the next are also
compared so that the consistency of slopes can be compared. The mean fairway levels are measured
over various widths which adds to the variability of slope in some places (particularly around the
bridges), however, this factor was not thought significant enough to warrant recalculation using
consistent widths.
The table includes the proposed slopes and change in slope. These have been calculated following
comparison of the criteria in Section 21 and analysis (Appendix 1) and using the bed levels
recommended in Section 22.
87
Table 20.1
Fairway slopes and change in slope
Slope between sections (m/km)
Location
Distance
(km)
Waikuku
0.80
SH1
Toppings Rd
1960/
62
Change in Slope between sections (m/km)
2009
Average
of 4
surveys
Proposed
1960/62
2009
Average
of 4
surveys
Proposed
1.61
1.1
1.3
1.4
1.1
2.41
2.9
2.4
2.5
1.9
1.8
1.1
1.0
0.8
2.96
2.0
1.3
2.1
2.2
-1.0
-1.1
-0.4
0.3
3.22
1.1
3.0
1.2
3.0
-0.9
1.7
-0.9
0.8
4.02
3.7
3.5
3.5
3.8
2.7
0.5
2.4
0.8
4.83
2.5
2.7
2.7
2.8
-1.2
-0.9
-0.8
-1.0
5.63
4.1
3.6
3.8
3.2
1.6
0.9
1.1
0.4
Tulls Rd
6.44
2.9
3.2
3.0
2.7
-1.2
-0.4
-0.8
-0.5
Smart Rd
7.24
3.7
3.9
3.9
4.3
0.8
0.7
0.9
1.6
Marchmont
Rd
8.05
3.8
3.2
3.6
3.9
0.1
-0.6
-0.3
-0.4
4.5
4.6
4.4
4.4
0.7
1.3
0.7
0.6
-0.5
8.85
9.66
4.3
3.7
4.0
4.0
-0.1
-0.8
-0.4
Rail Br
10.46
4.1
4.5
4.3
4.1
-0.2
0.8
0.2
0.1
RTB
11.27
4.4
3.9
4.2
4.3
0.3
-0.6
0.0
0.2
12.07
4.5
4.7
4.6
4.8
0.1
0.8
0.4
0.5
Groyne 1
12.87
4.8
4.0
4.6
4.1
0.3
-0.7
0.0
-0.7
Lehmans Rd
13.68
5.4
5.9
5.6
5.7
0.6
1.8
0.9
1.6
Groyne 2
14.48
4.5
5.4
4.9
5.0
-0.9
-0.5
-0.7
-0.7
0.3
Airstrip
15.29
5.1
5.6
5.3
5.3
0.6
0.2
0.4
Groyne 3
16.09
5.1
5.6
5.4
5.5
-0.1
0.0
0.0
0.2
Dalziells Rd
16.90
5.7
5.0
5.3
5.2
0.6
-0.6
0.0
-0.3
Groyne 6
17.70
5.3
5.5
5.3
5.4
-0.4
0.5
0.0
0.1
Groyne 8
19.31
5.4
5.3
5.4
5.3
0.1
-0.2
0.1
0.0
Okuku Confl
20.92
5.5
5.4
5.3
5.4
0.0
0.1
-0.1
0.0
Average 0 to 8.05km
2.77
2.82
2.76
2.89
4.59
4.47
4.52
4.51
Average 14.48 to
20.92km
5.23
5.40
5.29
5.30
0.96
0.86
0.76
0.67
Standard deviation
88
Slopes between cross-sections
7
6
Slope (m/km)
5
2008/09
1960/62
Average of 4 surveys
Proposed
Trendline 2008/09
Trendline 1960/62
4
3
2
1
0
0
5
10
15
20
River distance (km)
Figure 20-1



Slopes between cross-sections
Slopes generally decrease in the downstream direction from about 5.3 m/km at the Okuku
confluence to 4.6 m/km at the Rangiora Traffic Bridge to 1.7m/km around 1km from the coast.
Apart from ups and downs near SH1 the most significant changes in slope are a general
flattening of the river in the 8.05 to 12.87 km reach and a steepening of the river over the
12.87 to 16.09 km reach.
Between the 1960/62 and 2008/09 surveys there was a slight decrease in the standard
deviations of the change in slope between sections. This is likely to be indicative of a more
stable channel. The proposed slopes have significantly less variation so are expected to
encourage channel stability.
Not evident in the table above is the variation in slope that has been recorded between surveys
which has been greatest downstream of the 3.22 km cross-section. The relatively flat slope and
narrow channel probably mean this area is the most susceptible to sediment build-up but because
these sediments are relatively fine they can be remobilised in floods relatively easily. To maintain
flood capacity in this area it is important to monitor this area relatively frequently and adjust the
management accordingly.
89
20.2 Channel invert slopes
Table 20.2
Slope of the channel invert
Location
Distance
(km)
Waikuku
0.80
1.61
SH1
Toppings Rd
Invert Slope (m/km)
Change in Invert Slope
1960/62
1997
2009
1.7
2.0
0.9
1960/62
1997
2009
2.41
2.3
3.1
2.9
0.6
1.1
2.0
2.96
2.0
1.9
0.2
-0.3
-1.2
-2.8
3.22
0.7
1.5
3.3
-1.4
-0.4
3.1
4.02
3.4
3.0
3.9
2.7
1.5
0.6
4.83
3.5
3.6
2.9
0.1
0.6
-1.0
5.63
2.6
3.5
2.6
-1.0
-0.1
-0.3
Tulls Rd
6.44
3.3
2.5
3.7
0.8
-1.0
1.1
Smarts Rd
7.24
4.4
4.2
2.5
1.1
1.7
-1.1
Marchmont
Rd
8.05
3.7
3.5
4.8
-0.6
-0.7
2.3
8.85
4.9
4.4
5.1
1.1
0.9
0.3
9.66
4.3
4.1
3.0
-0.6
-0.3
-2.1
Rail Br
10.46
4.1
4.0
4.1
-0.2
0.0
1.1
RTB
11.27
3.9
3.9
5.0
-0.1
-0.2
0.9
12.07
5.0
4.4
4.3
1.1
0.5
-0.7
Groyne 1
12.87
4.7
5.7
5.0
-0.3
1.2
0.6
Lehmans Rd
13.68
4.8
4.7
4.6
0.1
-0.9
-0.4
Groyne 2
14.48
4.5
5.4
5.4
-0.4
0.7
0.9
Airstrip
15.29
5.5
5.2
6.3
1.0
-0.3
0.9
Groyne 3
16.09
3.9
5.6
4.3
-1.6
0.4
-2.0
Dalziells Rd
16.90
6.4
5.0
6.1
2.5
-0.6
1.8
Groyne 6
17.70
5.9
5.3
4.4
-0.5
0.4
-1.8
Groyne 8
19.31
5.6
5.8
5.6
-0.3
0.5
1.3
Okuku Confl
20.92
4.3
4.8
5.0
-1.3
-1.0
-0.6
Average 0 to 8.05km
2.76
2.86
2.76
4.53
4.45
4.44
5.14
5.29
5.31
1.11
0.83
1.52
Standard deviation


While more variable the trends generally mimic the trends in fairway slopes.
The relatively steep slopes and/or large changes in slope at some locations (SH1, 8.059.66 km and 15.29-16.09 km) in the 2008/09 survey may be indicative of channel instability in
these areas.
21 Criteria
To establish reasonable minimum bed levels four main criteria are proposed:


90
Maintaining a minimum fairway and berm capacity to meet scheme objectives. The mean
fairway level required to achieve the design flood capacity sets the highest level that a
minimum bed level can be set at. Note: This report is recommending a change in the
Scheme Objectives, namely an increase in the design flow from 2400 m3.s-1 to
3250 m3.s-1.
Limiting berm edge heights to enable erosion to be managed and to protect existing


Reducing the variability of mean bed level slope without introducing large changes in slope to
encourage the stability of the river channels and slope.
Maintaining a consistency of capacity over the length of the scheme. The flood damage
benefits of reduced bed levels are only likely to be achieved if capacity is increased
throughout the system. It is important to retain equity throughout the scheme so that some
areas are not overly advantaged at the expense of others so that, for example, the likelihood
of overtopping of stopbanks remains similar throughout the scheme. This is the main criteria
where a significant degree of judgement needs to be exercised, in particular, balancing the
risk between areas of wide river bed and wide berm with areas of narrow river bed and narrow
berm.
The 1995 Technical Investigation Report for the Ashley River Floodplain Management Regional Plan
3 -1
noted that, for much of its length, the river system could carry 3000 m .s (plus the design freeboard
3 -1
of 600 mm) even though the scheme was designed to carry 2400 m .s , although there remained the
risk of stopbanks being undermined at lower flows. In 1976, 2400 m3.s-1 was estimated to be the 1%
Annual Exceedance Probability (AEP) or 1-in-100 year event.
With the inclusion of more data, the recent recommendation for a conservative estimate of the 1%
AEP event is 3250 m3.s-1 (rounded) (Griffiths, 2009, refer also Section 12).
With the exception of 7 of the 25 cross-sections, a capacity of 3500 m3.s-1 (plus freeboard) has now
been achieved through much of the river.
The general increase in flood capacity that has occurred over time is seen as positive in this regard. It
is reflected in the trends of lowered fairway levels and increasing berm minus fairway differences
which are representative of bank heights. However, bank heights have also increased in some areas
as a result of berm aggradation. There is a limit to how far this trend can continue with the current
berm edge management which largely relies on willow trees anchored to the river bank with wire
ropes. If the banks are allowed to increase in height further more costly measures such as rock
protection may be required in more places.
Effective bank protection works (including willows and rock) can (almost by definition) lead to deeper
channels as the bank protection works resist lateral erosive forces and confine the channel. Some of
the lateral erosive forces are converted to vertical forces leading to deeper scour holes adjacent to
protective works.
As set out in Section 26, in order to restore and retain a consistency of channel capacity (the fourth
3 -1
point above) it is recommended that a flow of 3250 m .s plus 600 mm freeboard is now adopted as
the minimum capacity for the scheme. The bed levels proposed in section 22 allow for this
recommendation to be achieved over time in an orderly manner.
21.1 Indicators











Adequate/inadequate freeboard in design flood.
Fairway capacity to berm level.
Berm-mean bed level.
Berm-invert level.
Berm difference (left/right).
Mean bed level.
Invert level.
Structures.
Slope.
Consistency of capacity over length.
Berm aggrade.
The indicators for each cross-section are listed in the tables in Sections 13-20. For each cross-section
the relative importance of each indicator is considered and a dominant influence and proposed change
in bed level is listed in Appendix 1. The resultant proposed minimum bed levels are listed in Section
22.
91
22 Recommended bed levels
The following bed levels are recommended as target levels for the bed of the Ashley River.
Table 22.1
Recommended bed levels
Location
Distance
Mean
fairway
level
2008/09
Waikuku Beach
0.80
1.61
2.41
1.11
2.20
4.16
Proposed
Rise/Fall to
recommended
level (relative
to 2009)
0
-0.2
-0.6
SH1
2.96
4.89
-0.1
4.79
3.22
5.67
-0.1
5.57
Rail bridge
4.02
4.83
5.63
6.44
7.24
8.05
8.85
9.66
10.46
8.51
10.67
13.54
16.11
19.21
21.84
25.49
28.52
32.12
0.1
0.2
-0.1
-0.5
-0.2
0.3
0.2
0.4
0.1
8.61
10.87
13.44
15.61
19.01
22.14
25.69
28.92
32.22
RTB
11.27
35.29
0.4
35.69
12.07
39.05
0.5
39.55
12.87
42.27
0.6
42.87
13.68
47.01
0.5
47.51
14.48
51.33
0.2
51.53
drop in bed level
15.29
16.09
16.90
17.70
19.31
20.92
55.85
60.34
64.38
68.77
77.35
86.09
0
-0.1
0.1
0
0
-0.1
55.85
60.24
64.48
68.77
77.35
85.99
fairway capacity low
aggradation, fairway capacity very low
slope, fairway capacity low
just adequate freeboard
capacity, fairway capacity very low
Toppings Rd
Tulls Rd
Smart Rd
Marchmont Rd
Groyne 1, Rossiter
Rd
Lehmans Rd
Groyne 2, Merton
Rd
Airstrip
Groyne 3
Dalziells Rd
Groyne 6
Groyne 8
Okuku confl
Recommended
Bed Level (m)
1.11
2.00
3.56
Dominant influences
modest aggradation
just adequate freeboard, slope
capacity
large berm-invert difference, bridge, slope,
capacity
large berm-invert difference, slope,
capacity
slope
slope
drop in bed level, slope
capacity, slope
slope
drop in bed level
high fairway capacity
large drop in bed level
large drop in bed level, capacity
large drop in bed level, large berm-fairway
difference
difference, capacity
Given the recent history of degradation and the general increase in flood capacity that has resulted it
would be prudent not to encourage further general degradation at least until future surveys show that
bed levels have stabilised or there is a strong community desire for further increases in flood capacity
with a recognition of the increased costs of erosion protection that would result. Accordingly, no
envelope to minimum bed levels is recommended – minimum bed level conditions on consents should
be the same as the recommended bed levels in the table above.
An exception can be considered in the lower reach from 3.22 km downstream where bed levels are
more volatile, the fairway width is relatively narrow and flood capacity is more likely to be
compromised. More frequent (5-yearly) survey is recommended in this area and, provided the levels
are appropriate for the SH1 bridge, minimum bed levels could be 200 mm below the recommended
levels in this area.
Gravel extraction should be limited to the few locations where drops in bed level have been
recommended. The volumes involved are small in relation to historic extraction rates and current
applications in process.
92
2008/09 bed level in relation to recommended bed level
0.8
Difference in level (m)
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
0
5
10
15
20
River distance (km)
Figure 22-1
2008/09 bed level relative to recommended bed level
23 Sediment budget
The change in volume of sediment stored in the fairway is compared between the various surveys in
Table 23.1, along with estimates of gravel extraction. Note that the reported extraction figures prior to
1992 are approximate only. Table 23.2 shows the same information in an average annual format.
Table 23.1
Volume changes and extraction returns 1960/62 to 2008/09
Volumes (thousands of cubic metres)
Change in fairway volume over period
Extraction returns (very approximate
before 1992)
1960/62 to
1976
1976 to
1986/88
1986/88 to
1997
1997 to
2005 LIDAR
(approx)
2005 to
2008/09
Whole
period
1960/622008/09
-518
-370
-170
-797
-798
-2653
440
283
2957?
1050
approx?
770 approx? 410 approx
Net supply to fairway (volume in minus
volume out)
532?
400?
240
-357
-515
304?
Berm aggradation over period
210
567
221
n/a
-66
932
Net supply to whole river
742?
967?
461
-357
-581
1236?
Table 23.2
Annualised volume changes and extraction returns 1960/62 to 2008/09
1960/62 to
1976
1976 to
1986/88
1986/88 to
1997
1997 to
2005 LIDAR
(approx)
2005 to
2008/09
Whole
period
1960/622008/09
-35
-34
-17
-100
-228
-56
70 approx?
70 approx?
41 approx
55
81
62?
Net supply to fairway
35?
36?
24
-45
-147
6?
Berm aggradation
14
52
22
-6
-6
20
Net supply to whole river
49?
88?
46
-51
-153
26?
Volumes per year (thousands of cubic
metres)
Change – fairway volume
Extraction returns (very approximate
before 1992)
93
A net loss of material within the fairway has been recorded at each resurvey.
There will be a natural inflow of gravel from upstream. Estimates suggest this averages 35-40,000 m3
per year (Sutherland, 2005), but this may well have dropped since 1997 owing to the paucity of floods.
The loss to the estuary is not quantified but has a limited area in which to spread. The loss to the
coast is expected to be very small given that gravel reaches the coast only occasionally (refer Section
9).
The approximate extraction returns up to 1997 exceed the net loss indicating there was a positive net
supply of around 30,000m3 during that period.
After 1997, the loss within the fairway significantly exceeded the extraction returns, either indicating
there was a net loss of material (downstream of the cross-section monitoring area, to the estuary or to
the coast) or there was significant under-reporting of returns, missing returns, or a combination of the
above. It appears unlikely from Figure 23-1 that large quantities of gravel found its way right through
the system to the coast; most of the change is at least 8km from the coast.
Sutherland (2005) estimated a small (1,000 m3/year) sustainable supply for the lower river from limited
data to 1997, discarding the approximate estimates of extraction prior to 1992.
Ashley River change and extraction 1997 to 2008/09
by cross-section
50000
Volume (m3)
0
-50000
-100000
-150000
-200000
80
0
16
10
24
10
29
60
32
20
40
20
48
30
56
30
64
40
72
40
80
50
88
50
96
60
10
46
11 0
27
12 0
07
12 0
87
13 0
68
14 0
48
15 0
29
16 0
09
16 0
90
17 0
70
19 0
31
20 0
92
0
-250000
Cross-section distance
Volume change 2008/09-1997
Figure 23-1
Smoothed returns Jul 1997 to Dec 2008
Location of changes in volume and reported extraction volumes 1997 to 2008/09
For the purposes of estimating gravel resource availability within the fairway, it would be prudent to
assume that there is no sustainable supply for the lower river although this assumption should be
revised as new information becomes available.
The berms have aggraded gradually as may be expected if they remain well vegetated. The minor
drop in berm levels to 2008/09 can be explained by some localised clearance of berm vegetation at
two cross-sections – these are probably not representative and the recorded drop is within the margin
of error. Most berm cross-sections show little or no change between 1997 and 2008/09.
Berm aggradation has been calculated at an average of about 20,000 m3 per year over the survey
record but this has not led to significant reductions in flood capacity (with no reduction in recent years).
The use of the berm as erosion protection in the form of a vegetated buffer between the active
94
channel and the stopbanks or terraces generally precludes the use of this material – if it were to be
removed, in most places there would be a significant increase in the risk of lateral erosion of the
stopbanks.
24 Implications for gravel extraction consents
There are currently 8 resource consents for gravel extraction on the Ashley River downstream of the
Okuku confluence with a further 3 expired consents allowed to continue operating until their renewal
application is decided. The total remaining volume on the 8 current consents amounts to 308,000 m3.
Only one of the existing consents has minimum bed levels specified.
There are 14 applications in process for a combined total volume of approximately 3 million cubic
metres of gravel.
The scale of the existing consents is small enough that their effect on bed levels and erosion risk is
not likely to be significant. However, the applications in process should be limited to the gravel
available above the recommended bed levels without allowance for any supply rate. Once the existing
consents are allowed for there will be very little available for either new or renewal applications.
Small additional volumes could potentially be available where it is desirable to widen the fairway but
these volumes may need to be placed in context with the overall degradation observed in the river.
In the case of the expired consents their applications should be decided as soon as possible to avoid
further degradation of the bed.
Table 24.1
Potential gravel extraction volumes
Location
Distance
Width
Waikuku Beach
0.80
1.61
2.41
2.96
3.22
4.02
4.83
5.63
6.44
7.24
8.05
8.85
9.66
10.46
11.27
12.07
398.8
195.1
231.8
231.6
136.3
309.1
211.9
202.4
231.6
413.7
476.0
422.9
361.0
359.3
275.1
519.6
2008/09
surveyed
level
above
min bed
level
0.2
0.4
0.8
0.3
0.3
-0.1
-0.2
0.1
0.5
0.2
-0.3
-0.2
-0.4
-0.1
-0.4
-0.5
12.87
287.6
-0.6
-172900
13.68
468.3
-0.5
-164700
100000
14.48
459.4
-0.2
-130400
100000
15.29
16.09
16.90
17.70
19.31
20.92
577.3
585.6
500.5
467.0
485.5
454.9
0
0.1
-0.1
0
0
0.1
-37200
23400
3400
-20000
0
36600
-595300
SH1
Toppings Rd
Tulls Rd
Smart Rd
Marchmont Rd
Rail bridge
RTB
Groyne 1,
Rossiter Rd
Lehmans Rd
Groyne 2,
Merton Rd
Airstrip
Groyne 3
Dalziells Rd
Groyne 6
Groyne 8
Okuku confl
Total
Volume
balance
63900
105400
70100
14300
4000
-29700
-8900
55100
79400
-24300
-91000
-92700
-72100
-59100
-147900
Potentially
available
for
extraction
Total
future
consented
volume
(20092015)
63900
105400
70100
66000
111000
3000
15000
35000
55100
79400
10000
10000
18000
10000
10000
10000
Potentially
available
for current
applications
67100
45100
69400
Applications
in process
20000
0
225000
225000
0
1000
208500
234000
234000
144000
111500
64000
29000
4000
0
100000
100000
23400
3400
36600
437300
23400
5000
5000
308000
36600
241600
220000
170000
170000
170000
170000
150000
2950000
95
440,000 m3 of gravel is currently available, a good proportion of which is already consented. Relatively
small amounts are consented in some areas where bed levels are below the recommended bed
levels. In these areas, the consequences of continued extraction at this level are probably not severe.
About 240,000 m3 is available for the current applications within the fairway.
25 Conclusion
The Ashley River has a long history of flooding combined with substantial sediment movement and
erosion. The Ashley River stopbanks have not been breached since 1953 although break-outs almost
occurred in 1986 and 2002.
The stopbanks were last upgraded in 1976 to contain floods up to 2400 m3.s-1 with a freeboard of
600 mm. Recent survey and modelling suggest the river has more than that capacity (3000 to
3500 m3.s-1). The revised 100 year flood estimate recommended for design purposes is 3250 m3.s-1.
A trend of degradation of the fairway which has accelerated markedly since the 1997 survey is evident
from the survey record. In the period since 1997 degradation far exceeds the reported gravel
extraction.
A range of indicators have been tabulated which together provide a record of how the capacity and
erosion risk has changed over time.
Recommended target mean bed levels are set out based on 4 criteria related to the indicators. The
criteria are intended to:




3. -1
Maintain a minimum fairway and berm flood capacity of 3250 m s plus 600 mm freeboard.
Limit berm edge heights to enable erosion risk to be managed and to protect existing
Reduce the variability of mean bed slope without introducing large changes in slope to assist
with river bed stability and consistency of bedload transport capacity.
Maintain a consistency of flood capacity over the length of the scheme.
26 Recommendations
96

Target levels set out in Section 22 be adopted as minimum bed levels with the exception that
bed levels be allowed to drop up to 200mm further in the reach downstream of SH1. Gravel
extraction should be limited to the few locations where drops in bed level have been
recommended. The volumes involved are small in relation to historic extraction rates and
current applications in process.

The Ashley River Rating District consider adopting 3250 m3.s-1 (plus 600 mm freeboard) as
the design flow for the river.

It is apparent that significant changes can occur in a relatively short time, therefore, where
large scale excavation is occurring the cross sections upstream of, through and downstream
of, the excavation areas should be surveyed annually. In the absence of extraction, the reach
downstream of SH1 should be surveyed at least 5-yearly so that the capacity of the reach can
be managed. Elsewhere, a maximum of 10 years between surveys (or more frequent if large
floods appear to have altered the bed substantially) is considered adequate.
27 Peer review
Dr. G. A. Griffiths (NIWA) has conducted a review of the draft report and his comments and
suggestions have been incorporated into this report. Dr. Griffiths has had a long association with the
braided gravel bed rivers of Canterbury. In a covering letter he states:
“I have read this long and detailed report carefully and in my view it is a sound piece of work. In
particular, I think the conclusions and recommendations are well justified by the background
material and the analysis.
The methodology is essentially that employed in a similar study on the Waimakariri River. I note
that the report from the Waimakariri study received extensive external review and stood up in a
related consent hearing concerning gravel extraction.
I have made numerous minor technical and editorial comments on the text which is enclosed as
hard copy because I believe it will be easier for you to understand my remarks in situ, rather
than in a long list given here at second hand.
Three suggestions of a more major nature for improvement are: first, a plot of recommended
bed levels versus actual (2008/09) levels to illustrate Table 22.1 would be helpful; second, the
executive summary should include the material given in the conclusions and recommendations,
and; third Table 23.1 needs to be revised as you cannot have a negative inflow or supply rate.
The only important, outstanding matter in the report is the apparent mystery of the missing
gravel. To my mind, at any rate, there is no mystery. The analysis clearly shows that the
proximate cause must be substantial under-reporting in the returns for shingle extractions
assuming returns have been made and not lost. I believe that is a technical fact, albeit a
politically unpalatable one perhaps….”
28 Acknowledgements
The late Roger McPherson researched the extensive history of flooding which has been reproduced in
part from the first Reference below; also the physical and structural settings sections.
Thanks to Bill Mecchia’s survey team for providing a reliable and comprehensive survey database to
draw from. Thanks also to Geoff Scholes for some long term perspectives, Michael Chivers for
information on resource consents and Ian Heslop and David Aires for their comments.
The various References below provided an excellent platform for this investigation.
29 References
Boyle A.J., May 1995, Ashley River Floodplain Management Regional Plan, Technical Investigation,
Report 95(6) ISBN 1-68937-280-8.
Boyle A.J., Surman, M.R., June 2007, Opihi River fairway bed level investigation, Report No. U07/13
ISBN 978-1-86937-643-7.
Boyle A.J., Surman M.R., June 2007, Pareora River fairway bed level investigation, Report No. R07/9
ISBN 1-86937-637-6.
Boyle A.J., Surman M.R., January 2009, Waimakariri River Bed Level Investigation, Report No.
R08/11 ISBN 978-86937-758-8.
97
Boyle A.J., April 2009, Waimakariri River design flow simulation (2007/2008), Report No. U07/105
ISBN 978-1-86937-945-2.
Environment Canterbury, Waimakariri District Council, February 2003, Waimakariri District Flood
Hazard Management Strategy, Report No. R03/2 ISBN 1-86937-476-2, N:\
daily:03021100035.
Little, R, 1991, Stability of the Ashley River Mouth, North Canterbury., University of Canterbury
North Canterbury Catchment Board and Regional Water Board, October 1989, Ashley River (Rakihuri)
and Catchment Land and Water Management Plan
Griffiths G., Pearson C., McKerchar, A., NIWA, July 2009, Review of Ashley River flood frequency at
Rangiora Traffic Bridge, NIWA Client Report: CHC2009-103
Single, M., MWH, December 2005, Pegasus Bay Report, Status of Gravel Resources and
Management Implications
Statistics New Zealand, 2003, Agricultural Statistics 2002
Sutherland N., MWH, December 2005, Ashley River Report, Status of Gravel Resources and
Management Implications
98
Appendix 1 Bed level decision matrix
99
SH1
Rail bridge
RTB
Groyne 1,
Rossiter Rd
Lehmans
Rd
Groyne 2,
Merton Rd
Airstrip
Groyne 3
Dalziells Rd
Groyne 6
Groyne 8
Okuku confl
yes
marginal
yes
marginal
3.22
marginal
yes
marginal
marginal
marginal
yes
marginal
yes
yes
left
yes
yes
yes
yes
marginal
yes
8.05
yes
yes
yes
yes
marginal
yes
yes
yes
marginal
yes
yes
yes
yes
yes
yes
yes
yes
marginal
yes
yes
yes
yes
yes
10.46
yes
marginal
yes
yes
very
12.07
marginal
yes
yes
yes
very
yes
yes
yes
yes
12.87
marginal
yes
yes
yes
very
yes
13.68
yes
marginal
yes
yes
very
yes
14.48
15.29
16.09
16.90
17.70
19.31
20.92
marginal
left
both
left
right
yes
right
yes
very
yes
yes
very
marginal
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
marginal
marginal
marginal
yes
marginal
marginal
marginal
yes
yes
yes
Dominant influences
Proposed Rise/Fall to
max level (m) relative to
2009
Fairway
Structures
Significant
berm aggrade
small drop
-0.1
widen (or drop)
-0.1
no change
0.1
slope
small rise
0.2
slope
small drop
-0.1
drop in bed level, slope
drop
small drop
-0.5
-0.2
capacity, slope
slope
bridge
narrow
yes
right
some
very
marginal
modest aggradation
just adequate freeboard, slope
capacity
large berm-invert difference, bridge, slope,
capacity
large berm-invert difference, slope,
capacity
yes
yes
marginal
yes
0
-0.2
-0.6
left
marginal
yes
marginal
small drop
small drop
large drop
yes
marginal
yes
right
some
some
left
marginal
yes
yes
yes
yes
8.85
9.66
11.27
Other considerations
Relative
fairway
capacity
moderate
Modest berm
difference
yes
yes
yes
marginal
marginal
left
marginal
Modest bermfairway
difference
Just adeqaute
freeboard
Relative
fairway
capacity high
Large drop in
invert level
Large drop in
bed level
marginal
marginal
4.83
6.44
7.24
Reasons for no change
yes
2.96
left
marginal
Large berm
difference
Large bermfairway
difference
Large berminvert
difference
yes
marginal
yes
5.63
Tulls Rd
Smart Rd
Marchmont
Rd
yes
1.61
2.41
4.02
Toppings
Rd
Relative
fairway
capacity low
0.80
Reasons for change - higher
Rise in bed
level
Waikuku
Beach
Distance
(km)
Very small
berm-fairway
difference
Location
Inadeqaute
freeboard
Reasons for change - lower
Recommended change
wide
wide
Rail
bridge
Road
bridge
u/s of
Bridge
narrow
rise
0.3
drop in bed level
small rise
rise
0.2
0.4
high fairway capacity
small rise
0.1
large drop in bed level, capacity
rise
0.4
difference
rise
0.5
bit narrow
rise
0.6
difference, capacity
some
bit narrow
rise
0.5
small rise
0.2
drop in bed level
some
wide
wide
small drop
drop
no change
small drop
small drop
widen/small drop
0
-0.1
0.1
0
0
-0.1
fairway capacity low
aggradation, fairway capacity very low
just adequate freeboard
capacity, fairway capacity very low
bit narrow
Notes
Fairway width: comments in comparison to the fairway width at nearby cross-sections (upstream and downstream).
Structures: nearby structures noted
Adopted thresholds and their associated descriptions
Freeboard in design flood (3250 cumecs)
Fairway capacity to berm level: assessed by the ratio of the minimum
average berm level minus fairway level to 600 cumecs flow depth:
Berm-mean fairway level:
Berm-invert level:
Berm difference (left/right):
Drop in mean fairway level:
Change in invert level:
Slope:
Consistency of capacity over length:
Significant berm aggrade:
Less than 0.5m = inadequate, less than 0.6m = marginal
Negative (berm lower than fairway) to 0.2 = very low, 0.2-0.7 = low, 0.7-1.1 = medium, 1.1-1.3 = med-high (marginal), 1.3 – 1.8 = high, greater than 1.8 = very
high.
Less than 0.4m = very small, 0.4-0.5m = marginal (small), 0.5-1.5m = modest, 1.5-2m = marginal (large), greater than 2m = large
2.5-3m = marginal, greater than 3m = large
0.8-1.0m = marginal, greater than 1m = large
0.4-0.6 = marginal (large), greater than 0.6m = large
Drop of 1-1.5m = marginal (large), drop of greater than 1.5m = large
Aim for the proposed slope to be in the range of the observed nearby slopes and for the standard deviation of slopes to be reduced.
See fairway capacity to berm level
Greater than an average of 0.5m on both berms or greater than an average of 0.6m on one berm “Some” = average of greater than 0.2m
100

Ashley River bed level investigation

Transcription

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