3. DATA 3.1 Historical Rainfall 3.1.1 Summary of Pluviometer Data

Transcription

Rose Bay Catchment Flood Study
3.
DATA
The first stage in the investigation of flooding matters is to establish the nature, size and frequency
of the problem. On large river systems such as the Hawkesbury River there are generally stream
height and historical records dating back to the early 1900's, or in some cases even further.
However, in small urban catchments such as Rose Bay there are no stream gauges or official
historical records available. A picture of flooding must therefore be obtained from an examination
of rainfall records and local knowledge. For this reason, a comprehensive data collection exercise
was undertaken.
3.1
Historical Rainfall
No detailed review of historical rainfall data was undertaken as part of References 1 or 2. Therefore
a brief summary of the information collected for the Rushcutters Bay Flood Study (Reference 5) is
outlined below.
3.1.1
Summary of Pluviometer Data from Reference 5
Pluviometer records (continuous record of rainfalls) provide a more detailed description of temporal
variations in rainfall than daily (24 hourly) records. Table 4 lists the maximum storm intensities for
the four largest recent rainfall events from the available pluviometers and daily read gauges.
Table 4:
November 1984, January 1989, March 1989 and January 1994 Maximum
Recorded Storm Depths (in mm)
Station
Location
Paddington
Observatory Hill*
Vaucluse
9 Nov 1984
20 min
30 min
38
54
57
80
43
50
Station Location
Royal Botanic Gardens
Rose Bay
Observatory Hill
Paddington
8 Nov 1984 (3)
37
44
71
6 Jan 1989
20 min
30 min
52
54
42
44
39
43
9 March 1989
20 min
30 min
38
43
24
30
19
24
26 Jan 1991
20 min
30 min
46
52
44
58
23
24
24 hour totals to 0900 hrs
9 Nov 1984 (3)
6 Jan 1989
9 Mar 1989
248
49
39
(1)
85
40
234
47
35 (2)
166
56
50
26 Jan 1991
59
53
65
54
Notes:
* approximate depths
(1)
(2)
(3)
Gauge washed away in flood.
Accumulated total over a four day period.
The November 1984 event consisted of two separate rainfall bursts (between 6:00am and 10:00am and
9:00pm and midnight). Both produced flooding but the second burst was the most intense. One possible
reason why there are so few recorded flood levels is that the second burst occurred at night and thus few would
have been outside to view the flood extent or record levels.
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3.1.2
November 1984
The 8th-9th November 1984 storm was a significant rainfall event across the Sydney and Wollongong
region. Table 5 (taken from Reference 5) shows that this storm had an approximate ARI of
100 years across several locations in Sydney. It consisted of two bursts with the latter the most
intense period and flooding was reported throughout the catchment.
ARI Estimates of the 8th November 1984 Rainfall
Table 5:
Station
Sydney - Observatory Hill
Mosman
Vaucluse
3.2
0.5 hour
100y
20y
100y
1 hour
100y
50y
100y
Rainfall Duration
2 hour
100y
100y
50y
3 hour
100y
20y
20y
6 hour
100y
10y
10y
Rose Bay Water Level Data
Water level variations in Rose Bay will impact on flood levels in the lower parts of the open channel
system. The variations are largely as a function of astronomic tides but may also be influenced by:
•
wind set up and the increased barometric effect,
•
wave set up,
•
wave runup,
•
the Greenhouse Effect (Climate Change).
The adopted design water levels (taken from Reference 6) in Sydney Harbour at Fort Denison are
given in Table 6.
Table 6:
Adopted Design Water Levels at Fort Denison
ARI (year)
20
50
100
Events >100y ARI
Water Level (mAHD)
1.38
1.42
1.45
Not known but assumed as 1.50
However, these elevated water levels are unlikely to occur in conjunction with a flood over the Rose
Bay catchment which is generated by a short duration (less than 2 hours) rainfall event. The
coincidence of rainfall and ocean level events has been assessed in many similar studies, including
Reference 5. The approach of using a static water level of 1.0 mAHD in conjunction with flooding
in the local catchment was adopted for this present study. This level approximates to a tide that is
only exceeded a few times in a year.
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3.3
Historical Flood Information
3.3.1
Overview
A data search was carried out to identify the dates and magnitudes of historical floods. The search
concentrated on the period since approximately 1980 as it was considered that data prior to this
date would generally be of insufficient quality and quantity for model calibration. Unfortunately there
is no stream height gauges in the catchment or other means of reliably determining the level of past
flood events. Reliance must therefore be made on the following:
•
Woollahra Municipal Council records,
•
Sydney Water database,
•
previous reports,
•
questionnaire issued in June 2007 as part of this study,
•
local newspapers,
•
local residents.
3.3.2
Reference 4
Reference 4 undertook a review of all available flood information for the Bellevue Hill and North
Bondi catchments including flood depths, types of flooding and locations, and dates of flooding.
For Reference 4 some 7000 questionnaires were distributed with approximately 2% returned. A
summary of the key responses are provided in Table 7.
Table 7:
Questionnaire Results (Reference 4)
Item
Ever experienced flooding?
Nature of flooding?
•
Above house floor
•
Under house
•
In yard
•
In street
Yes Response
77%
19%
10%
32%
37%
Most respondents agreed that November 1984 was the worst flood they had encountered. The list
of dates of flooding in Reference 4 included no events prior to the 1980s. However it is unrealistic
to assume that there were no prior flood events. For example, 10th/11th March 1975 produced
significant flooding throughout the Sydney and Wollongong region and produced a significant daily
total (over 200 mm at Centennial Park and Botanic Gardens). Reference 7 reported rainfall depths
at Sydney Observatory Hill for the 1, 2 and 3 hour durations equal to or in excess of the 100 year
ARI design rainfall. Though there appears to be no reports of flooding in the Rose Bay catchment
for this event.
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Reference 4 also provides a summary of the flooding problems within the Bellevue Hill
sub-catchment. The majority of these locations are east of Old South Head Road, in the North
Bondi (Waverley LGA) sub-catchment which is not within the study area of this current assessment.
There has also been works implemented following the recommendations made in Reference 4,
hence some of the flooding problems identified in the areas below may now be alleviated.
•
the low lying flat areas around Plumer and O’Sullivan Roads due to water ponding,
•
the weirs in the Woollahra Golf Course obstructing flows,
•
along Balfour Road and Powell Road due to large overflows,
•
isolated flooding problems in the upper catchment in streets such as Latimer Road, due
to overland flows in gutters discharging onto footpaths.
Collected historical flood levels from Reference 4 are shown in Table 8 with the locations shown
on Figure 2.
Table 8:
Historical Flood Levels (mAHD)
Location
House
Nov 84
No.
Bellevue Hill Sub-catchment:
Balfour Rd
49
72
BF
75
Street
104
Latimer Rd
22
AF in 1988 storm
55
Yard in 1988 storm
Marion Av
8
Street
12
18
20
Street
22
Street
O’Sullivan Rd
63
Street
Plumer Rd
20
Yard/Street
Salisbury Rd
60
74
AF
Key:
AF
BF
Yard
Street
Jan 89
Storm Date
Mar 89
Dec 90
Jan 91
AF
BF
Yard
Always in heavy rain
Street
Street
Street
Street
Yard
Street
AF
= above floor
= below floor
= flooding in yard
= flooding over road
The limited flood history that is available does suggest significant flood problems have occurred in
the past.
3.3.3
Reference 1
The results from the DRAINS model outputs analysed in Reference 1 suggest that problems may
exist in the following areas within the Bellevue Hill subcatchment:
•
11 and 13 Bundarra Road and 8, 10 and 12 Blaxland Road,
•
19, 21 and 23 Bunyula Road and 58 and 60 Boronia Road,
•
49 and 51 Boronia Road and 177 and 179 O’Sullivan Road,
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•
•
•
•
•
•
63 and 65 Victoria Road,
2 Beresford Crescent and 50, 52 and 56 Beresford Road,
24, 26, 19 and 21 Balfour Street,
7 Aston Gardens,
9 Cranbrook Lane and 13, 15 and 17-19 Cranbrook Road,
Cranbrook School.
Within the Rose Bay subcatchments problems may exist at:
•
482, 484-486 and 488-492 Old South Head Road,
•
properties along Spencer Street,
•
34, 36, 51, 53 and 55 Dover Road,
•
56, 58, 71 and 73a Wilberforce Avenue,
•
about half of the Rose Bay Shopping Centre, particularly at the intersection of Newcastle
Street and New South Head Road.
3.3.4
Newspaper Reports of November 1984 Event
The November 1984 storms occurred over several days and caused widespread damage across
Sydney, hence generating a large amount of media interest. Reports in the Sydney Morning Herald
(10/11/1989) and The Sun (16/11/1989) indicate that Rose Bay was one of the worst affected
suburbs. Photographs 11 to 14 show the extent of the damage.
Events described in the articles include:
•
A ‘wave’ of water ran down Newcastle Street and Wilberforce Avenue to the shops at New
South Head Road, where water smashed shop windows and stacked cars on top of each
other (shown in Photo 11).
•
This water then went through Percival Park, ‘tearing great gashes in the soil’, shown in
Photo 13.
•
Eight shops in the Rose Bay shopping centre were damaged, the worst being opposite
Newcastle Street on New South Head Road. Water was reported as 1.7 m deep inside
one of these shops, and 1.5 m deep in another.
•
A mudslide was caused by a burst pipe on the playing field at Scots College, shown in
Photo 12 sending mud down the gully to Cranbrook Road, and filling houses and cars with
sand (Photo 14).
•
Where the two flow paths of Newcastle Street and Wilberforce Avenue met, 2 m high
waves were created.
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Photo 11: Cars piled on top of each other, New
South Head Road (source - SMH, 10/11/1989)
Photo 12: A burst stormwater pipe causing a
mudslide at Scots College (source - SMH,
10/11/1989)
Photo 13: Flow coming from Rose Bay shopping
area damages park and foreshore (source - SMH,
10/11/1989)
3.3.5
Photo 14: Sand from mudslide fills car (source SMH, 10/11/1989)
Information from Council Archives
A search was carried out of Woollahra Municipal Council archives for documents relevant to
flooding in the catchment. The relevant properties, dates of flooding and description of events are
listed in Table 9. Some of the dates are events already mentioned previously (such as 6/1/1989,
8/11/1984 and 26/1/1991). Other dates are typically from local flooding caused by inadequate
household or street drainage and may not have occurred in conjunction with significant catchment
flooding (this being the main focus of this present study).
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Table 9:
Flood Related Data from Council Documents
Address
Beresford Road
Date
06/01/1989
49 Boronia Road
02/03/1981
01/09/2002
51 Boronia Road
11 Bundarra Road
November 1984
26/01/1991
01/09/2002
08/01/1973
09/04/1973
01/05/1973
13 Bundarra Road
Dover Road
09/04/1973
06/01/1989
November 1984
51 Dover Road
June 1986
06/05/1986
72 Salisbury Road
2001
45 Salisbury Road
1986
Description
Water entered properties from Beresford Road, moved
through No. 75 and 77 Beresford Road and then into No.
86 Balfour Road.
Flooding of property, retaining wall collapsed into
property. This resulted from water ponding against the
retaining wall due to the reverse fall of the footpath.
Runoff from street entered property - went down pathway,
into backyard.
Rubble from road dumped in backyard.
Flooding at property.
Runoff from street entered property - went down pathway,
into backyard.
Flooding 3 mm - 19 mm in house (area received 152 mm
rain in 2 hours). Flow went over the concrete driveway
across the footpath, through the gate and down the
pathway to the front entrance door.
Water entered property though not as extensive as
8/01/73.
Flow entered property, into house (area received 25 mm
of rain in 20 mins).
Overflow from footpath entered property.
Water half way across nature strips outside houses
No’s 49 - 63.
Number of houses on west side of this section of Dover
Road were flooded from short duration, heavy storm.
Water ponding in front of No’s 51, 53 and 55 Dover Road,
perhaps aggravated by the camber of the road itself.
Faint water marks on the brick wall at the fence alignment
of No. 51. These marks were located approximately
60 mm above the back of the concrete footpath. Water
flowed into property under roller door and into the
building. Interviews with other residents living in the
vicinity of the subject property (No’s 55 and 32) indicated
that at the peak of the flood, water completely covered
the road.
Dover Road is on a flat grade with gutter grates no
greater than 0.5% grade which may contribute to the
flooding problem.
Front footpath had lifted and was diverting street water
into property. During heavy storms water flows into
property.
Water from gutter flow finds its way across the driveway
at No. 47. This has resulted from a kerb height that has
been reduced consecutively by pavement re-seals.
The above record provides minimal information on the November 1984 event which caused
extensive damage throughout the catchment.
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3.4
Results from July 2007 Resident Questionnaire
A total of 3146 questionnaires were sent out, of which 328 (10%) were returned. The results are
shown on Figure 3. The main outcome of the survey was the relatively low level of response and
the absence of detailed flood height data. Figure 3 shows that many residents have only recently
moved into the area, and thus would not have experienced the significant floods of November 1984
and the early 1990s. The location of the respondents to the various questions are provided on
figures in Appendix B.
3.5
ALS Survey
The Airborne Laser Scanning (ALS) data was provided to Council under separate cover and used
for establishing the 2D hydraulic model. This survey was flown in December 2005 and comprised
ground levels at approximately 1 m to 2 m intervals. These data have a vertical accuracy of the
order of ±0.15 m. The ALS levels are shown on Figure 4.
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3. DATA 3.1 Historical Rainfall 3.1.1 Summary of Pluviometer Data

Transcription

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