Gualan Island Study

GUALAN ISLAND STUDY, SOUTH FORD, OUTER HEBRIDES
Final Report
Ref: TSE/SA/865/68-PE/03
Alastair Dawson, Cristina Gómez and William Ritchie
Aberdeen Institute for Coastal Science and Management (AICSM)
e-mail: [email protected]
University of Aberdeen
February 2010
TABLE OF CONTENTS
Page
1. INTRODUCTION
1.1 Scope of requirement
2
2. METHODS
2.1 Location
4
2.2 Data used and techniques
5
2.3 Field work
8
3. RESULTS
3.1 Geomorphological Background
9
3.2 Landscape Changes between 1984 – 2005
9
3.3 Changes in High Water Mark of Ordinary Spring Tides (HWMOST)
11
3.4 Coastal Change over last 200 years
12
3.5 Recent Changes
15
3.6 Southern Gualan – South Uist area
17
3.7 Volumetric and Area Changes
19
3.8 Discussion
22
3.9 The Future of Gualan Island
25
3.10 Recommendations
27
4. REFERENCES
30
5. APPENDICES
Appendix 1
31
Appendix 2
35
Appendix 3
45
i
GUALAN ISLAND STUDY, SOUTH FORD, OUTER HEBRIDES
Alastair Dawson, Cristina Gómez and William Ritchie
Aberdeen Institute for Coastal Science and Management (AICSM)
University of Aberdeen
1
INTRODUCTION
The future evolution of the South Ford area, between Benbecula and South Uist, will
to a large extent be influenced by what happens to the barrier island of Gualan, which
almost closes the 2.7 km “gap” on the Atlantic side of South Ford. Gualan is a
remnant strip of sand and a vulnerable barrier which, in the future, could disappear
and open-up the west side of South Ford to the full range of Atlantic wave and tidal
forces. The aim of this research project is to attempt to reconstruct the evolution of
Gualan Island with an emphasis on recent trends and possible future changes. The
report concludes with a number of recommendations for future courses of action. This
Gualan Island project represents a contribution to other projects that are integral to the
overall study of the South Ford area and with which Comhairlie are familiar.
1.1 Scope of Requirement
Five scopes of requirement issues are considered here:
1.
Arising from previous research on coastal evolution in Scotland by the
Aberdeen team, the special nature of Gualan will be assessed partly by a
revision of this pre-existing body of relevant knowledge. This research
element makes use of former maps of the area (e.g. the Ordnance Survey maps
of AD 1878) that define earlier positions of the island.
2.
When the Aberdeen team undertook ground based LiDAR and GPS coastal
surveys during winter 2008, additional GPS control data was gathered for
Gualan in order to secure a number of ground control points. BAE
SOCETSET with its algorithms (see the South Ford survey project details) for
sets of aerial photographs of Gualan together with LiDAR data were analysed
to produce Digital Terrain Models (DTMs) as well as maps of change.
2
3.
The coring and sediment dating and analysis undertaken by Dr Rowan
(Dundee) have a relevance to this work since it provides information on the
nature of recent sediment dynamics adjacent to Gualan Island.
4.
Fieldwork was undertaken during spring 2009 in order to assess the
vulnerability of the coastal dunes in Gualan to erosion and to washover
processes that have taken place over the 2008-2009 winter. The centre – north
of the island is very narrow and low and an assessment of this vulnerability is
an important part of the proposal.
5.
Meteorological records for the Monach Isles lighthouse (records kept between
1867-1942) have been analysed. This work has focused on air pressure data
associated with extreme storms. An attempt has been made to answer the
question if there have ever been times during the latest 19th century (when
storminess was at its highest) where air pressure fell to lower levels than it did
during January 2005.
3
2. METHODS
To analyse the actual condition of Gualan Island and the changes occurred during the
last decades and centuries, various geospatial technologies have been applied: from
basic observation and drawing to more sophisticated LiDAR and GPS.
Comprehensive knowledge of coastal geomorphology and processes were required for
interpretation of all data collected.
2.1 Location
Gualan Island is a barrier island located in the transition area between Benbecula and
South Uist Islands (Outer Hebrides, Scotland). It is centred at (77537, 847762) OS
GB36.
Figure 2.1 Location of the study area. Gualan Island is between Benbecula and South
Uist.
4
2.2 Data used and techniques
a. Digital Terrain Model
Two Digital Terrain Models (DTMs) derived for the quantification of the physical
changes in the coastal landscape of the South Ford area were the main sources of data
for evaluation of changes occurred in the last two and a half decades. The DTMs were
derived for years 1984 and 2005. The main original sources of data for construction of
these models were two sets of vertical colour aerial photography and XYZ data
acquired with laser scanner in terrestrial and aerial field campaigns.
All photography was acquired from the CUCAP (Cambridge University
Collection of Air Photos) catalogue in digital format. Some characteristics of the
photos are resumed in Table 2.1.
Table 2.1 Characteristics of the data sources for derivation of DTMs
DTM 1984
DTM 2005
Number of frames
16
14
Scale
1:15,000
1:10,000
Date
24/04/1984
09/06/2006
TLS
Date
-
09/09/2008
ALS
Date
-
16/11/2005
Aerial photography
Aerial laser scanner (ALS), also known as LiDAR (Light detection and range)
data were available for the Western part of the South Ford including Gualan Island
from an aerial survey undertaken in 2005 by SNH (Scottish Natural Heritage). LiDAR
data has in this case 1 m resolution in the XY plane; its accuracy is normally in the
range 0.2-0.3 m (Maune, 2006).
Terrestrial Laser Scanner (TLS) data were collected during fieldwork in
September 2008. Areas of special interest identified on photography and with
previous knowledge of the area were surveyed in detail. These areas are the most
susceptible of change and might be re-surveyed in the future for monitoring purposes.
The most up to date equipment from Trimble®, the GX 3D Scanner (Trimble, 2009),
was used for this purpose.
5
Global Positioning System (GPS) points were measured with Trimble® 5800 GPS
receivers with TSC2 data loggers. The methods used to measure these points
depended on the telephone signal available. When the reception was good, corrections
on real time (RTK) method with VRS (Virtual Reference Stations) was used. In areas
were the telephone reception was not good DGPS (Differential GPS) method with
post processing was applied in static mode.
Figure 2.2 Digital Terrain Model of the South Ford area used in the study. Left: DTM 1984; right:
DTM 2005
Geographical Information System (GIS) techniques permit the identification of
explicit locations where changes have occurred in the morphology of coastal
formations (Dawson et al., 2007) and the analysis of change quantities, with a defined
level of accuracy. Evaluation of planimetric change as well as change in height is
readily done, making the estimation of volumetric changes possible. An accurate
registration of all data in the same coordinate reference system (OS GB36) enabled
correct results when algebraic operations of the raster DTMs were implemented.
DTMs spatial resolution for these operations was 5x5 m, small enough to capture the
detail of coastal formations and changes (Gómez et al., 2008).
6
b. Historical maps
For analysis of longer term changes occurred in the last centuries, historical maps
dated 1805, 1878 and 1965 were valuable sources of information. These maps were
scanned and relevant elements for analysis, such as MHWL (Mean High Water Line)
digitized. Comparison of the planimetric location of the MHWL at different dates
allowed interpretation of changes and processes going on in the Gualan Island area.
Figure 2.3 Historical map of Benbecula—AD 1805
c. Historical aerial photography
A third source of data consists of various sets of historical photography (Appendix 2)
which were found in TARA (The Aerial Reconnaissance Archive) in Edinburgh.
Digital copies of this photography were obtained from the archive and visually
interpreted and compared with recent photography. Table 2.2 shows a list and some
details of the photography used. These photo images were not subject to quantitative
analysis in this study but they were used as supplementary sources of information to
enable checking of the validity of other map and photography measurements.
7
Table 2.2 List of historical photography used for interpretation in the study. Source:
TARA (The Aerial Reconnaissance Archive)
Photo code
Date
Scale
CPE_UK_0189_1376
10/10/1946
1:10000
CPE_UK_0189_3390
10/10/1946
1:10000
CPE_UK_0189_3391
10/10/1946
1:10000
CPE_UK_0191_4051
10/10/1946
1:10000
CPE_UK_0191_4053
10/10/1946
1:10000
OS_63_062_035
24/05/1962
1:27000
OS_63_146_018
01/07/1963
1:27000
OS_65_072_001
01/05/1965
1:5000
OS_65_072_074
01/05/1965
1:5000
OS_65_090_067
13/05/1965
1:5000
2.3 Field work
Field work is always necessary for a correct interpretation and for verification of the
results obtained in the laboratory. In August 2009 coastal geomorphologists (AD and
WR) made a field trip to Gualan Island to get a better understanding and to correctly
interpret the outcomes previously obtained with GIS techniques on the computer
(field photographs in Appendix 3).
8
3. RESULTS
3.1 Geomorphological Background
Gualan Island represents a classic barrier island that effectively closes off the South
Ford basin from the full force of Atlantic waves. The island is separated from South
Uist by a small channel that is only operative during high tide. For the most part, the
southern part of the island is separated from South Uist by low sandbanks. To the
north, the island is separated from Benbecula by a tidal channel, the north channel that
is the main conduit for water flow between the Minch and the Atlantic. To the lee of
the northern and central parts of Gualan, flood tide sedimentation has resulted in the
development of a large intertidal delta (Figure 3.1; 4). Similarly, on the Atlantic side
of Gualan seaward of the exit point of the north channel, a large ebb tidal delta is
present (Figure 3.1; 6).
3.2 Landscape Changes between 1984 - 2005
Comparison of the 1984 and 2005 DTMs reveals some significant changes that have
taken place. These are listed below:
1. The seaward edge of Gualan appears to have retreated to the east. This is
shown by the strip of red on the change map (Figure 3.1). The greatest amount
of retreat has been in the north (shown as red and purple) the amount of retreat
decreasing from north to south. At the very north there is a recurve shown as a
concavity of the change map and corresponding to the area of red and purple
(Figure 3.1; 3).
2. There appear to have been two areas of sediment accretion. The first of these
is at the extreme northern end of the island (Figure 3.1; 2). Here, a gravel,
sand and boulder spit extend to the NE towards the edge of the north channel.
This spit shows itself as an area of convexity. To the south it is attached to
high (5-10 m) vegetated dunes that make up a complex area of undulating
marram-clad topography. The second area of accretion is at the extreme south
of the barrier island. This area consists of two distinct areas of landscape. To
the rear of the southern end of Gualan, there is an area of sand accretion that
effectively blocks the movement of tidal waters between Gualan and the
9
northern coastline of South Uist (Figure 3.1; 5). The second area of accretion
here consists of the sand dunes themselves. In this area, the coastal dunes have
accreted vertically since 1984 forming a stable barrier of vegetated sand. This
is shown as a thin green line indicating that despite the west to east retreat of
the shoreward face of Gualan, there has also been vertical dune accretion.
3. In the extreme north of the area, complex changes have taken place with areas
of high erosion lying adjacent to areas of accretion. In this area, the most
extreme sediment loss has been on the Benbecula side of the north channel
(Figure 3.1; 1). This appears to be related in some way with tidal flooding in
this area and the construction of a flood tidal delta in the lee of the northern
and middle parts of Gualan Island. In the lee of Gualan, these processes may
be linked to numerous small areas of sediment loss marked on the map by a
series of anastomosing channels and sandbanks.
Figure 3.1 DTM change map between 1984 - 2005
10
3.3 Changes in High Water Mark of Ordinary Spring Tides (HWMOST)
The relative positions of High Water Mark of Ordinary Spring Tides (HWMOST)
were superimposed onto the DTMs in order to investigate patterns of change. For this
we used the Admiralty Tide Tables manual value of 2.3 m OD. The plots show
significant change in some areas and negligible change in others. The greatest
changes have occurred in the northern area of Gualan and the adjacent coastal area on
the north side of the channel. In the latter area, the reconstruction shows a huge loss of
land since 1984. In the northern area of Gualan, these data replicates the descriptions
listed above except that in this case a planimetric change can be observed. Hence the
greatest changes have been along the seaward face of Gualan, where a lateral
recession of HWMOST is indicated between 20 – 30 m (Figure 3.2).
Figure 3.2 HWMOST positions for 1984 and 2005
11
3.4 Coastal Change over last 200 years
In this section we refer firstly to the Benbecula Estate map for AD 1805 (see also
section 2). Although this map covers the area due north of Gualan and only shows the
southern coastal of Benbecula together with a limited part of the South Ford basin, it
shows some features of fundamental importance to the present study (Figure 3.3).
Figure 3.3 Area of southern Benbecula, AD 1805. Note the pink area showing the
Lionaclete area as shown as bare sand. Note also the tidal inlets that presently exist
as freshwater lochs.
Some remarkable changes can be observed comparing the map a recent one
(Figure 3.4). The first is that it shows the area presently north of Lionaclete school as
being characterised by bare sand. This area is the only such area shown as bare sand
for southern Benbecula in 1805. Second, it shows a radically different coastal
geography for the area south of the school and as far east as the road junction past the
Isle of Benbecula Hotel. For the area adjacent to Lionaclete School, a small tidal
creek is shown extending inland (Figure 3.4). Another tidal creek is shown for the
loch area east of the Isle of Benbecula Hotel (Figure 3.4). By contrast, the position of
12
Hestimul island as well as the route of the north channel appear to be in precisely the
same positions as they are today.
Figure 3.4 Coastal line in 1805 drawn over recent map. Note two right hand arrows
indicate locations of two tidal creeks while left arrow shows approximate position of
HWMOST located much further seaward than today
In all the later maps of this area, the tidal creek adjacent to Lionaclete is shown
as a freshwater loch. In addition, all maps for the late 19th and 20th centuries show a
linear ditch extending from this loch to the sea. At present this ditch crosses the main
road ca. 140 m east of the Dark Island Hotel. The purpose of this ditch was to create a
drainage outlet for the loch and surrounding fields. Together, this information points
to a large inundation of blown sand into this area sometime after the start of the 19th
century (i.e. post AD 1805) but prior to the latter part of the 19th century when the
first Ordnance Survey map was produced for this area. To what extent, this period of
sand blow also affected Gualan is not clear. However, we can presume a huge
seaward advance of the Lionaclete coastline at this approximate time. All tidal creeks
became blocked by blown sand. Subsequently, coastal erosion along the flank of the
13
north channel adjacent to Gualan began to remove large quantities of the large
volumes of blown sand.
This change represents one of two reasons why there appears to have been so
much coastal erosion south of Lionaclete School. This coastal erosion has served to
remove and redistribute the volumes of blown sand that were deposited in this area
soon after 1805 and which had caused the coastal edge to advance seawards.
Further information on the nature and rate of coastal change is available from
inspection of the Ordnance Survey 1:10,560 map of 1878 (Figure 3.5). Remarkable
changes in the position and shape of Gualan are evident. For example, the northern
end of Gualan was then located nearly 500 m south of its present position. Given the
more southerly position of the Lionaclete coastline at this time, one can envisage a
200-250 m wide tidal strait through which Minch and Atlantic waters were
exchanged. Not only was the northern end of the barrier island located much further
south but the Atlantic shoreface of the island was located ca. 100 m west of its present
position (Figure 3.5). The coast line of Ordnance Survey map for 1965 is also plotted
on Figure 3.5. The position of HWMOST for this period shows that continued erosion
along the northern flank of the north channel had taken place between 1878 and 1965.
It is inferred, therefore, that during this time, the position of the north channel was
located where the most northern spit occurs at present.
14
Figure 3.5 Coastline change during last 200 years.
3.5 Recent Changes
The most recent changes to have affected Gualan are shown on an oblique DTMgenerated image (Figure 3.6). In this image, the scale of the erosion adjacent to
Lionaclete can clearly be seen in addition to the loss of sediment on the seaward flank
of the northern section of Gualan Island. The newly-formed gravel spit at the northern
end (Figure 3.6; 2) can also be seen very clearly. The change between the vegetated
dunes at the northern end of Gualan and the gravel spit to the north represents a major
change in the style of sedimentation for this area. Since the 2005 imagery was flown
after the January 2005 great storm, it is not possible to tell if this change was a direct
consequence of the storm or if this change represents the effects of more gradual
coastal changes that took place between 1984 and 2005. We favour the former
explanation for no other reason that the change from sand deposition across an area of
coastal dunes to the deposition of a coarse-grained gravel spit containing boulders,
would seem to represent a marked change in wind and wave conditions.
15
Figure 3.6 Oblique view of height changes in Northern Gualan Island, 1984-2005
The areas of red in the lee of Gualan island are intriguing (Figure 3.6; 5). It has
been argued earlier that this land area, when submerged during high tide, represents
the general site of a flood tide delta (Figure 3.6; 6). We are of the opinion that much
of the sediment accretion adjacent to this area represents part of this intertidal delta.
The red areas, all submerged during high tide, may represent ephemeral channels
adjacent to this delta area. The areas of sediment accretion associated with the flood
tide delta lie landward of the tributary channel (Figure 3.6). Towards the centre of the
basin and to the east and south of the main channels, the overwhelming pattern of
change is that of sediment gain. A significant part of this may represent parts of the
flood tidal delta sedimentation.
16
3.6 Southern Gualan – South Uist area
The southern part of Gualan Island and adjacent areas between the island and South
Uist exhibit evidence for complex changes between 1984 and 2005. Contemporary
accounts for the early 1980s describe the presence of an open water channel
separating Gualan from South Uist that enabled an exchange of tidal waters between
the Atlantic and the South Ford basin. The 1984 air photographs show this channel as
a clear feature. Since then, the channel has been subject to aggradation, having been
filled by sediment sufficient to reduce this channel to a broad low sandbank through
which very little water flows.
Figure 3.7 Southern section of Gualan Island showing HWMOST positions
In addition to this area of sediment infill in the area of the channel, there has
been a more wide-scale infill of sediment in the lee of southern Gualan. The DTM
shows this change very clearly (Figure 3.8; 1). This area of sediment infill has
extended southwards as far as the culvert at the northern end of Loch Bi. The channel
that takes fresh water from Loch Bi is still a clear feature in the landscape. However,
17
it is surrounded by a broad low sand plain that represents the southern section of this
broad area of sediment infill.
The positions of HWMOST show significant changes to have taken place
between 1984 and 2005. For southern Gualan the most notable changes have been
along the Atlantic shoreface of the barrier island. The HWMOST plots show
significant recession over this time interval, these having occurred mostly along the
stretches of the barrier where the coastal dune areas are scarce and fragmentary
(Figure 3.7). For the extreme southern section of the barrier island there has been
negligible change in the position of HWMOST. As supplementary information, there
have been some notable changes along several areas of the adjacent shoreline of
South Uist. Here, areas shown as red in 1984 appear to have been subject to a degree
of additional tidal inundation by 2005 (Figure 3.8).
The topographic changes that have taken place between 1984 and 2005 across
southern Gualan are varied and complex (Figure 3.8; 3). The majority of the shoreface
of the island has been subject to uniform erosion and retreat with the amount of retreat
decreasing to the south. An exception has been along the extreme area where there
has been no change has been along the extreme southernmost section of the barrier. In
the latter area, there is also clear evidence of dune accretion and upwards growth
(Figure 3.8; 2). In addition to the area of sediment accretion in the lee of the southern
end of the island, there is also a linear strip of green (accretion) that extends from the
extreme south of the island ca. 300 m northwards. This is an important observation
since it appears to indicate that this area was more susceptible to marine inundation
during storms ca. 1984 than it is today. A consequence of this change is that the basin
area in the lee of the southern end of Gualan is essentially protected due to the
combined effects of this sediment accretion and the near complete closure of the tidal
channel at the southern end of Gualan.
18
Figure 3.8 Southern end of Gualan Island
3.7 Volumetric and Area Changes
The DTM change maps have enabled the calculation of volumetric changes of
sediment gain and loss between 1984 and 2005. In attempting to understand what
these changes mean, one has always to be aware that the volume changes measured
represent simply the changes that have taken place between the two dates during
which remote sensing was undertaken. As a result we know nothing about the nature
of change that took place at various times within the intervening period. Specifically,
since the January storm took place prior to the 2005 photography, it is not possible to
tell how much of the volume changes were attributable to this one extreme event.
Notwithstanding this caveat, Figure 3.9 shows the volume changes that have
taken place at the northern end of Gualan adjacent to the north channel. By far the
largest area where volume change has occurred, ca. 48,500 m2, has been eroded over
this 20 year period. It is inferred here that the key process responsible for this change
have been the flood tide currents that enter the South Ford basin at this point from the
Atlantic. On the opposite side of the channel, the northern end of the island spit has
19
advanced to the NE and has resulted in the deposition of an additional ca. 4300 m2 of
sediment as far as the southern edge of the channel.
Figure 3.9 Red transparency shows eroded area, green transparency shows accreted
area at the northern end of Gualan island
The patterns of sediment volume loss and gain across the basin are shown in
Figure 3.10. This diagram highlights the key areas within the basin where the greatest
sediment gain has taken place as well as areas along the open coast where there has
been loss. For this report we focus on change at Gualan Island. Coastal erosion has
been dominant along the shoreward Atlantic face of the barrier island along virtually
all of its length except for the northern spit area. In real terms this loss is most
probably due to shoreline retreat. This does not mean, however, that the entire feature
is being eroded away by the sea. It should be noted that while the shoreface of the
southern section of the island has undergone retreat, there has been a synchronous
vertical accretion of the coastal dunes in this area.
20
Figure 3.10 Areas where volumetric change has been calculated. Areas named L have
suffered loss of volume; areas named G have gained volume. Note that region L1 extends
further north and it is truncated by the limits of this Figure.
The data shown in Figure 3.10 and Table 3.1 indicate that over ca. 20 years the
shoreface of Gualan (Figure 3.10; L4) has experienced an areal loss of ca. 88,000 m2
equivalent to an approximate loss of 7,800 m2 per year.
Table 3.1. Areas and volumes of sediment gain and loss shown in Figure 3.10
Area
Surface (m2)
G1
G2
G3
G4
G5
L1
L2
L3
L4
152,739
499,308
43,438
17,995
44,075
55,674
12,830
108,897
88,259
Total Volume
(m3)
180,494
307,878
35,382
26,561
61,835
-64,602
-10,598
-245,785
-194,794
Volume per
surface (m3/m2)
1.182
0.617
0.814
1.476
1.403
-1.160
-0.826
-2.257
-2.207
Total Volume per
year (m3/y)
7,219.76
12,315.12
1,415.28
1,062.44
2,473.40
-2,484.08
-423.92
-9.831.40
-7,791.76
21
3.8 Discussion
This section is prefaced by a map showing the extent of coastal flooding that took
place during the January 2005 storm (Figure 3.11). This map was generated by taking
the observational data and superimposing this survey map information onto the DTM.
The map shows not only the extent of flooding but also spatial variations in the flood
levels. The map should always be regarded as an incomplete map since not all flooded
areas were surveyed – the spread of the data is only as good as the amount of
observational information that was entered onto the original flood map.
Figure 3.11 DTM reconstruction of the areas flooded during the 2005 storm
For the purpose of this report, only one area of Gualan is shown as having
been overtopped by floodwaters. This area is the central section of the barrier island.
It coincides with the area where coastal dunes are fragmented or absent and the
protective backshore shingle and cobble ridge is relatively low and often eroding.
Field inspection of this area during August 2009 shows that this area is essentially
dune-free. On the ground, the barrier in this area is represented by a ridge of coarse
gravel. The beach shingle is here locally mantled by seaweed that extends over the top
22
of the ridge and onto areas in the lee of the ridge – showing that recent storms are
regularly overtopping the ridge section in this area (see Appendix 3 – ground
photographs).
Thus, Gualan can be usefully divided into 4 distinct sections. In the extreme
north there is a well-defined recurved gravel and shingle spit. Its origin and shape are
clearly due to strong flood tide currents and distal end longshore transport. This spit is
relatively new feature –and it is speculated here that due to the distinct character of
this feature, it may have been mostly produced during the 2005 storm. The second
area lies to the south of the spit. This area is characterised by high (over 10 m)
vegetated coastal dunes. To all intents and purposes, this section of the barrier island,
owing to its greater width (50-100 m) and height (up to 15 m) has no likelihood of
being breached by the sea. Locally the dunes also show at least 2 phases of
northwards extension, it probably having kept pace with the developing spit. These
northerly extensions have either narrowed the flood –ebb tidal channels or pushed
them northwards towards Lionaclete. Nevertheless, a concave plan profile at the
shoreface points to recent severe erosion having taken place (possibly during the 2005
storm). The third section is the central area (described above) where the cover of
coastal dunes is fragmentary and absent in some areas. This is the area presently being
overtopped during winter storms and is also where a future storm might easily breach
the barrier island (Figure 3.12). The fourth section is in the southern area of the
barrier island. This area, like those further north, has also experienced shoreface
retreat in recent decades. However, this is also the area where the coastal dunes have
accreted during the last two decades and where the coastal dune topography has
increased in elevation – thus making this area less susceptible to wave overtopping
than was the case in 1984. The former main exit for the drainage from Loch Bi at the
south end of Gualan is now almost closed and filled with sand.
23
Figure 3.12 Locations 1 and 2 mark the northern coastal dune limit in 1984. Site C
identifies a dune area presently susceptible to limited degradation. Areas A, B and D
represent the areas present susceptible to the greatest erosion and wave overtopping.
Within area A, the coastal stretch shown as B defines the area where a future breach
is likely (under a ‘do nothing’ strategy).
24
3.9 The Future of Gualan Island
Lessons from the Past
In relation to any attempt to make recommendations in respect to what ought to
happen to Gualan Island in the future, it is important to understand past changes and
how Gualan has responded to past episodes of climate change and extreme weather.
In the engineers’ causeway plan document (cited in the University of Dundee
report), a borehole adjacent to the causeway route recovered peat deposits occurring
beneath ca. 5 m of sands and silts dated to ca. 4-5000 years before present. This
observation is consistent with the long-held view that during the last several thousand
years there has been a sustained rise in relative sea level and accompanied by general
coastal retreat in this area during which a former land surface (indicated by the peat
deposits) was buried by marine sediments.
Field inspection of some of the small islands located 3-400 m east of Gualan
island reveal the presence of machair sediments. This observation, together with local
accounts of machair on the islands of neighbouring Hestimul, point to the conclusion
that at some stage in the past, presumably after relative sea level had reached near
present, much of the South Ford basin was covered in machair. For a number of
reasons (including sea level rise, possible climatic change and, latterly, changes in
land use) this machair landscape became degraded to the extent that now there are few
areas left. It is this context of past changes that provides a setting for the future
evolution of Gualan Island. The Ordnance Survey map for AD 1878 makes it clear
that the island has migrated eastwards over the last ca. 140 years. Reconstruction of
the position of HWMOST for this time indicates that a landward recession in the
order of 100 m has taken place - equivalent to an average recession rate of ca. 0.7 m
per year. Comparison of the 1984 and 2005 maps points to a similar rate of recession
over the last ca. 20 years. In this case, the recession across the central area has been in
the order of 20 m equivalent to a retreat rate of ca. 1 m per year. This having been
said, the southern section of Gualan Island has risen in elevation by ca. 1 m since
1984 due to the vertical accretion of wind-blown sand on the crest and lee of the
coastal dune ridge.
25
From these observations, it becomes clear that the key area of concern for
Gualan is the central section where the cover of coastal dune sediments has all but
disappeared. The presence of fragments of the seaweed, Fucus vesiculosis, on the
crest of the gravel ridge here demonstrates that this area of the barrier island has been
regularly inundated by Atlantic waves during recent winters. The 2005 DTM also
provides clear evidence that this area was breached by floodwaters during the 2005
storm. In terms of geomorphology, this process is described as washover, when
sediment is transported onto the flat intertidal area of South Ford as a sand-splay –
with some gravel and shingle. In the long term, these areas of overwash could be
stabilised by vegetation but this is here considered as unlikely to happen. Crucially
however, there is little chance of this sediment ever being returned to the Atlantic side
of the barrier island.
Field inspection of this area shows evidence that some measures might have
been undertaken locally to prevent further erosion in this area. In the lee of the beach
ridge lie numerous accumulations of discarded fishing nets. These have been placed
randomly in preparedness for unravelling these across the ridge surface in order to
serve as means of trapping sediment. However, none have been put in place while
many are non biodegradable. Other measures have been taken in this area to alleviate
erosion. The most disturbing of these is provided by vehicle tracks in the central area
that seem to indicate the effort of an individual to excavate gravel from the beach and
deposit it on top of the ridge. At the present time, therefore, there appears to have
been no measures undertaken to shore up coastal defences in this area.
Inspection of weather data for the Monach Isles shows clearly that, in the past,
the Outer Isles have experienced many severe storms some of which have been
associated with lower air pressure than that which occurred during the January 2005
storm. In particular, we draw attention to the storm of January 14-18, 1871 when air
pressure fell to a record low of 938 mb (compare with 955 mb during the January
storm of 2005). We quote the contemporary lighthouse keepers’ report, ‘…on the 16th
at 2 am, wind from South, heavy falls of sleet, at noon the wind shifted to the SW, the
barometer fell to 27.70 inches of mercury (938 mb) – this is the lowest seen on the
barometer since coming to this station..’. From the perspective of ca. 150 years of
weather history for the Outer Isles, there indeed have been other storms in the past
26
that have had a magnitude and intensity comparable to the 2005 storm. These appear
to be represented in past weather records by individual storms during 1869, 1871,
1872 and 1921 (Dawson et al. 2007). We therefore interpret the 2005 storm as being
‘..the worst in living memory..’, yet beyond living memory there have been a small
number of other highly destructive storms. Despite the UKCIP09 Climate Impact
report prediction of no appreciable increase in winter storminess across northern
Scotland over the next 2 decades, the coastal communities of the Outer Isles are
advised here to prepare for another highly destructive storm at some time in the future
(Lowe et al. 2009).
3.10 Recommendations
The above observations demonstrating a slow (ca. 1 m per year) landward rollover of
the barrier island system together with recent breaching during storms points to the
existence of a dynamic and fragile barrier island complex. If we add to this an
estimate of a long-term rise in relative sea level in the order of 2 mm per year
(DEFRA data cited in Dawson et al. 2008), one is bound to envisage that, in the
future, the barrier island will continue to experience shoreface erosion and landward
retreat together with occasional breaching during storms.
Apart from the functioning of the barrier island itself, Gualan Island performs
another key role as acting as a barrier that separates the full force of Atlantic waves
from the sheltered coastline that surrounds the South Ford Basin. Waves generated
within the basin at the present time during storms are never high owing to the limited
fetch environment in which they can develop. As a consequence, wave action along
the shoreline surrounding the intertidal basin of South Ford is relatively minor. This
opinion was confirmed during a field survey undertaken 10 weeks after the storm,
when it was very noticeable that evidence of severe storm erosion (dislodged
boulders, eroded coastal cliffs, etc.) was absent within the South Ford basin (Dawson
and Dawson, 2005). The key argument in respect of the future of Gualan has to be
whether the central section should be allowed to breach during future storms or if
measures should be taken to prevent breaching of the central section.
One option is therefore to do nothing. For purposes of cost-benefit analysis
this has to represent the baseline for this study. If nothing is done in the future, it is
27
highly probable that the central part of Gualan will breach during a severe winter
storm. His breach may, in time, widen to an unknown size. The main consequence of
such a change would be to increase the erosive strength of wind-generated water
waves within the South Ford basin and, presumably, lead to an acceleration of coastal
erosion around the basin. If such a change was to happen, a new second tidal channel
would therefore open up enabling the diurnal exchange of Atlantic waters with those
in the South Ford basin. This report cannot predict the exact consequences of such a
change. We therefore await the results of the modelling group to inform us of what
such an effect might be. One might expect, however, that part of the present tidal flow
through the north channel might be re-routed through this new breach.
Whether or not two channels would continue to exist due to the exchange of
tidal waters is open to question. Certainly pattern of ebb and flood channels in basin
would alter significantly as would the pattern of tidal drainage channels. The position
and size of individual sandbanks would also change. Would this reshaping be more or
less of a risk to flooding processes around the bay in the future? Would such a change
also reduce the tidal flow through the culvert at the north end of the South Ford
causeway? These questions are probably most appropriately answered by the results
of the modelling study.
The width of such a breach would be very important in determining wave
regimes within the South Ford basin. It is suspected that a narrow channel would have
a limited effect on wave state. A wider channel on the other hand might allow the
movement of wind-generated water waves into the basin and, by this process, increase
rates of coastal erosion around the edges of the basin. Although the causeway is
almost 3 km East of Gualan, it might be exposed to increased attack from windgenerated water waves. Sediment dynamics within the basin would almost certainly
change also. Perhaps key in such a set of processes of change would be the issue of
how wide such a Gualan barrier breach might become. It is entirely possible that a
relatively narrow channel (ca. 20 m wide) might develop over time into a channel or
tidal strait several hundred metres in width changing the basin into an embayment in
which storm waves could develop.
If such a breach were to develop and widen the consequences are difficult to
foresee. For example, even though the Gualan barrier failed in 2005 it remains unclear
28
what the direct effects of this breach were within the basin. It might be prudent,
therefore, to embark on a reconstruction of the coastal dune ridge along the central
section of the barrier island through the provision of an appropriate amount of sand
sufficient to create a coastal dune ridge at least 20 m in width and up to 10 m in
height. Such a ridge should have embedded within it, sufficient lengths of
biodegradable hessian (jute) matting that would have the purpose of binding the sand
together and diminishing its susceptibility to future erosion by storms and high tides.
We would recommend that the defences should be reinforced by gabions embedded
within the dunes, these serving as fail safe devices. A supplementary action should be
the planting of marram grass using the sprigging method that, in conjunction with
hessian matting, should be sufficient to establish a continuous cover of vegetation
along the length of coastal ridge presently at risk from erosion and wave overtopping.
29
REFERENCES:
Dawson, A.G. and Dawson, S. (2005) Western Isles Coastal Zone Assessment
Survey, Benbecula and South Uist, Commissioned Report for EASE Archaeology and
Historic Scotland, 125pp.
Dawson A.G., Dawson, S and W Ritchie (2007) Historical Climatology and coastal
change associated with the 'Great Storm' of January 2005, South Uist and Benbecula,
Scottish Outer Hebrides, Scottish Geographical Journal, 123, 2, 135 – 149.
Dawson, A.G., Ritchie, W.A., Green, D., Wright, R., Gomez, C. and A Taylor (2008)
Assessment of the rates and causes of change in Scotland’s beaches and dunes –
Phase 2, Commissioned Report for Scottish Natural Heritage, Account No. SM002
RGC1479.
DEFRA report and data:
http://www.defra.gov.uk/environ/fcd/pubs/pagn/climatechangeupdate.pdf
Gómez, C., Taylor,A., Green,D., Ritchie,W., Dawson,A., and R.Wright (2008)
Terrain 3D modelling for the assessment of coastal change in beach and dune systems
in Scotland. SOC Bulletin, n.42
Lowe, J.A. et al. (2009) UK Climate Projections science report: marine and coastal
projections. Met. Office Hadley Centre, Exeter, UK, ISBN 978-1-906360-03-0.
Maune, D.F. (2006) Digital elevation model technologies and applications: The DEM
users manual, 2nd edition
Note: all maps used within this report were printed under licenses held by the
University of Aberdeen for research purposes.
30
Appendix 1.
Monach Lighthouse, Gales :1868 -75
Note: this record is incomplete due to problems associated with the quality of the
data and the recording of gale occurrence. The most importance issue arising is the
occurrence of a severe gale during January 1871 when air pressure fell as low as 938
mb – this value compares with the lowest pressure reading for the January 2005 storm
of 955 mb. A similar severe storm during January 1872 had a recorded air pressure
minimum of 946.5 mb.
Sept 1868
4th at 5 am – ceased at midnight
19th at 10 pm - until 20th at 3 pm
29th at 10 am - storm continues till midnight 30th
October 1868
5th at 2 pm - ceased at midnight of 7th
14th at 3 pm - ceased 8 am 17th
18th at 10 pm - ceased at 3 pm of 19th
20th at 6 am – ceased at 6pm on 20th
22nd at 3 pm – ceased at 2 pm on 24th
Midnight of 25th – ceased at 8 pm 27th
28th at 10 am – ceased midnight on 30th
November 1868
3rd at midnight – ceased 8pm on 6th
21st at 6 pm – ceased at 4.30 pm on 22nd
22nd at 7.30 am – ceased at 6 am on 23rd
28th at 10 am – ceased at 10 pm on 28th
29th at 2 pm – ceased at end of month
December 1868
5th at 2 am – ceased at 4 pm on 6th (964 mb)
9th at 9 am – ceased at midnight
Heavy gale: 10th at 4 am – ceased at midnight (970 mb)
12th at 4 am – ceased 6pm same day (963 mb)
17th at 7 pm – ceased at 6pm on 18th (979 mb)
22nd at noon – gale 8 – until 11 pm (964 mb)
26th at 8 am – gale 8 – ceased 2pm on 27th (962 mb)
27th at 8 pm – gale 9 – ceased 9 am 29th (953 mb)
January 1869
28-29th - strong gale
February 1869
14th – storm from NNW
26th – storm from SW
September 1869
17-19th – gale
December 1869
12-14th – severe gale
31
January 1870
7-8th – heavy ground swell with gale
14th – tremendous ground swell at high water
May 1870
11-12th – gale
August 1870
10th – heavy ground swell – but no gale
September 1870
9th – gale
October 1870
18-20th – gale
23-25th – strong gale
November 1870
20-22nd – gale
January 1871
15-17th - SW severe gale – air pressure falls to minimum of
938 mb noted by keepers as lowest recorded since lighthouse
was first manned in 1868. Keeper (James Burnett) leaves to be
replaced by William McLellan
March 1871
6th – gale
August 1871
4th – gale
22nd – gale
December 1871
16th – 21st – prolonged gales
24-27th – prolonged gales
January 1872
3-7th – heavy gales
8-12th- heavy gales
17-19th - severe gale – barometer to 946.5 mb
February 1873
1-3rd – gale
April 1873
4th – gale
September 1873
18th – gale
October 1872
1-2nd –gale
23-24 – gale
November 1872
1-2nd – gale
5-6th – gale
19-20th – gale
22-26th – prolonged gale – barometer to 957 mb
29-30th – gale
December 1872
5-9th – prolonged gale from NW
23-25th – gale from WSW
32
Jan 1873
1-3rd – gale
8-9th – gale
18-21st – prolonged gale barometer to 948 mb
March 1873
1st – gale
April 1873
5th – gale
May 1873
5-6th - gale
June 1873
9th – gale
24th – gale
October 1873
9-10th – gale
21-24th – prolonged gale
November 1873
1-2nd – gale
December 1873
30-31st – gale
January 1874
1-2nd – gale
3rd – gale
5-7th – gale
11-13th –gale
18-24th – prolonged gale
30th – gale
February 1874
11th – gale
14th – gale
24-27th – prolonged and heavy gale
March 1874
8-9th – gale
19th – gale
28-29th – gale
31st – violent gale
April 1874
1-4th – prolonged gale
August 1874
1st –gale
5th – gale
September 1874
1-2nd – gale
14th – gale
30th – gale
October 1874
2-3rd – gale
8-9th – gale
16-17th – gale
20-21st – gale
33
November 1874
29-30th – gale
December 1874
10-11th – gale
January 1875
1-3rd – gale
March 1875
25-27th – gale
May 1875
21st – gale
August 1875
26th – gale
September 1875
24-25th – gale
October 1875
4-6th – strong gale
November 1875
20th – gale
December 1875
21-23rd – severe gale
34
Appendix 2.
Historical photography from TARA archive
Photo 1
Date: 10/10/1946
Scale: 1: 10000
35
Photo 2
Date: 10/10/1946
Scale: 1: 10000
36
Photo 3
Date: 10/10/1946
Scale: 1: 10000
37
Photo 4
Date: 10/10/1946
Scale: 1: 10000
38
Photo 5
Date: 10/10/1946
Scale: 1: 10000
39
Photo 6
Date: 24/05/1962
Scale (1/m): 1: 27000
40
Photo 7
Date: 01/07/1963
Scale (1/m): 1: 27000
41
Photo 8
Date: 01/05/1965
Scale: 1: 5000
42
Photo 9
Date: 01/05/1965
Scale: 1: 5000
43
Photo 10
Date: 13/05/1965
Scale: 1: 5000
44
Appendix 3.
Ground photographs taken during August 2009 on Gualan Island
Recurved spit at northern
end of Gualan island
View looking south of main
coastal dune ridge
South-facing view of
unvegetated shingle ridge
45
Eroding coastal dune ridge facing north
towards northern end of Gualan
Panorama view of unvegetated section of central
Gualan ridge looking north
Lee of Gualan barrier showing areas of salt marsh
and overwash deposits from storms
Seaward face of southern Gualan barrier island
showing vegetated coastal dune ridge
46
View north along shingle ridge
and overlying dune sediments.
Note low area in foreground
where marram cover is
fragmentary. Note also seaweed
fragments from recent storms on
gravel surface.
Close up of section of degraded
ridge showing modern seaweed
cover
Fishing net material in lee of
southern Gualan ridge
47