Barrier Island Geomorphology, Hydrodynamic Modelling ... - BioOne

Journal of Coastal Research

28

6

1462–1476

Coconut Creek, Florida

November 2012

Barrier Island Geomorphology, Hydrodynamic Modelling, and Historical Shoreline Changes: An Example from South Uist and Benbecula, Scottish Outer Hebrides Alastair G. Dawson{, Cristina Go´mez{, William Ritchie{, Crispian Batstone{, Mark Lawless{, John S. Rowan1, Sue Dawson1, Jason McIlveny{{, Richard Bates{{, and David Muir11 {

Aberdeen Institute for Coastal Science and Management (AICSM) Department of Geography and Environment School of Geoscience University of Aberdeen Aberdeen AB24 3UE, Scotland, UK [email protected]

{ JBA Consulting South Barn Broughton Hall Skipton, North Yorkshire BD23 3AE, UK

1

{{ Environmental Research Institute UHI Millennium Institute Thurso KW14 7JD, UK

{{ Department of Earth Sciences Irvine Building University of St Andrews Fife KY16 9AL, Scotland, UK

11

www.cerf-jcr.org

School of the Environment University of Dundee Dundee DD1 4HN, UK

Integrated Coastal Zone Management Comhairle nan Eilean Siar Balivanich Isle of Benbecula HS7 5LA, Scotland, UK

ABSTRACT Dawson, A.G.; Go´mez, C.; Ritchie, W.; Batstone, C.; Lawless, M.; Rowan, J.S.; Dawson, S.; Mcilveny, J.; Bates, R., and Muir, D., 2012. Barrier island geomorphology, hydrodynamic modelling, and historical shoreline changes: an example from South Uist and Benbecula, Scottish Outer Hebrides. Journal of Coastal Research, 28(6), 1462–1476. Coconut Creek (Florida), ISSN 0749-0208. A partly quantitative reconstruction is provided of the evolution of Gualan Island, a barrier island located between South Uist and Benbecula in the Scottish Outer Hebrides, using historical maps, aerial photographs, and Lidar (light detection and ranging) data. Geomorphological changes over the last approximately 200 years are described together with quantitative changes in the dimension of the barrier island, including rates of shoreline retreat. A series of digital terrain models (DTMs) provided the boundary conditions for a two-dimensional (2D) ocean circulation tide-surge model simulating water level and wave conditions associated with a highly destructive storm that took place during January 2005. During this storm event, the central part of the barrier island was overtopped by waves. Validating the hydrodynamic model against eye-witness and field evidence obtained after the 2005 storm allowed simulation of a range of potential future breaching scenarios. Thus with the same storm conditions a large barrier breach 500 m wide would result in wave heights rising by 0.8–0.9 m on hitherto sheltered shorelines. Barrier island, Gualan Island, Scottish Outer Hebrides, geomorphological reconstruction, January 2005 storm in Atlantic Scotland, digital terrain modelling, hydrodynamic models.

ADDITIONAL INDEX WORDS:

INTRODUCTION Current scenarios of sea-level rise imply that barrier islands are becoming increasingly susceptible to erosion and overtopping by storm waves (Cooper, Lewis, and Pilkey, 2007). The Atlantic coastal margin of the Scottish Outer Hebrides is particularly vulnerable to the destructive effects of winter storms and sea-level rise. Throughout the island chain there are several barrier islands affording shelter to coastal settlements and important infrastructure. The islands have a long history of coastal erosion and flooding as a result of winter storms. Within the last century the most destructive of these

DOI: 10.2112/JCOASTRES-D-11-00184.1 received 13 October 2011; accepted in revision 19 January 2012. Published Pre-print online 13 April 2012. ’ Coastal Education & Research Foundation 2012

storms took place during January 11–12, 2005, when a frontal cyclone tracked northward along the Atlantic seaboard of Scotland. Southerly winds associated with this storm reached maximum velocities in the order of 170 km/h destroying coastal properties, damaging roads, and producing an approximately 2 m storm surge that inundated some coastal areas (Dawson, Dawson, and Ritchie, 2007a). The storm coincided with spring tide, and the duration of the event, over 15 hours, led to exceptionally high water levels during the evening of the 11th and again on the morning of the 12th. In the area described in this paper, five people lost their lives in the floodwaters. Here we use historical maps, aerial photographs, and Lidar (light detection and ranging) to reconstruct the recent geomorphological evolution of a barrier island (Gualan Island) located on the Atlantic seaboard of the Scottish Outer Hebrides (Figure 1). The narrow island effectively shelters the sandfilled tidal basin of South Ford from strong Atlantic waves.

Barrier Island Geomorphology, Modelling, and Shoreline Change

Figure 1.

Location map of Gualan Island and South Ford.

Thus, the preservation of this barrier function is central to protecting the low and vulnerable coastlines surrounding the South Ford from future flooding and damage similar to the inundation associated with the 2005 storm. The position, size, and morphology of Gualan Island have clearly altered and led to changes that, arguably, have increased its vulnerability to storm damage. Thus, although exact measurement of change is not possible, these three lines of investigation provide evidence of historical trends that might be extrapolated forwards and help address the fears and misgivings of local communities that are focused on the question, ‘‘What happens if Gualan Island is breached by the sea and divided permanently into two smaller islands?’’

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concluded that much of the postglacial (Holocene) sea-level rise in the Uists probably occurred before ca. 5000 years ago and had thereafter remained relatively unchanged. More recently, Jordan et al. (2010) argue on the basis of stratigraphic evidence from neighbouring Harris that this rise in relative sea level may have continued until as recently as ca. 2000 years ago before reaching its present position. Field investigations by the authors in the South Ford area have shown that small areas of machair (an ancient sand dune system, largely composed of aeolian shell-rich sand deposits) presently exist on some of the rocky islands in the South Ford basin. This observation points to the likelihood that the low-lying machair coastal plain was considerably more extensive in the South Ford area until relatively recently, and that rising sea levels and extensive erosion along the low-lying machair coast have led to the almost complete removal of machair from this area except for Gualan Island, which could therefore be regarded as the remnant coastal dune ridge of a once more extensive machair system (cf. Angus and Elliott, 1992). Complex but important flows of tidal water and some fresh water discharges flow into and out of South Ford from both the Atlantic Ocean in the west and the Minch in the east. Ebb and flood water movements flow through the gap at the north end of the causeway. These flows are gravity driven, and, in theory, the Minch tidal wave is thought to be 15 minutes ahead of the Atlantic cycle; however, this is often disrupted by wind and wave conditions. Ebb and flood water movements are important for sedimentation in South Ford, but, at this time, no detailed knowledge is available to describe both the changing tidal ranges in different parts of the basin and the consequent water movements, the most important of which is the substantial flow between Benbecula and Gualan Island through the North Channel (Figure 1).

Recent Changes

BACKGROUND The islands of South Uist and Benbecula are separated by a broad tidal basin that extends between the Minch and the Atlantic Ocean (Figure 1). On the Atlantic seaboard, the 2-kmlong N-S trending barrier island of Gualan protects coastal areas within South Ford from the destructive effects of Atlantic storm waves. The north end of the barrier island is separated from Benbecula by a narrow (approximately 100-m-wide) tidal channel (the North Channel). By contrast, the southern end of the barrier island is separated from South Uist during low tide by a broad area of sandbars and during high tide by a shallow stretch of water (the South Channel). Tidal flow between the Minch (to the east) and the Atlantic Ocean (to the west) is maintained by a 15m-wide bridge section located at the northern end of a causeway approximately 2 km east of Gualan Island (built in 1983 to replace an earlier bridge) that connects Benbecula and South Uist. At low tide the area between the causeway and Gualan Island, known as the South Ford basin, is almost completely dry and filled by sandbanks except for the North Channel (Figure 1).

Long-Term Coastal Changes Relatively little is known of past patterns of relative sea-level change in the Outer Hebrides. Ritchie (1966, 1979, 1985)

During the storm of January 2005, Gualan Island was breached at several locations by Atlantic storm waves. Local communities fear that future storms and sea-level rise will lead to the complete breaching of Gualan Island causing neighbouring coastal areas to be subject to flooding and to the full force of winter Atlantic storm waves. Little is known, however, of the historical landscape evolution of Gualan Island and how it has changed in size, shape, morphology, and position over time. The geomorphology of the island is characterised by marramcovered coastal dunes that decrease in height from north (over 10 m) to south (less than 5 m high). The northern part of the barrier island is typically around 100 m wide and tapers to its southern end where it is approximately 30 m wide. In the extreme north there is a well-defined recurved gravel and shingle spit that terminates at the edge of the North Channel. The central coast of Gualan is almost bereft of protective dunes, and the shingle and gravel ridge is susceptible to wash-over when high tides and onshore storms coincide. Typical splays of gravel are deposited on the leeward side and provide good evidence of the frequency of these events. Using historical evidence in the form of old maps and aerial photographs (the latter dating from 1946), it is possible to recognise the most significant geomorphological trends in the evolution of Gualan

Journal of Coastal Research, Vol. 28, No. 6, 2012

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Table 1.

Dawson et al.

Factors reducing the reliability of measures of coastal change. Factor

Source

Rapidity and variability of changes in beach and coastal edge Inadequate historical base information Reliance on specific tidal elevation lines (e.g., high water mark, spring tides) Episodic and random nature of surveys (i.e., actual coastal condition at the time of survey or exposure of aerial photography) Typicality or representativeness of coastal condition at time of survey Intrinsic survey and cartographic errors Identification and definition of coastal edge on maps and aerial photographs Longshore variations in nature of coastline (e.g., contiguous erosional and accretional zones) Sampling (e.g., number and location of profiles) Failure to distinguish between point, section, or extensive longshore measurement Time intervals between surveys/data acquisition Lack of appreciation of normal seasonal changes

Island. Notwithstanding the appreciation of the range of difficulties that are associated with using both map and photographic evidence to quantify coastal changes, both sources are combined here along with field survey to attempt to provide some quantitative measure of change over the historical period (Table 1).

Natural Data Technique Data Natural Data/technique Technique Natural Data Technique Data Natural

study, the 1805, 1825, 1878, and 1965 map positions of HWMOST were digitised and transferred onto a terrain model. Whereas the 1805 and 1825 high water mark positions are regarded as inexact estimates, the 1878 and 1965 mapped positions are considered as being relatively accurate.

Historical Aerial Photography

METHODS Historical Map Data Several historical maps were used to estimate the recent evolution of Gualan Island. The most important map material was accessed through the National Map Library of Scotland, Edinburgh, where a significant dataset is available online (http://www.nls.uk/collections/maps). Despite the generalized representations of the earliest maps (and some known inaccuracies), they remain an invaluable source of information to reconstruct and understand landscape change in this area (Dawson, Go´mez, and Ritchie, 2009; Go´mez et al., 2008). The earliest detailed historical map of South Uist and Benbecula that exists prior to ca. 1750 are the classic topographic maps of Blaeu (1654). Two later maps (Moll, 1745; Tiddeman, 1730) provide additional information for the early 18th century. Three more detailed recent maps cover the period from 1776 to 1832 and are internally consistent in the coastal landscapes that they depict (Bald, 1825; Bowen, 1776; Huddart, 1794). Of these, the first one constitutes a highly detailed estate plan for South Uist for 1825. A contemporary estate map, dated 1805, depicts the coastal landscape of southern Benbecula and, although not including Gualan Island, shows the position of the channel that separates Gualan from Benbecula, as well as the contemporary position of high water mark of spring tides (HWMOST). Detailed topographic and offshore maps were later produced by the Ordnance Survey (OS) and the Hydrographic Office at a scale of 1 : 10,560. The oldest OS topographic map, dated 1878, contains accurate information based on trigonometric field survey and accurate levelling to bench marks. This map delineates Gualan Island very clearly. It also includes a surveyed line showing the position of high water mark of ordinary spring tides (Wood, 2009). This line position is also incorporated in a later 1965 OS map of the area (scale 1 : 10,560). Both OS maps are directly comparable with each other as the latter is based on repeated trigonometric measurements and levelling to the 1878 bench marks. For this

A range of historical aerial photography was investigated at the TARA archive at the Royal Commission of Ancient Monuments, Edinburgh. The oldest air photos are derived from the RAF series of 1946; later sets exist for 1962, 1963, and 1965 (Table 2). The lack of information concerning the acquisition parameters of the photographs, such as flying height or camera attributes, precluded a rigorous quantitative analysis; therefore, we used the aerial photography data as indicators of change. These historical photographs, which have an intrinsic value in providing supplementary sources of information to enable checking of the validity of other map and photographic measurements, were thus visually interpreted and compared with more recent photography (Table 3). Geographical Information Systems (GIS) software was used to display the air photo images at the same scale and appropriate orientation. Each image is framed by the same latitude and longitude coordinates, and all are oriented to grid north. These procedures enable direct comparisons between images and thus make it possible to draw inferences on patterns of coastal change and relate these to the later imagery. While reasonable, direct comparison of photography acquired at different scales requires careful interpretation of change and a good understanding of the accuracy of detected transformations.

Digital Terrain Models (DTMs) Digital terrain models of the South Ford area were produced for 1984 and 2005. The main sources of data to generate these DTMs were vertical colour aerial photography acquired from the CUCAP (Cambridge University Collection of Air Photos) catalogue in digital format. Deriving the DTMs from aerial photography posed the usual difficulties associated with coastal environments, mainly related to the scarcity of reliable ground control points and to the presence of light reflections from waves and water bodies. Seasonal vegetation height covering some areas was another factor that needed consideration when interpreting change.



Table 2. Historical photography used for interpretation in the study. Source: TARA Photography Archive, Edinburgh. Photo Code

CPE_UK_0189_1376 CPE_UK_0189_3390 CPE_UK_0189_3391 CPE_UK_0191_4051 CPE_UK_0191_4053 OS_63_062_035 OS_63_146_018 OS_65_072_001 OS_65_072_074 OS_65_090_067

Date

10/10/1946 10/10/1946 10/10/1946 10/10/1946 10/10/1946 24/05/1962 01/07/1963 01/05/1965 01/05/1965 13/05/1965

Scale

1:10,000 1:10,000 1:10,000 1:10,000 1:10,000 1:27,000 1:27,000 1:5000 1:5000 1:5000

Because ground-control coordinates support the derivation of DTMs, it is not possible to use the actual water levels on the imagery to define the shoreline. Wave and tidal levels at the time of exposure are unknown variables. Specific elevations, e.g., HWMOST, can be superimposed, however, as contours and can be used for comparative purposes with large scale maps and aerial photography from different dates. Terrestrial laser scanner (TLS) and aerial laser scanner (ALS) data (Table 3) were incorporated into the recent DTM, enhancing its quality and accuracy. Aerial laser scanner data, commonly known as Lidar, was available for the western part of the South Ford area (including Gualan Island) from an aerial survey commissioned by the Scottish Natural Heritage in June 2005. The Lidar data has a 1 m resolution in the XY-plane with accuracy in the range 0.2–0.3 m (Maune, 2006). Terrestrial laser scanner data was collected during fieldwork in September 2008: areas of special interest identified on photography were surveyed using a TrimbleH GX 3D Scanner, providing dense clouds of points for which location accuracy is related to Global Positioning System (GPS) measurements. The accuracy of the more recent DTM (60.5 m in height) was validated with GPS points measured with TrimbleH 5800 GPS receivers and corrected with Real Time Kinematic (RTK) or postprocessing. Once the DTMs were prepared, GIS techniques enabled the identification of locations where significant changes in coastal morphology had taken place (Dawson et al., 2007b; Moore et al., 2005). Volumetric and height changes across Gualan Island and surrounding areas were determined using algebraic operations of DTMs in raster format. The spatial resolution for these operations was 232 m2, small enough to capture the detail of local changes in the coastal geomorphology (Go´mez et al., 2008). However, since volumetric calculations are dependent on the accuracy of the DTM’s height and on the pixel size, these values are the most error prone. The position of the present HWMOST line elevation, 2.3 m above ordnance datum (AOD), was identified in the 1984 and 2005 DTMs, thus enabling a thorough comparison of positioning and identification of morphological changes.

Field Survey During February 2005, field mapping of the coastal geomorphology of Benbecula and South Uist was undertaken in order to evaluate and measure changes in morphology caused by the January 2005 storm (Dawson and Dawson, 2005; Moore et al.,

Table 3.

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Details of data used in DTM construction. DTM 1984

DTM 2005

16 1:15,000 24/04/1984 -

14 1:10,000 09/06/2006 09/09/2008 16/11/2005

Aerial photography Number of frames Scale Date TLS Date ALS Date

2005). This was followed by a series of geomorphological surveys and terrain mapping supported by a hand-held GPS undertaken between 2005 and 2010 (Dawson, Go´mez, and Ritchie, 2010a,b).

Hydrodynamic Modelling The hydrodynamic processes of astronomical tide, storm surge, and waves were simulated in the South Ford using a fully coupled tide-surge-wave modelling system (Batstone and Lawless, 2009). Variations in sea levels and currents attributable to both tidal and meteorological processes were simulated using the 2D ocean circulation model ADCIRC (Luettich, Westerink, and Scheffner, 1992). The transformation of offshore waves to the coastline was performed using the nearshore wave transformation model STWAVE (Smith, Sherlock, and Resiol, 2001). This spectral wave model simulates transformation processes such as depth-induced refraction and shoaling, steepness-induced breaking, wavewave interaction, and wave growth caused by wind forcing. Coupling of these models during run time allowed for the simulation of wave set-up, which was found to be a significant contributor to the extreme sea levels that occurred during the January 2005 storm (Wolf, 2007). The model grid extended from the deep Atlantic across the Gualan–South Ford area as far as the Minch. The variable model grid resolution increased from 4 km cells in the Atlantic to 25 m cells in the South Ford for increased accuracy in the shallower waters. The model bathymetry was derived from gridded GEBCO data supplied by the British Oceanographic Data Centre and digitised chart data supplied by Seazone Solutions Limited. These data were combined with the 2005 DTM to provide a complete bathymetry data set for the model. Wind speed and air pressure data used to force the ocean model were derived from model reanalyses, available from the National Centers for Environmental Prediction (NCEP). Additional data for the January 2005 storm were obtained from the U.K. Meteorological Office. Offshore wave conditions for the January 2005 storm were supplied by the NOAA WaveWatch III operational wave model (validated against a recently deployed CEFAS WaveNet Directional Waverider Buoy situated approximately 50 km to the west of South Uist [Batstone and Lawless, 2009]). Calibration and validation of the model made use of multiple records of water level and current velocities taken in the North Channel during spring 2009. The root mean square error between observed and modeled tide levels was 0.2 m at Corran during the SpringNeap tidal cycle period of February 2009 (see Figure 2). At this narrow western entrance to the South Ford, peak currents


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Figure 2. Relationship between tidal gauged and modelled elevations at Corran.

during this time reached to 0.9 m/s. The model typically reproduced the peak currents during ebb and flood times to within 10% of observed values.

RESULTS Geomorphological Changes: 1946–2006 For simplicity, the area of the barrier and adjacent landmasses is divided here into three sections: (1) the area of North Gualan Island, North Channel, and the adjacent Lionaclete coastline of Benbecula; (2) the central part of Gualan Island breached during the 2005 storm; and (3) the southern Gualan Island, the South Channel, and the adjacent coastline of South Uist.

North Gualan, the North Channel, and Lionaclete Specific aerial photographs illustrate changes on both sides of the tidal inlet between Gualan and Lionaclate. The sequence 1946 to 2006 in Figure 3 shows significant reduction in the areas of dunes and machair on the north side of the channel, and these have been associated with a NE shift in the course of the channel. Research at Lionaclate in the 1960s recorded some of the highest and most active dunes in Benbecula as occurring along this short coastal section. Thus the map of planimetric change does not give the full picture of the large volume of sand that was removed. The question remains, however, as to whether this quantity of sand was carried into South Ford, out into the Atlantic, or retained in enlarged flood and ebb tidal deltas. Air photographs confirm that significant coastal change occurred in the Lionaclete area between 1984 and 2006

Figure 3. Coastline reconstructions for the Lionaclete area, 1946–2006. A line marks the position of high water mark in 2006. Note the field areas of crops shown in 1946 and 1965 that appear later to have been abandoned, in some areas mantled by blown sand.

(Figure 3). Fields evidently being cultivated in the 1946 and 1965 photographs appear abandoned by 1984. By 2006 less than half of the original coastal plain remained. These changes were associated with the erosion of the south-projecting sand spit and the northward migration of the North Channel. The morphological changes of the northern tip of Gualan Island are shown in Figure 4. This time series graphically illustrates the progressive retreat and realignment of the Atlantic coastline between 1946 and 2006. The sequence shows clearly that while morphological changes were active during the period 1946 to 1965, such changes were modest by comparison with those that took place since 1984. The six images represent snapshots of the coastal terrain over a long period of time and thus point to the need for caution in any quantitative description of coastal change. Thus, for example, whereas in the 1946 imagery the Atlantic coastline exhibits a markedly convex profile in plan, this appears to have been straightened somewhat and also elongated by 1962. Whereas the 1962 and 1963 images show little change, a clear change is evident in the 1965 imagery where a new spit appears to have formed at the northern end of the island. Most distal ends of active spits exhibit relatively rapid planimetric changes, and the north end of Gualan is no exception. Similarly, there are notable changes in the alignments of upper and lower tidal limits and the coastline as defined by the beach-dune interface. The sequence of aerial photographs can only provide snapshots of these planimetric



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predating the severe event of January 2005 (Figure 5). In 1946 the dune edge on the Atlantic coast appears well defined and vegetated by marram grasses with only occasional areas of bare sand. In marked contrast, the 1962 photograph shows the majority of the central section stripped of vegetation and dominated by bare sediment attributable to the combined effects of washing by storm waves and wind erosion. Fast ecological recovery is indicated in the succeeding 1963 and 1965 photographs, which point to the restoration of a nearcontinuous cover of coastal vegetation. This suggests that the episode of storminess that impacted central Gualan most likely occurred during the winter of 1961–62. In 1984 this area appeared vegetated and stable but again was washed out and covered by sand and gravel in 2005. Finally, the persistence of the strip of marram-clad coastal dune (shown as C in the 2006 image), which was in the same position in the 1946 image, suggests that the position of the high water mark has not changed significantly within this time frame.

Southern Gualan Island, South Channel, and Adjacent Coast of South Uist

Figure 4. Evolution of the northern Gualan Island coastline, 1946–2006. Note the northward extension of the barrier island since 1946 culminating in the development of the spit in the 2006 image.

changes, but the evidence clearly illustrates the sensitivity of the position of the north end of Gualan. It is the interplay of tidal and wave-generated forces that determine these transient positions. Nevertheless the balance of evidence appears to demonstrate that this net NE shift since 1946 can be correlated with the substantial erosion of the Lionaclate shoreline. This observation poses further questions. Is there a limit to this direction of movement? Could there be a cyclic diversion south of the present position akin to the breakthrough of a fluvial meander? Field mapping in 2010 revealed an old dune escarpment running at right angles to the coastline about 80 m south of the end of the present dune ridge that might be associated with a former north limit of the spit. If correct this might support a cyclic hypothesis.

Central Breached Section of Gualan Island The photographic evidence from the central section of Gualan Island suggests a history of periodic storm breaching

The southern section of Gualan Island evidences some of the most significant coastal changes within the envelope of photographic evidence. These include a marked reduction in width of the South Channel since 1946 together with a displacement of the most southern part of the spit (Figure 6). The southern area of marram-covered dunes was separated from an area of salt marsh (validated in the field) by an area of bare sand (including embryo dunes) until 1984 (shown as a pink line on the 1963 image). The line is replotted on the 2006 photo and demonstrates significant shoreline recession in this area between 1963 and 2006. The South Channel remains well defined in the 1962 and 1963 photos before narrowing and becoming shallower by 1984 (Figure 6). Low-relief sand bars are also visible within the channel on the 1984 photograph. Between 1965 and 1984 the channel changed direction seaward of Gualan Island from NWSE to N-S. These changes are intimately linked to the localised accretion of sediment within the intertidal zone in the southern flanks of the South Ford (Rowan, Dawson, and McIlveny, 2010). Nevertheless, it is not possible to quantify these changes in width and depth due to the different states of the tide at the time of the exposure of the aerial photographs.

Changing Basin Dimensions Detected With DTM Analysis (1984–2005) The acquisition of Lidar data during the summer of 2005 was intended to provide a terrain model of the coastal landscape in the aftermath of the January 2005 storm. Comparison of the 1984 and 2005 DTMs thus incorporates a significant element of morphological change caused by this storm. This creates a difficulty in interpreting terrain changes between 1984 and 2005 because it is impossible to determine how much of the change was caused by gradual (low-magnitude) changes over the 21-year time interval and how much was due to highmagnitude changes caused by the 2005 storm. Since 1984, the position of high water mark along the Atlantic shoreline of Gualan Island appears to have migrated to the east (Figure 7).


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Figure 5. Central breached area of Gualan Island. Note the marked increase in bare sand cover between 1946 and 1962 (A) and the later reappearance of a vegetation cover by 1963. A similar marked change occurs between 1984 and 2006. (B) An area of conspicuous change over time. The strip of marram-clad coastal dune (C) in the 1946 and 2006 images hints at little change in the position of high water mark. Arrows in the 1962 photography point to local areas of wash-over breakthrough and deposition.

The greatest amount of retreat has been in the north with the amount of retreat decreasing from north to south. Significant intertidal sediment accretion has taken place at the extreme northern end of the island (Figure 7). Here, a recurved gravel, sand, and boulder spit extends to the NE and terminates at the edge of the North Channel. At its southern end the spit is attached to high (5–10 m) vegetated dunes. Sediment accretion has also occurred at the extreme south of the barrier island. Landward of the south end of Gualan Island, a broad intertidal sandflat now blocks the movement of tidal waters through the South Channel between Gualan Island and the northern coastline of South Uist (Figure 7). In the southern part of Gualan Island the coastal dunes have also accreted vertically since 1984 forming a stable barrier of vegetated sand (Figure 7).

Figure 6. Southern Gualan Island, South Channel, and adjacent coast of South Uist. Note the marked reduction in width of the South channel since 1946 and particularly since 1984. The area of vegetated dunes was separated from an area of salt marsh by an area of bare sand until 1984 (marked with a line on the 1963 photo and replotted on the 2006 image). The line is replotted on the 2006 photo demonstrating a net shoreface recession between 1963 and 2006. Area A in 1984 and 1965 highlights the channel change of direction; area B highlights temporary sand deposition in 1984.

An oblique DTM image of Gualan Island illustrates the scale of the changes that have taken place between 1984 and 2005. In addition to the loss of sediment on the seaward flank of the northern section of Gualan Island and the formation of a new spit (Figure 8), in the north, coastal erosion has taken place adjacent to Lionaclete. 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 pattern of sedimentation for this area. Whereas the southern end of Gualan also experienced an increase in volume over this time period (approximately 44,800 m3 gain), the central area everywhere was subject to sediment loss of which the greatest volumetric loss (approximately 127,900 m3) was from within the coastal dunes at the northern end (Figure 8; area L3). Because the 2005 data was acquired 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 represented the



Figure 7. Mapped change in the position of HWMOST from 1984 to 2005. The two insets provide clear evidence of progressive narrowing of sections of the barrier island and eastwards displacement of the HWMOST. (Color for this figure is available in the online version of this paper.)

effects of more gradual coastal changes that took place between 1984 and 2005. Contemporary accounts for the early 1980s describe the South Channel as a stretch of open water separating Gualan Island from South Uist, characterised by a tidal current 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. There has also been a wide-scale infill of sediment in the lee of south Gualan Island, where sediment aggradation has resulted in the impeding of the tidal exchange of water between Loch Bee and the South Ford basin (Rowan, Dawson, and McIlveny, 2010). Thus the majority of the Atlantic coastline of Gualan Island experienced erosion and retreat since 1984, with the exception of the south end of the island where there has been widespread beach and dune accretion. A consequence of this change is that the area in the lee of the southern end of Gualan Island is presently protected from Atlantic waves due to the combined effects of this sediment accretion and the near complete closure of the South Channel. Although not a part of the current investigation, it has been suggested subsequently that some of the changes to water levels and tidal flows within South Ford could be a result of sand build-up inside the basin to a sufficient level to affect the volume of the tidal prism west of the causeway (Figure 1). Although difficult to assess, this could be a useful line of research that would help the understanding of the tidal flows, episodes of flooding, and coastal changes in the general area of the strand that have been identified in this study.

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In summary, different parts of Gualan Island have experienced distinctive patterns of coastal change since 1946. In the north, significant spit evolution has taken place, presumably through the combined influence of strong tidal currents, extreme storm events, and trains of powerful Atlantic waves. The central section of the island has been very susceptible to wave-induced changes that have led to a series of wash-over events associated with a narrowing of the sand barrier over time. The most reliable evidence of wash-over as an important process of coastal change consists of multiple splays of reworked sand, gravel, and shingle on the lee side of the central part of the island (Figure 5), which was confirmed by eye-witness accounts following the 2005 storm. There is, however, the problem of distinguishing between cause and effect as to whether erosion and retreat of the Atlantic coastline have facilitated increased wash-over activity, or whether washover is a driver of coastline recession especially in the centre of the barrier. The local importance of wash-over at Gualan accords with comparable studies into barrier island morphodynamics elsewhere, e.g., the Gulf Coast of the United States (Hayes, 2005; Ritchie, 2005). In contrast, the southern area appears to have become more sheltered, possibly because of a combination of short-term changes in wave climate, sediment accretion, reduction in channel flow, and coastal dune accretion.

Coastal Change over Last 200 Years The available historical map data for South Uist and Benbecula depict contrasting scenes of geomorphological change in the coastal landscape. In general, the maps that exist for the period prior to ca. AD 1750, however inaccurate in detail, are consistent in their portrayal of a broad tidal strait separating South Uist and North Uist. In two key maps (Moll, 1745; Tiddeman, 1730), Benbecula is shown as a relatively small island area with broad tidal channels to both the north and south through which water flowed between the Minch and the Atlantic. By contrast, the oldest and best-known map of Blaeu (1654) shows a large inlet between Benbecula and South Uist that terminates eastward at a narrow land connection joining the two land areas. Three more detailed maps covering the period 1776 to 1832 are internally consistent in the coastal landscapes that they depict (Bald, 1825; Bowen, 1776; Huddart, 1794). Of these, the first and last are the most detailed (Figures 9 and 10). For example, the Huddart map (1794) shows a well-defined tidal strait separating Benbecula and South Uist with the western part of this area stipple marked and denoting the presence of sand flats. To the west, Gualan Island is clearly marked (Figure 9). The island is shown as being separated from South Uist by a narrow channel that leads out of Loch Bee. The outlet from this loch into the tidal strait would appear to have been at least 200 m wide, implying that Loch Bee was considerably more open to tidal influences than it is today: its outlet is now constrained by a dam over which there is a road, and the outlet/inlet for water exchange is, in effect, a relatively narrow stone culvert. The later map of Bald (1825) also shows Gualan Island (referred to on the map as Shoulder) separated from South Uist by the same narrow channel as shown on the Huddart (1794) map


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Figure 8. Left: Oblique DTM-derived change map, 1984–2005. 1: land loss along Lionaclete coastline; 2: recurved gravel spit; 3: land loss on northern Gualan coastline; 4: Lionaclete and areas of vertical sediment accretion within South Ford. Right: areas of volume change between 1984 and 2005. (Color for this figure is only available in the online version of this paper.)

(Figure 10). There is thus reason to believe that Gualan Island was in existence in its approximate present location since at least 1794, and that an extensive tidal strand existed between Benbecula and South Uist since at least this time. The Bald (1825) estate map also lists Gualan Island as then occupying an area of 11.8 ha compared to 22 ha in 2005. In addition to the Bald (1825) map of South Uist showing the location of Gualan Island (Figure 10, top), the Benbecula Estate map for AD 1805 (Figure 10, bottom) shows the Lionaclete coastal area together with a limited part of the South Ford basin. This map shows the area at Lionaclete as being characterised by bare sand. This area is the only such area shown as bare sand for southern Benbecula in 1805. The map also shows a radically different coastal geography for Lionaclete with one prominent tidal creek (dashed line in Figure 10, bottom). At present, this marine inlet is infilled with aeolian sediment, while the inland area has been converted into a freshwater loch. By contrast, the course of the North Channel appears to have been in approximately the same position as it is today. The positions of high water mark lines for different map and photograph dates that have been superimposed on the 2005 DTM show that by 1878 the north end of Gualan Island was

located approximately 500 m south of its present position (Figure 11). Given the more southerly position of the Lionaclete coastline at this time, one can envisage that during the highest tides, a 200–250-m-wide tidal strait (the North Channel) existed during the 19th century 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 coast of the island was located approximately 100 m west of its present position. The position of HWMOST for 1965 shows that erosion along the northern flank of the North Channel continued to take place between 1878 and 1965. Assuming that HWMOST can be used as a reference line in measuring the retreat of a coastline as defined by the beachdune interface, comparison of the OS map for 1878 with the most recent maps shows clearly that the island has migrated eastwards over the last approximately 140 years by about 100 m, which is equivalent to an average recession rate of roughly 0.7 m per year. Comparison of the 1984 and 2005 maps (Figure 12, right) points to a similar rate of recession over the last approximately 20 years. Thus, the recession across the central area has been in the order of 20 m, which is equivalent to a retreat rate of about 1 m/yr or an average loss of about 0.78 ha/yr of shoreface over approximately 20 years. The



Figure 9. Historical map of the South Ford area showing the reconstructions of Huddart (1794) (courtesy of the National Map Library of Scotland).

formation of a new roughly 0.43 ha gravel spit at the north end of the island (Figure 12, right) is inferred to have resulted from the 2005 storm. This pattern of accretion is further associated with an additional land area of 4.1 ha across northern Gualan created since 1878 (Figure 11, pointed area). An attempt was also made to compare geomorphological changes between 1825 and 2005 using the Bald (1825) map and the most recent Lidar data. The latter map identifies the contemporary HWMOST. Using a digitised best estimate of HWMOST for 1825, we estimate that the Atlantic coastline of Gualan Island during this 180 year period has retreated between 105–125 m, equivalent to an average retreat of approximately 0.5 m/yr (compare with the calculated retreat since 1878 of about 100 m). Further, since 1825, the northern end of the island has prograded northwards by approximately 780 m (Figure 12, left). Comparison of imagery for 1984 and 2005, however, points to a retreat of up to 50 m during this period coupled with an approximate 150 m progradation at the northern end of the island and the deposition of approximately 27,900 m3 of sediment (Figure 12).

Modelling the 2005 Storm Event Wolf (2007) modelled the 2005 storm in 100 m deep waters offshore of South Uist using a fully developed sea with a sustained wind speed of 23 m/s (83 km/h) generating 14 m high waves. Wolf (2007) maintained that this is significantly lower than the 17 m wave height determined for the 50 year return period and suggests that the January 2005 storm event may indeed correspond closer to an event with a 10-year recurrence. The long-term evolution of the Gualan barrier island points to

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it having increased in size over time, but until the storm of 2005 evidence for breaching has been equivocal. There are multiple working hypotheses that individually or by interactions may account for an increased susceptibility of the barrier to erosion and breaching (e.g., relative sea-level rise, change in coastal sediment budgets, land management practices, or autogenic thresholds being exceeded). All, however, imply a geomorphological future that is likely to feature breaching during extreme storm events. During the 2005 storm, only the central section of Gualan Island was overtopped by floodwaters. This area coincides with a zone where coastal dunes are presently fragmented or absent together with a low and actively eroding backshore shingle and cobble ridge. Field inspection of this area during August 2009 shows this area as essentially vulnerable to further wash-over damage. It is dune-free and characterised by a low ridge of coarse gravel. The beach shingle is locally mantled in this area by stranded seaweed that extends over the top of the ridge and onto areas in the lee of the ridge, showing that recent storms are regularly overtopping this section of the barrier ridge. The breach of Gualan Island during January 2005 highlights the prospect of large waves propagating directly into the South Ford in the future if wash-over events become more frequent, which is in conjunction with the development of a proto/ permanent breach. The extent to which Gualan protects the South Ford from storm waves together with scenario modelling of a future breach was investigated by performing multiple simulations of the January 2005 storm using the hydrodynamic model (Figure 13). First, a model using the 2005 DTM was made to simulate flooding associated with the 2005 storm. Second, three scenarios of barrier island breaching were run using the same model in order to investigate the hydrodynamic effects that such changes would have on the wave regime in the lee of Gualan Island.

Method for Simulating Future Breach Scenarios for Gualan Island The inundation extent and water depths modelled for the January 2005 storm provides good agreement with witness accounts of flooding extent (Figure 13). The model performs well in identifying those sections of the South Uist coastline that were subjected to the most damaging incursions of flood water. The simulation also indicates that the central section of Gualan Island would be overtopped, resulting in a higher storm surge propagating into the South Ford basin, which again accords with eye-witness accounts and subsequent field observations. The modelled breach is, however, significantly smaller than the field evidence suggests, most likely because the erosional effects of strong current flow are less well represented in the tide and surge model (Figure 13A). The model simulated three breach scenarios (Table 4) in which the central section of Gualan Island is removed, allowing the tidal exchange of water between the Atlantic and South Ford (Figure 13B–D). The scenarios represented different breach configurations, each comprising a single 2–3 m-deep channel running E-W and centred over the site of failure during the 2005 event but varying in width between 100 m (B), 300 m (C), and 500 m (D). The breaches were created by


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Figure 10. Historical map of South Ford and Gualan of Bald (1825) (courtesy of the National Map Library of Scotland) and Benbecula Estate map for AD 1805 (bottom). (Color for this figure is only available in the online version of this paper.)

appropriately editing the elevation values on Gualan Island in the model domain before model simulation. Oceanic forcing generated by the WaveWatch model was coupled to the 25-mhigh resolution grid-wave model to simulate the propagation of offshore waves into the South Ford. In order to capture the

minimum flood risk from wave-driven inundation of the shorezone, these scenarios used peak wave heights derived from a 2005 ‘‘no breach’’ model run, which indicated that internally generated waves could attain heights of 0.8 m in the lee of the island. This is likely to under-represent wave-wave



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interactions and local-scale variations in bottom-friction coefficient values (Batstone and Lawless, 2009). For the scenario of moderate erosional breaching, waves and tidal flow were allowed to propagate through a 100-m-wide channel through central Gualan Island (Figure 13B). The model calculates wave heights of 2 m during the peak of the storm 100 m to the west of Gualan Island. As these waves translate into shallower water they break and dissipate, resulting in an elevated water surface level and wave heights of 1 m propagating into the South Ford basin. Wave energy is quickly dissipated within the South Ford because of depthlimitation effects (Figure 13C). In the severe erosion scenario wave heights exceeding 1 m propagate much further into the tidal basin, and breaking waves of 0.5 m add problems in addition to the elevated water level. In the extreme scenario a 500-m-wide breach is represented, allowing waves of 0.6 m to compound the inundation of the South Uist shoreline, rising to 0.8–0.9 m in the eastern section of the South Ford (Figure 13D). Figure 11. DTM-enabled reconstruction of changes in the position of high water mark for northern Gualan Island and Lionaclete over last approximate 200 years. (Color for this figure is only available in the online version of this paper.)

Figure 12.

DISCUSSION Notwithstanding the uncertainties introduced by inaccuracies in early maps, especially from 1825, and the problem of

Estimates of Atlantic shoreface retreat (m), Gualan Island for AD 1825–2005 (left) and AD 1984–2005 (right).


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Figure 13. (A) Hydrodynamic model showing breaching of central Gualan area during 2005 storm. Diagonal shading denotes areas excluded from the model. (B) Modelled wave heights (m) associated with a moderate breach of central Gualan. (C) Modelled wave heights associated with a severe breach of Gualan island (m). (D) Modelled wave heights (m) associated with an extremely severe breach of central Gualan. Note that in (B), (C), and (D), wave heights above 1 m are not detailed. (Color for this figure is only available in the online version of this paper.)

defining what is meant by the coastline, it would appear that the analysis presented here describes a barrier island that has increased in area from approximately 14.9 ha in 1825 to 22.1 ha in 2005. The digitised history of former positions of high water mark demonstrate that, in addition to its long-term growth, most of Gualan Island has been subject to a slow (approximately 1 m per year) landward rollover over at least the last approximately 140 years. Best estimates of comparing HWMOSTs for AD 1825 and 2005 point to a retreat of the Atlantic coastline of Gualan during this period of between 105–125 m, equivalent to an average retreat of approximately 0.5 m/yr. During this time interval the northern end of the island has prograded northwards by approximately 780 m. Comparison of imagery for 1984 and 2005 points to a retreat of

up to 50 m during this period coupled with an approximate 150 m progradation at the northern end of the island and the deposition of roughly 27,900 m3 of sediment. It is likely that most of these latter changes took place during the storm of January 11, 2005. Lowe et al. (2009) and Jordan et al. (2010) argue that the long-term rise in relative sea level during historical times (last several centuries) across the Scottish Outer Hebrides has been in the order of 2 mm per year. If correct, it is therefore not unreasonable to argue that since 1825, relative sea level has risen in this area by approximately 0.4 m and that the approximate 105–125 m of shoreline retreat at Gualan should be considered within this context. At the same time, one may interpret recent coastal change at Gualan as having been dominated by a single extreme storm event. Yet



Table 4. Model Run

1

2

3

4

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Example correlations of model runs with actual changes. Simulation Description

Results

2005 storm: Inundation modelling of the January 11–12, 2005 storm.

There is a good representation of broad areas of flooding on South Uist and the breaching of Gualan Island. The model predicted flooding depths on South Uist to be within 40 cm of post-storm surveyed levels at five distinct locations. Gualan Island moderate breach: Modelling of Waves of 1 m height propagate into the South Ford basin but largely dissipate. A moderate waves during the peak water level of the (0.3 m) increase in wave heights along the northern shoreline of South Uist occurs. Wave January 2005 storm, assuming a 2–3m energy propagating through the breach has dissipated before reaching the South Ford deep breach in the middle of Gualan causeway (wave energy here is entirely generated within the South Ford basin). Island of 100 m length. Gualan Island severe breach: as in 2, for a Wave heights along the northern shoreline of South Uist can reach 0.5 m due to the increased wave 300-m-long breach. energy propagating through the larger breach. However, this additional energy (above that generated within the South Ford basin) dissipates before reaching the South Ford causeway. Gualan Island catastrophic breach: as in 2, Wave heights along the northern shoreline of South Uist can reach 0.6 m. Wave energy for a 500-m-long breach. propagating through the breach adds approximately 10% to wave heights experienced at the South Ford causeway (0.9 m).

in this context, recent trends in winter storminess frequency and intensity for this area are unclear (Dawson, Dawson, and Ritchie, 2007a). The change during recent winters from a positive to a negative North Atlantic Oscillation and a switch from higher to lower winter storm frequencies is inconsistent with the popular view that the North Atlantic region has experienced an increase in winter storminess during recent decades (e.g., Gunther et al., 1998). Aerial photo evidence confirms significant sediment accretion since 1984 in the lee of south Gualan Island and the near closure of the South Channel. The southern part of the barrier island has also experienced vertical dune accretion since 1984, and because of this it was sufficiently high to have resisted wave overtopping during the January 2005 storm. In this respect the variable height of the dune barrier behaves like most types of barrier islands worldwide (Ritchie, 2005) in that the height and width of the coastal dune ridge provide the first line of defence against wash-over, subsequent breaching, and possible long-term fragmentation. The modelling results for the combination of factors that result in storm breaching presents coastal planners with a dilemma. A consequence of each of the breaching scenarios is that the South Ford area, together with the adjacent coastal areas of Benbecula and South Uist, is likely to be subject to increased wave erosion during storms. Some coastal areas without dunes within the tidal basin are also at very low altitudes and are therefore also vulnerable to flooding. Yet if the 1825 mapping of the island is approximately correct, the long-term evolution of the island points to it having increased both in size and length over time (thus enhancing its effect as a barrier) and that, despite the numerous winter storms of the last approximately 200 years, there is no evidence of the island ever having been previously divided in two, as was nearly the case during January 2005. Looking forwards, the key coastal protection issue is the total volume of beach and dune sand in the Gualan barrier and its susceptibility to erosion and redistribution during extreme storm events. With the introduction of the Flood Risk Management (Scotland) Act 2009 there is a requirement to promote sustainable solutions, which are therefore likely to integrate limited structural interventions to protect the central section of Gualan, coupled with better flood warning and more proactive planning to restrict

new developments in the most vulnerable low-lying areas around the South Ford shoreline.

ACKNOWLEDGMENTS We are grateful to Alison Sandison for cartographic help and to Jennie Morrison for preparing the revised manuscript. The paper is a contribution to the Scottish Alliance for Geoscience, Environment and Society (SAGES) and to NPP Project CoastAdapt.

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