SECTION 3: MAIN CATCHMENTS, COASTAL AREAS & SURFACE WATER MANAGEMENT WITHIN OUTER HEBRIDES LOCAL PLAN DISTRICT

CHAPTER 4.4: COASTAL FLOODING

Benbecula, South Uist & Barra Coastal Area Local Plan D istrict Local Authorit y Outer Hebrides - 02 Comhairle nan Eilean Siar

The Benbecula, South Uist & Barra Coastal Area (Figure 1) has a coastline with an approximate length of 830km. It comprises the southern portion of the Outer Hebrides Local Plan District (LPD) and includes the islands of Benbecula, South Uist, and Barra. This coastal area contains three of the eight Potentially Vulnerable Areas (PVAs) in the Outer Hebrides including those which have the greatest flood risk. The three PVAs are: Benbecula (02/06); Lochs Bi and Druidibeag (02/07); and, Bornish to Boisdale (02/08).

The east coast is typically embayed with many inlets, and sea lochs. By contrast, on the west coast, machair grasslands often flanked with sand dunes are the predominant land form extending up to 2 kilometres inland from the foreshore. Many of the settlements are located close to the coastline while others are situated at the landward limit of the machair where it joins with inland land forms such as glacial deposits, rock or peat.

02 South Uist, Benbecula and Barra coastal Page 1 of 12 area Version 1.0

Figure 1: Benbecula, South Uist and Barra Coastal area

02 South Uist, Benbecula and Barra coastal Page 2 of 12 area Version 1.0

4.4.1 Coastal Flooding Impacts

Main urban centres and infrastructure at risk

There are between 101 and 250 residential properties and between 11 and 50 non- residential properties at medium to high risk of coastal flooding. Approximately 95% of properties at risk in the medium to high risk event are located within the three PVAs.

Figure 2 shows the locations of the residential property at risk of coastal flooding. The location with the largest number of residential properties at medium to high risk of coastal flooding and with the highest level of AAD is Balivanich, Table 1. There is also coastal flood risk in the Dalabrog and Iochdar areas on South Uist and Gramasdail, Creagorry and Lionacleit on Benbecula.

Table 1 – Main areas at risk of coastal flooding No of residential properties at Annual Average Damages Locations flood risk (at the 1 in 200 year flood event) Balivanich 50 to 100 £392000

There are approximately 150 road segments at risk of coastal flooding in the medium to high risk event. The main routes affected include the A865, B891, and B892.

History of Flooding

In January 2005 a severe storm hit the west coast of Scotland including the Outer Hebrides. During the storm five people from the same family died when their cars were swept from a coastal road in Iochdar, South Uist as they were trying to escape from flood waters. Widespread flooding occurred in many locations, particularly on the west coast, with roads, agricultural land, houses and other buildings being inundated with sea water.

Although there are no other officially recorded flood events in this coastal area coastal flooding has occurred on numerous occasions when storm surge and high tides coincide.

Economic activity

The annual average damages (AAD) from coastal flooding in the Benbecula, South Uist and Barra coastal area are estimated to be approximately £577,000. This accounts for 41% of the total damages for the LPD from all flood sources.

These AADs are made up from a variety of damage categories including residential property, non-residential property, agriculture, roads, damage to vehicles, and emergency service costs. For the Benbecula, South Uist and Barra Coastal Area the vast majority of the damages (65%) are to residential properties.

The locations where economic damages are likely to occur due to coastal flooding are shown in Figure 3. This includes damages around the majority of the coastline as a consequence of flooding of approximately 20km 2 of agricultural land at medium to high risk of coastal flooding. The areas that have the highest damage densities are around Balivanich on Benbecula and Kilpheder on South Uist.

02 South Uist, Benbecula and Barra coastal Page 3 of 12 area Version 1.0

Areas of Environmental and Cultural Importance at risk of flooding

The Western Isles has hundreds of archaeological sites situated along the coast and the low lying areas beyond. Many of these are in a relatively good condition with minimal disturbance. The main threat to these sites is coastal erosion.

In relation to the many cultural heritage sites at risk of coastal erosion, assessment surveys and excavations such as those run by the SCAPE Trust (Scottish Coastal Archaeology and the Problem of Erosion), and information recorded by other agencies have helped to highlight this issue.

There are 16 cultural heritage sites at medium to high risk of coastal flooding in this coastal area. All of these are Scheduled Ancient Monuments and the majority are prehistoric domestic and defensive sites, however there is also a standing stone at Pollachar, and three secular sites including Caisteal Bheagram and the castle at Drimsdale.

There is approximately 50km 2 of environmentally designated areas at medium to high risk of coastal flooding.

A study conducted shortly after the January 2005 storm has shown that machair habitats are resilient to saline incursion possibly due to the high level of interaction between freshwater/brackish water in the inland machair loch system and the water table (Angus S. & Rennie A.F. 2012). The ability to restore surface water salinity levels through a unified subsoil/surface water table may be an advantage in respect of short duration flooding, but if saline flooding is more persistent, either due to failure of drainage systems or to more frequent marine inundation, this dispersal effect may become a liability as it would result in saline contamination equally of ground and surface water.

Management of drainage systems is important for flood water dispersal and for maintaining current levels of salinity in the water table. Adjustments may be required to cope with increased saline inflow caused by rising sea level, but as sea level rises still further it will become more difficult for the drainage system to cope.

Ground nesting birds may also be adversely impacted by coastal flooding.

02 South Uist, Benbecula and Barra coastal Page 4 of 12 area Version 1.0

Figure 2: Benbecula, South Uist and Barra coastal area residential properties at risk of coastal flooding

02 South Uist, Benbecula and Barra coastal Page 5 of 12 area Version 1.0

Figure 3: Annual average damages from coastal flooding

02 South Uist, Benbecula and Barra coastal Page 6 of 12 area Version 1.0

4.4.2 Managing Flood Risk along the Coastline

Existing flood protection schemes

There are numerous coastal defences throughout the Benbecula, South Uist and Barra coastal area, some of which provide flood protection in addition to protection against erosion. Details of the defences present in the three PVAs can be found in the individual PVA reports.

Comhairle nan Eilean Siar is the planning authority for the Outer Hebrides and is required by Scottish Planning Policy to consider flood risk issues when determining planning applications. The Comhairle adopts differing approaches for proposals depending on the level of risk and developers may be required to provide a Flood Risk Assessment.

The Scottish Environment Protection Agency (SEPA) is Scotland's flood warning authority. It provides flooding updates and advice on being prepared for flooding. SEPA operates a general flood alert scheme for the Outer Hebrides but there are no specific flood warning target areas.

The Local Authority Emergency Planning Unit works with SEPA in helping raise awareness of flood risk and the dissemination of warnings to the community. They work with emergency services in response to severe flooding events, in providing shelter in the event of evacuations and with communities and businesses on resilience and recovery.

SEPA operates a general coastal flood alert scheme for the Outer Hebrides but there are no specific flood warning target areas identified.

Awareness raising campaigns & community flood action groups

There are two community flood action groups, the Iochdar Flood Action Group and the Middle District Flood Action Group which were formed in the immediate aftermath of the January 2005 storm. Public meetings and workshops have been held in recent years to discuss flooding and coastal erosion issues particularly in Benbecula and South Uist. These events have been well attended reflecting the importance of coastal issues to the local population and the willingness of local people to be involved in decision-making and implementation of schemes.

Organisations that have been involved include the Comhairle; Scottish Natural Heritage; Coast Hebrides, the local coastal partnership ( www.coasthebrides.co.uk ); Stòras Uibhist, the community land owner; the CoastAdapt project (www.coastadapt.org); and Oxfam Scotland who provided community support.

Comhairle nan Eilean Siar was lead partner in the CoastAdapt EU project which aimed to safeguard people living in North Atlantic coastal communities and help them adapt to the impacts of climate change.

Flood Risk Management Studies

There are number of studies into flood and coastal erosion risk for this Local Plan District. The main studies are listed in Table 2.

02 South Uist, Benbecula and Barra coastal Page 7 of 12 area Version 1.0

Table 2. Flood and Coastal Erosion Risk studies relevant to the Benbecula, South Uist and Barra coastal area. Study Title Source Survey of Coastal Erosion in the Western HR Wallingford, 1995 Isles (Management Units 31 to 44) Coastal Zone Assessment Study (for SCAPE, 2005 Archaeological Assets) Report on Condition of Drainage Systems Stòras Uibhist, 2006 in Benbecula and South Uist An Ataireachd Ard: the Uist storm of Angus S. & Rennie A.F. (2012). January 2005 Scottish Natural Heritage Commissioned Report Wave and Tidal Modelling Studies related HR Wallingford, 2009 to the Western Isles Coastal Protection Scheme Programme South Ford Hydrodynamic Study Comhairle nan Eilean Siar, 2011 Vulnerability and Adaptation to Extreme Muir D., Dawson A.G., Gagnon A.S. & Coastal Flooding: an Example from the O’Mahony C., 2013. ICE Coasts, Marine South Ford Area, Scottish Outer Hebrides. Structures and Breakwaters

Community driven coastal management: Young E., Muir D., Dawson A. G. & an example of the implementation of a Dawson S., 2014. Ocean & Coastal coastal defence bund on South Uist, Management (in press) Scottish Outer Hebrides

4.4.3 Climate Change and Future Flood Risk

Climate change is predicted to result in sea level rise according to the most recent UK Climate Projections (UKCP09) report. There are inherent uncertainties in the projections of climate change which arise from three causes; natural climate variability, modelling uncertainty and the future level of emissions of greenhouse gases.

Taking a “worst case” scenario (i.e. assuming high emissions in future, and the 95 percentile confidence limit for modelling), a sea level rise relative to the land in the Benbecula, South Uist and Barra coastal area of around 0.52 metres by 2080 is predicted. Under this scenario, the number of residential properties at medium to high risk of flooding will almost double and non-residential properties will more than double.

4.4.4 Coastal Processes

The dominant wave conditions on the west coast are south-west to west (between 210°N and 290°N). Significant wave heights recorded by the West of Hebrides WaveNet site can exceed 10 metres offshore. However, due to the wide and shallow coastal slope, and the rocky and irregular bathymetry, wave energy is dissipated and wave heights on the beaches are significantly reduced and are controlled by water depths at the shoreline itself. As a consequence, wave action is greatest during spring tides and / or during storm surges.

The wave climate on the east coast is highly variable due to the irregular nature of the coastline. Islands, rock outcrops and reefs are common and cause significant wave energy dissipation. In addition to waves generated in the Sea of the Hebrides

02 South Uist, Benbecula and Barra coastal Page 8 of 12 area Version 1.0

itself, swell waves generated in the North Atlantic will diffract into the area resulting in significant wave height of over 8 metres occurring offshore in the Barra region.

The tidal range varies from 3.2m at Barra Head to 3.7m at Castlebay. The offshore current velocities are generally low except between islands and around headlands where peak speeds of up to 1ms -1 can be found.

On the west coast, rock outcrops and headlands control the location and shape of the extensive, wide sandy beach areas that are often backed by dunes and flat coastal plain known as machair. Sand and gravel derived from glacial deposits have been greatly supplemented by calcareous sand formed by the breakdown of shells.

There are also a large number of shingle beaches, in some cases forming ridges lying above sandy beaches or forming the core of the sandy beaches. However, due to a sediment deficit much of the coastline is eroding and over 30% of beaches lack a protective dune cordon. In these areas the coast is susceptible to over wash and flooding. Coastal flooding is also a major risk to the delicately balanced machair ecosystem. Large areas of the machair are near mean sea-level in elevation making drainage of these areas challenging.

The east is dominated by a hard rocky coast with intertidal rock platforms and only sparse beaches.

The Coastal Erosion Susceptibility Model (CESM) has been developed to assess the susceptibility of the Scottish coastline to erosion. It is based on characterising the coast using available national data and has been calibrated against a range of case studies around the country with varying levels of erosion to check that the model is representative.

The natural susceptibility to coastal erosion for the Benbecula, South Uist and Barra coastal area is shown in Figure 5.

There are numerous short isolated sections of coast which are classed as more susceptible to erosion. Examples can be found, in the vicinity of Benbecula and Barra airports, along the west coast of South Uist, and on the islands of Pabbay, Mingulay and Berneray.

02 South Uist, Benbecula and Barra coastal Page 9 of 12 area Version 1.0

02 South Uist, Benbecula and Barra coastal Page 10 of 12 area Version 1.0

02 South Uist, Benbecula and Barra coastal Page 11 of 12 area Version 1.0

4.4.5 Potential for Natural Flood Management

A national screening exercise to identify areas where there are opportunities for alteration or restoration of natural features to help manage flood risk has been undertaken. The outputs provide a high level strategic assessment of those areas along coastlines where implementation of certain types of natural flood management measures would be most effective and merit further investigation.

The purpose of the screening exercise carried out for the coastline in the Outer Hebrides was to help identify areas where measures such as regenerating saltmarshes or sand dune restoration could be used to reduce wave energy arriving at the shore. The maps showing the results of the natural flood management screening can be view on the SEPA website.

Beach systems in the Outer Hebrides, whether sand, shingle or dunes, provide a natural coastal defence function thereby reducing the risk of flooding. A notable feature of the inshore seabed to the west of the islands is its shallow gradient, which together with vast kelp forests which grow on it, play a critical role in protecting the soft coastline of the Uists from wave action. The combination of shallow gradient and roughness created by the kelp forest greatly dissipate wave energy.

The main areas of potential for wave energy attenuation tend to be on the soft sandy western coastlines of Benbecula, South Uist, and Barra coastal area. Very little potential has been identified along the eastern coastlines.

Due to sediment deficit much of the western coastline is eroding and a large proportion lacks a protective dune cordon. In these areas the coast is susceptible to over wash and flooding which is of particular concern due to the negative gradient inland from the coastal edge. Potential exists for rebuilding gaps in dune systems and for development of methods to counter undercutting of the low machair edge by wave action.

On Benbecula the entire western coastline from the airport to Luib Bhan bay has potential for wave energy dissipation. There are also small areas of potential at Rubha ma-thuath on the south coast, Rairnis and Orasaigh Uisgeabhagh on the east coast, and the area around the end of the causeway from Grimsay. On South Uist the entire western coastline has potential for wave energy dissipation.

The South Ford Hydrodynamics Study has identified beach recharge at Gualan Island together with dune stabilisation at Lionacleit as a means of wave energy dissipation for the South Ford area and the south west coast of Benbecula.

4.4.6 Links with River Basin Planning

This section will present information on pressures and actions identified in River Basin Management Plan. Information to complete this section is not yet available but will be included in the documents issued for public consultation in December 2014.

02 South Uist, Benbecula and Barra coastal Page 12 of 12 area Version 1.0