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SECTION 3: MAIN CATCHMENTS, COASTAL AREAS & SURFACE WATER MANAGEMENT WITHIN OUTER ... · The Outer Hebrides has hundreds of archaeological sites situated along the coast and in the

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    SECTION 3:

    MAIN CATCHMENTS, COASTAL AREAS & SURFACE WATER MANAGEMENT WITHIN OUTER HEBRIDES LOCAL PLAN DISTRICT CHAPTER 4.2: COASTAL FLOODING Lewis & Harris Coastal Area Local Plan District (LPD) Local Authority Outer Hebrides - 02 Comhairle nan Eilean Siar

    The Lewis & Harris Coastal Area (Figure 1) has a coastline with a length of approximately 940km. It comprises the northern portion of the Outer Hebrides Local Plan District (LPD) and includes the island of Lewis and Harris. This coastal area contains three of the eight Potentially Vulnerable Areas (PVAs) in the Outer Hebrides: Ness, Isle of Lewis (02/01); Stornoway, (02/02); and, Southern Harris (02/03). The coastline is heavily embayed with many inlets and sea lochs. For the most part the coastline is rocky in nature with cliffs, reefs and rocky outcrops. Locally shingle and sandy beaches form at the heads of inlets and in bays. Some of the sandy beaches are extensive for example at Uig, Luskintyre and Borve on the west coast. On the west coast of Lewis in the district of Uig machair grasslands are the predominant land form extending to around 2 kilometres inland from the foreshore.

    davmuiTypewritten textAppendix 3

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    Figure 1: Lewis and Harris Coastal Area

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    4.2.1 Coastal Flooding Impacts Main urban centres and infrastructure at risk There are between 11 and 50 residential properties and between 51 and 100 non-residential properties at medium to high risk of coastal flooding. Approximately 69% of properties at risk are located within the three PVAs. Figure 2 shows the locations of the residential property at risk of coastal flooding. There is coastal flood risk at Stornoway, Ardvourlie, Gearraidh Bhaird, Calanais, and Griomarstadh on Lewis, Scalpay, and Leverburgh and Fionnsabhagh on Harris. Table 1 shows the main areas at risk of coastal flooding in the Lewis and Harris Coastal Area.

    There are approximately 170 road segments at medium to high risk of coastal flooding. The main routes affected include the A857, A859 and A866. Minor roads are also impacted. There are 13 energy production/electricity utility sites and a telecommunications utility site at medium to high risk of coastal flooding as well as an emergency services facility. History of Flooding Stornoway was affected by a coastal flood event in February 1990 when the airport, streets and Town Hall were flooded, the bus station was cut off by waves and there was a loss of power. This is the most significant coastal flood event on record although there have been a number of localised, less significant coastal flood events. A more recent event occurred in early January 2014 when roads in Stornoway were closed due to flooding caused by a combination of high spring tides and unusually low atmospheric pressure. Fortunately, surge was not accompanied by wind and wave action, flooding was solely by still water and flood water did not enter properties. Economic activity The Annual Average Damages (AAD) from coastal flooding in the Lewis and Harris coastal catchment are estimated to be approximately £315,000. This accounts for around 22% of the total damages for the LPD from all sources of flooding. These annual average damages are made up from a variety of damage categories including residential property, non-residential property, agriculture, roads, vehicles, and emergency service costs. For the Lewis and Harris Coastal Area the majority of the damages (57%) are from non-residential properties, and 22% from residential properties.

    Table 1 – Main urban areas at risk of coastal flooding

    Locations No of residential properties at flood risk (at the 1 in 200 year

    flood event) Annual Average Damages

    Stornoway 11 to 50 £135,000

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    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 medium to high coastal flood risk to approximately 8km2 of agricultural land. The areas that have the highest damage densities are around Stornoway, Leverburgh, and Scalpay. Areas of Environmental and Cultural Importance at r isk of flooding The Outer Hebrides has hundreds of archaeological sites situated along the coast and in the adjacent low lying areas. Many of these sites are in a relatively good condition and have minimal disturbance. The main threat to these sites is from 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 32 cultural heritage sites including Lews Castle and Lady Lever Park at risk of coastal flooding in this catchment in the medium to high risk event, the majority of which are Scheduled Monuments. There are 18 prehistoric domestic and defensive sites, five prehistoric ritual and funerary sites, four ecclesiastical sites, two industrial and transport sites, and two secular defensive sites. There is approximately 7km2 of environmentally designated areas at risk of coastal flooding even in the extreme events. The main impacts are from temporary inundations of saltwater however due to the dynamic nature of coastal environments in the Western Isles it is unlikely that permanent damage is caused. Over the longer term certain habitats may suffer from a reduction in extent or species distribution due to saltwater inundation or a migration to other sites. Ground nesting birds may also be adversely impacted by flooding.

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    Figure 2: Lewis and Harris coastal area residential properties at risk from coastal flooding

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    Figure 3: Lewis and Harris coastal area annual average damages

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    4.2.2 Managing Flood Risk along the Coastline Existing flood protection schemes There are numerous coastal defences throughout the Lewis and Harris coastal area; details of the defences present in the three PVAs can be found in the 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. Flood Risk Management Studies Comhairle nan Eilean Siar is carrying out a Strategic Flood Risk Assessment (SFRA) for the Stornoway area designed to inform the Development Planning Process, e.g. local plans. A number of studies into coastal flooding and erosion have been undertaken. These are listed in Table 2. Table 2 – Coastal flooding and erosion risk studies

    Study Title Source Survey of Coastal Erosion in the Western Isles (Management Units 24 to 30)

    HR Wallingford, 1995

    Coastal Zone Assessment Study SCAPE, 2005 4.2.3 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 Lewis and Harris coastal area of 0.55m is predicted. Under this scenario the number of residential and non-residential properties at medium to high risk of coastal flooding will increase by around 100% and 50% respectively.

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    4.2.4 Coastal Processes Offshore wave heights in excess of 12 metres can occur under extreme conditions. The local coastal shape and bathymetry will dictate how much of this energy actually reaches the shoreline. For example, the wave climate affecting the west coastline of Harris is less severe than for Lewis due to the dissipative influence on waves of the more gently sloping nearshore bathymetry. There are numerous examples of inlets and bays which are sheltered from both Atlantic storm waves and from swell. The tidal range varies between three metres and four metres, but can be amplified slightly within the sea lochs such as East Loch Tarbert. The coastline of Lewis and Harris is predominantly hard rock cliffs with occasional shingle/cobble fringe beaches particularly along the inner areas of the sea lochs and in bays. Extensive sandy beaches form in more sheltered areas such as in Luskintyre and Uig, which are backed by machair grasslands. The hard bedrock along the western coast of Lewis is overlain by glacial deposits of boulder clay generally between one metre and five metres in height. Erosion of this soft material was the main source of the material which forms the beaches along the coast. There is currently limited sediment supply from erosion of the remaining glacial deposits however broken shells provide an important additional source of sand sized sediment. 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. Based on the outputs from the CESM, the natural susceptibility to coastal erosion for the Lewis and Harris coastal area is shown in Figure 5. While the majority of the coastline has a low natural susceptibility to erosion there are numerous short isolated sections of coast which are classed as more susceptible. Examples of these more susceptible sections of coast can be found in the vicinity of Stornoway airport and Mealabost.

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    4.2.5 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 for wave energy dissipation is to help identify areas where salt marshes or other measures such as artificial reefs 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. There are a number of relatively short sections of the coastline on Lewis and Harris that have potential for wave energy dissipation. For example, along the western coast of northern Lewis. Further examples can be found along the southern end of Broad Bay and along the strip of land connecting with the Eye Peninsula. 4.2.6 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.