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

CHAPTER 4.3: COASTAL FLOODING

North Uist and Berneray Coastal Area Local Plan D istrict Local Authority Outer Hebrides - 02 Comhairle nan Eilean Siar

The North Uist and Berneray Coastal Area (Figure 1) has a coastline with a length of approximately 350km. It comprises the islands of North Uist and Berneray which form the central part of the Outer Hebrides Local Plan District (LPD). This coastal area contains two of the eight Potentially Vulnerable Areas (PVAs) in the Outer Hebrides: Lochmaddy & Trumisgarry (PVA 02/04); and North Uist (PVA 02/05).

The coastline is typically embayed with inlets and sea lochs particularly on the east and south coast. On the north and west coasts machair grasslands are the predominant land form extending to around 2 kilometres inland from the foreshore. The majority of 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.

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Figure 1: North Uist and Berneray Coastal Area

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4.3.1 Coastal Flooding Impacts

Main urban centres and infrastructure at risk

There are between 11 and 50 residential properties and less than 10 non-residential properties at medium to high risk of coastal flooding. Approximately 42% of properties at medium to high risk are located within the PVAs.

Figure 2 shows the locations of the residential property at risk of coastal flooding. There is coastal flood risk to residential property at Borgh on Berneray, Port nan Long, Malacleit, Cnoc a Lin, Lochmaddy on North Uist, and the island of Grimsay.

There are approximately 60 road segments at risk of coastal flooding in the medium to high risk event. The main routes affected include the A865, A867, B893, and B894. Minor roads are also impacted.

History of Flooding

In January 2005 a severe storm hit the west coast of Scotland including the Outer Hebrides. Sections of the causeway that joins North Uist to the neighbouring islands of Grimsay and Benbecula were virtually destroyed. Causeways at Baleshare and at Ard Heisgeir suffered severe damage, and significant coastal flooding occurred in many locations including the main road junction at Clachan. The Baleshare causeway is overtopped on a regular basis and rocks, stones and seaweed are washed onto the carriageway.

Localised flood events are not uncommon around North Uist and Berneray.

Economic activity

The Annual Average Damages (AAD) from coastal flooding in the North Uist and Berneray coastal area are estimated to be approximately £84,000. This accounts for around 6% of the total damages for the LPD from all sources of flooding.

These annual average damages are a result of coastal flooding to residential property, non-residential property, agriculture, roads, vehicles, and emergency service costs. For the North Uist and Berneray Coastal Area, residential properties contribute the largest proportion, around 49%, to the total damages, and damages to roads contribute around 24%.

The locations where economic damages are likely to occur due to coastal flooding are shown in Figure 3. There are likely to be damages around the majority of the coastline as a consequence of flooding to agricultural land of which there is approximately 6km 2 of agricultural land at medium to high risk of coastal flooding.

Areas of Environmental and Cultural Importance at risk of flooding

The Outer Hebrides has hundreds of archaeological sites situated along the coast and the low lying areas beyond. Many of these 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

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Archaeology and the Problem of Erosion), and information recorded by other agencies have helped to highlight this issue.

There are 11 cultural heritage sites at medium to high risk of coastal flooding in this coastal area, all of which are designated as Scheduled Monuments. The majority of the sites are prehistoric domestic and defensive sites such as the promontory fort at Caisteal Odair. There is also a stone circle at Sornach Coir’Fhinn.

There is approximately 30km 2 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. 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.

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Figure 2: North Uist and Berneray coastal area residential properties at risk of coastal flooding

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Figure 3: North Uist and Berneray coastal area annual damages

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4.3.2 Managing Flood Risk along the Coastline

Existing flood protection schemes

There are no formal coastal defences within the North Uist Coastal Catchment.

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.

The European funded CoastAdapt project (www.coastadpat.org ) investigated means of adaptation to a changing climate with Comhairle nan Eilean Siar as lead partner, and coastal erosion and flooding have been discussed by the North Uist Community Council on a number of occasions.

Flood Risk Management Studies

A number of studies into coastal flooding and erosion have been undertaken. These are listed in Table 1.

Table 1 – Coastal flooding and erosion risk studies Study Title Source Survey of Coastal Erosion in the Western HR Wallingford, 1995 Isles (Management Units 24 to 30) Coastal Zone Assessment Study SCAPE, 2005 An Ataireachd Ard: the great sea surge: The Angus S. & Rennie A.F. (2007) Outer natural heritage impact of the storm of 11 Hebrides Scottish Natural Heritage January 2005 in the Uists and Barra Commissioned Report

4.3.3 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 (based on UKCP09 high emissions and the 95 percentile confidence limit for modelling) a sea level rise relative to the land could be up to 0.53m by 2080. Under this scenario, the number of residential and non- residential properties at medium to high risk of flooding will approximately double with

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between 11 and 50 residential and between 11 and 50 non-residential properties at risk.

4.3.4 Coastal Processes

The dominant wave conditions on the west coast are south-west to west (between 210°N and 290°N). Extreme swell wave heights can be in excess of 10m offshore. However due to the wide and shallow coastal slope, and the rocky and irregular bathymetry, wave energy is dissipated before the waves reach the coastline. Wave heights on the beaches are controlled by the water depths at the shoreline itself and 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 and the Minch, swell waves generated in the North Atlantic will diffract into the area through the sound of Harris.

The tidal range varies from 3.1m at Scolpaig to 3.8m at Shillay. The offshore current velocities are generally low except between islands and around headlands where peak speeds of up to 2ms -1 can be found for example between North Uist and Berneray.

On the west coast, rock outcrops and headlands control the location and shape of the extensive, wide sandy beaches 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.

Many of the beaches on the west coast are sheltered by offshore islands, rock reefs and shoals. The beaches are relatively stable with minimal net sediment losses. Transport by wind is probably the most dynamic erosive process occurring at present. The east is dominated by a hard rocky coast with intertidal rock platforms and occasional 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 North Uist and Berneray coastal catchment 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. Along the west coast of Berneray, the Corran Aird a Mhorain spit, and on the west coast from Aird an Runair to Balashare there are significant areas of coast which are more susceptible to coastal erosion. Wave erosion is most likely to occur during storm conditions at high tide elevations.

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4.3.5 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.

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 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 northern and western coastlines of North Uist and Berneray have potential for wave energy dissipation. The main area of potential for wave energy dissipation is on the northern coastline around the Beinn Mhor peninsula, particularly along the machair on the western coastline. There are significant lengths of potential for wave energy dissipation around the Aird a Mhorain peninsula, Traigh Bhalaigh bay, and the entire west coast from Aird an Runair to Carinish including Kirkibost Island and Baleshare. There is little potential along the eastern and southern coastline.

4.3.6 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.

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