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

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1 SECTION 3: MAIN CATCHMENTS, COASTAL AREAS & SURFACE WATER MANAGEMENT WITHIN OUTER HEBRIDES LOCAL PLAN DISTRICT CHAPTER 4.5: SURFACE WATER FLOODING The purpose of this section is to identify the main surface water flooding issues within the Outer Hebrides LPD and to help identify any priority areas where the preparation of a surface water management plan would be required Surface Water Flooding Impacts Main urban centres and infrastructure at risk There are less than 10 residential properties at medium to high risk of surface water flooding in the Outer Hebrides Local Plan District (LPD). This represents less than 5% of those at risk from all sources of flooding. The number of non-residential properties at medium to high risk of surface water flooding is between 10 and 50 again representing a small proportion of the total number at risk of flooding from all sources in the Outer Hebrides. The main areas of surface water flood risk are in Tarbert on Harris and Tobha Mor on South Uist, (Figure 1). History of Flooding There have been no recorded surface water flood events that have had a high impact in the Outer Hebrides although there have been a number of localised flood events. These flooding events often occur during periods of tidal surge combined with high rainfall and can result in flooding of land upstream of river estuaries, such as at the Howmore River (Tabha Mor) in South Uist. Economic activity The annual average damages (AAD) from surface water flooding for the whole of the LPD are estimated to be 59,000. This accounts for just 4% of the total flood damages for the LPD. The Sir Edward Scott School on the Isle of Harris and a number of utility sites are at medium to high risk of surface water flooding. Approximately 300 road segments and small sections of the airport runways at Stornoway and Benbecula are also at medium to high risk of surface water flooding. The locations where economic damages are likely to occur due to surface water flooding are shown in Figure 2 (a & b). These are scattered across the islands of the Outer Hebrides with the greatest concentration in the vicinity of Stornoway. There are also damages in the vicinity of Tarbert and Tabost on Harris, Breacleit on Great Bernera, Leverburgh on Harris, Tobha Mor, Lochboisdale, and Gearraidh na Monadh on South Uist.

2 Areas of Environmental and Cultural Importance at risk of flooding There are 45 cultural heritage sites at medium to high risk of surface water flooding in the Outer Hebrides LPD including Lews Castle and Lady Lever Park. The majority of the cultural heritage sites at risk are Scheduled Ancient Monuments and include prehistoric domestic and defensive sites, prehistoric ritual and funerary sites, and secular sites such as the castle at Drimsdale (Caisteal Bheagram) Managing Surface Water Flood Risk There are no known flood protection schemes specifically for surface water flooding and no known surface water management studies for the Outer Hebrides. Surface water flooding, taken on its own, has a relatively low impact on overall flood risk in the Outer Hebrides. However, when combined with high tide levels, when surface water discharge to sea outfalls is reduced or blocked, localised surface water flooding can occur as a consequence. Management of surface water flooding is therefore closely linked to river and coastal flood management Climate Change and Future Flood Risk The levels of precipitation over northern Europe have significantly increased since There have been more frequent spells of wet weather and an increase in total precipitation. The intensity of rainfall has increased particularly during the winter and to a lesser extent during the spring and autumn. Climate change is predicted to lead to a further increase in the amount of rainfall in the Outer Hebrides 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 20% increase in rainfall for modelling purposes (this assumes a scenario of high emissions in future), the number of residential and non-residential properties at medium to high risk of flooding does not change significantly. However this does not take account of the combined impact of sea level rise and a consequential rise in groundwater and loch levels Links with River Basin Planning This section will present information on pressures and actions identified in River Basin Management Plans. Details are not available at this time but will be added ahead of public consultation on the Flood Risk Management Strategies.

3 Figure 1: Main areas of surface water flood risk in the Outer Hebrides

4 Figure 2a: Annual Average Damages from surface water flooding

5 Figure 2b: Annual Average Damages from surface water flooding