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March 2010 DURHAM COUNTY COUNCIL Level 2 SFRA Submitted to: Durham County Council REPORT Report Number: P9514100072-2-A02 Distribution: Environment Agency Northumbrian Water DURHAM LEVEL 2 SFRA Table of Contents 1.0 CONTEXT ................................................................................................................ 2 2.0 METHODOLOGY .................................................................................................... 2 2.1 Site Credentials .......................................................................................... 3 2.2 Risk of Flooding from Rivers ...................................................................... 3 2.3 Impact of Climate Change .......................................................................... 5 2.4 Risk of Flooding from the Sea .................................................................... 5 2.5 Risk of Flooding from Surface Water .......................................................... 5 2.6 Risk of Flooding from Groundwater ............................................................ 5 2.7 Residual Risk of Flooding ........................................................................... 6 2.8 PPS25 Constraints to Development ........................................................... 6 3.0 LEVEL 2 SFRA FINDINGS & RECOMMENDATIONS ............................................ 7 Appendix A Level 2 Assessment of All Housing Sites Appendix B Detailed Level 2 Assessment of Housing Sites within Zone 3 Appendix C Level 2 Assessment of Employment Sites Appendix D Level 2 Assessment of Minerals & Waste Sites October 20090 Report No. P9514100072-2-A02 i DURHAM LEVEL 2 SFRA 1.0 CONTEXT Durham County Council was given unitary status in April 2009, replacing the district authorities of Easington, Durham City, Chester-le-Street, Wear Valley, Derwentside, Teesdale and Sedgefield. Under the Planning and Compulsory Purchase Act (2004), the newly formed County Council is required to prepare a Local Development Framework (LDF), setting out their strategy for future development within the County to 2026. The LDF will replace the previous district Local Plans and emerging Local Development Frameworks, taking a fresh view of the County as a whole. The Durham County Level 1 SFRA was prepared by Golder Associates in August 2009, following the flooding that affected in the County in July 2009. The Level 1 SFRA is intended to provide an overview of the risk posed by flooding throughout the County from rivers, the North Sea, groundwater, and surface water runoff. The findings and recommendations of this investigation have informed the application of the Sequential Test as part of the planning process. Where a balanced view has been taken (following the application of the Sequential Test), and it is determined by the Council that further consideration of areas at risk of flooding may be warranted on planning grounds, a Level 2 SFRA is required in accordance with PPS25. The Level 2 SFRA involves a more detailed (local) investigation of the risk of flooding within an emerging development area to ensure that the requirements of the Exception Test can ultimately be met. This document represents the Durham County Level 2 SFRA. The SFRA will form part of the evidence base that supports the Durham County Local Development Framework (LDF). The SFRA has been prepared in accordance with Planning Policy Statement (PPS) 25: Development and Flood Risk (December 2006) and the PPS25 Practice Guide (June 2008) for this purpose. 2.0 METHODOLOGY The methodology adopted to inform the appraisal of flood risk within emerging development areas is set out in the discussion below. Clear recommendations have been provided to guide Council decision making in relation to the allocation of land, and mitigation measures that should be sought as an outcome of the development management (control) process. These recommendations have been drawn from the recommended Council responses to flood risk set out within the Level 1 SFRA. It is reiterated that this Level 2 SFRA has been developed specifically to inform the planning process, and the level of detail adopted in the analyses is commensurate to this objective. A more detailed site based assessment of flood risk (from all sources) will be required from developers at the planning application stage, in accordance with Section 7 of the Level 1 SFRA. For those sites within which there is a risk of fluvial flooding, a hydraulic model should be used to determine more definitively the depth, velocity and extent of flooding within the site. March 2010 Report No. P9514100072-2-A02 DURHAM LEVEL 2 SFRA It is strongly recommended that the developer opens discussion with the Council and the Environment Agency as early as possible to agree the scope of the modelling required as part of the Flood Risk Assessment (FRA) for the specific site that is under consideration. The Environment Agency will often also be able to provide (at a cost) existing modelling and/or useful information to inform the assessment of flood risk within the site. 2.1 Site Credentials Emerging sites that are under consideration for the future allocation of housing, employment and minerals & waste development were provided by Durham County Council in September 2009. The topography and geology of the area was interrogated on the basis of the county-wide digital elevation model (DEM)1 and published 1:625,000 geological maps utilised in the development of the Level 1 SFRA. Incidents of historical river and surface water flooding have been collated from the County Council, the Environment Agency, and Town & Parish Councils, as described in Section 4.1 of the Level 1 SFRA. 2.2 Risk of Flooding from Rivers The risk of fluvial flooding has been assessed in the following manner: 1. Flood Level The extent of river flooding in the 5% (1 in 20), 1% (1 in 100) and 0.1% (1 in 1000) design events is represented within the Level 1 SFRA as Zone 3b Functional Floodplain, Zone 3a High Probability, and Zone 2 Medium Probability respectively. The predicted maximum flood level for each site (for each design event) has been estimated by interrogating the ground level at which the predicted flood extent intersects the digital elevation model (DEM). 2. Flood Depth The predicted maximum flood depth for each site (for each design event) has been estimated by interrogating the minimum ground level within the site from the digital elevation model (DEM), and subtracting this from the predicted flood level. The average flood depth has been calculated by subtracting the mean site level (taken from the DEM) from the predicted flood level. 1 Durham County Council, 2009 March 2010 Report No. P9514100072-2-A02 DURHAM LEVEL 2 SFRA 3. Flow Velocity The velocity of floodwaters through the site has been approximated in accordance with first principles. The cross sectional area (A), wetted perimeter (P) and longitudinal gradient (S) of the floodplain is assessed from the digital elevation model (DEM). The flow velocity is then calculated using Manning’s Equation (assuming a manning’s roughness (n) of 0.050), i.e.: This methodology provides an average velocity across the floodplain, which is considered representative for planning purposes within the overbank areas. 4. Flood Defences The presence of formal flood defences has been informed by the Environment Agency, as described in Section 4.2.2 of the Level 1 SFRA. Those areas of the County that may be susceptible to a residual risk of flooding due to defence failure has been interrogated, as described in Section 5.8 of the Level 1 SFRA. Where the proposed development site falls within an area that is highlighted within the Level 1 SFRA as within a danger (or hazard) zone due to the presence of raised defences, a more detailed interrogation of the potential risk of breach failure is required as part of this Level 2 SFRA. It is noted however that there are no sites that have been identified by the Level 2 SFRA as benefitting from the presence of formal flood defences. Informal flood defences are described as structures that alter the path of floodwaters, and in doing so provide a degree of protection to properties that would otherwise be at risk. They have not been designed and/or constructed to retain floodwater however. Informal flood defences may include (for example) boundary walls, fences, or road embankments. The presence of informal flood defences has been investigated on a site-by-site basis as part of the Level 2 SFRA, however once again no sites have been identified that benefit from the existence of an informal flood defence. March 2010 Report No. P9514100072-2-A02 DURHAM LEVEL 2 SFRA 2.3 Impact of Climate Change The Level 1 SFRA provides an indication of the likely impact of climate change (over a 100 year period) upon the anticipated extent of the 1% (1 in 100) flood event. This information has been used to assess the potential increase in the risk of fluvial flooding, over the lifetime of the proposed development, as a result of climate change. An approximation of the anticipated increase in the extent and depth of flooding in the 1% (1 in 100) event has been made in accordance with the methodology set out in the ‘Risk of Flooding from Rivers’ above. The assessment of climate change impacts upon the risk of flooding within the County of Durham is described in Section 5.9 of the Level 1 SFRA report. 2.4 Risk of Flooding from the Sea The risk of flooding from the sea has been evaluated on the basis of the flood zones set out within the Level 1 SFRA. It is noted