Aylesbury Town Level 2 Strategic Flood Risk Assessment Final Report
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Aylesbury Town Level 2 Strategic Flood Risk Assessment Final Report Aylesbury Vale District Council 9 April 2009 9T5436 Strategic Flood Risk Assessment Level 2 for Aylesbury Town – August 2012 Update This note has been prepared to advise the reader of the updated context for this evidence base in terms of the work on the Vale of Aylesbury Plan and planning applications around Aylesbury. The SFRA Level 2 was prepared by Royal Haskoning and published by AVDC in 2009 following detailed watercourse modelling to inform the planning of potential major development site options around the town. The report identifies the locations of potential flood storage areas, the potential impact of breaches to flood defences, potential improvements to the existing Aylesbury Flood Alleviation Scheme at Broughton and the extent of the functional floodplain. The evidence base is still valid for both assessing planning applications in Aylesbury and to inform the Vale of Aylesbury Plan or any Neighbourhood Planning work. However it should be noted that there have been significant legislative, regulatory and National planning policy changes since the publication of the SFRA Level 2 in April 2009. These include:- the Flood and Water Management Act 2010 Localism Act 2011 Town and Country Planning (Local Planning) (England) Regulations 2012 National Planning Policy Framework (2012) and Technical Guidance to the National Planning Policy Framework 2012 The SFRA Level 1 for Aylesbury Vale (August 2012) provides a full coverage of current legislation, regulations and the impacts of the National Planning Policy Framework. Where the SFRA Level 2 for Aylesbury Town is inconsistent with the SFRA Level 1 on current legislation, regulations and the NPPF, those in the SFRA Level 1 are prevalent. EXECUTIVE SUMMARY Background The local development documents (LDDs), which make up the Local Development Framework (LDF) must be based on a robust and credible evidence base. This Level 2 Strategic Flood Risk Assessment (SFRA) informs the preparation of the Aylesbury Vale Core Strategy with regards to flood risk. It will assist the Aylesbury Vale District Council in applying the Sequential Test and the Exception Test, providing a method of managing flood risk while still allowing necessary development to occur. It is recognised that this is a live document which should be updated accordingly with changes to flood risk management and planning policies. In order to pass the Exception Test, a development must pass all the following aspects: (a) it must be demonstrated that the development provides wider sustainability benefits to the community that outweigh flood risk, informed by a SFRA where one has been prepared. If the Development Plan Documents (DPDs) have reached the ‘submission’ stage – see Figure 4 of PPS12: Local Development Frameworks – the benefits of the development should contribute to the Core Strategy’s Sustainability Appraisal; (b) the development should be on developable previously-developed land or, if it is not on previously developed land, that there are no reasonable alternative sites on developable previously-developed land. (c) a Flood Risk Assessment must demonstrate that the development will be safe, without increasing the flood risk elsewhere, and, where possible, will reduce flood risk overall. The assessment will refer to the joint Defra and Environment Agency document “Flood Risks to People Phase 2, Guidance Document, FD2321/TR2 (March 2006)” to demonstrate that the development is safe. If the Growth Arcs can maintain the runoff at the Greenfield rates, reduce flood risk to the town centre and surrounding areas, such as at Watermead, and keep built development out of the present day 0.1% annual probability residual risk areas, then they would pass aspect (c). This Final Level 2 SFRA satisfies the amplified guidance within the Living Draft Practice Guide Companion to PPS25 by: Defining the functional floodplain; Considering the nature of the flood hazard, taking account of the presence of flood management measures e.g. flood defences and defacto defences; Considering the nature of flood hazard if defences fail; Appraising the defence infrastructure condition and maintenance in light of flood risk; Informing Emergency Planning Procedures (access and egress to development sites); and Providing guidance on the preparation of Flood Risk Assessments and the consideration of managing surface water (Appendix A). All the growth arc allocations include functional floodplain and Flood Zones 2 and 3 to ensure the Council has maximum control of these areas. The Council’s policy is that Aylesbury Town Level 2 SFRA 9T5436/D1/R003/302072/PBor Final Report - i - 9 April 2009 there should be no built development within any of these areas. This approach is in line with the selected policy from the Thames Catchment Flood Management Plan which is to “accept flood risk, but in the longer term take action to ensure that the risk does not increase from current level”. The functional floodplain The functional floodplain has been defined using a flood with an annual probability of 5% (1:20 years) plus the areas within the existing flood storage reservoirs (Stocklake FSR and Bear Brook FSR), where water is stored in times of flooding. In this floodplain only water-compatible development (e.g. amenity open space) should be permitted, together with essential infrastructure if it passes the Exception Test. The present and future 5% annual probability flood events affect small areas of each Growth Arc: Eastern Growth Arc –14.4 hectares (3.4%) and 15.4 hectares (3.6%) respectively; Southern Growth Arc – 1.4 hectares (0.3%) and 1.8 hectares (0.4%) respectively; and Combined East and Southern Growth Arc – 1.4 hectares (0.3%) and 1.8 hectares (0.4%) respectively. Generally the flood hazard to people is low except in the north east Major Development Area (MDA) where it is moderate to significant to people. Under the 5% annual probability flood event, the flood storage reservoirs and defences protect the urban areas through the town except small areas of Fairford Leys and Fowler Road. The River Thame functional floodplain extends to some properties at Quarrendon (north of Jackson Road) and also parts of the MDAs at Berryfields and Weedon Hill. The east part of Aston Clinton Road MDA is within the West End Ditch functional floodplain (next to Broughton Lane and below Old Manor Farm) and the Wendover Brook functional floodplain runs through the north east portion. All of the proposed strategic link roads are within the present day and future functional floodplains and in accordance with PPS25 Table D.1 they should be designed to remain operational and safe for users in times of flood. However, the link roads provide the opportunity of multiple benefits, such as flood defence and increased biodiversity. Buckinghamshire County Council should take account of the level of flood risk and design the link roads in consultation with Aylesbury Vale District Council and the Environment Agency to make them sustainable and ensure they do not increase flood risk elsewhere. Residual flood risk and flood hazard Small areas of all the Growth Arcs are within the defended present day and future 1% and 0.1% annual probability flood events. However, the Council is not proposing any built development in any of these areas. As there is enough land outside of the present day residual risk areas, only water-compatible development (e.g. amenity open space) should be permitted, together with essential infrastructure if it passes the Exception Test. 9T5436/D1/R003/302072/PBor Aylesbury Town Level 2 SFRA 9 April 2009 - ii - Final Report For areas of previously developed land in the town and where there is no available land outside of the 0.1% residual flood risk area, the Flood Hazard maps should be used to steer water compatible and less vulnerable development to low hazard areas (shown as blue depth-velocity combinations). Development in areas affected by an allowance for climate change should be adaptable e.g. where ground floor use can be changed to less vulnerable/ water compatible types to make space for water. Key findings relative to areas at risk are: There is not much difference in the River Thame flood extent between the defended 5%, 1% and 0.1% annual probability events in the north east MDA of the Eastern Growth Arc as the valley sides are reasonably well defined. The flood hazard is moderate to significant; • The east MDA at Broughton Crossing (between the A418 Aylesbury Road and A41 Aston Clinton Road) is not within the future defended 0.1% annual probability flood event. Although the strategic link road from the A41 Aston Clinton Road crosses the floodplain, the flood hazard is generally low or moderate even considering climate change; • The Aston Clinton Road impedes floodwater causing flooding to occur in the south east MDA. Less Vulnerable land use (Table D.2 in PPS25) is appropriate in the affected area but More Vulnerable land use would have to pass the Exception Test. Also if the Aston Clinton Road is designated as a defacto defence, then the Environment Agency should be aware of the implications; At the Aston Clinton Road and Broughton Lane junction there is a pumping station that is partially flooded by the 0.1% annual probability event. It is recommended that the function of the pumping station is reviewed to assess if it should be considered as critical infrastructure during a flood event and consequently kept operational; and Only the northern edge of the sewage treatment works is within the present day defended 1% annual probability flood extent and the flood hazard is low to people. The northern and western edges are partially within the present day 0.1% flood extent. However, the future 0.1% flood event results in significant flood hazards in some of the areas. As none of the modelled flood storage reservoirs significantly reduce flood risk at the sewage treatment works, other mitigation measures should be considered.