Strategic Flood Risk Assessment (SFRA)

Runnymede 2030 Strategic Flood Risk Assessment (SFRA) January 2018 This page is intentionally blank EXECUTIVE SUMMARY Introduction (Chapter 1) This 2018 Strategic Flood Risk Assessment (SFRA) provides an update to the Council’s 2009 SFRA carried out by ENTEC and seeks to provide a robust assessment of flood risk across the Borough from all sources of flooding. The report seeks to take into account updates in modelling available, planning policy and legislation since the 2009 SFRA was produced. Policy background (Chapter 2) The NPPF establishes a presumption in favour of sustainable development, where development that accords with a Borough’s development plan (or when judged against the policies in the NPPF if the development plan for an area is absent, silent or relevant policies are out of date) should be approved without delay. It requires each Council to produce a Local Plan for its area that addresses the spatial implications of economic, social and environmental change. The NPPF states that a Local Plan should also set out the strategic priorities for the area. The NPPF also confirms that each local planning authority should ensure that its Local Plan is based on adequate, up-to-date and relevant evidence about the economic, social and environmental characteristics and prospects of its area. Paragraph 100 goes on to state that, ‘Local Plans should be supported by Strategic Flood Risk Assessment and develop policies to manage flood risk from all sources, taking account of advice from the Environment Agency and other relevant flood risk management bodies, such as lead local flood authorities and internal drainage boards’. This SFRA will form part of the evidence base that underpins the Council’s new Runnymede Local Plan. One of the key aims of the NPPF in relation to flooding and flood risk is to ensure that flood risk is taken into account, both in the plan making and decision taking processes to deliver sustainable development. Central to the NPPF is a sequential risk-based approach to flood risk which aims to steer new development to areas with the lowest probability of flooding. Development should not be allocated or permitted if there are reasonably available sites appropriate for the proposed development in areas with a lower probability of flooding (from any form of flooding). The NPPF states in paragraph 101 that a Council’s SFRA should provide the basis for applying this sequential test. The NPPF sets out the need to consider all sources of flood risk (such as groundwater, surface water and foul sewer flooding) in addition to the main fluvial and tidal sources in a SFRA. The implications of climate change on flood risk also require consideration so as to ensure that the Local Plan properly takes into account future risk and promotes sustainable development. When considering this point further, it is worthy of mention that the Government has also published legislation on climate change in the form of The Climate Change Act (2008), legislating for climate change mitigation and adaption. The Act imposes a duty on the UK for carbon emissions to be reduced by 80 percent by 2050 from a 1990 baseline. To be balanced against the need to direct development to areas at the lowest risk of flooding, is the requirement of the NPPF to boost significantly the supply of housing and promote sustainable Runnymede Strategic Flood Risk Assessment 2018. Executive Summary Page 1 economic growth. LPAs are being tasked with making provision for the delivery of a wide choice of high quality homes, widening opportunities for home ownership and creating sustainable, inclusive and mixed communities. There is a range of other policy documents at National, County and Local level that are considered relevant in the preparation of this SFRA. The Environment Agency has also published a number of relevant documents. A comprehensive summary of the most relevant documents can be found in chapter 2 of the main document. Local context (Chapter 3) Runnymede is located in North West Surrey and is split into 14 wards. It contains three main towns; Addlestone, Egham and Chertsey, alongside a range of smaller settlements at Englefield Green, Virginia Water, New Haw, Woodham, Ottershaw, Lyne, Longcross and Thorpe (the last three settlements listed are all located in the Green Belt). In total approximately 79% of the Borough is located in the Green Belt. The remainder of the Borough including its town centres is located in the Urban Area. The topography in the Borough ranges from approximately 5-15m AOD along the eastern edge of the Borough in the floodplain of the River Thames. Levels very across the Borough rising to a high point of approximately 80AOD in the vicinity of Englefield Green in the north western corner. The predominant bedrock geology in Runnymede is the Bagshot Formation (which underlays approximately two thirds of the borough). It is also noteworthy that Egham and parts of Thorpe are underlain by the London Clay Formation (Clay). There is a covering of superficial deposits across approximately 50% of the Borough. This is mainly in the eastern half of the Borough although pockets also exist in the Englefield Green and Virginia Water areas. The bedrock underlying the majority of the borough (with the exception of the north eastern quadrant of the borough which covers the Egham and Thorpe areas) is defined as a secondary A aquifer. The remaining north eastern quadrant of the Borough is designated as unproductive strata. The superficial deposits present in the Borough are classified in places as principal aquifers and in others, secondary aquifers (primarily Secondary A aquifers but in some places Secondary (undifferentiated) aquifers). There are 12 main rivers that run through Runnymede. The River Thames is the principal main river within the Runnymede administrative area. Its main tributaries within this area are the River Wey and the Chertsey Bourne and the Addlestone Bourne, all main rivers. There are an additional 8 main rivers which are, in turn, tributaries of these latter three rivers. Subsidiary to the main rivers there is an extensive network of ordinary watercourses across Runnymede. These watercourses drain into the main rivers. The detailed river network in Runnymede can be viewed in figure 8. Overview of fluvial flood risks in Runnymede (Chapter 4) Fluvial flooding from the River Thames and its main tributaries; the Chertsey Bourne, the Addlestone Bourne and River Wey, are the primary sources of flooding in Runnymede. The floodplain of the River Thames is fairly extensive on its eastern side within Runnymede, due to the flat, low lying Runnymede Strategic Flood Risk Assessment 2018. Executive Summary Page 2 nature of the land, and presents the greatest fluvial flood risk for the Borough. Areas potentially at risk from flooding from the Bourne and the River Wey include Woodham, New Haw, Addlestone and Hamm Moor. Other sources of fluvial flooding include the Hurst Ditch, Meadlake Ditch and the Moat which are all tributaries of the Chertsey Bourne system that flow southwards through Egham, Egham Hythe and Thorpe. Although the risk of flooding is more constrained within these river valleys, because their floodplains are much less extensive and carry smaller volumes of flow, where the Hurst Ditch flows parallel to the Chertsey Bourne near Thorpe, the combined flood zone extents create a larger area of potential flood risk. Meadlake Ditch and the Moat also lie within the floodplain of the River Thames. Furthermore, there are thought to be integrated flood risks from the tributaries of the Chertsey Bourne and River Thames, due to backing up effects as well as high groundwater levels when the Thames is high. The ditches in the Egham Hythe to Chertsey area therefore not only present a risk of flooding due to local rainstorms but also from flood water backup from the Thames/Chertsey Bourne. The Borough contains no formal flood defences; a formal defence being classified as a structure that is specifically built for the purposes of flood defence. Informal flood defences include structures that may act to contain flood water but were not originally constructed for that purpose. While there are certain structures such as the M3 and M25 motorways and railway lines in the Borough that could act as informal flood defences, they are not widely recognised as flood defences themselves as they were not constructed for the purpose of protecting properties from flooding. There is a long record of flooding from rivers in the Borough, and in particular from the River Thames. Major recorded flood events occurred in 1898, 1947, 1968, 2003 and late 2013-early 2014. The EA also holds records of fluvial flooding in Runnymede for the years 1929, 1954, 1974, 1988, 1990 and 2000. Mapping of the historic flood events in figures 10 a, b, c and d demonstrates that much of Thorpe Park experiences flooding and is an important area of the Thames floodplain. Furthermore the urban areas that have been most affected include parts of Egham Hythe and Chertsey from the Thames and the Chertsey Bourne, and New Haw from the Addlestone Bourne. As well as flooding from the Thames, other watercourses in the Borough have been sources of flooding mainly after heavy rainfall and as a result of culverting. The PPG defines 4 different flood zones. These refer to the probability of river and sea flooding, ignoring the presence of defences and not accounting of the possible impacts of climate change and consequent changes in the future probability of flooding. The fluvial flood zones can be defined as follows: Flood Zone Definition Zone 1-low probability Land having a less than 1 in 1,000 (0.1%) annual probability of river flooding in any given year. (Shown as ‘clear’ on the EA Flood Map – all land outside Zones 2 and 3) Zone 2-Medium Land having between a 1 in 1,000 probability (0.1%) and 1 in 100 (1%) annual probability of river flooding in any Runnymede Strategic Flood Risk Assessment 2018.

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