Final Report Upper Lavant Valley Flood Risk Management Study Prepared for West Sussex County Council January 2015 Burderop Park Swindon, Wiltshire SN4 0QD Contents Section Page Acronyms and Abbreviations ................................................................................................................... iv Introduction ............................................................................................................................. 1-1 1.1 Project context ....................................................................................................................... 1-1 1.2 Background to Upper Lavant Valley study ............................................................................. 1-1 1.2.1 Objectives ................................................................................................................. 1-1 1.2.2 Scope ......................................................................................................................... 1-2 1.2.3 Study area ................................................................................................................. 1-3 1.2.4 Key stakeholders ....................................................................................................... 1-3 1.2.5 Data collected for study ............................................................................................ 1-4 Flooding History and Impacts .................................................................................................... 2-1 2.1 Flooding History ..................................................................................................................... 2-1 2.2 Flooding Impacts .................................................................................................................... 2-1 Analysis of flooding ................................................................................................................... 3-1 3.1 Introduction ........................................................................................................................... 3-1 3.2 Catchment boundary and characteristics .............................................................................. 3-1 3.3 Analysis of groundwater flooding .......................................................................................... 3-1 3.4 Analysis of sewer flooding ..................................................................................................... 3-5 3.5 Setting thresholds and trigger levels ..................................................................................... 3-7 Options and Action Plan for Main Settlements ........................................................................... 4-1 4.1 Introduction ........................................................................................................................... 4-1 4.2 Initial options considered ...................................................................................................... 4-2 4.3 Short-listed options for main settlements ............................................................................. 4-2 4.3.1 Mid and East Lavant .................................................................................................. 4-3 4.3.2 West Dean and Chilgrove ......................................................................................... 4-4 4.3.3 Singleton ................................................................................................................... 4-6 4.3.4 Charlton .................................................................................................................... 4-8 4.3.5 East Dean .................................................................................................................. 4-9 4.4 Environmental considerations ............................................................................................. 4-10 4.4.1 Environmental Baseline .......................................................................................... 4-10 4.4.2 Potential environmental impacts ........................................................................... 4-12 Appendix A Roles and Responsibilities ........................................................................................... 4-16 Appendix B Study Boundary .......................................................................................................... 4-17 Appendix C Technical analysis of flooding in the Lavant Study area ................................................ 4-18 Appendix D Initial Measures Considered ........................................................................................ 4-19 Appendix E Drawings .................................................................................................................... 4-20 Appendix F Environmental Constraints Plan................................................................................... 4-21 ACRONYMS AND ABBREVIATIONS Acronyms and Abbreviations Defra Department for Environment, Food and Rural Affairs EA Environment Agency FEH Flood Estimation Handbook GIS Geographic Information System IRP Infiltration Reduction Plan MAFP Multi Agency Flood Plan SEA Strategic Environmental Assessment SW Southern Water SWMP Surface Water Management Plan WSCC West Sussex County Council SECTION 1 Introduction 1.1 Project context This Surface Water Management Plan (SWMP) has been undertaken as part of a commission to develop SWMPs for five areas of West Sussex which have a history of significant flooding from surface water, groundwater and drainage systems. The five study areas were: Easebourne; Lancing; Manhood Peninsula; Upper Lavant Valley, and; West Chichester, including Fishbourne and Parklands. These areas were selected as part of West Sussex County Council’s (WSCC) response to the severe flooding in the summer and winter of 2012, although it is recognised that many of these have suffered flooding on multiple occasions. A SWMP is described as a framework through which key local partners with a responsibility for surface water and drainage in their area work together to understand the causes of surface water flooding and agree the most cost effective way of managing that risk. The purpose is to make sustainable surface water management decisions that are evidence based, risk based, future proofed and inclusive of stakeholder views. Managing surface water flooding requires a range of partners, organisations and individuals to work together. The roles and responsibilities for those involved in helping to manage surface water flooding are described in Appendix A. 1.2 Background to Upper Lavant Valley study Within this context the Upper Lavant Valley consists of the main settlements of Charlton, Chilgrove, East Dean, East and Mid Lavant, Singleton and West Dean. The River Lavant flows through all of the settlements (with the exception of Chilgrove). Due to the underlying chalk the River Lavant is primarily a groundwater fed stream, and therefore responds to changes in groundwater levels through the year. It frequently runs dry during summer months when groundwater levels within the chalk catchment are lower. Flooding within the Upper Lavant Valley occurs primarily when high groundwater levels cause over topping of the River Lavant and the local ditch networks which connect to it. Flooding also occurs after heavy rainfall when the catchment is already saturated. In addition, the foul sewer network can become overloaded by high groundwater levels which inundate and infiltrate the sewer network affecting its operation. Within the last 20 years flooding in the Upper Lavant Valley has occurred in in the winters of 1993/1994, 2000/01, 2012/13 and 2013/14. Further information on the flooding mechanisms and history is provided in Section 2. WSCC has drafted a Lavant Valley Groundwater Multi-Agency Flood Plan (MAFP) to outline the actions and responsibilities for each identified organisation in response to high groundwater levels. This SWMP has provided a further, more detailed analysis of groundwater levels and their impacts on flooding within the catchment. The purpose has been to provide additional evidence to inform the MAFP. The findings of this study will be used to update the MAFP, and provide locally specific capital, maintenance and emergency management approaches to minimise the impact of flooding to people, properties and infrastructure within the Upper Lavant Valley. 1.2.1 Objectives The primary objectives of this study are to: UPPER LAVANT STUDY FINAL REPORT (WORD) 1-1 SECTION 1 INTRODUCTION understand the flooding issues within the catchment, including flooding from the River Lavant, the foul sewer network, and high groundwater levels; consider current flood risk management practices within the Upper Lavant Valley, including how actions are taken in response to groundwater levels at Chilgrove; identify localised capital and maintenance measures which can be taken to reduce flood risk to people, property and infrastructure, and; provide evidence to support WSCC, the Environment Agency, Southern Water and local residents to prepare for, and respond to, flooding incidents. 1.2.2 Scope The scope for this SWMP was established during the early part of the overall project programme through discussions with WSCC, a rapid assessment of available data, and early identification of the flooding issues and mechanisms. A scoping document was prepared in March 2014 and agreed by WSCC. The scope is outlined in detail below.