6*r>y EA - South W e s t*» 3 tt E n v ir o n m e n t H A g e n c y REVIEW OF FLOOD DEFENCE PRACTICES ON THE SOMERSET LEVELS AND MOORS THE REPORT July 1999 CONTENTS DOCUMENT 1 THE REPORT 1. Executive Summary.....................................................................................................3 2. Introduction...................................................................................................................6 3. History.............................................................................................................................8 4. Overview.........................................................................................................................9 4.1 Geology and Topography 4.2 The Uplands 4.3 The Levels and Moors 4.4 The Coastal Belt and Sea Defences 4.5 The Operating Authorities 5. Benefit/cost analysis....................................................................................................24 5.1 Aim 5.2 The Study Area 5.3 Flood Defence Project Appraisal Procedures 5.4 Approach 5.5 Agriculture results 5.6 Wildlife and Conservation Results 5.7 Urban Results 5.8 Other Results 5.9 Costs 5.10 Conclusions 6. Parrett and Sowy Operational Model...................................................................35 6.1 Introduction 6.2 Description of the Model 6.3 Operating scenarios 6.4 Future Programme 7. Effects of Development............................................................................................. 39 7.1 Introduction 7.2 Hydrology and Flooding 7.3 Catchment Descriptions, Tone, Isle and Yeo 7.4 Hydrological Analysis 7.5 Conclusions ENVIRONMENT AGENCY 021869 DOCUMENT 2 THE APPENDICES A The Tidal Reaches B The River Parrett Above Langport C The River Parrett System Below Langport D The River Brue and Huntspill System E The Upper and Lower Axe System F Maintenance Summary G Benefits Agriculture H Benefits Conservation I Benefits Urban J Benefits Other K References DOCUMENT 3 MAPS AND FIGURES Map 1 The Parrett System Map 2 The Brue and Axe System Map 3 1 in 100 year Fluvial, 1 in 200 year Tidal Flood Plains Figs 4.1 Topography 4.2 Profile of Strata 6.1 Calibration Event 15-16 December 1994 6.2 Calibration Event 12-13 December 1997 6.3 Effects of Raising and Lowering Spillway Levels at Allermoor and Beazley 6.4 Effect of Dredging the River Tone 6.5 Effect of Dredging the River Parrett 6.6 Effect of different openings of Dunball Sluice 6.7 Effect of Raising Hook Bridge Spillway 6.8 Effect of different openings at Monks Leaze Clyse B1 Flooded Moors above Langport B2 Details above Langport B3 Details above Langport B4 Flood Compartments in Moors Upstream of Langport Cl Monks Leaze Clyse Inlet to the Sowy River C2 Curry Moor and Adjacent Moors D1 Details at Gold Comer and North Drain Pumping Station Fl - FI 1 Bradshaw Weedcutting and Tractor Flailing Specifications REVIEW OF FLOOD DEFENCE PRACTICES ON THE SOMERSET LEVELS AND MOORS 1. EXECUTIVE SUMMARY 1.1 Review Documents and Consultation Process A commitment to review flood defence practices is a key part of the Environment Agency’s Somerset Levels and Moors Water Level Management Strategy. This is the first report to be produced as part of that review. It is in three parts: i) A scoping document presenting information on current practices ii) Supporting Appendices iii) Maps and Figures This first report sets out the present flood defence practices as understood by the Environment Agency. Comments are invited on the information provided. Once a common information baseline is agreed the review will then proceed to its next stage. This will involve consideration of any changes to flood defence practices and the drawing up of future programmes of work. Suggestions on these aspects of the review are welcome too. To ensure momentum can be maintained and to allow consideration by the Agency’s Flood Defence Committee, it would be helpful for initial responses to be made by 31 October 1999. To aid the consultation process and to help ensure an understanding of the complex issues involved the Agency is happy to respond to requests for further information and to accept invitations to address meetings. 1.2 Basic Principle The basic principle of Flood Defence in the area is that to protect the urban areas of Taunton, Bridgwater, Langport etc, excess flood waters from rivers or high tides is stored on the low lying moors until it can be evacuated without increasing flood risk downstream. 1.3 Overview The report first provides a factual overview of the complex drainage system, including a history of the drainage and how it evolved, and the topography and geology that moulded it. The sluices, pumping stations, flood banks and channels are detailed, and their roles in summer water supply and flood defence outlined. Guidelines within which the Agency operate when conditions allow are stated, and the importance of structures to strategic and local drainage highlighted. The overview section ends with a summary of the roles and responsibilities of the various Operating Authorities affecting flood defence within the Levels and Moors. 1.4 Benefit/Cost Analysis The Benefit/Cost Analysis looks at all the identifiable benefits of the Agency’s flood defence work in catchment, the area which feeds the Levels and Moors. It does this by considering flood damages under the existing standard of service, under a “do nothing” scenario and with a reduced standard of service. Whilst economic values are calculated for agricultural and urban benefits, no attempt has been made to assign such values to wildlife, habitat, landscape, archaeology, tourism, recreation and socio-economic benefits, but these are described. The Agency’s current revenue costs and capital costs over the last ten years are used for annual costs. The agricultural benefit analysis includes a financial analysis reflecting the impact on farming incomes, including Environmentally Sensitive Area (ESA) payments, and a comparative economic analysis involving net margins under Treasury rules. The economic analysis recognises ESA payments as a measure of the nation's valuation of the land environmentally. With the assumptions made, the benefits to the nation of the current flood defence regime are significantly less than the financial benefit to farmers. The analysis also concludes that agriculture is heavily dependent on flood defence. Reduction in flood defence standards would increase economic benefit to the nation, whilst reducing financial benefit to farmers. If reduced standards of Flood Defence resulted in a significant reduction in agriculture on the Somerset Levels and Moors the economic benefit would not be realised, due to the lack of farmers to manage the land. The cost of the current standard of service of flood defence can only be justified by the protection afforded to the major urban areas by storage of floodwater on the Moors. All three scenarios offer some nature conservation interest. There is a need for a consensus as to the relative merits of the different habitats resulting from each scenario. The area’s natural assets most highly prized nationally and internationally are reliant on the current standard of flood defence, and would be lost if that service reduced. 1.5 Parrett and Sowy Operational Model A hydraulic computer model first developed in 1993 has been significantly enhanced to help the testing of options for operating the Parrett, Sowy, Tone and King’s Sedgemoor Drain system. Flood events since 1997 have been used to calibrate flows, and observed levels against model simulation results are presented. Some limited scenarios changing spillway levels, dredging regimes and sluice operations have been tested. Even where these show improvements, further work on the cost effectiveness and economic justification of such changes is necessary. Further calibration during different flood events will give greater confidence in model predictions and updated survey information needs to be collected and input. 1.6 Effects of Development In July 1995, the Agency presented a series of studies on flooding on the Somerset Levels and Moors. This included a presentation of the insignificance of the runoff from Taunton compared to that from the other 96.5% of the River Tone catchment. To further illustrate this, and the impact of development on other rivers draining to the Levels and Moors, studies have been undertaken using catchment computer models with and without urban areas. These studies conclude that the total urban area of the River Tone contributes just a 2.8% increase on a 1 in 100 return period flood, rising to 4.1% on an annual flood. Similarly the urban contribution of the River Isle is just 1% for a range of return periods, and for the River Yeo 2% for a 1 in 100 year event. These are so small, that the impact of recent increased development is almost unmeasurable, as all recent developments have included an element of detention as recommended by the Agency. 1.7 Maintenance Summary The Appendices accompanying this report generally give further detailed information to back up that in the Overview and Benefit/Cost Analysis. However, Appendix F} the Maintenance Summary is a spreadsheet presenting routine grasscutting, weed control and dredging for all main river reaches in the Overview study area, and the specification for this work. It gives a reason for the work being undertaken, and shows changes in specification introduced since the 1980s. These details will help identify current and past practice on lengths where there may be some concern about