The Use of Wetlands for Flood Attenuation FINAL REPORT - An Taisce

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The Use of Wetlands for Flood Attenuation Aquatic Services Unit, UCC Report by: Lauren Williams Aquatic Services Unit (ASU) University College Cork (UCC) ERI Building Lee Road Cork Ireland Dr. Simon Harrison School of Biological, Earth & Environmental Sciences (BEES), UCC. Dr. Anne Marie O’Hagan Law, Policy and Environment, Hydraulics and Maritime Research Centre (HMRC), UCC. For: An Taisce National Trust for Ireland Tailors Hall Back Lane Dublin 8 Reference as: Williams, L., Harrison, S. and O’Hagan A M. (2012) The use of wetlands for flood attenuation. Report for An Taisce by Aquatic Services Unit, University College Cork. Front cover photograph: Groundwater fed depression wetland (fen), Watergrasshill, Co. Cork. FINAL REPORT, February, 2012 2
The Use of Wetlands for Flood Attenuation Aquatic Services Unit, UCC TABLE OF CONTENTS EXECUTIVE SUMMARY .......................................................................................................... 6 PART I The role of wetlands in Flood Attenuation ......................................................... 11 1. Introduction ....................................................................................................... 11 1.1 Flooding ............................................................................................................................... 11 1.2 Wetlands and flood attenuation ......................................................................................... 12 1.3 Issues concerning the use of wetlands in flood attenuation............................................... 13 1.4 Policy implications ............................................................................................................... 14 2. How do wetlands attenuate flooding? ............................................................... 15 2.1 Overview .............................................................................................................................. 15 2.2 The process of flood attenuation ........................................................................................ 15 3. Wetland hydrology and flood attenuation properties ....................................... 19 3.1 Alluvial floodplains .............................................................................................................. 20 3.2 Peatlands ............................................................................................................................. 24 3.2.1 Raised and blanket bogs ............................................................................................. 24 3.2.2 Fens ............................................................................................................................. 26 3.3 Karstic landscapes ............................................................................................................... 29 3.4 Coastal wetlands .................................................................................................................. 30 3.5 Function specific constructed wetlands .............................................................................. 32 3.5.1 Integrated Constructed Wetlands (ICWs) ................................................................... 32 3.5.2 Sustainable Drainage Systems (SuDS)......................................................................... 33 4. Management of wetlands .................................................................................. 36 4.1 Floodplain management ...................................................................................................... 36 4.2 Peatland management ........................................................................................................ 38 4.3 Variables affecting wetland management and flood attenuation ...................................... 40 5. Conflicts between flood attenuation and other wetland functions.................... 42 5.1 Biodiversity .......................................................................................................................... 42 5.2 Water quality ....................................................................................................................... 43 6. Methods to enhance and mimic natural drainage processes ............................. 45 6.1 Overview .............................................................................................................................. 45 6.2 Restoring alluvial floodplain function .................................................................................. 45 6.3 Managed coastal realignment ............................................................................................. 46 6.4 International examples ........................................................................................................ 46 6.4.1 Floodplain restoration: Southlake Moor, UK. ............................................................. 46 6.4.2 Polder creation: Altenheim Polders, Germany ........................................................... 48 6.4.3 Wetland storage: Whangamarino wetland, New Zealand ......................................... 48 FINAL REPORT, February, 2012 3
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