Catchment Scale Fine Sediment Dynamics and Its Implications for Flood Management
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Catchment scale fine sediment dynamics and its implications for flood management Sarah J Twohig School of Architecture Building and Civil Engineering A Doctoral Thesis Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University © by Sarah Twohig 2018 Abstract The impact of fine sediment on catchment flood risk is often neglected when designing and implementing catchment managements plans. Yet, the accumulation of fine sediment can diminish channel capacity, causing an increase in flood risk. To successfully transition away from costly and invasive sediment management methods towards an integrated Natural Flood Management (NFM) approach, the dynamics of fine sediment and its implications for flood risk management must be explored. This thesis employs a novel geomorphological, hydrological, engineering and social approach to explore the influence of fine sediment on flood risk. Hydrological modelling of connectivity and erosion risk was conducted to determine the potential sources using SCIMAP. The model found the River Eye has relatively low connectivity, suggesting sources of sediment in- channel are likely derived from local sources such as river banks. The sub- catchments of Burton Brook and Langham Brook as the areas of highest connectivity and erosion risk indicating likely source areas. To explore patterns of spatial and temporal fine sediment transport an intensive field monitoring campaign was conducted using Time Integrated Mass Sediment samplers (TIMS) across the River Eye catchment. The suspended sediment samples found sites upstream of the silt trap had higher sediment loads than downstream, indicating the silt traps effectiveness in reducing fine sediment. Sediment yield analysis found the Burton Brook tributary is a significant contributor of fine sediments delivering 17.4 g km2 day-1. The TIMS were analysed for their relative efficiency and found sediment load between samplers to vary by 4%-171%. However, analysis of physical sediment properties was found to be consistent. At a reach scale, three flood defences were evaluated to determine their effectiveness in terms of sediment transport. Water level monitors installed upstream of the silt trap observed flow backing-up behind the silt trap >200m in high flow events. TIMS were installed up and downstream of two silt traps and found the silt traps were effective in reducing sediment load by 59.7- 98.0% on 7/8 occasions (Burton Brook) and 32.5-71.9% on 5/8 occasions (Ham Bridge). A complementary qualitative study was undertaken to assess the non-technical barriers to changes in flood risk and sediment management. A catchment wide survey was used to investigate attitudes of awareness, resilience and responsibility. Response analysis identified potential barriers to future NFM installation such as preference to traditional engineering structures and dredging to manage sediment delivery. The outcomes from all four approaches were used to create a series of recommendations on future flood risk and fine sediment management for catchment managers in lowland catchments. Acknowledgements I would like to start by thanking my supervisor Dr Ian Pattison for his continued support and guidance throughout the PhD process, always having an open door to ask questions and for his valuable feedback, helping me to improve as a researcher. I would also like to thank my second supervisor Prof. Graham Sander for his advice, often whilst stood on a river bank. This research project would not have been possible without NERC CENTA funding, gaining the approval of land owners and catchment managers within the River Eye. I would like to thank the laboratory technicians in Civil Engineering for their support in the construction of stilling wells, TIMS and laboratory assistance. I thank Dr Philip Soar and Dr Peter Downs my undergraduate and masters supervisors for their excellent teaching, supervision and enthusiasm for rivers. Without you I would have never had the confidence or passion to embark on this journey. To my friends, many of whom have spent cold days in rivers with me: Faye, Harry, Alan, Milly, Cardo, Jen and Kate thank you for making the last four years so enjoyable, the daily coffee trips, helping on fieldwork and being so supportive. To my family; thank you for always believing in me and the endless love and support. My final thanks go to my husband, Mike. You are my inspiration I would not have been able to come this far without you. Contents Table of Figures ........................................................................................... viii Table of Tables ............................................................................................ xvi 1. Chapter One: Context, aims and objectives ............................................ 1 1.1 Research context .............................................................................. 1 1.1.1 Hydrological contributions to flood risk ....................................... 2 1.1.2 Geomorphological contributions to flood risk .............................. 3 1.1.3 Engineering component to flood risk .......................................... 5 1.1.4 Social contributions to flood risk ................................................. 6 1.2 Research aims .................................................................................. 6 1.3 Thesis structure ................................................................................ 9 2 Chapter Two The role of fine sediment in managing catchment flood risk 11 2.1 Chapter Scope ................................................................................ 11 2.2 Fine sediment ................................................................................. 11 2.3 Fine sediment sources .................................................................... 12 2.3.1 Geology .................................................................................... 12 2.3.2 Slope and relief......................................................................... 12 2.3.3 Climate ..................................................................................... 13 2.3.4 Land cover ................................................................................ 13 2.3.5 Bank erosion ............................................................................ 15 2.4 Fine sediment detachment .............................................................. 16 2.5 Spatial variation in suspended sediment ......................................... 16 2.6 Fine sediment connectivity .............................................................. 18 2.6.1 Hydrological connectivity .......................................................... 19 2.6.2 Geomorphological connectivity ................................................. 22 i 2.7 Methods of identifying fine sediment connectivity ........................... 24 2.7.1 Modelling sediment connectivity ............................................... 24 2.7.2 Field methods for sediment connectivity ................................... 27 2.8 Fine sediment delivery in river channels ......................................... 32 2.8.1 Hydraulic implications of sediment deposition .......................... 33 2.8.2 Geomorphological implications of sediment deposition ............ 36 2.8.3 Future implications of sediment delivery in a changing climate 40 2.9 Interactions between traditional engineering techniques and hydrology .................................................................................................. 41 2.9.1 Traditional engineering techniques: Dams ............................... 42 2.9.2 Traditional engineering techniques: bank protection ................ 43 2.9.3 Traditional engineering techniques: dredging ........................... 43 2.10 Natural Flood Management engineering: working with natural hydrological processes ............................................................................. 45 2.10.1 Instream NFM structures ....................................................... 47 2.10.2 Silt traps ................................................................................ 47 2.10.3 Riparian buffer strips ............................................................. 48 2.10.4 Natural Flood Management: limitations ................................. 48 2.11 Social consideration in flood risk management ............................ 49 2.11.1 Flood awareness ................................................................... 51 2.11.2 Flood resilience ..................................................................... 52 2.11.3 Flood responsibility ............................................................... 55 2.12 Chapter Summary ........................................................................ 58 3 Chapter Three Study Site ..................................................................... 60 3.1 Chapter Scope ................................................................................ 60 3.2 The River Eye Catchment ............................................................... 60 3.2.1 Geology .................................................................................... 64 ii 3.2.2 Soil Properties .......................................................................... 65 3.2.3 Land Cover ............................................................................... 66 3.2.4 Site of Special Scientific Interest (SSSI) ................................... 68 3.3 River Eye Flood Risk Problem .......................................................