HYDROECOLOGICAL MONITORING AND MODELLING OF RIVER-FLOODPLAIN RESTORATION IN A UK LOWLAND RIVER MEADOW By Hannah Marie Clilverd UCL A thesis submitted for the degree of Doctor of Philosophy 1 SIGNED DECLARATION I, Hannah Marie Clilverd confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. 2 ABSTRACT Channelization and embankment of rivers has led to major ecological degradation of aquatic habitats worldwide. River restoration can be used to restore favourable hydrological conditions for target processes or species. This study is based on rarely available, detailed pre- and post-restoration hydrological data collected from 2007–2010 from a wet grassland meadow in Norfolk, UK. Based on these data, coupled hydrological/hydraulic models were developed of pre-embankment and post-embankment conditions using the MIKE-SHE/MIKE-11 system. Fine-scale plant and chemical sampling was conducted on the floodplain meadow to assess the spatial pattern of plant communities in relation to soil physicochemical conditions. Simulated groundwater levels for a 10-year period were then used to predict changes in plant community composition following embankment-removal. Hydrology was identified as the primary driver of plant community composition, while soil fertility was also important. Embankment removal resulted in widespread floodplain inundation at high river flows and frequent localised flooding at the river edge at lower flows. Subsequently, groundwater levels were higher and subsurface storage was greater. The restoration had a moderate effect on flood-peak attenuation and improved free drainage to the river. Reinstatement of overbank flows did not substantially affect the degree of aeration stress on the meadow, except along the river embankments where sum exceedance values for aeration stress increased from 0 m weeks (dry- grassland) to 7 m weeks (fen). The restored groundwater regime may be suitable for more diverse plant assemblages. However the benefits of flooding (e.g. propagule dispersal, reduced competition) may be over-ridden without management to reduce waterlogging during the growing season, or balance additional nutrient supply from river water. The results from this study suggest that removal of river embankments can increase river- floodplain hydrological connectivity to form a more natural flood-pulsed wetland ecotone, which favours conditions for enhanced flood storage, plant species composition and nutrient retention. 3 ACKNOWLEDGEMENTS I would like to thank my adviser Julian Thompson, for his input, insight and support throughout my graduate career at UCL. In addition, I would like to thank the other members of my committee, Carl Sayer, Kate Heppell, and Jan Axmacher, whose ideas and guidance were invaluable. I would like to thank my family particularly my partner, Dayton Dove, who provided endless love and encouragement, and my children, Rowan and Annabel, who provided laughter and diversion, and my parents Karen and Anthony who inspired in me a love of the natural environment. Funding and other support for this study was provided by the Environment Agency, UCL Department of Geography, University of London Central Research Fund Grant, and the UCL Graduate School. I thank Ross Haddow and Adel MacNicol of the Stody Estate for their practical support and encouragement, Richard Hey for designing the river restoration, the River Glaven Conservation Group, Environment Agency, Wild Trout Trust, and Natural England for their technical support during the embankment removal. Special thanks are extended to Tony Leach, Derek Sayer, Chabungbam Rajagopal Singh, Victoria Sheppard and Helene Burningham for their assistance in the field, and Simon Dobinson, Laura Shotbolt, Charlie Stratford, and Ian Patmore for assistance in the laboratory. 4 TABLE OF CONTENTS TITLE PAGE .................................................................................... 1 SIGNED DECLARATION ................................................................. 2 ABSTRACT ...................................................................................... 3 ACKNOWLEDGEMENTS ................................................................. 4 TABLE OF CONTENTS ................................................................... 5 LIST OF TABLES ........................................................................... 10 LIST OF FIGURES ......................................................................... 12 Chapter 1: River-floodplain habitats and functions ......................... 20 1.1 Introduction .............................................................................................. 20 1.2 Research rationale, aims and objectives ................................................. 23 1.3. Thesis structure ...................................................................................... 26 Chapter 2: Hydrological, chemical, and ecological characteristics of floodplain environments .............................................................. 29 2.1 Introduction .............................................................................................. 29 2.2 Floodplain and riparian zone hydrology ................................................... 29 2.2.1 Conceptual models of floodplain and riparian zone hydrology .......... 29 2.2.2 The hyporheic zone .......................................................................... 35 2.2.3 Hydrological connectivity and its importance .................................... 36 2.3 Riparian zone biogeochemistry ............................................................... 45 2.3.1 Biogeochemical transformations in riparian sediments ..................... 45 2.3.2 Nitrogen and phosphorus biogeochemistry....................................... 47 2.3.3 Water quality functions and management of floodplains ................... 50 2.4 Riparian zone community composition .................................................... 54 2.4.1 The effects of waterlogging on plant community-composition ........... 54 2.4.2 Fertilisation of riparian zones ............................................................ 60 2.4.3 Grazing impacts on plant diversity .................................................... 64 2.4.4 Flooding disturbance and propagule dispersal ................................. 66 2.4.5 Human pressures on lowland wet grasslands ................................... 68 2.4.6 Management of lowland hay meadows ............................................. 70 5 2.5 River regulation, and restoration ............................................................. 71 2.5.1 Channel modification ........................................................................ 71 2.5.2. Restoration techniques .................................................................... 74 2.6 Hydrological modelling ............................................................................ 79 2.6.1 Model classification and representation of hydrological processes ... 79 2.6.2 Surface water-groundwater modelling and river restoration .............. 88 2.7 Conclusions ............................................................................................. 94 Chapter 3: The River Glaven and Hunworth Meadow ..................... 95 3.1 Introduction .............................................................................................. 95 3.2 Location and climatology ......................................................................... 95 3.3 Geology of the River Glaven catchment ................................................ 100 3.4 Surface water quality ............................................................................. 105 3.5 Flora, fauna, and conservation value of the River Glaven ..................... 106 3.6 Modification of the River Glaven and land management at Hunworth Meadow ....................................................................................................... 110 3.7 The River Glaven restoration project ..................................................... 114 Chapter 4: Methods Part I – hydrological and chemical monitoring117 4.1 Introduction ............................................................................................ 117 4.2 Study design .......................................................................................... 117 4.3. Hydrological monitoring ........................................................................ 123 4.3.1 Groundwater and surface water levels ........................................... 123 4.3.2 Base flow index and flow exceedance values ................................. 124 4.3.3 Evapotranspiration .......................................................................... 124 4.3.4 Hydraulic conductivity ..................................................................... 128 4.4 River-floodplain biogeochemical monitoring .......................................... 132 4.4.1 Soil structural and physical properties ............................................ 132 4.4.2 Water chemistry sampling and analysis .......................................... 133 4.4.3 Oxygen concentration in soil pores ................................................. 134 4.4.4 River and floodplain topography ..................................................... 137 4.5 Flood prediction ..................................................................................... 139 4.5.1 Bankfull capacity ............................................................................