Catchment-scale fine sediment dynamics and its implications for flood management

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 of fine sediment using SCIMAP. The model found the River Eye has relatively low hydrological 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 the sites upstream of the silt trap had higher sediment loads than downstream, indicating the silt traps effectiveness in reducing fine sediment transfer. 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 fine 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.