The deposition and storage of suspended sediment in contemporary floodplain systems : a case study of the River Culm, Devon
This thesis documents the rates and patterns of contemporary and recent overbank deposition along the lower reaches of the River Calm, S. E. Devon, England. Two temporal perspectives are adopted: (i) short-term rates based on contemporary flood events, using sediment traps, the sampling of in-channel suspended sediment and qualitative field observation of inundation processes; and, (ii) recent (over the past 35 years) rates of vertical accumulation, using the caesium-137 technique. These techniques also enable lateral and downstream trends to be identified. Longer-term implications are also addressed using a simple lithofacies classification of alluvial sections, supplemented by historical evidence of channel change and anthropogenic impact along the study reach. Overbank deposition rates are generally low along the study reach, averaging 0.7 mm year-1 (using estimates of suspended sediment conveyance losses), 0.4 mm year-1 (sediment traps) and 0.5 mm year-1 (caesium-137 studies). In general, the highest deposition rates are associated with the levee, depressions and breaches. Deposition rates, however, are temporally and spatially highly variable, being dependent upon the microtopographic relief and vegetation of the floodplain, the mode of inundation, and the extent of retention pondage. Anthropogenic impact on the floodplain is also noted, in particular the role of ditches in distributing floodwaters across the floodplain and the compartmentalisation effect of barriers to flow. These have important implications for the rates of overbank deposition and the sedimentological characteristics of deposits. The role of scour remobilisation of sediment is also assessed. The spatial detail afforded by the caesium-137 technique has been used to identify areas of the floodplain which may be locally susceptible to scour. This study highlights the potential for lowland floodplains undergoing regular flooding to suffer contamination from radionuclides and other pollutants, and leads to an improved understanding of the spatial and temporal rates and trends in deposition and sedimentological characteristics of overbank deposits on the floodplain of a suspension load-dominant river system.