Evidence of Non-contiguous Flood Driven Coarse Sediment Transfer and Implications for Sediment Management

We investigate gravel redistribution and morphological response of three headwater streams following the Storm Desmond floods of December 2015. Management of the watercourses following the flooding concentrated on removal of gravel and clearing of vegetation, perceived as having been significant causes of local flooding. Aerial LIDAR and ortho-photography were employed to critically assess the location, type and magnitude of sediment mobilization, using an sUAV to capture imagery of lake sediment fans across the area. Three key findings throw some insight into the geomorphic response to this event: 1) volumes of sediment mobilisation were not as high as anticipated, with minor changes of lake alluvial fans indicative of limited sediment delivery; 2) upland piedmont zones appear to have stored large volumes of coarse sediment discharged from headwater channels; 3) reactivated wandering channel zones appear to have acted as buffers to large scale sediment movement rather than acting as large supply zones. Sediment accumulation through towns and villages was harder to quantify due to the rapid clean-up operation, however, the authors suggest that volumes may not be as large as assumed and the widespread dredging is likely to have caused more problems than it has solved as the bed of many watercourses are now highly susceptible to mobilisation following mechanical disruption of the previously strongly armoured surface.