Modelling hysteresis in sediment transport and continuous distributions of eroded sediment

Sediment transport hysteresis occurs due to the different sediment fluxes for the same discharge on the rising and falling limb of the overland flow hydrograph. Experimental field data has shown that the common shapes of the hysteresis loops encompass (i) clockwise, (ii) anti-clockwise and (iii) figure 8 with both flow orientations. It has been suggested that the shape of these loops can be used to identify the different processes of runoff and sediment transport and the sediment source area. We present simulations carried out using the Hairsine-Rose (HR) erosion model which reproduces all of the above hysteresis loops for flow conditions that are straightforward to establish within a laboratory flume. As both agricultural chemical and biological pathogens bind preferentially to the clay and silt sized particles, estimating contaminant fluxes to surface water bodies requires knowledge of the size distribution of transported sediment. We also present some recent work on extending the HR model from discrete sediment size classes to a continuous distribution of sizes along with some corresponding solutions.