Raindrop Size and Flow Depth Control Sediment Sorting in Shallow Flows on Steep Slopes

Raindrop size and flow depth interactions determine the energy available for erosion processes. A detailed laboratory study was conducted to investigate the influence of raindrop size on sediment sorting in shallow flows on steep slopes. Raindrops with sizes of 1.34, 2.45, and 4.04mm were applied to three inflows (0.65, 1, and 2L/min) with the same rainfall intensity (90mm/hr). The De Brouckere mean diameter (D[4,3]) was enhanced with increases in the ratio between flow depth and raindrop size (; p<0.05), indicating that more raindrop energy dissipated in deeper flows. Moreover, a greater proportion of energy was dissipated from small raindrops than from large ones with an increasing flow depth. Particles >0.5mm were slightly enriched by rolling under 1.34-mm raindrop impact, whereas enrichment did not occur under 2.45- and 4.04-mm raindrop impact, probably because the latter induced more severe aggregate disintegration, resulting in fewer coarse particles. The mean sediment concentrations under inflows of 0.65, 1, and 2L/min were 101.65, 89.32, and 79.02g/L, respectively. Shallow flows on steep slopes effectively carried detached particles; moreover, coarse particles could roll along the slopes, which was inconsistent with their behavior on gentle slopes. This study highlights the need to understand the influence of the interaction between raindrop size and flow depth on erosion processes associated with sediment size selectivity for better erosion modeling and water quality prediction. m