Soil erosion processes.II. Settling velocity characteristics of eroded sediment

Abstract

Settling velocity characteristics of sediment eroded by overland flow only, or from a combination of rainfall (100 mm h-l) and runon were measured under controlled conditions in a simulated rainfall tilting-flume facility. Two contrasting soil types were studied: a cracking clay (black earth or vertisol), and a slightly dispersive sandy clay loam (solonchak or aridisol). For a constant volumetric flux (1.0x10-1 m3 m-1 s-1) at exit from the 5.8m long flume and a slope of 0.5%, sheet erosion prevailed, whilst for the same flux at a steeper slope of 5%, rill erosion prevailed. Settling velocity characteristics of eroded sediment were found to be dependent on erosion process, flow hydraulics, soil type, and time in the erosion event. For both soil types, there was a progressive change in settling velocity characteristics with time, this change being less pronounced for sediment eroded dominantly by rill flow as opposed to sheet flow. Temporal changes in settling velocity characteristics were attributed to the development of a deposited layer of coarser, faster settling sediment on the soil surface. The net outcome of rill erosion was less size-selectivity compared with sheet erosion, as determined by the measured settling velocity characteristics of eroded sediment. This outcome was associated with the greater erosive power of rill flow compared with sheet flow. Rainfall was found to influence the settling velocity characteristics of eroded sediment substantially when sheet flow predominated. This was thought to be due to lower flow velocities under rainfall (and therefore smaller contribution to soil loss by entrainment). The findings reported in this study have important implications when assessing nutrient losses from eroded sediment, and in predicting the spatial redistribution of eroded sediment.