RAINFALL DETACHMENT AND DEPOSITION - EXPERIMENTS WITH LOW SLOPES AND SIGNIFICANT WATER DEPTHS

Rainfall impact is probably the dominant erosion process on very low slopes where water depths can be significant. This study was conducted to investigate the concentrations and settling velocity characteristics of sediment eroded under simulated rainfall for situations where very low slope and significant constant depths of water prevail. Two contrasting soils (Vertisol and Aridisol) were subjected to rainfall at two constant rates of 56 and 100 mm h-1 in a 5.8 by 1 m tilting-flume apparatus. Experiments were carried out with three different mean depths of ponded water (2, 5, and 10 mm), sustained by adjusting the flume slope (0.1 to 1%). Sediment concentrations and the settling velocity characteristics of the eroded sediment were determined from samples of runoff water. Sediment concentration was higher for the Aridisol than for the Vertisol, increased with rainfall rate and decreased with mean water depth in all experiments. For each rainfall rate and mean depth of water, sediment concentration decreased with time until a steady-state concentration was reached. Settling velocity characteristics indicated that eroded sediment was initially finer than the original soil, but became coarser with time, approaching an equilibrium distribution. The temporal change in experimental data is thought to be due to the development with time of a coarser deposited layer that shields an increasing fraction of the original soil surface until an equilibrium fraction less than unity is achieved. The concepts presented provide a basis for understanding the interaction of rainfall detachment and deposition of cohesive soils composed of a range of sizes and densities of aggregates and particles.