SOIL EROSION BY SHEET FLOW

The soil erosion process in the interrill overland flow range is governed by soil detachment from raindrop impact and the transport of soil materials by shallow overland flow. A mathematical model of soil erosion is developed which includes the effects of varying overland flow depths on soil detachment and transport. Overland flow depth is estimated by assuming a steady state profile with an empirical expression for the the Darcy-Weisbach friction factor.The sediment transport capacity of runoff is represented as the product of a soil transport factor, bottom shear stress and flow velocity. The sediment transport capacity relation allows satisfactory predictions of soil losses from surfaces with small slopes. Both the soil detachment and transport capacity factors must be obtained from experimental data.The mathematical model is tested using soil loss data collected from a laboratory rainfall simulator. The laboratory study includes measurements of the effects of varying discharge and slope steepness on soil erosion. The mathematical model is shown to allow adequate predictions of the experimentally measured soil loss values.The laboratory study also includes measurements of the size distribution of transported sediment as influenced by slope. No significant difference in sediment size distribution is found due to slopes varying from 1 to 20 percent.Soil detachment by raindrop impact is represented as the product of a soil detachment factor and the maximum point pressure at the soil-water interface caused by raindrop impact. Point pressure is estimated from an empirical equation involving drop impact velocity, drop diameter and water layer depth. This equation is shown to provide adequate predictions of soil detachment for widely varying rainfall conditions.