Effect of rainfall intensity, slope and antecedent moisture content on sediment concentration and sediment enrichment ratio

Abstract

Abstract The transportability of sediment by runoff and its potential for subsequent deposition is dependent on the size and distribution of the transported material and the sediment concentration. The size distribution of the eroded sediment is also expected to vary depending on which erosion process is predominant. This study discusses the results of laboratory analysis to evaluate the effect of rainfall intensity, slope steepness, soil types and antecedent moisture content on sediment concentration, runoff coefficients, and sediment enrichment ratios. To achieve the objectives, laboratory experiments were conducted using FEL-3 rainfall simulator and erosion pan. Rainfall intensity of 120, 70, and 55 mm/h was applied sequentially at 9, 25 and 45% slope for three soil types found in Alemaya watershed, eastern Ethiopia Alemaya Black soil (Soil A), Regosols (Soil B), and Cambisols (Soil C) varied from clay to sandy clay loam in texture with wet and dry antecedent moisture contents. The rate of sediment concentration significantly varied with moisture contents. For Soil A, sediment concentrations (24.3 g/l) from an initially air-dry surface and (18.7 g/l) from an initially wet surface were observed. Wetting decreased sediment concentration of Soil A, B and C by 23, 45.7 and 1.3%, respectively. Sediment concentration didn't show any trend with rainfall intensities. For Soil B, the highest average silt enrichment ratio of 2.15 was observed followed by average clay enrichment ratio of 1.61. For Soil A and Soil C, average clay enrichment ratios of 1.13 and 1.91 were observed, respectively. Initial moisture contents had effect on the enrichment ratio, however, the actual effect varied with soil type and the size of the particle. Results of the study indicated that the actual effect of slope as well as rainfall intensity on sediment concentration and sediment yield vary with soil types and moisture contents.