Variability of particle size characteristics of sheetwash sediments and fluvial suspended sediment in a small semiarid catchment, Kenya

Abstract

The variability of particle size characteristics for sheetwash sediments and fluvial suspended sediments are presented. Data for three representative storm events are described for a small 0.30 km 2 erodible semiarid catchment. Suspended sediment output per event ranged from 340 to 810 t km −2. Ninety-eight percent of the suspended sediment was less than 63 μm, and was supplied from remobilization of stored colluvium. Discharge and particle size class relationships were complex for individual storm events, but were best described by third-order polynomial functions. Results of a one-way analysis of variance showed that concentrations of total suspended sediment, silt and sand varied significantly between storm events. Temporal variation of clay concentration in runoff were not significant, suggesting that a single hillslope source area contributed to the channel subsystem. The mean grain size and sorting of sheetwash sediments were significantly different from fluvial suspended sediment for individual storm events. Sheetwash sediments were coarser and more poorly sorted than fluvial suspended sediments. Enrichment ratios indicated selective erosion and transport of silt and clay, and preferential deposition of sands.