Flow Dynamics and the Variability of Suspended Sediment in a Semiarid Tropical Stream, Baringo District, Kenya

ABSTRACT

Limited data are available on flow dynamics and suspended sediment transport in ephemeral, semiarid drainage systems, particularly in tropical regions. Data for three representative low magnitude flow events were chosen to examine the temporal variability of suspended sediment particle-size and flow dynamics. Data were obtained from a one year monitoring programme in a 0.30 km² ephemeral drainage system in Kenya. Surface sealing of sodium rich units produced a rapid runoff response, with hydrograph rise-times of <35 min. Suspension was the primary mode of fluvial transport in the Katiorin basin under unsteady, non-uniform, turbulent flow conditions. Maximum suspended sediment concentrations occurred at peak discharge. Suspended sediment yields were high >3401 km^-2 event^-1, and represented mean denudations >0.23 mm event^-1. Particle-size analysis of suspended sediment indicated that hillslopes were the main source areas for sediment production. Variations in suspended sand, silt and clay concentrations with discharge were complex for three flow events. Regression models determined for individual events were not always significant (P = 0.05). However, if they were significant the coefficient of non-determination (1 - r²) was generally low especially when grain size concentration data for the three storm events were grouped. The high unexplained variation resulted from temporal lumping of all storm data which is an inherent problem of rating curves. Values predicted from rating curves are not absolute, and must be used cautiously when attempting to predict sediment, nutrient and contaminant loads from catchments.