Soil Erosion and Conservation in Western Kenya

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In the Lake Victoria Basin in Western Kenya, soil erosion is developing into large gullies while are seriously threatening the inhabitants. This research project aims at understanding the basic process of the gully erosion and proposing ideas of assessment for catchment-scale conservation in the area.(1)Excessive concentration of overland flow during heavy rains has incised large gullies on both topsoil and underlying Quaternary sediments that infill the Kavirondo rift.(2)The annual erosion rate is as high as 20m head cut retreat.(3)Based on the lithostratigraphic evidence, the Quaternary sediments are classified into upper Awach member and lower Sondu member. Hardness values of the Sondu member sediments are relatively higher than the Awach member sediments and this would be due largely to the different content of ferric-oxide that cements the detrital grains and to the different porosity.(4)Gully erosion varies in its physiognomy such as eroding patterns and head cut retreat rate. These characteristics are the result of complex combinations of eroding forces like overland flow concentration and suspended load density, and lithological properties. These factors are also controlled by upslope catchment areas, vegetational coverage, land uses, precipitation and geological backgrounds.(5)Three types of image analysis have been carried out for three purposes. Analyzed image data are topographic maps, remotely sensed satellite imagery and aerial photographs. Results of the analyses of these image data were combined to produce the distribution map, gullying patterns and information of the upslope catchments. Overlay analysis has finally been made to produce hazard warning maps for eroding/contributing zones.