Reservoir sedimentation and catchment land use

Abstract

The thesis is based on a study of two silted up reservoirs and the surrounding catchment in a semi-arid part of Machakos District, Kenya.The catchment (11. 4 km2) is located near Salama market. 80 km on Nairobi - Mombasa road. The area has annual rainfall in the range 500 - 800 mm and annual temperature between lS·C and 20·C. The catchment area under study has two silted up reservoirs. First is the Kwa Hiui on the lower part and whose catchment area is 4.5 Km2. Secondly, the Ndoloni reservoir on the upper part and whose catchment area is 3.2 Km2. The remaining area (3.7 km2) was in the upper part of the catchment. wi th good grass cover 1 where minimal sediment was trapped by small reservoirs. The whole catchment is part of what was a ranch. belonging to Frank O'Brien Wilson_ The workers of the ranch were settled on the lower part in 1965 and the upper portion was purchased by the Aimi Ha Kilungu Cooperative Society in 1977. The objectives of the study were to: (i) reconstruct the history of reservoir sedimentation and catchment erosion; (ii ) evaluate the past and present conservation measures; (iii) assess the feasibility of desilting the two reservoirs or constructing new reservoirs. The investigations carried out included measurements of volumes of sediments deposited in Kwa Miui and Ndoloni reservoirs to arrive at sedimentation rates, volumes of soils eroded from gullies and footpaths/animal tracks. Catchment erosion rates were estimated by use of micro-erosion features. An evaluation on soil conservation measures was carried out in the lower part of the catchment (settled area). Estimates of costs of desilting the two reservoirs and constructing new small reservoirs was done. The results obtained were on sediment accumulation in reservoirs whereby during the period 1955 - 1975 about 36,108m3 of sediments were deposited in Kwa Miui reservoir and 10,896m3 in Ndoloni during the period 1954 -1984. From this, the annual sedimentation rates were 401m3/Km2 and 114 m3/Km2 respectively and assuming that the bulk densi ty of the sediments in the reset-voir were similar to the catchment soi Ls , These annual sedimentation rates correspond to a soil denudation rate between 0.1 mm The total amount of soils from gullies and footpaths/animal tracks in Kwa Miui catchment were 9,731m3 and 7,014m3 accounting in total for 16,745m3 (46%) of sediments deposited in the Kwa Miui reservoir. There was one gully in Ndoloni catchment which had a volume of 387m3 accounting for 4% of the sediments deposited in Ndoloni. From this study the conclusions l.•e.•t-e that the rates of sedimentation were high leading to rapid siltation of reservoirs and thus denying the inhabitants and livestock the availability of water_ Though the opened fields for cultivation were terraced. there were inadequacies which led to erosion. Likewise on grazing land erosion was observed because of overstocking. The activities associated with settlement have led to erosion within the catchment. followed byt reservoir siltation. To rehabilitate these, heightening of the Kwa Miui dam is a feasible solution. Second in feasibility is desilting of the Kwa Miui and Ndoloni reservoirs.