Reservoir sedimentation and a case study related implications: Masinga dam

Abstract

There are many reservoirs which can no longer serve their design functions because much of their life storage volume have been filled by sediment. The need, therefore, to consider reservoir sedimentation in respect of both existing and proposed reservoirs, topredict the loss of available storage isparamount as a function of time, the distribution of deposition, trap efficiency and discharge - sediment relationship. The practical problems arising from reservoir sedimentation can be 1 . rI eXp.1.alneu by reference to examples and include; 1} loss of storage and consequent loss of yield; 2} increased upstream flooding; 3} blockage of intakes; 4} abrasion of turbines; and 5) impact on downstream river regime. The study of reservoir sedimentation was carried out with reference to Masinga darn, a project of Tana and Athi River Development Authority (TARDA), Ministry of Land Reclamation, Regional and Water Development, Government of Kenya. The life of a project is likely to be seriously affected during the early part of its useful life as a result of disruption of fLow equilibrium resulting from damming of a channel. This may lead to aggradation in the reservoir at e rate more than predicted during design period and hence reduce the storage capacity of the project thus shortening its life period. Aggradation may also lead to increased upstream flooding and therefore endanger not only the existing stru capacity of the project thus shortening its life period. Aggradation may also lead to increased upstream flooding and therefore endanger not only the existing structures but also human and animal life. It is inview of the many problems arising from rapid sedimentation that this project was chosen to investigate the changes insediment accumulation rate which might have resulted from the reservoir impoundment eight years ago. The study revealed that; 1) The rate of sediment accumulation in the reservoir is of the order of 4.21x105 m3j annum. This rate is higher than that used during design of , ~ 3.0x10o m"j annum. Therefore, the reservoir life will be 370 years assuming the current rate of siltation persists. This is less than 500 years used as a design value. Thus the high rate of siltation has the effect of reducing the life span of the project by 130 years. 2) The reservoir trap efficiency ranges from -28.82 to 99.9 % . 3) Aggradation has taken place more significantly at the sections where the channel enters the reservoir. 4) Rivers TanajSagana and Maragua are heavy sediment laden with naturalfbackground levels reaching high levels of 1500 tonnes/day for Sagana and 100 tonnes/day for Maragua. 5) During any given runoff event, suspended sediment concentrations on the rising limp of a storm hydrograph are usually greater than those measured at equivalent flows on the falling limp.