Abstract
WEAP (Water Evaluation And Planning) is an integrated water resources planning model developed for more than 25 years by the Stockholm Environment Institute (SEI). In the WEAP model, dam and the water demands (upstream and downstream) are all schematized as an interconnected system ruled by allocation priorities (e.g., dam operation rules, priority for competing demands such as water supply, irrigation, hydropower, environmental flow, water storage in the reservoir). Thwake Reservoir is a multipurpose reservoir for hydropower generation, water supply and irrigation development in Kitui and Makueni Counties. The counties are well known as semi-arid hence the key objective of the study is to estimate the safe yield of the reservoir to enhance effective allocation of the water for different competing demands. Safe yield is based on projected future conditions that include a repeat of the most severe drought of record. Because the definition refers to projected future conditions, safe yield is inherently an estimated characteristic of future conditions that cannot currently be observed or measured. Thus any safe yield is only theoretically available at the time of the estimate. However, the reference to a repeat of the most severe drought of record indicates that past conditions are to be used to project future conditions. In this regard, 1000 years’ synthetic flows were generated using SWAT Model. In the model, 61 years (1952-2012) historical flows were used and the projected flows input into the WEAP model. The monthly reservoir balance for the base scenario and upstream dams development scenario were simulated. The performance of the Thwake Reservoir under different scenarios was assessed. Different sets of scenarios were explored for the Base Case and upstream development dams. These sets were tuned to the operational rules of the Thwake reservoir. Eventually, the dependable yield of the reservoir under different operating rules was estimated for both the base case and the upstream dam development scenarios. Thwake reservoir has an estimated dependable yield when it will be in operation before and after the development of the upstream dams.
Highlights
The results showed that Athi River at RGS 3F02 has an annual mean discharge of 34.04 m3/sec
Thwake reservoir has an estimated dependable yield when it will be in operation before and after the development of the upstream dams In practical terms of reservoir operation, these rules translate as follows: For the base case scenario, at any water level, the water released through the turbine should be at minimum equal to the environmental flow
Water is only partly released to the Konza domestic water supply demand and to the remaining hydropower water demand above the environmental flow requirement
Summary
The proposed Thwake dam site is located approximately 1km downstream of the confluence of rivers Thwake and Athi. The axis of the proposed dam runs across the Athi River and when implemented will connect Makueni and Kitui Counties. The altitude of the site is approximately 840m above sea level (m.a.s.l). The highest point of the catchment is at 2,600 m.a.s.l. Athi Catchment upstream of the dam site extends across four administrative provinces – Central, Nairobi, Eastern and Rift Valley. The following counties are within the catchment i.e Nairobi, Kiambu, Machakos, Kajiado, Makueni and Kitui. Included within the catchment is Nairobi City, Athi River Township and large parts of Kajiado County.
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