Environmental Water Requirements Rapid Assessment (Downstream of the Hydropower Plant of Lower Kihansi)
Raphael M. TSHIMANGA#, and Preksedis M. NDOMBA#
The present study was carried out to assess the hydrological and hydraulic characteristics that control the flow dependent river ecosystem, necessary to maintain the adequacy of the Environmental Water Allocation downstream of the Kihansi Dam. The indicative parameters of Environmental Water Requirements were assessed using three independent methodologies namely, Hydrology-based, Hydraulic rating and Habitat simulation. The adopted approach was meant to ensure information sharing between the results of the three Environmental Flow Assessment (EFA) methods. This is partly because the data required for holistic methodology could not be obtained. The analysis of the research outputs showed that: the index of flow variability appears to be representative of frequency and magnitude of flood and low flow events at Q5 (42.6 m3/s for 1KB28 and 34.6 m3/s for NC3) and Q70 (11.8 m3/s for 1KB28 and 9.9 m3/s for NC3). The average hydraulic parameters are function of discharge and vary as the discharge increases, with a break point for the wetted perimeter beyond which the increase in the stream discharge causes minor increase of the considered hydraulic parameter. The break point varies at a discharge rate of 11.8 m3/s, 11.8 m3/s and 10 m3/s, respectively for the three cross sections. These values appear to be much higher than the three scenarios proposed by previous researchers for flow restoration downstream of the Kihansi Dam (Do nothing scenario, 1.83 m3/s and 7 m3/s). The habitat analysis was done using the Percent Average Annual Flow, which is a surrogate for habitat quality, through three physical stream parameters: depth, velocity and percent width. The values showed that the river contains 16-28% fewer depth than those prescribed by Tennant (1976) at the interval of 10 -200 % of AAF. However, the study was conducted using one flow sampling and it is required to undergo a complete sampling involving low, medium and high flows. An ecological database of the catchment is needed to set prescriptions for Environmental Water Requirements.
Key words: EFA, Hydrology, Hydraulic rating, Habitat simulation, Lower Kihansi Hydropower
1 Lecturer in the department of Natural Resource Management, University of Kinshasa
2 Ph.D. Lecturer in the department of Water Resources Engineering, University of Dar Es Salaam
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