Remaining Storage of the Zaria Impounded Reservoir and Rate of Erosion in Its Drainage Basin
Kudamnya, E. A.; Schoeneich, K.; Garba, M. L.

Abstract
Zaria Impounded Reservoir is the only source of piped water supply to Zaria and its environs since its impoundment in 1974 to date. However, its useful life is being threatened by continuous siltation with sediments originating from its drainage basin – the Galma Drainage Basin as a result of improper land use practices. Owing to the great importance of the reservoir in the supply of potable water to Zaria and environs, an attempt was made to study the reservoir’s bathymetry using boat, metric tape with a heavy object attached to it and satellite navigator. Result of this measurements shows that the present year, 2010, average weighted depth has been reduced to 2.82 metres from 15.00 metres at the day of impoundment in 1974. Also, the remaining storage was reduced to 21,129,242 m3, about 46 percent of the original volume, 45,800,000 m3 at the day of impoundment. Furthermore, the rate of siltation between years 1974 to 2010 was calculated to be 685,299 m3/year. It corresponds to about 1.5 percent per year of the original storage. Further, the measurement has shown that only the active storage is left in year 2010. The dead storage had been completely silted up about 23 years after the impoundment and active storage has been reduced by 8,670,758 m3 in year 2010 from the original 29,800,000 m3. By year 2041 the reservoir will be completely silted up if preventive measures are not taken. Additionally, rate of erosion depth in the drainage basin has been calculated as 0.3 mm/a.

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