AN ASSESSMENT OF SEDIMENT INPUT TO THE LOWER USUMA RESERVOIR, ABUJA, NIGERIA.
Abu, R. D*, Iguisi, E. O., Dewingong C. L, and Halilu, A. S
Many dams and reservoirs have been constructed in Nigeria to provide potable water for municipal consumption, irrigation, industrial among other uses and to address the issues of water management. However, many of the dams and reservoirs are losing their storage capacity as a result of siltation. The goal of this study is to estimate the annual siltation rate for the Lower Usuma reservoir using conventional hydrographic survey and acoustic echo sounding technique (bathymetric method). Lower Usuma reservoir had a gross storage capacity of 100 × 106m3 (100 million cubic metres), a maximum operational level of 587.440m a.s.l., with a maximum depth of 49 metres and a catchment area of 241km2 at dam site with a daily designed production capacity of 10,000 cubic metres per hour. The reservoir is 1,300 metres long and became operational in 1986. The result obtained with the conventional hydrographic technique shows a current maximum depth of 35.3m, depth loss of 13.7m (1986-2012), annual siltation rate of 0.53m while the acoustic echo sounding technique records a current maximum depth of 37.0m, depth loss of 12.0m (1986-2012) and 0.46m annual siltation rate. The result of the Z–test statistics indicates that there is no significant difference between reservoir depth measurement by conventional hydrographic technique and integrated acoustic echo sounding technique. The reason for the lower sedimentation depth measurement obtained using the integrated acoustic echo sounding technique can be explained on the basis of accuracy in the depth measurement. The reservoir has lost 12.0m depth to siltation; indicating 24.5% loss in installed storage capacity. The implications of the siltation problem and the consequent loss of depth of the Lower Usuma reservoir is that the dam is gradually losing its function and capacity for controlling flooding and provision of potable drinking water to the FCT inhabitants.
Key words: Acoustic echo sounding technique, Conventional hydrographic survey, Siltation depth, Storage capacity loss, Dams and Reservoirs.