Hydraulic Performance Evaluation of Diversion Weir and Canal Structures: Case Study of Basaka Small Scale Irrigation Scheme, Wayu Tuka, East Wallaga, Oromia, Ethiopia

Hydraulic Performance evaluation of the irrigation system is important to conserve scarce water resources by improving the performance of the existing irrigation structure. However, there was no study in the past performance evaluation of the Basaka scheme, which is used for irrigation purposes. This study was to evaluate the hydraulic performance of the diversion weir and the canal structures by using hydraulic performance indicators, conveyance indicators, and maintenance indicators. The primary data collected were direct field measurements in the canals and direct field observations of function & mal-function structures. The secondary data was also taken from the designed document scheme and the journals. The study was carried out during the one irrigation season from February to April 2023. The data collected was analyzed using empirical equations. A diversion weir structure is unsafe for the passage of high floods because the discharge capacity of the weir (227.8m3/s) is relatively lower than the peak design discharge (234m3/s) that is generated from catchment areas. The current intake gate water withdrawal to the main canal was 97%. The discharge measurement was performed at three measuring stations at the head, middle, and tail on the main and secondary canals respectively by using the area-velocity method. Also, the velocity of discharge flow through both canals was examined by using floating material (tennis ball). The causes of malfunctioning structures were investigated through field observation. And also, there is a sedimentation problem in the diversion weir due to the under-sluice gate structure is not constructed at diversion head-work, whereas seepage problems in the main and secondary canals due to the many parts of the canals are unlined. The overall average main and secondary canal's conveyance efficiency was 57% & 59%, and water conveyance loss was 43% and 41% respectively, which is below recommended values. This conveyance efficiency is reduced due to breakage occurring on the main canal, seepage, siltation and vegetation growth within the canal interfere. The maintenance indicators investigation of the scheme was examined. This means that; the value of the effectiveness of infrastructure was 58%, the sustainability of the irrigated area was 71%, the delivery duration ratio was 200%, and the water surface elevation ratio (58% for MC and 62% for SC) respectively. Based on the results of the maintenance and conveyance indicators, Basaka small irrigation scheme requires a high level of maintenance.