PERFORMANCE ANALYSIS OF SMALL SCALE IRRIGATION SCHEME OF RAMIS RIVER, IN THE CASE OF DIVERSION HEAD WORK, EAST HARARGHE, ETHIOPIA

Abstract

Many irrigation schemes have been designed and built in Ethiopia recently; about 90% of small-scale irrigation projects are underperforming. This study aimed to assess the performance analysis of three selected diversion weirs through performance indicators of hydrological, hydraulic and structural analysis. Secondary and primary data are collected and analyzed for the determination of peak flood design discharge and design of diversion weir. For peak, flood runoff estimation using 23-year rainfall from Chelenko, Kullubi and Bedeno metrological stations analyzed for the future 50 years, return period rainfall was 136mm for Lega Kosta and Chafe Ramis, whereas for Gololcha Ramis 117mm were used. SCS unit hydrograph Methods were used for peak flood analysis based on Land use/cover and sub watershed analysis done by GIS 10.4.2. The design discharge performance evaluation for the diversion weir corresponding to a return period of 1 in 50 years Lega Kosta, Chafe Ramis and Gololcha Ramis peak design flood are 203m3/sec, 289m3/sec and 352m3/sec respectively, whereas from the designed document represent 189m3/sec, 191.44m3/sec and 231.4m3/sec. The results indicate that total discharges passes over of existing diversion weir structures, 100.83m3/sec, 131.58m3/sec, and 256.25m3/sec are less than peak discharges, which implies the structures flood passage capacities are 50%, 54.4%, and 27.3% of the actual design discharges below the standard discharges passes and flood passage capacity of the existing under-sluice sections were 5.64%, 8.9%, and 9.12%, also below recommended for Lega Kosta, Chafe Ramis, and Gololcha Ramis diversion headwork structures respectively. The hydraulic performance analysis's main problems observed in the Lega Kosta, Chafe Ramis and Gololcha Ramis scheme were sedimentation of headwork, damage of intakes and sluice gates, clogging of intakes, and damages to distribution systems and weir bodies. This result shows that the cause’s failures were due to wrong design discharge estimation. The structural stability analyzed indicates stability analysis of the weir body and its appurtenance structures was analyzed and compared with standard safety factor. Finally, the major causes of the diversion headwork 48.7%, 28% and 23.3% in hydrological, hydraulic and structures underperform recently, which will hinder their performance in the future too. Therefore, the proper hydrologic, hydraulic and structural design of the diversion headwork structures is a very important task that contributes to the good performance and sustainability of the structures.