ASSESSMENT OF DESIGN PRACTICES AND PERFORMANCE OF DIVERSION WEIR IN SMALL SCALE IRRIGATION (CASE STUDY FROM PROJECTS IN WESTERN OROMIA, ETHIOPIA)

Abstract

Diversion weir built across the river or canal to divert water from the original course. Assessment of design practice and performance evaluation of case study carried out in western Oromia, Ethiopia on six selected diversion weir and analysis the design method, parameters and assumptions that make change the weir dimension and cause low performance. Secondary and primary data are collected and analyzed for determination of peak flood design discharge and design of diversion weir. Four meteorological stations were used for design runoff estimation by g umbles‟ distribution formula of 50 years return period. Sibu Sire rainfall station 88.7 mm for Indris and Jalale, Baco 107 mm for Gibe Lamu and Gambela Tare, Boneya Boshe 118.54 mm for Chokorsa and Nekemt 133.07 mm designed rainfall used for Fite diversion weir design. Land use/cover and sub watershed analysis done by GIS 10.1 and peak flood design determined by unit hydrograph for Indris 158 m 3 /s, Jalale 79 m 3 /s, Chokorsa 106 m 3 /s, Fite 50 m 3 /s, Gibe Lamu 105 m 3 /s and Gembela Tare 72 m 3 /s. The weir dimensions top and bottom width, cutoffs, creep length, seepage head and apron thickness were determined for all selected weirs. The seepage head and apron thickness determined by Bligh‟s, Lane‟s and Khosla‟s . After hydraulic dimension of weir design the estimated exit gradient is safe under Bligh‟s, Lane‟s and Khosla‟s with compared to natural soil exit gradient. Structural stability analyzed indicates Indris, Fite and Chokorsa are safe at the hydraulic design and in the case of G/Lamu, G/Tare and Jalale the stable value obtain ed after further stability analyzed done by changing the weir bottom width. As performance evaluation revealed that the designed weir cross section of In dris, G/Tare, Fite and Chokorsa were accommodating peak flood while Jalale cannot and flow shift occurred out of G/Lamu weir cross section. Piping, uplift pressure, scouring, sliding, overturning and over stress did not occurred on the five selected weir however, in the case of Chokorsa weir downstream floor totally scoured and burst due to uplift pressure occurred. In general the constructed weirs have no more engineering design problems which cause low performance but lack of well schemes administration the major problems occurred as field observation result indicates.