ASSESSMENT OF DESIGN PRACTICES AND PERFORMANCE OF DIVERSION WEIR IN SMALL SCALE IRRIGATION (A CASE STUDIES IN OFFIYA IRRIGATION PROJECT)

Abstract

Many irrigation scames have been design and built in ethiopia in recent year; about 90% of small scale irrigation projects are under performance. Offiya small scale irrigation project is one of non-functional after it constructed due to destruction of different head work structure components. This study aimed to assess the design practice and performance of Offiya diversion weir through technical performance indicator of hydraulic and structural analysis. The main input data used were collected from Ethiopian metrological services agency, minister of water irrigation and energy office department of GIS, Kaffa zone water mine and energy department, Chena woreda water mine and energy department and direct field survey. The tools that used in this study were Tape, Leveling, GPS Garmin 72, GIS 10.4.1, and Excel spread sheet, DEM, Bentley flow master, HEC-HMS and Geostudio2018. Peak rainfall from Chena metrological station analyzed for feature 50 years return by using Gumbel extreme value distribution method was 140.5846mm. The analysis resulted the river peak design flood of 133.08m3/s and the discharge capacity of the weir over the crest was 80.77988m3/s where about 60.7% of design flood computed by HEC HMS.The weir dimensions top and bottom wi dth, cutoffs, creep length, seepage head and apron thickness were determined or Offiya weirs . The stability analysis of weir body and its appurtenance structures was analyzed and compa red with standard safety factor. The overturning and sliding stability factor for weir body were 2.4 and 1.43 this shows the weir was safe against overturning but not sliding. The upstream retaining wall and wing wall was safe against overturning and sliding but not safe for overstress and bearing capacity of foundation as analysis result. The downstream retaining wall was safe for stability against overturning, but not for sliding, overstress and bearing capacity. The quantity of seepage flux for water at pond level and high flood condition were 1.3479 x 10^-5 m3/s/m and 1.2131 x 10^-5 m3/s/m respectively. The safe exit gradient of silt soil weir foundation was 0.17 whereas the computed was 0.225 which indicate the foundation soil not safe. The thickness of impervious floor for static and dynamic case were than the provided thickness of impervious floor these show the under performance the weir structure to the design period. The other underperformance indicator w as the absence of river training, energy dissipater and up and downstream protection work