ASSESSMENT OF DESIGN PRACTICES AND PERFORMANCE EVALUATION OF RIVER DIVERSION STRUCTURES FOR IRRIGATION: A CASE STUDY OF RASSA AND WODESA SMALL SCALE IRRIGATION SCHEMES IN WONDO GENET WOREDA, SIDAMA REGION, ETHIOPIA.

Abstract

Diversion headworks play a vital role in diverting water from rivers or channels for irrigation. However, many such structures face hydrological, hydraulic, structural, operational, and management challenges, leading to reduced functionality and high maintenance costs. Some recently constructed diversion structures have been reported to work properly while others have what could be termed as partial failures; parts of the structures get damaged as time passes. This study focuses on assessing the design practices and performance evaluation of the Rasa and Wodesa diversion headwork structures, which were constructed for irrigation purposes. The research evaluates the current conditions of these structures, analyzes the design practices, and identifies key factors contributing to their underperformance. Primary data, including field observations and measurements, were collected, along with secondary data from sources such as the National Meteorology Agency, the Ministry of Water and Energy, and the Sidama Regional State Irrigation Development Agency. The tools used include GPS, GIS 10.3.1, Excel, DEM, Bentley Flow Master, and HEC-HMS. The hydrological analysis, conducted using the HEC-HMS model, was calibrated and validated with observed streamflow data from the Tukur Wuha River for the periods 1991–2000 and 2001–2007, respectively. The model achieved good performance, with Nash-Sutcliffe Efficiency (NSE) values of 0.671 and 0.672, R² values of 0.79 and 0.8, Percent Bias (PBIAS) of 7.26% and 7.41%, and RMSE (Stdev) of 0.6 for both periods. The optimized parameters from the calibrated model were transferred to the ungauged Rasa and Wodesa watersheds using the regionalization technique. Design storm analysis of the model presented the extreme flows of 59.2m³/s for Rasa and 98.2m³/s for Wodesa which are beyond the weir design capacity of 44.2m³/s and 84m³/s respectively which results in flooding. Poor flood passage capacity and deposition of sediment were also recorded, which hindered efficiency. More specifically, structural evaluations indicated that floor thicknesses of concrete were low, cutoffs inadequate, and weir dimensions too small to provide adequate protection against uplift pressures and piping. Despite these problems, the stability for both weirs in terms of sliding, overturning, overstressing, and contact pressures were determined to be safe. The retaining walls were also safe from a structural perspective although contact pressures were higher than the recommended safe limits and therefore required further assessment and strengthening. This paper points out key aspects that would enable a small-scale diversion weir to function with greater durability and efficiency in irrigation systems