PERFORMANCE ASSESSMENT OF URBAN DRAINAGE SYSTEMS OF YIRBA TOWN SIDAMA REGIONAL STATE, ETHIOPIA

Abstract

Developing countries experience accelerated urbanization without adequate investment in infrastructure. Yirba town, like other towns of Ethiopia, have a lot of problems including inadequacy and poor-quality drainage infrastructure. The objective of this study is to assess the hydraulic performance of stormwater drainage systems of the study area, to identify stormwater management issues and existing urban drainage system problems, and to identify the best management practices for drainage problem mitigation measures and to achieve these specific objective SWMM5.2 model and LID structural measures were used in the study. Rainfall, infiltration, and physical characteristics of the catchment area were organized as three main inputs for the model. Rainfall depths of thirty one years (1992-2022) of Yirba station were obtained from National Meteorological Agency. According to chi-square test of the best fit probability is Log Pearson type III distribution and that is used to developed IDF curve. Infiltration of the model area is represented by Curve number method. Physical characteristics of the sub catchments including topography of the model area was analyzed using Goggle Earth and ArcGIS software. The calibration and validation of the SWMM5.2 model were done and its performance was tested by the goodness of fit using the coefficient of determination (R2) =0.97 and Nash Sutcliffe Efficiency Coefficient (NSE) =0.90. The total simulation area of sub catchment is 80 ha; the model area is divided in to 45 sub catchments with a drainage network of 38 conduits, 38 junctions and 2 outfalls. From model result 50% of conduits and 55% of junctions are flooded; at the outfall total sub-catchment runoff is 4.35m 3 /s average flow rate, 10.84m 3 /s maximum flow rate and 91.10*103 m3 total volumes of both outfalls. Among the various low impact development (LID) structural measures, infiltration trench and vegetative swale are incorporated and simulated in the model. Based on the simulation of LID's 10-year return period, improvements in terms of reduced peak discharges and increased peak discharge times were observed in sites where LID was used. In terms of a 10-year period, the peak runoff from particular sub catchments of drains is reduced by a minimum of 1.82% and a maximum of 19.91%. In general, the Yirba town urban drainage system performance infrastructure overflow was assessed and managed for the problem occurring using the possible mitigation techniques by improved LID control for all study areas.