HYDRAULIC PERFORMANCE EVALUATION OF URBAN DRAINAGE SYSTEM AND PROPOSED MITIGATION MEASURES: A CASE STUDY AT KOFE KEBELE IN DILLA TOWN

Abstract

This thesis aims to provide a comprehensive evaluation of the hydraulic performance of an urban drainage system and propose effective mitigation measures to address the negative impact of flooding in Kofe Kebele of Dilla town using the Stormwater Management Model (SWMM) 5.1. The study area covers 180 hectares of land; it has three outlets, the catchment is classified into 34 sub-catchments, and the drainage system has 144 junction nodes and 191 conduit links. Flow routing was computed using kinematic waves and Green-Ampt was used for the infiltration model. Rainfall data for Dilla were obtained from the National Meteorological Agency and twenty-five years of data were used (1997–2021). The normal ratio method was used to fill in missing data. A double mass curve was used to check consistency, outlier identification was performed to check for higher and lower outliers, and rainfall frequency analysis and frequency distribution fitting were performed to evaluate the goodness of fit. A goodness-of-fit test was performed, and it was found that Gumbel, or GEV (I), has a good fitness value, and IDF curves were developed based on the ERA standard. A 25-year return period was used as the design storm. The United States Environmental Protection Agency’s Storm Water Management Model (EPA SWMM 5.1) and the Geographic Information Systems (ArcGIS) tool were used to model and analyze stormwater characteristics. recorded water levels and velocities were used to calibrate and validate the EPA SWMM 5.1 model. Three conduits were selected for calibration and validation. The results obtained for the calibration of the performance indicator of the Nash-Sutcliffe efficiency (NSE) for each conduit (C78, C97, and C184) were 0.87, 0.91, and 0.85, respectively, and for the coefficient of determination (R2 ) were 0.75, 0.84, and 0.77, respectively. The other indicator considered was the relative error, where the errors were 8.3%, 4%, and 7%. Referring to the validation, the performance of the model for Nash Sutcliffe efficiency (NSE) for the aforementioned conduits was 0.91, 0.88, and 0.88, respectively; it was 0.88, 0.99, and 0.78 for the coefficient of determination (R2 ), with a relative error of 3.7%. These results show that the model has a good performance and can perform the analysis for the study area. Low-impact development (LID) techniques were investigated based on land use and land cover conditions to sustainably solve the flooding problem. From various LID alternatives, three LID techniques were chosen based on site conditions: bioretention cells, infiltration trenches, and rain barrels. The simulation results indicated that the combination of bioretention cells, infiltration trenches, and rain barrels had a better capacity to minimize runoff. This study suggests using low-impact development (LID) to address flooding issues in a sustainable manner.