A THESIS SUBMITTED TO THE FACULTY OF WATER SUPPLY AND ENVIRONMENTAL ENGINEERING, INSTITUTE OF WATER TECHNOLOGY, SCHOOL OF GRADUATE STUDIES, ARBA MINCH UNIVERSITY, IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN WATER SUPPLY AND SANITATION ENGINEERINg

Abstract

Rainfall-induced flooding during the rainy season is a regular phenomenon in Dilla Town. Rapid urbanization, change of rainfall intensity, and inadequate existing drainage system increasing runoff in the study area. Stormwater damage to infrastructure, human life, and the spread of water-borne infections is caused by flooding on highways, inlets, and stormwater threshing near residential areas. The aim of this research is to examine the existing stormwater drainage network and assess the impact of climate and urbanization (impervious area) to develop a stormwater management model for Dilla town. PCSWMM2022, Arc GIS, and various simulations for baseline conditions and Representative concentration pathways RCP4.5 & RCP8.5 (future scenarios) were integrated with EPA SWMM5.2. The stormwater drainage system in an average of six kebeles has been assessed as poor at 55.52%, which means inadequate and undersized to carry the generated runoff from urbanization and climate change. The 15-minute interval time series of a design storm with 2-year, 5-year, 10-year, 25-year, 50-year, and 100-year return periods were determined by generating a rainfall hyetograph from an IDF curve using the Chicago approach and the alternating block methodology. The IDF results indicate that rainfall intensity declines with storm duration; and that a long return period will result in high intensity rainfall for any given duration. In the future climate change scenario between 2022 and 2053, the RCP 8.5 IDF Curve shows the greatest relative change (74.33%) in precipitation intensity for 2 hours and a 100-year return period, while RCP 4.5 shows the greatest relative change (16.82%) for 30 minutes and a 50-year return period. The impervious area is 27.83% and 65.88% in 2002 and 2022, respectively. The total sub-catchment peak runoff increased from 105.92 m3 /sec to 187m3 /sec at 2-year return period and 232.7m3 /sec to 363.2m3/sec at 100-year return period between 2002 to 2022 years. Peak runoff under RCP4.5 and total volume at the outfall were estimated in the climate change scenario for 2, 5, 10, 25, 50, and 100 years return periods as 1.34, 1.42, 1.46, 1.49, 1.51, and 1.53 times more on the combined scenario, respectively. When compared to the baseline scenario the total subcatchments runoff under combined scenario increases by 45.7% for RCP4.5 and 11.4 times the baseline scenario total peak runoff for RCP8.5. It would be possible to make more informed judgments on stormwater runoff mitigation in the study area by simulating urban stormwater drainage systems for the present and future periods, taking climate change and urbanization impacts on runof