MODELING OF URBAN STORM WATER MANAGEMENT ON RUNOFF PRODUCTION: THE CASE STUDY OF ARBA MINCH TOWN(SUBCITY OF SIKELA), ETHIOPIA

Abstract

The drainage systems are expected to function effectively in an urban area to manage the runoff flow along or across their alignment. Proper urban storm water management plays a vital role in the effective operation of urban drainage systems. Inefficient urban storm water management often leads to an increased surface runoff which induces a significant impact on public life and infrastructures. Hence, studies that focus on evaluating urban storm water management are necessary. However, studies evaluating the impacts of such activities are rarely available in Arba Minch town, where this study has been conducted; street flooding has become a recurrent problem. Overflowing on the main road surface and water stagnation has become common in rainy seasons. The aim of this study, the magnitude of runoff for various recurrent intervals, capacity of existing storm water drainage systems, and spatial variation on runoff production due to urbanization in sub city of Sikela were assessed. In order, to achieve this objective, the storm water management model (SWIM) was used. In this study, a field survey was conducted to collect primary data on the drainage systems, % imperiousness estimate from LULC data, and soil textures in the laboratory. The catchment was divided into 47 sub- catchments depending upon the building block, flow direction, and drainage networks. The result of simulations showed the insufficient capacity of the drainage system of the town to handle the heavy designed rainfall, with most of the nodes flooded and conduits surcharged. Sub catchment surface runoff was found to increase with the return period. From the total 47 sub-catchments, the average surface runoff was 0.0651m3/sec, 0.1366m3/sec, 0.2545m3/sec, 0.6459m3/sec, and 0.7455m3/sec for different return period’s 2-yr, 5-yr, 10-yr, 25-yr, and 30-yr respectively. Simulation results of 2-year return period design rainfall show that maximum nodes flooded occurring at junctions J7, J8, J24, and J25, and conduits C14, C18 and C27, have attained maximum (full depth). The Simulated peak runoff result was greatest in sub-catchment No.8, No.1, No.6, No.30, and No.9 for different return periods 2, 5, 10, 25, and 30-year respectively. This study concluded that the Surface runoff is a rapid increase for the above 2-year design rainfall returns period mostly due to the impermeable surface and the inadequate urban drainage systems of the study area, which causes damage to infrastructures, and public (private) properties