ADEQUACY ASSESSMENT OF STORM WATER DRAINAGE SYSTEMS UNDER CLIMATE CHANGE: THE CASE OF CHIRO TOWN, WEST HARARGHE ZONE, ETHIOPIA

Abstract

The failure of urban drainage systems due to extreme precipitation could cuase massive damage to infrastructures and great impact on socio-economic activity as a whole. The main goal of this research is to determine whether the existing roadside storm water drainage system in Chiro City is adequate or not for the incoming runoff from the within catchment, under both current and future climate change scenarios, using a hydrological model called the storm water management model (SWMM version 5.1.015). The study was focused on three specific objectives that aligned with this general objective. These include developing IDF curve for the study area, which was used to calculate rainfall intensity across various return periods. This rainfall intensity was utilized to generate design storms of various durations, which were then used as rainfall time series in the SWMM model. The second specific objective was to determine the capacity of existing drainage systems and evaluate their performance, while the third specific objective was to determine the adequacy of existing drainage systems under climate change. Basic data such as land use, DEM, drainage system, and precipitation data were collected and preprocessed to fulfill the requirements of the SWMM construction. For this particular study, the catchments discretized based on the natural topography taking into account the rivers and natural runoff channels as boundary lines. Sub-catchment was defined with the help of ArcGIS software. The intensity of rainfall derived from the IDF curve were converted to design hyetograph for 5,10, 25, 50, 100 year return periods.2hour duration at 10 minute time step was used to develop the design hyetograph. Rainfall pattern were gathered from the design storm hyetograph for all five return periods with a two-hour duration and used as an imput for SWMM. IDF curves for observed rainfall show an 88.60% increase in rainfall intensity from 5 year to 100 year return period, whereas IDF curves for projected rainfall show 95.09% increase in rainfall intensity. The rainfall- runoff modeling results for the conveyance system reveal that the majority of the rainfall in the study area were converted to surface runoff. The Maximum inflow is noticed at junction j101 were 4.25, 5.57, 6.12, 6.69 and 7.31m3/s under 5,10, 25,50,100 year return period respectively and flooding noticed under all storm events. Outfall 21 has the largest peak discharge under all five different return period. The results clearly reveal that in general more than 50% of existing storm drainage network are capable of withstanding rainfall for all return periods under both observed rainfall and climate change scenario. However, these doesn’t mean that the city have no flooding problems at all. Because, even though the percent of overloaded drainage line for all return periods are less than 50% of the total drainage line, there are overloaded ditches, and also others are filled with debris of solid wastes. Hence the area around those drainage line experiences high flooding problems.