EVALUATION OF URBAN DRAINAGE SYSTEM BY SWMM: A CASE STUDY AT ALABA KULITO TOWN, SNNPRS, ETHIOPIA

Abstract

The general objective of the study was Evaluation of Urban Drainage System of Alaba Kulito Town by Storm Water Mmanagement Model (SWMM). The study employed the collection of primary data like measuring the existing drainage size of the flood-prone region and asking stakeholders and secondary data which was obtained from National Meteorology Agency (NMA) of Ethiopian (33 years meteorological data), Ethiopian Map Agency (topographic map and soil map data), Ministry of Water, Irrigation and Energy (soil data) and Alaba Town Municipality (historical data of flood and organizational structure of city administration). 3.7% of Missed RF data of Alaba Kulito station was filled by Normal Ratio Method (NRM). The Generalized Pareto Distribution (GPD) method which was followed by Peak Over Threshold (POT) extreme value determination method was used to predict the probability of flood occurrence due to the best fit and approach of study. For analysis of hydrology and hydraulics done by the Soil Conservation Service Curve Number (SCS-CN)/rational method and Stormwater Management Model (SWMM). It has been found that Alaba-Kulito town is geographically nearly plain which was between class 1 and class 4 slope classification and located in foothill which contributes much amount of runoff to the town and some of the drainage lines are incapable to convey runoff generated from rural catchment. In addition to this, limited landscape based mitigation strategies in the study area with insufficiency of drainage canals, limited collector and feeder drainage lines, lack of awareness of community while disposing of household wastes together have worsened the impacts of flooding. The overall result of the study is terminated by distinguishing and pointing both structural methods: diverting the upper catchment(which shares more than 64% runoff load ), providing collector and feeder drainage lines through the flood-prone section of town and constructing a lined canal at the common outlet to Bilate river which is about 1.5km from ST.Gabriel church ; and non-structural managing systems depending on the degree of the flood. The peak runoff load of each junction and nodes are obtained by summing up the runoff magnitude of all upper contributing catchments and accordingly the outlet point received about 49.45m 3 /s and 29.1m 3 /s without divertion work and if diversion work was provided for 10 years return period respectively. The coefficient of correlation between simulated and estimated peak discharge becomes greater than 0.99.