ASSESSMENT OF URBAN STORM WATER DRAINAGE SYSTEM USING STORM WATER MANAGEMENT MODEL: THE CASE OF BONGA TOWN, ETHIOPIA

Abstract

Storm water drainage is the systematic way of transporting surface runoff from urban environment to protect the risk of flooding and damage. Bonga town has a problem of storm drainage system due to lack of suitable topography and properly design of drainage system. Objective of the study is to assess storm drainage system of Bonga town of selected area in affected sites through storm water management model version 5.1. The outputs with this study is simulation of the drainage system from initial node to outlet point without flooding problem at flow routing of drainage network and with the permissible percent of error to surface runoff. Both primary and secondary data’s were collected from the field observation, municipality office and national metrology agency of Ethiopia respectively. Estimation of the storm water runoff peak discharge in the drainage network was carried out with available rainfall data and Log Pearson Type III probability distribution method was selected for frequency analysis in the study according to the value of coefficient determination. The affected part of the town around study site by existing poor drainage system was identified on roadway, ditches and residences. For comparison and evaluation of performance of model as well as its fitting, rational method was used to calculate peak discharges in all sub catchments. The study catchment was divided based on existing road access and elevation difference into twenty seven individual sub catchments and on those 121 conduits/channels, 116 node/junctions and 2 outlets were set on the model for simulation. Validation of the storm water management model 5.1 in the study was carried out through the continuity percent error of surface runoff and flow routing. The model results were 1.6 for surface runoff and zero for flow routing respectively. The performance of model was evaluated with the value that determined through statistical equations such as coefficient determination, Nash Sutcliffe and relative error and its values were 0.895, 20.5, and 0.86 respectively. The total amount of determined peak runoff discharge from study area in storm water management modeling 5.1 was 12.32 cms and whereas 9.31cms in rational method respectively. Therefore, redesign has to be required with stake holder experts to manage the storm water drainage system of study area in terms of peak runoff concentration that generated from sub catchments.