HYDRAULIC PERFORMANCE ASSESSMENT OF STORM WATER DRAINAGE SYSTEMS OF DEJEN TOWN USING STORM WATER MANAGEMENT MODEL

Abstract

Stormwater drainage problem is a major challenge facing most of the Cities and Towns of the world including Cities and Towns in Ethiopia. As Dejen Town has stormwater drainage problems, this study was conducted to assess the current performance of the Town, to estimate and predict flood amount, and to design drainage structure for a design period of 10-years to solve such problems. To do this, first meteorological data such as rainfall data of the study area from (1987-2018), drainage line, and plan of the Town was collected from concerning bodies, and field survey of the study area was done to measure the drainage canal dimensions, and Global Position System of the Junctions. Then missing rainfall data was filled; data consistency was checked and analyzed using Microsoft-excel. Assessment of the current drainage condition of the study area was performed and the current drainage capacity of existing structures was calculated using measured canal dimensions by Manning’s formula. Nevertheless, most of the existing drainage structures were inadequate to dispose runoff to the outfall area and most drainage structures in study area were poor. The goodness of fit test indicates General Pareto distribution fits for flood frequency analysis and Intensity-Duration- Frequency curve was developed by using this distribution method. By using rainfall intensity calculated from the developed Intensity Duration-Frequency curve, the amount of peak discharges were estimated by the rational formula for each sub-catchments for a 10-years design period. Then, Storm Water Management Model was applied for the prediction of discharge in each sub-catchment. The Storm Water Management Model output indicated that the discharge resulted from the sub-catchments was greater than the existing capacity in most sub-catchment. This implies in most of the canals, Junctions, and outfalls the flood level was greater than the designed water level, and over-flooding occurs at drainage canals and most of the junctions were over flooded. To solve these problems the following mitigations were recommended; construction of additional drainage structures with proper dimension especially for secondary roadsides with no drainage structures, design and construction of well connected structures, adopting the culture of clearing sediment and periodic repairing of drainage structures before total failure.