Assessment of Storm Water Drainage System Performance of Mojo Town, Central Ethiopia
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Date
2020-12
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Addis Ababa University
Abstract
The main objective of this study was to assess the existing storm water drainage performance of Mojo town and to provide the possible recommendations or engineering measures that enables to alleviate the prevailing problems. Storm Water Management Model (SWMM), which is a dynamic rainfall runoff simulation model widely applied for urban drainage, was chosen for the performance evaluation to assess the triggering factors as well. The model was developed based on a selected area of 354 ha, that has been a flood prone area mainly due to the construction of two mega projects. The model area is divided in to 45 sub catchments with a drainage network of 94 conduits, 74 junctions and 2 outfalls. Three important inputs for the model were organized: Rainfall, infiltration and physical characteristic of the model area. 1. Rainfall depths of twenty-one years (1997-2017) of Mojo metrology station were obtained from National Metrological Agency. An IDF curve for different return periods (2,5,10,25,50,100yrs) formulated by Log person type III distribution method was used as an input for the model due to its slightly better coefficient of determination than Gumbel method.2. Infiltration of the model area is represented by Green-Ampt equation.3 Physical characteristics of the sub catchments including topography of the model area was analyzed using Global mapper, ArcGIS software. Besides, three successive Google earth images were used to analyze the land use land cover change of the model area through 11 years (2009-2019) period. Based on the inputs the model is built and model parameters were derived based on characteristics of the sub catchments and drains infrastructures. According to the sensitivity analysis, catchment width and Manning roughness coefficient for pervious areas were found the most sensitive parameters. Three drains (D_48, D_89 & D_90) were selected for calibration and validation. Three rainfall event observations for calibration and one event for validation were used on the two most sensitive parameters until the simulated and observed values reached at acceptable level. The performance of SWWM is insured based on an evaluation that is made by coefficient of determination (R2) and Nash-Sutcliffe Efficiency Coefficient (NSE). The model is simulated for one rainfall event, for different return periods and 3 hours rainfall duration. According to 10 years return period of simulation regarding storm water discharge, 25 conduits and 28 junctions were flooded which is 27.0 % and 37.8% respectively. An increment in terms of Peak runoff, total runoff & peak discharge is observed due to LULC change. The other important issue is a significant proportion of the drainage system is subjected to below 0.9m3/s flow velocity that insists deposition of solid material in drains which affects the system performance. Among the various low impact development (LID) structural measures, Bio retention and Infiltration trench are incorporated and simulated in the model. Based on 10 yrs & 25 yrs return period simulation for LID, improvements in terms of peak discharges decrease and elongated peak time discharge were observed where LID is applied. Regarding 10 yrs period ,the magnitude of peak discharge of drains is decreased with the minimum of 1.5% and maximum of 35.1.Generally, the drainage system of the town is not suffice for the runoff generated from the town because of intensive land use/land cover (LULC) change as well as lack of green areas in required proportion. The major source of overflow of drains is highly related to the construction of the two mega projects. Minimum flow velocity, which is below cleansing velocity, is the other problem that affects the drainage system performance. In terms of quality, different studies verified that storm water of Mojo town is highly polluted due to mainly industrial pollution. This study mainly recommends proper implementation of the town master plan in such a way to regulate reasonable land use land cover. Redesigning of drains and related drainage infrastructure is critically required in such a way to meet the minimum limit of flow velocity and to use as remedy for the problem of overflow of drains. Diversified application of structural and nonstructural measures of LID help to alleviate the prevailing drainage problem of the town.
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Urban drainage, SWMM, LID, calibration, validation