EVALUATING HYDRAULIC PERFORMANCE OF URBAN STORMWATER DRAINAGE SYSTEM OF HOSSAINA TOWN, ETHIOPIA

Abstract

Urban flooding is basically due to the inadequate and inefficient urban drainage systems, which causes damage to public and private properties and disrupts public life. This has initiated simulation for identifying problems related to the urban drainage system and management. In this study, the existing states of storm water drains of typical sub-catchments in Hossaina town were evaluated with the aid of storm water management model and rational method under different working conditions. The sub-catchments and drainage line were defined using percentage of imperviousness, elevation and building blocks. The maximum daily rainfall was disaggregated into hourly events using reduction formula. Thereafter, storm water management model was used to simulate the response of catchment to rainfall and compared with result from rational formula for validation. Model calibration and validation result shows that there was good agreement between observed and simulated runoff with Nash-Sutcliffe efficiency and Root Mean Square Error values 0.809 and 0.824 respectively. The simulation result shows that most of the rainfall in the study area is converted into a surface runoff. Consequently the capacities of the storm drains are insufficient for intense rainfall ended in flooding. The vulnerable places are identified and mitigation measures are suggested to reduce the flood problems. Two low impact development controls were applied to assess the model performance at a catchment scale. For smaller rainfall amounts of up to 70 mm, the model performed well, and the peak runoff reduction was 34.25% while for rainfall measures in the range of (70-95) mm, the model performance was reasonably good, and the reduction of peak runoff was 24.8%. The model is more powerful in peak runoff reduction during the shorter rainfall events and is not as satisfactory as expected for longer events more than 95mm rainfall. Therefore, it would be more efficient to combine the low impact development techniques with other conventional stormwater management practices to control urban flooding in case of high intensity storm events.