Assessment Of Urban Storm Water Drainage :( A Case Study Of Mizan- Aman Town, Southern Ethiopia)

Abstract

One of the many complex problems resulting from increased urbanization is related to management of storm water from urban areas. Mizan Teferi town is surrounded by high forest which receives highest annual rainfall in every year. Due to this, severe floods occur frequently in every rainy season in some parts of the town. But the Existing drainage system is inadequate and it is not properly managed to carry the surface runoff. Therefore, this study focuses on assessment of urban storm water drainage conditions of Mizan Teferi town and then to propose the appropriate drain size with effective drainage capacity for the study area. Daily precipitation data, Land use land cover (LU/LC) and topographic data were the input data used for this study. The daily precipitation data from year 1995 to 2017 was obtained from the National Metrological Agency of Ethiopia.LU/LC and topographic data were obtained from Mizan Teferi town municipality service offices. Field observation was under taken in order to identify the existing drainage system problems of the town. The rational formula was used to estimate the peak discharge of the catchment which requires runoff coefficient, catchment area and rainfall intensity as an input data. The study area was delineated in to 10 sub-catchments using Arc GIS 10.1. The existing drainage outlets of the town were followed to delineate the catchments. The runoff coefficient for each sub catchment was computed form the LU/LC of each plot of land. The Intensity Duration Frequency (IDF) curve was developed for 2, 5,10,25,50 and 100 years of return period. The visual information indicates that the observed major problems of the drainage system are Overflow from the channel, clogging of the channel by sediment, grasses and domestic solid waste. The performance value indicates that only 18.75% of the existing channels are ranked under very god condition. The capacity of the existing channels was computed using the Manning roughness formula and it was compared with the 25 years return period peak discharge. The result shows that only 30% of the existing channels can accommodate the 25 years return period.70% of the channels dimensions need modification to accommodate the 25 years return period peak discharge. Based on this, appropriate channel dimension has been designed based on the 25 years return period peak discharge for each catchment. The result indicates that, the existing peak discharge has occurred at catchment 4 which was 13.17 m3 /s whereas the 25 years return period of catchment 4 will be 20.339 m3 /s which requires to modify the channel depth, bottom width, and side slope. The finding can be used as an input data for Mizan Teferi town municipality for decision regarding modification and management of the existing drainage system. Finally, it is recommended to continuously monitor the existing or the modified drainage systems of the town.