Performance Assessment of Storm Water Drainage System The case of Gelgel Beles Town, Ethiopia

Abstract

Urbanization of an area invariably leads to increase in overall imperviousness of the area. Storm water drainage problem is one of the major challenges facing many countries in Ethiopia in general and Gelgel Beles town in particular. The objective of this study is to evaluate the current storm water drainage systems and associated problems. This research involves collection of both primary data collected from interview field surveying, while the secondary data for the study has been obtained from gelgel beles municipality office, gilgel beles town finance office, minster of metrological agency and from responsible organizations for further interpretation and analyses. The land use land cover image was processed for the years and 2020 by using Arc GIS.10.1 and Google earth pro then for each land use/land cover, annual storm water runoff volume was calculated. The Intensity Duration Frequency (IDF) curve was developed by Gumbel method to analyze rainfall and the consequent peak runoff for different return periods using a rational method. The existing storm drainage systems of Gelgel Beles town were: lack of well-connected drainage lines, solid and liquid wastes were directly disposed into the storm drainage system, which results in decreasing the efficiency of the system, unavailability of drainage systems at the proper place. Additionally, the awareness of the community towards such an issue was absent; the major percentage of drainage system performs severely degraded. Most of survey results obtained in three kebele can shows the majority of the drainage systems were inadequate to carry the design flood based on intensity with a return period it designed. The other challenges of storm water management were related to lack of city-wide drainage master plan, absence of hydrologic data considerations during designing drains, and weak enforcement on solid waste dumping into drains system. The Rainfall depth was calculated by different probability distribution methods and the value is 123.68mm, 133.64mm, 125.26mm, 151.21mm for Normal, Log Normal, Person Type III, Gumble (EVI) respectively and by ERA it was 102.45 mm for 100-year return period. The value for annual runoff volume in the study area was 10089.09m 3 , 2321.245m 3 , 2263.234m 3 , and 557.7444m 3 for Built-up area, Grass cover, Vegetation cover and bare land respectively. Finally, before the urban drainage becomes a permanent socio-economic nuisance and brings irreversible damage to the city; this study strongly recommends immediate implementation of best management practice that is supported by strong institutional setup, policy framework, and the public at large