MODELLING URBAN STORM WATER FLOODING IN HOSSANA TOWN: THE CASE OF ADDIS SUBCITY

Abstract

Urban flooding is a condition where storm water cannot enter a storm drain or it is beyond the capacity of the urban drainage system. Such urban flooding is mainly caused by inefficient urban drainage systems, and it cause damage to public and private buildings and disrupts public life. Therefore, modeling urban floods were very essential to mitigate flooding related problems. The main objective of this research was modeling urban storm water flooding in Hossana town specifically in Addis sub-city. There were many tools for urban storm flood modeling, but out of limited free tools, Storm water management model (SWMM) has a good record of applicability in many locations elsewhere. Various rainfall and spatial data were collected to model urban floods. Daily maximum rainfall data from Hossana station for 28 years (1990-2017) was used to identify parent probability distribution and enabled to develop the Intensity duration frequency curve (IDF) for study area. SWMM was implemented to simulate the flood depth in the drainage channels and Arc GIS is used to delineate the extent of the flood contributing area. The hydrology and hydraulic routing of the watershed was estimated using the SWMM at the total of 1-hour time of concentration daily time series data of different year return periods. The inputs of the models (nodes, conduits and sub- catchment area property) were assigned with the help of Auto-CAD, Arc-GIS 10, Google earth pro, and field survey. The result obtained from the model showed that (39%), (41%) drainage manholes encountered flooding for 10 and 25 years return periods respectively. Hence, approximately (40%) of the drainage system of the city previously designed was inadequate to discharge the system runoff to final point. Discharge of the catchment was also determined by using Soil conservation service (SCS) curve method and rational method that was used to relate and check the SWMM model output value. In general, the results of this study will be useful for further design and planning of the sub cities in the Hossana town. Moreover, further research on urban flooding which accounts two-dimensional flow is recommended to account the flood extent and risks associated with it.