Flood Prediction and Modeling: A Case Study of Jimma Town

Abstract

Nowadays, Floods are the most critical among all natural calamities to many parts of Ethiopia that causes tremendous losses in terms of property and life, particularly in the lowland areas. Jimma town, around bishishe because of heavy rainfall flash floods was occurred it damages homes, and market places with their property. Hence, the core objective of this study was to model and predict flood with respect to rainfall and to find depth and extent flood inundation. Modeling was done by integrating Arc view GIS, Hydrologic Engineering Center-River Analysis System, and related software’s. The data used to accomplish those task were secondary data such as (daily stream flow and Rainfall of 15 years), (Soil map and Landuse land cover) was collected from Ministry of Water Resource, irrigation and Electricity, National Meteorological Service Agency, and Ethiopia Mapping Agency respectively. Normal Ratio Method was used for filling missing value of precipitation and using double mass curve consistency of rainfall was checked. Other parameter like Curve Number and basin lag time were generated using Hydrologic Engineering Centers Geo-Spatial Hydrological Modeling System which is an interface between ArcGIS and HEC-HMS. Soil Conservation service curve number, Soil Conservation Service Unit-hydrograph, Monthly constant, and Muskingum were chosen for precipitation loss modeling ,excess precipitation transformation to direct runoff, base flow modeling, and flood routing respectively. Among collected 15 years hydro-meteorological data, 8 events (1995-2002) were used for model calibration and 7 events (2003-2010) were used for model validation. The model performance was evaluated using Nash Sutcliffe Efficiency (NSE) and Coefficient of Determination (R2 ).Nash Sutcliffe during calibration and validation was 0.75 and 0.7 respectively where as Coefficient of determination during these two processes are 0.94 and 0.89 respectively. Flood frequency analysis was conducted using HEC-HMS frequency storm method for2,5,10,25,50,and 100 year return period and corresponding results are 19.6m3 /s,28.8m3 /s,35.4m3 /s,44.3m3 /s, 51.2m3 /s, and 58.5m3 /s for respective return period. Flood inundation mapping was modeled for peak discharge of each return period using HEC-RA S and inundation area 71.46ha, 71.66ha, 71.92ha, 90.46ha, 90.68ha, and 90.73ha respectively. Based on the result downstream dwellers of the stream were found to be vulnerable area.