ESTIMATION OF FLOOD, USING HYDROLOGIC MODELING SYSTEM (HEC-HMS), BILATE RIVER WATERSHED, ETHIOPIA

Abstract

In Ethiopia, particularly in the study area, flooding have occurred frequently and lead to displacement of people from their place, and properties have been damaged. The general objective of this study is estimation of flood using Hydrologic Modeling System (HEC-HMS) for Bilate river watershed, Ethiopia. Understanding the complex relationships between rainfall and runoff processes is necessary for the proper estimation of the quantity of flood generated in a watershed. The input data used were the spatial data, meteorological data and hydrological data. The Bilate watershed initially delineated as 150 sub basins and then merged into six sub basins using the combination of Arc hydro tool version 10.4 and HEC-GeoHMS extension of ArcGIS 10.4. The model calibrated and validated using daily rainfall and stream flow recorded data for 14 years (1996 to 2009) and 7 years (2010 to 2016) for calibration and validation respectively at Bilate Tena gauging station, at the outlet of the watershed. Curve number were more sensitive parameter in the Bilate watershed. The Nash-Sutcliffe Efficiency, Coefficient of determination and percent bias was used to check performance of the model. During calibration the model performance resulted in Coefficient of determination= 0.96, Nash-Sutcliffe Efficiency = 0.855, Root Mean Squared error=6.3, Mean Absolute error =10.3 Also, during validation Coefficient of determination =0.97, Nash- Sutcliffe Efficiency =0.947 Root Mean Squared, error =4 and Mean, Absolute error =10.3. These values showed that the model performed well during calibration and validations. Hydrologic modeling system was used for determining the peak flow discharge for return periods of 5, 10, 25, 50 and 100 years and the result was found to be 391.7m3/s, 698.1m3/s, 1359.1m3/s, 1685m3/s and 1912m3/s respectively in HEC-HMS. Using flood frequency analysis, the peak flow discharge of 126.43m3/s using Eve Type-I flood frequency analysis for 5 and 10, 25, 50 and 100 years return periods,187.92m3/s, 217.93m3/s, 239.59m3/s and 262.53m3/s using Eve Type-I flood frequency analysis respectively and the highest peak flow recorded was 262.53m3/s. Finally, to select mitigation measure, it recommended undertaking further flood studies instead of the traditional way of design to protect the area from flooding