DAM BREAK MODELING BY USING HEC-HMS AND HEC-RAS: THE CASE OF MINDU DAM TANZANIA

Abstract

Floods resulting from dam failure have extremely devastating and irreversible environmental, social, and economic implications. Modeling a dam break is crucial in assessing downstream propagation conditions and flood risks and devising contingency strategies to mitigate the consequences of dam failure. In this study, the Hydrologic Engineering Center’s Hydrologic Modeling System (HEC-HMS) and the Hydrologic Engineering Center’s River Analysis System (HEC-RAS) were used to simulate the hypothetical dam failure under overtopping and piping scenarios for the Mindu Dam in Tanzania. It was necessary to carry out a rainfall-runoff relationship analysis using the Hydrologic Engineering Center’s Hydrologic Modeling System (HEC-HMS) and estimate the inflow design by selecting design storms of various return periods. The maximum 24- hour design storm with a 10,000-year return analyzed from rainfall data observed between 1990 and 2019 was used to generate a hydrograph in HEC-HMS using the SCS Curve Number method. The hydrological model in both calibration and validation, showed satisfactory results with NSE values of 0.692 and 0.603 respectively while the coefficient of determination R2 yielded values of 0.73 and 0.69. Breach parameters estimation was carried out using the Froehlich (2008) regression equation and hydrodynamic modeling was performed in HEC-RAS 2D to generate flood inundation maps. During our analysis, the peak discharge of 2319.3m3/s and 2225.12m3/s were observed in both overtopping and piping failure respectively. Flood inundations are predicted to threaten transport due to inundation of the highways connecting Morogoro, Dodoma, and Iringa regions including the houses near the Ngerengere River. Flood waves will take less than two hours to reach the Mafiga Water Treatment Plant which supplies 65% population with water in Morogoro town, and six hours to reach 20km downstream which is the ending section under investigation. Inundation depths of up to 6.8 meters are expected in the Uwanja wa Ndege and Uwanja wa Taifa wards, particularly in places near the river channel. The results obtained under this study provide a road map for comprehensive flood risk assessment and the creation of an emergency action plan for the safeguarding community and their properties in case of floods resulting from a dam break.