DAM BREACH ANALYSIS AND DOWNSTREAM FLOOD MAPPING USING HEC-RAS AND HEC-FIA (A CASE STUDY OF GERHU-SIRNAY DAM)

Abstract

Dam breach analysis is a science that quantifies the hazard sourced from dam failures. On downstream of Gerhu-sirnay dam there are developments including water treatment plant and multi owner seasonal agricultural areas. The objective of this study was to estimate breach outflow hydrograph to the downstream reach for overtopping and piping scenarios henceforward to assess flood damage for the worst scenario. The quality of rainfall data was tested for outlier, adequacy, homogeneity using Pettitt (1979) test and consistency tests. The probable maximum precipitation calculated by Hershfield (1965) method was 129.7 mm. The inflow hydrograph was obtained by using soil conservation service (SCS) method and its probable maximum flood (PMF) reaches 329.1 m 3 /s. The breach parameters for both scenarios were early calculated by the four regression equations integrated with Hydrologic Engineering Center River Analysis System (HEC-RAS) breach calculator tab. Due to comparison in both scenarios, the final breach parameters were taken from Von Thun and Gillette (1990). For overtopping the resulting parameters were 54 m breach bottom width, 85.75 m breach top width, 0.5 side slope (H: V) and 0.73 hrs breach formation time whereas in case of piping these parameters were 32 m, 57.75 m, 0.5 and 0.53 hrs respectively. Then HEC-RAS version 5.0.3 has used to model the dam breach analysis with two dimensional unsteady flow conditions of inflow hydrograph as upstream boundary condition and normal depth as downstream boundary condition. The peak breach outflow for both scenarios were 2,239.7 and 1,282.68 m 3 /s respectively as overtopping was the worst scenario. From the prepared inundation maps by GIS for about 521.1 ha area was inundated under the worst scenario with a maximum depth 24.07 m and duration of 1.0 hrs. By using Hydrologic Engineering Center Flood Impact Analysis (HEC-FIA) version 2.2, the flood damage was estimated in terms of direct economic damage and life loss. The total ex-ant direct economic damage result shows 63,650,493.3 Birr. From the report of LIFE-Sim dynamic model integrated with HEC-FIA, about four persons could be lost their life. To minimize the quantified damage, well operation of emergency action plan would be an important tool. The water treatment plant of Gerhu-sirnay town must be displaced from its current place to at least 0.073 km apart from both sides of the river banks until to do this the stakeholders of the plant must be under the age of 65 years to have an efficient warning mobilization