Abstract
Analysing and simulating dam breach events and the resulting floods is critical for differentiating and reducing impendences due to possibility failure of dams. Hydrological Engineering Centre (HEC-RAS) River Analysis System was used for developing dam failure model. HEC-GeoRAS was used to extract geometric information from a Digital Elevation Model (DEM) and then imported into HEC-RAS version (5.0.7).
The process of dam breach modelling is to analyse the downstream flooding risks of the dam by the simulating of possible failure of dam scenarios. The prediction of geometry for dams breach is important in studies of dam breach. Flood hydrograph characteristics that result from dam breach depend on breach geometry and time required for the breach forming. For investigating the effect of breach parameters on breach maximum outflows, five empirical equations are used in order to predict dam breach parameters by using HEC-RAS model. Hamrin dam was selected as a case study in this research. The dam has been checked for overtopping and piping failure modes by the use of two dimensional river analysis model HEC-RAS and for use in the model. The unsteady flow calculations performed in HEC-RAS.
For Hamrin dam, we used five empirical equations for predicting breach geometry and breach formation time. The equations used are Singh and Snorrason (1982), MacDonald and Langridge-Monopolis (1984), Bureau of Reclamation (USBR) (1988), Von Thun and Gillette (1990) and Froehlich (2008). Three water levels for each method were considered. Peak discharge value and time of peak discharge were analysed and discussed for each scenario. The results showed that Froehlich approach was most adequate empirical equation to estimate the breach parameters for Hamrin dam. Overtopping failure mode tends to provide higher peak discharge (61909.64 
) comparison with piping failure mode (52532.33 ) for Froehlich approach.
Sensitivity analysis was performed in order to assess the influence of each parameter on resulting flood hydrograph. HEC-RAS model was used to calculate the influence of each parameter on flood hydrograph that resulted. The breach width ( 
), side slope (z) and breach formation time ( 
) increased respectively by 25%, 50%, 75% and 100% and decreased respectively by 25%, 50% and 75%. Sensitivity analysis was performed with Froehlich approach with the overtopping failure mode and maximum operating level at 107.5 meter above sea level. Flood hydrograph was estimated at the dam site for each case. Result of sensitivity analysis show that peak discharge and time to reach it is adequately sensitive to breach side slope, it has high sensitivity to breach formation time and less sensitivity to breach width.
