Two Dimensional Flood Inundation Mapping Under Overtopping Failure of Umrar Dam—Effect of Terrain Resolution and Sensitivity Analysis

In this study, a hypothetical dam break analysis is carried out at Umrar Dam, Madhya Pradesh, India to map the extent of flood and sensitivity analysis is conducted to find out the impact of dam break parameters on water depth and water velocity at four locations on the downstream side of the dam. An overtopping failure mode is selected based on the type of dam and historical failures of the dam. Different regression equations are used to estimate dam break parameters. The Hydrological Engineering Centre's River Analysis System 2D (HEC-RAS 2D) model was used to model and analyze the dam break flood based on the parameters of the dam break. To determine the effect of terrain resolution on the extent of flood inundation, the differential global positioning system (DGPS) was used to generate the following three different resolution terrains: SRTM-30 m, ALOS PALSAR 12.5 m, and 8 m resolution DEM. The terrain with high resolution (8 m) gave the maximum inundation area of 4.383 km2 which is 13.84% higher when compared with the low-resolution DEM of 30 m cell size. The effect of this dam failure is very high inundating 9 villages on the downstream fully or partially within 1.5 h of dam break failure with a maximum inundation depth of 16 m. The sensitivity analysis conducted by varying the dam break parameters (dam break bottom width, side slopes, break formation time) by ± 25%, + 50%, + 75% showed that their impact on water depth and water velocity is minimal on downstream side and limiting to only immediate downstream area, concluding that the dam break parameters estimated using the regression equations developed by Froehlich DC (2008) are optimum. However, the results at 0.4 km downstream of dam site shows that the water depth and water velocity values increase with the dam break bottom width.