ROLES OF DEM ON 1D HYDRAULIC MODEL IN SIMULATING FLOOD INUNDATION MAPPING: THE CASE OF BLATE RIVER ETHIOPIA

Abstract

Flood is one of the natural calamities that cause huge losses and damages. Flood hazard zonation has been widely produced to face the impact of the disaster. Mapping flood hazard is an effective tool in determining the extent and depth of floods associated with hazard level in specified areas that need to be prioritized during flood occurrences. Topography plays an important role in the accuracy of flood inundation maps. Topographic data, such as digital elevation models (DEMs), are essential input in flood inundation modeling. The objective of this study is to determine the response of hydraulic model with different DEMs to generate the river geometry data in hydraulic simulations and It’s role on the generating flood inundation mapping. The outcomes of our study show that the use of different DEMs and how the predicted extents produced by flood models can be affected by the DEM has serious implications to the results of hydraulic models. In the present study, the performance of 4 DEMS having different sources and resolution (ALOS, 30m), (NASDEM, 30m) and (SRTM with 30m and 90 m resolution) evaluated with the aid of flood modeling software hydrologic engineering Centre-river analysis system. Peak discharge was assigned to calculate among the 17-years of series data, Flood inundation mapping begins with the calculation of the statistical parameters required for a proposed probability distribution by the method of moments from the given data. Based on the result of the distribution of the model estimated peak flow using General extreme value distribution with different return period’s interval 5, 10, 25, 50, 100, and 200 with the result of 129.9124 m3s-1, 146.2383 m3s-1, 156.2048 m3s-1, 159.5893m3s-1, 161.308 m3s-1 and, 162.1806 m3s-1,respectively. The terrain model was constructed in ArcView GIS as a triangulated irregular network (TIN), which served as the base for all later modeling. The hydraulic modeling was done as one-dimensional unsteady flow in HEC-RAS flow routing software. The results of hydrodynamic modeling done in HEC-RAS were again exported to HEC-GeoRAS for simulating and mapping flood inundation area in GIS environment. Flood inundation modeling has different DEM Sources and resolution for 100 return periods and different characteristics of maps with Blate River for return period’s analysis for RAS mapper flood depth. The results show that the hydraulic modeling outputs obtained from coarser resolution DEM using this approach are comparable to the results obtained using high resolution accurate DEMs. High-resolution elevation data resulted in better inundation delineation than did low resolution elevation data