Determining the Optimum Map Scale for use in Hydraulic Modeling of Flood (Economic-Hydraulic Approach)

One the most important methods of presenting the river geometry is the using of Digital elevation models (DEMs). Among the several factors that affect the quality of a terrain model, DEM resolution and the scale of topographic maps are the most important ones. This research addressed the sensitivity of terrain model and 1D hydraulic parameters of HEC-RAS model to different map scales in two different rivers. Besides, investigating the role of DEM resolution on the quality of cross-sections and hydraulic outputs are another aspects of this study. Results show that using 1:2000 and 1:5000 maps instead of 1:1000 and 1:2000 maps in SojasRood and Sarbaz rivers, respectively lead to relative error lower than 8 % in simulating inundated extents, while the reduction of land surveying cost in both rivers is more than 77%. Also, evaluating the effect of DEM resolution on flood’s hydraulic properties indicates that when DEM resolution getting coarser the water surface elevation (WSE) and inundated extents will be increased in both rivers. However, it should be noted that the effect of DEM resolution on inundated extents is more considerable than WSE. In other words, if the only objective of hydraulic simulation is limited to estimation of WSE, DEM resolution plays a minor role and therefore low resolution DEMs can be used.