Influence of the concept of subgrid variability and computational mesh dimensions on the performance of HEC-RAS 2D model in simulating river floodplains (Case study: Sarbaz River)

The 2D HEC-RAS model uses a new concept called subgrid-variability to include elevation changes within a computational cell and thus has very little dependence on computational cell dimensions. The purpose of this study was to evaluate the effect of the concept of subgrid-variability on the simulated flood area in the Sarbaz River. The other purposes of this study is to investigate the effect of map scale on the performance of 1D and 2D HEC-RAS model. The results showed that with increasing computational cell dimensions, the model error in simulating flood zones was relatively low compared to small cell dimensions. For example, if using a computing grid with dimensions of 500 meters instead of a 20 meters grid, the simulated flood area in different parts of the Sarbaz River would be less than 15 percent. The run time of the 2D model in the cell dimension of 500 meters is approximately 45 times less than the run time of the model in case of using cell dimensions of 20 meters. The findings clearly indicate to what extent the use of the concept of subgrid variability in modeling can be effective when implementing numerical models, especially in large and complex rivers. The calculations also show that the difference between 1D and 2D models in the simulation of hydraulic parameters in small scales is relatively small and by increasing the map scale, the difference between the two models increases.