Impact Of Dem Grid Size On Estimation Of Flood Depth And Flood Level Using 1d-2d Coupled Flow Model

Abstract

A hydrodynamic coupled 1-D and 2-D flow model has been developed in a rural terrain of northern India where an industrial plant has been proposed. The source of flooding in the area includes breach in canal in addition to local site rainfall for which flood simulation has been carried out. The design value of the local site rainfall is computed based on PMP value obtained from IMD and frequency analysis of daily rainfall of 6 raingauges in the area. The flow in canal and breach section has been simulated in 1-D flow model, MIKE 11 while the flow in floodplain and local site rainfall has been simulated in 2-D model, MIKE 21 and both are combined through MIKE FLOOD. Three cases of flood simulation has been carried out representing the floodplain by three different DEMs namely; DEM1, DEM2 and DEM3 of grid size of 180 m, 180 m and 10 m respectively. In DEM1, the floodplain is represented by 180 m grid size using the topographical information available in Survey of India (SOI) toposheet at 1: 50,000 scale. The detailed topographical maps (with CI <=1 m)/ surveys are available for selected areas only, mainly for the project sites. For the study area the topographical survey has been carried out at 2m CI and DEM at 10 m regular grid (DEM3) has been generated from it. The DEM2 is generated by resampling the DEM3 at 180 m grid size to save computational time and computing resources. Hence, three cases of floodplain representation have been developed and the flood inundation depth and flood level due to rainfall estimates of 100 yr return period have been computed. The results are analyzed to study the effect of DEM grid size over the flood parameter. It is found that the inundation depth is increased while the flood level is decreased for finer DEM. This is due to better representations of ponds/ terrain irregularities in finer DEM. The presence of depressions in the area produces more flooding depth while it also provides larger storage causing the overall decrease in flood level. The impact of DEM grid size on computing resources has also been analysed.