Flood hazard assessment for the coastal urban floodplain using 1D/2D coupled hydrodynamic model

In the current study, the one-dimensional/two-dimensional (1D/2D) coupled hydrodynamic model is used for the development of flood hazard maps for the frequently flooded coastal urban floodplain of the Surat city, India. The releases from the Ukai dam and tidal levels at the Arabian Sea are considered as upstream and downstream boundary conditions, respectively. The floodplain roughness was estimated using the existing land use land cover (LULC) classification, and the performance of the developed coupled hydrodynamic model was evaluated against the past flood data of year 2006 and 2013. The flood frequency analysis was carried out for peak inflow into the Ukai reservoir, and subsequently, the design flood hydrographs for different return periods have been developed. Finally, the simulated model output has been used to develop multi-parameter flood hazard maps defining the stability of people, vehicles, and buildings. More than 80% of the entire coastal urban floodplain of the Surat city is submerged during 100-year return period flood, with West and North zone of the city being the worst affected regions. Out of the total flooded area, nearly 20% area is under significant hazard for adults. The 27% area offers instability hazard to large four-wheel drive vehicles, whereas 14% area is affected with moderate to high hazard for buildings. The instability index for specific vehicle types is dominated by floating of small and large cars over 90% of the flooded area. Further, the combined hazard maps revealed that 14% of the flooded area is under very severe hazard category, posing a threat to the stability of people, vehicles, and buildings. The developed hazard maps will work as an effective non-structural measure for local administrative agencies to minimize the losses and better future planning.