Cyclone-induced storm surge flooding in the Ganges-Brahmaputra-Meghna delta under different mean-sea level rise scenarios

Abstract Bangladesh coast located in the lower part of the Ganges-Brahmaputra-Meghna delta (GBMD) regularly faces different flooding events from tidal flooding, pluvial flooding, and storm surge flooding. Cyclone-induced storm surge flooding is the most devastating flood event among all the flooding events in the GBMD in terms of human death toll and economic loss. Moreover, different climate change scenario studies suggest that the Bangladesh coast will face adverse impact from mean sea level (MSL) rise. In present study, an online wave-current coupled numerical model (Deflt3d and SWAN) is applied in the GBMD to assess cyclone-induced flooding under different mean seal rises including 0.5 m, 1 m and 1.5 m. The model setup is calibrated and validated for Cyclone Sidr. Results show that a 1.5 m increase in MSL will cause flooding of up to 33% of the total coastal area of Bangladesh. Current cyclones (e.g., Cyclone Sidr) will cause more flooding in the coast in the future because of MSL rise alone. Ignoring the earthen embankments in the GBMD in numerical model may results error up to 1.25 m in the storm surge height estimation. Sensitivity tests of different cyclonic parameters such as radius of maximum wind speed, maximum wind speed and landfall location in terms of cyclone-induced storm surge event are conducted by applying the model for different idealized scenarios. An increase in the cyclone maximum wind speed is more likely to increase flooded area rather than increase storm surge height at river stations in the GBMD because the protective embankments are overtopped releasing the waters into the floodplain.