River-tide interaction and cyclone-induced storm surge in the Ganges-Brahmaputra-Meghna delta

The estuarine circulation is produced by the combination of tide, wave, wind, and river discharge. During a cyclone, the strong wind along with tide-surge interaction and wave-current interactions produces a cyclone-induced storm surge event that may cause damage to life and property. Hence, the understanding of river-tide interactions and other physical processes related to the generation of storm surge height is vital for coastal zone disaster management. The Ganges-Brahmaputra-Meghna delta (GBMD), in the Bay of Bengal, is a perfect laboratory to study river-tide interaction as well as cyclone-induced storm surge events due to its hydrodynamic and geographical features. A barotropic model (Delft3D) is established for the GBMD that can reproduce the varying river discharge influence on the tides. Model results show that the tide can propagate up to 205 km inland from the estuary mouth in low river discharge periods. The balance between tidal dissipation and generation depends on the residual velocity generated by the river discharge and the velocity of the principal tides. For the first time, a two-fold influence of river discharge on tides is reported in an estuarine system. Critical river discharge thresholds produce optimal dissipation of semidiurnal tides and generation of quarterdiurnal tides through friction at the upper and middle estuary. River discharge above the critical river discharge amount dissipates both semidiurnal and quarterdiurnal tides more rapidly than it generates quarterdiurnal tides from nonlinear interactions. Next, the model is converted into a wave-coupled model to investigate wave-current interactions (WCI) during the Cyclone Sidr-induced storm surge event. Results show that the wave-current interaction can increase significant wave height by 1 m (59%) at the cyclone landfall location although the WCI has varying influence depending on the amplitudes and directions of the waves and currents. Wave energy dissipation is one of the key factors affecting the wave height variations and white capping dissipation dominates the wave dissipation processes. Finally, the model is used to study inundation scenarios using different mean sea level rise scenarios under cyclonic conditions. Results demonstrate the importance of considering the floodplain area in numerical modelling studies to estimate storm surge height during a cyclonic event