Tidal characteristics affected by dynamic morphology change in the Meghna estuary

摘要

The Meghna Estuary of Bangladesh experiences dynamic morphological changes due to heavy sediment discharge from the Ganges, Brahmaputra and Upper Meghna Rivers with annual rate of nearly billion tons. While morphology change strongly depends on various physical factors such as: river discharge, wind, waves and tides, tide-induced circulation current dominantly determines the sedimentation process. On the other hand, the tidal current itself is also strongly affected by the dynamic morphology changes. It is thus essential to understand interactive features of tides and morphology change. The present paper focuses on the tidal characteristics of the Meghna Estuary. A 2D depth-average hydrodynamic model based on non-linear shallow water wave equations was applied to investigate the tidal characteristics. The tidal water level at the southern boundary of the computation domain was determined by the global tidal model (NAO.99b). The hydrodynamic model was first validated by the observed tidal water level data from the Meghna Estuary Study during 1997. Analyses of tidal water levels show that both at meso (2m~4m) and macro (>4m) tidal zones tidal asymmetry increases with proximity to the coast. Larger phase difference was observed in the north-south direction rather than the one in the east-west direction. The tide from the Indian Ocean travels faster with larger amplitude along the eastern coast. Also near the southern coast of Khulna and western coast of Chittagong and along the narrow tidal channels the estuary is ebb-dominated. Sensitivity analysis of the tidal characteristics to changing bathymetry was also investigated and it was found through the analysis that asymmetry, skewness and phase of the tide especially far inside the estuary was significantly affected by the morphology.