Tidal Hydrodynamics in the Interconnected Channel Network of the Southwestern Ganges-Brahmaputra-Meghna Delta, Bangladesh
Bain, Rachel Louise
We quantify discharge exchange and tidal waveform propagation in the interconnected Shibsa-Pussur tidal channel network on the southwestern Ganges-Brahmaputra-Meghna Delta, with the goal of characterizing the natural system’s response to anthropogenic landscape modification. GIS analysis indicates that the construction of earthen embankments or “polders” in the 1960s immediately eliminated 20.9 to 30.2x10^7 m^3 of intertidal storage volume from the combined Shibsa and Pussur basins (48 to 53% volume reduction). The predicted morphodynamic response to this perturbation is channel siltation and abandonment; however, the mainstem Shibsa has widened and scoured over the past half-century, which is consistent with an increase in discharge. Modern field measurements of the local tidal prism suggest that the Shibsa River has maintained or slightly increased its pre-polder flow volume by capturing a large region of the Pussur’s former tidal basin. To further enhance our understanding of tidal dynamics in interconnected, large-scale tidal delta networks, we evaluate tidal deformation and water surface gradients as two possible mechanisms for the basin-capture event identified in the first phase of the study. Although the Pussur River displays an enhanced flood asymmetry (rising limb shorter than falling limb) relative to the Shibsa, the majority of measurement sites have ebb-dominant velocities due to the looping character of the network and/or the monsoon-season influx of river discharge from upstream. The available data are thus insufficient to determine whether spatial variations in tidal asymmetry drove a net import and export of sediment along the mainstem Pussur and Shibsa Rivers, respectively, leading to basin capture. Scaling analysis of the momentum equation indicates that the momentum and friction terms have the same order of magnitude, allowing instantaneous flow velocities to oppose an adverse surface gradient for up to three hours in the study area. Additionally, the subtidal (time-averaged) flow velocity opposes the subtidal surface gradient in the upper Shibsa River during monsoon-season spring tides. Any relationship between flow velocity and surface gradient is therefore complicated by additional parameters which we have not yet identified.