Flow Reorganization in an Anthropogenically Modified Tidal Channel Network: An Example From the Southwestern Ganges-Brahmaputra-Meghna Delta

We examine variations in discharge exchange between two parallel, 1- to 2-km-wide tidal channels (the Shibsa and the Pussur) in southwestern Bangladesh over spring-neap, and historical timescales. Our objective is to evaluate how large-scale, interconnected tidal channel networks respond to anthropogenic perturbation. The study area spans the boundary between the pristine Sundarbans Reserved Forest, where regular inundation of the intertidal platform maintains the fluvially abandoned delta plain, and the anthropogenically modified region to the north, where earthen embankments sequester large areas of formerly intertidal landscape. Estimates of tidal response to the embankment-driven reduction in basin volume, and hence tidal prism, predict a corresponding decrease in size of the mainstem Shibsa channel, yet the Shibsa is widening and locally scouring even as the interconnected Pussur channel faces rapid shoaling. Rather, the Shibsa has maintained or even increased its pre-polder tidal prism by capturing a large portion of the Pussur's basin via several "transverse" channels that are themselves widening and deepening. We propose that an enhanced tidal setup in the Pussur and the elimination of an effective Shibsa-Pussur flow barrier are driving this basin capture event. These results illustrate previously unrecognized channel interactions and emphasize the importance of flow reorganization in response to perturbations of interconnected, multichannel tidal networks that characterize several large tidal delta plains worldwide.