A comparison of the planforms of meandering tidal and fluvial channels on the Ganges-Brahmaputra-Jamuna Delta, Bangladesh

Abstract

The planform geometry of tidal channels is controlled by bidirectional flow, a fundamentally different process from that controlling the shape of meandering river channels. However, prior studies do not reach any consensus on how channel planforms differ in these two environments. We use Google Earth imagery to digitize 130 meandering channels on the Ganges-Brahmaputra-Jamuna Delta in Bangladesh, from which we produce a data set of meander and channel width statistics for tidal and fluvial channels. Our data indicate that the tidal meanders are more symmetrical than the river meanders, which are both downstream and upstream skewed. The fluvial meanders are also more sinuous than the tidal meanders. We observe some evidence of longer intrinsic and Cartesian meander wavelengths in the tidal environment, although this could be a consequence of anthropogenic modifications to the fluvial channels. Despite these differences, we find that the relative strength of the first through third curvature harmonics is the same for the tidal and fluvial meanders. Our data set also provides insight into the funnel-shaped geometry of tidal channel mouths; we find that there is a positive correlation between channel mouth width and e-folding length. For those channels with a fluvial connection upstream, there is also a positive correlation between mouth width and upstream width. We propose several causes for the observed trends in our data, but future study will be necessary to test these hypotheses.