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Observations and scaling of tidal mass transport across the lower Ganges-Brahmaputra delta plain: implications for delta management and sustainability

Preprint published in 2018 by Richard Hale, Rachel Bain, Steven Goodbred, Jim Best
This paper is available in a repository.
This paper is available in a repository.

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Preprint: policy unknown
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Postprint: policy unknown
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Published version: policy unknown


The landscape of southwest Bangladesh, a region constructed primarily by fluvial processes associated with the Ganges and Brahmaputra Rivers, is now maintained almost exclusively by tidal processes as the fluvial system has migrated to the east through the Holocene. In natural areas such as the Sundarbans National Forest, year-round spring-tide inundation delivers sufficient sediment for vertical accretion to keep pace with relative sea-level rise. However, recent human modification of the landscape in the form of embankment construction has terminated this pathway of sediment delivery for much of the region, resulting in a startling elevation imbalance, with inhabited areas often sitting > 1 m below mean high water. Restoring this landscape, or preventing land loss in the natural system, requires an understanding of how rates of water and sediment flux vary across time scales ranging from hours to months. In this study, we combine time-series observations of water level, salinity, and suspended sediment concentration, with ship-based measurements of large tidal channel hydrodynamics and sediment transport. To capture the greatest possible range of variability, cross-channel transects designed to encompass a 12.4-h tidal cycle were performed in both dry and wet seasons, during spring and neap tides. Regional suspended sediment concentration begins to increase in August, coincident with a decrease in local salinity, indicating the arrival of the sediment-laden, freshwater plume of the combined Ganges-Brahmaputra-Meghna rivers. We observe profound seasonality in sediment transport, despite somewhat modest seasonal variability in the magnitude of water discharge, indicating the importance of this seasonal sediment delivery. On tidal time-scales, spring tides transport an order of magnitude more sediment than neap tides in both the wet and dry seasons. In aggregate, sediment transport is flood-oriented, likely a result of tidal pumping. Finally, we note that rates of sediment and water discharge in the tidal channels are of the same scale as the annually averaged values for the Ganges or Brahmaputra rivers. These observations provide context for examining the relative importance of fluvial and tidal processes in what has been defined as the quintessential tidal delta in the classification scheme of Galloway. These data also inform critical questions regarding the timing and magnitude of sediment delivery to the region, which are especially important in predicting, and preparing for, future change under changing environmental conditions.

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