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Earth Surface Dynamics An interactive open-access journal of the European Geosciences Union
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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 19 Sep 2018

Research article | 19 Sep 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Earth Surface Dynamics (ESurf).

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

Richard Hale1, Rachel Bain2, Steven Goodbred Jr.2, and Jim Best3 Richard Hale et al.
  • 1Dept. of Ocean, Earth, and Atmos. Sci., Old Dominion University, Norfolk, VA, USA
  • 2Earth and Environmental Sciences Dept., Vanderbilt University, Nashville, TN USA
  • 3Departments of Geology, Geography & GIS, Mechanical Science and Engineering and Ven Te Chow Hydrosystems Laboratory, University of Illinois, Champagne, IL USA

Abstract. 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 >1m 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.

Richard Hale et al.
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Status: final response (author comments only)
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Richard Hale et al.
Richard Hale et al.
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Short summary
In Bangladesh, we measure how water and sediment move through tidal channels in an area heavily impacted by both sea level rise and the strategies that humans use to adapt. We found that the amount of water moving through the system is largely controlled by the strength of the tides, while the mass of sediment being transported is strongly connected to the season, and to a lesser extent the tides. These results are important for developing strategies to protect both human and natural landscapes.
In Bangladesh, we measure how water and sediment move through tidal channels in an area heavily...