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

Richard Hale,Jim Best,Rachel Bain,Steven Goodbred Jr.

doi:10.5194/esurf-7-231-2019

Richard Hale, Jim Best + Show 2 more

Open Access

https://doi.org/10.5194/esurf-7-231-2019

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Abstract

Abstract. The landscape of southwest Bangladesh, a region constructed primarily by fluvial processes associated with the Ganges River and Brahmaputra River, is now maintained almost exclusively by tidal processes as the fluvial system has migrated east and eliminated the most direct fluvial input. In natural areas such as the Sundarbans National Forest, year-round inundation during spring high tides delivers sufficient sediment that enables 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 timescales 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 comparatively modest seasonal variability in the magnitude of water discharge. These observations emphasize the importance of seasonal sediment delivery from the main-stem rivers to this remote tidal region. On tidal timescales, 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 as 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 and Brahmaputra rivers. These observations provide context for examining the relative importance of fluvial and tidal processes in what has been defined as a quintessentially tidally influenced delta in the classification scheme of Galloway (1975). 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 responses of the natural system to ongoing environmental change.

Highlights

At our long-term station deployed in a secondary tidal channel (Fig. 1), recorded water-level variations show tidal-period excursions with a range of 1.8 to 4.8 m over the 12 months of observation (Fig. 3)
We have measured tidal and seasonal variability associated with water discharge and suspended sediment concentration (SSC), and we used these observations to compute the magnitude of water and sediment exchange through a single tidal channel
As has been suggested previously, the wet season is found to exert a strong control on the timing and magnitude of sediment transport in this system, despite seemingly modest changes to the hydrodynamics

Summary

Introduction

The world’s great deltas are currently threatened by a variety of factors, including global sea-level rise (Overeem and Syvitski, 2009), overpopulation (Ericson et al, 2006), changes in sediment supply (Syvitski, 2003; Syvitski and Milliman, 2007; Anthony et al, 2015; Darby et al, 2016; Best, 2019), and other human-related activities such as water diversions, flood control structures, and groundwater and hydrocarbon extraction (Syvitski et al, 2009). Restoration of land-surface elevation in many populated areas in the GBM delta is already necessary due to the relative loss in elevation that has occurred since the widespread construction of embankments during the 1960s to 1980s Both planned (tidal river management) and unplanned (embankment failures) flooding of local polders (the embanked islands) has demonstrated the capacity of the natural system for effective sediment transport and deposition, with decimeters of annual accretion observed during recent breach events (Khadim et al, 2013; Auerbach et al, 2015; Kamal et al, 2018; Darby et al, 2018). Protecting the Sundarbans National Forest is of critical importance, as coastal wetlands and mangroves provide irreplaceable ecosystem services including storm-surge buffering (Uddin et al, 2013; Marois and Mitsch, 2015; Hossain et al, 2016; Sakib et al, 2015), serving as effective carbon traps (Mcleod et al, 2011; Alongi, 2014; Pendleton et al, 2012), and perhaps even helping to combat the impacts of ocean acidification (Yan, 2016)

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