The Stable Equilibrium of the Ganges-Brahmaputra-Meghna Delta: Balancing Elevation with Changes in Sea Level, Land Subsidence, Tides, and Fluvial Sediment Supply

Sundarbans

Abstract:

Relative sea-level rise, RSLR (i.e., the combined effect of increasing sea level and land subsidence), increases in mean tidal range, MTR (i.e., tidal amplification), and changes in available sediment threaten the precariously flat Ganges-Brahmaputra-Meghna (GBM) Delta. Much of this region, which is home to ~57 million people, lies within a few meters of sea level and is dominated by strong semi-diurnal tides that extend >200 km inland. While these tides convey the threat of flooding due to rising water levels, they also deliver ~330 Mt of sediment annually offering hope that sedimentation can offset increases in water level. This sediment supply is threatened by upstream damming which complicates an already uncertain future.

Understanding the long-term outlook of the GBM Delta requires a generalizable approach capable of exploring these vast uncertainties. Using a simple mass balance model of elevation change, we find that present rates of aggradation across the delta (1 to 4 cm/yr) coupled with expected increases in MTR (annual factor of 0.011) easily keep pace with RSLR (2 to 4 mm/yr). We explore the potential long-term behavior of the delta by calculating equilibrium elevation of the delta as a function of a range of values of MTR, RSLR, and suspended sediment concentration (SSC). We find much of the delta to be quite stable and robust to RSLR with MTR being the dominant control assuming adequate SSC. However, equilibrium elevations decrease rapidly below SSCs of 0.1 g/L. We find at lower sediment loads (≤25%) and higher RSLR (≥15 mm/yr), parts of the western delta begin to drown and the extent of tidal influence shifts seaward which may increase the potential for upstream catastrophic river avulsions. While these results offer optimism for the future of the GBM Delta, they also identify problematic thresholds within the range of possible outcomes driven by climate change and the potential curtailment of sediment delivery due to upstream damming.