STABLE EQUILIBRIUM OF THE GANGES-BRAHMAPUTRA-MEGHNA DELTA: BALANCING ELEVATION WITH CHANGES IN SEA LEVEL, LAND SUBSIDENCE, TIDES, AND FLUVIAL SEDIMENT SUPPLY

Abstract

The Ganges-Brahmaputra-Meghna (GBM) Delta faces a combination of climate and geopolitical factors that threaten to undermine elevation stability across the coastal region. 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. Rising tidal levels due to sea-level rise and subsidence, i.e., relative sea-level rise (RSLR), combined with expanding mean tidal ranges (MTRs) from tidal amplification imperil this precariously flat landscape. Conversely, these tides also convey ∼330 Mt of sediment annually offering hope that sedimentation can offset increases in water level. However, upstream damming jeopardizes this sediment supply by preventing the flow of sediment downstream.

Understanding the long-term outlook of the GBM Delta requires a generalizable approach capable of navigating these vast uncertainties. Using a simple mass balance model of elevation change, I find that present rates of aggradation across the delta (1 to 4 cmyr−1) coupled with expected increases in MTR (annual factor of 0.011) easily keep pace with RSLR (2 to 4 mmyr−1). I explore the potential long-term behavior of the delta by calculating equilibrium elevation over a characteristic range of values for MTR, RSLR, and suspended sediment concentration (SSC). I 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 gL−1. At sediment loads ≤25 % and RSLR ≥15 mmyr−1, parts of the western delta begin to drown. Furthermore, the extent of tidal influence shifts seaward which may increase the potential for upstream catastrophic river avulsions. These findings highlight both the potential resilience and vulnerabilities of the delta, while emphasizing the challenges posed by climate change and the geopolitics of sediment management. To ensure the long-term sustainability and resilience of the GBM Delta, it is imperative to employ both adaptation measures against rising water levels (e.g., flood barriers, infrastructure reinforcement, tidal river management) and collaborative geopolitical efforts to better manage sediment flows across the region.