Elevation Dynamics Between Polders and the Natural Sundarbans of the Ganges-Brahmaputra Delta Plain

Abstract

The Ganges-Brahmaputra Delta (GBD) in Bangladesh exists at a nexus of stability and vulnerability, as the rivers annually carry ~ 800–1000 MT of sediment from the Himalayan Mountains, yet coastal poldering and sediment extraction within the rivers remove elevation capital from the low-lying delta plain. Recent research in the GBD has begun to unravel how the world’s largest fluvio-deltaic mangrove forest—the Sundarbans—is keeping pace with sea level rise (SLR); however, this is contingent on adequate sediment supply delivered to the platform during semi-diurnal tides and the seasonal monsoon. Little is known about the elevation dynamics within human-modified polders by comparison, other than an elevation deficit of 1–1.5 m exists. In this study, seasonal data from Rod Surface Elevation Tables (RSETs) installed within a polder in the southwest region (Polder 32) are compared to the Sundarbans. Over ~ 8 years, results show that surface elevation is gaining within the Sundarbans at a more significant rate (~ 58.4%), and this is due to the higher vertical accretion rates measured in the Sundarbans (~ 67%) from abundant sources of allochthonous material. Elevation gain in the polder, particularly close to the embankment, appears to be attributed to sediment supplied from eroded embankments and local sluice gates, in addition to seasonal subsurface clay swelling during the monsoon. Shallow subsidence within both study areas appears to take place seasonally, but with less delivery of new sediment, the rate of shallow subsidence is lower in the polder compared to the Sundarbans. Despite seasonal shallow subsidence, the elevation change is net positive in both study areas if taken as a whole; however, interior poldered regions exhibit net elevation loss. This comparison in change of elevation, vertical accretion, and shallow subsidence shows how human modification has drastically changed the natural processes. Furthermore, our results are compared to rates of relative and effective SLR, which show that the Sundarbans is keeping pace in this region, while Polder 32 is not. These results are vital to inform embankment mitigation and flood risk in this dynamic delta system.