ELUCIDATING SEA-LEVEL RISE, SUBSIDENCE, AND SEDIMENT ACCRETION IN THE GANGES-BRAHMAPUTRA (G-B) TIDAL DELTAPLAIN

Coastal environments, at the interface between terrestrial and marine processes, can be extremely dynamic sedimentary systems that operate through a series of feedback loops responding to natural and anthropogenic perturbations. In the Ganges Brahmaputra tidal deltaplain, century-scale relative sea-level rise obtained from tide gauges is on the order of 4-6 mm/yr, however recent work has shown that the extensive embankment system has greatly reduced the regional tidal prism (i.e., volume of water moving on/off the landscape), led to an expansion of the tidal range by as much as 1 meter, and altered sediment distribution patterns. As a result, elevation loss due to embankment construction severely increases the risk of coastal communities to storm surge or even regular tidal flooding, while the natural mangrove forest appears to be in equilibrium with hydroperiod and sediment supply conditions.

We present an overview of modern subsidence and sedimentation rates measured within both embanked and natural regions of the lower G-B delta using tide gauges, GPS, Surface Elevation Tables, sediment tiles, shallow cores, and fiber optic compaction meters. We compare these results to long-term Holocene trends obtained through deeper borehole coring methods. In general, short term accretion rates can be quite large on connected platforms and within channels (3-6 and 10-20 cm/yr, respectively), while long term accretion is an order of magnitude less (average ~0.5 cm/yr). Modern subsidence is spatially very heterogenous and depends upon underlying stratigraphy (peat vs mineralgenic soils; rates ranging from 3-15 mm/yr), however radiocarbon dates from cores suggest that subsidence throughout the Holocene averages 1-3 mm/yr. While it is apparent these rates vary widely over a range of spatial and temporal scales, combining both short and long-term measurements is beneficial for assessing the future sustainability of these areas. Results show sedimentation remains sufficient to offset even modest subsidence under natural conditions, and suggest amelioration of elevation deficits is physically feasible should tidal exchange be restored to low-lying embanked areas.