Acute morphodynamic responses of the Ganges-Brahmaputra tidal deltaplain to natural and anthropogenic perturbations

Abstract:

The Ganges-Brahmaputra river system is largely free-flowing and has relatively few large engineering structures. However, ~5000 km² of the delta’s intertidal landscape was embanked in the 1960s and 1970s to increase arable land for food production, since leading to major morphodynamic changes. One response has been the siltation of ~1000 km of large tidal channels now completely infilled or <30 m width. This infilling has accounted for ~2 km²/yr of new land, offsetting half of the erosion taking place at the coast. In the remaining tidal channels, one of the most significant changes has been an amplification of the tidal range by >1 m, with mean high water rising 3× faster than mean sea level at ~1.5 cm/yr. Also ongoing has been a major tidal basin capture resulting from the rerouting of 1000s of m³/s of tidal discharge in response to the loss of intertidal area and channel closures. Such a basin capture is enabled by the system’s looping (i.e., interconnected) channel network, which allows mass exchange between adjacent tidal basins. This flow reorganization also caused several tidal channels to widen, thereby weakening embankments and leading to their failure during a 2009 cyclone. Together, these changes reflect a strongly coupled human-natural coastal system with significant feedbacks and societal consequences. Remarkably, no comparable changes have occurred in the adjacent Sundarbans mangrove forest, a portion of the deltaplain that remains largely natural and un-embanked. It is notable that such acute morphodynamic responses occur in the absence of direct fluvial input to this part of the delta. Rather, mass exchange between the inner shelf and the tidal channels effectively couples the tidal deltaplain with the remote river plume. In fact, the tidal exchange exceeds the ~1000 km³ of water annually discharged by the main rivers, with single-channel discharge 80 km inland regularly reaching 20-40 × 10³ m³/s with associated sediment transport rates of 10-15 × 10³ kg/s. This routine exchange of water and sediment with the inner shelf ultimately sustains a net import of >100 × 10⁹ kg of sediment to the tidal deltaplain and sedimentation rates >1 cm/yr. Such abundant sediment supply and transport energy are thus sufficient to offset relative sea-level rise in the natural environment, but not for those areas that remain embanked.