In the Balance: Natural vs. Embanked Landscapes on the Ganges Brahmaputra Tidal Delta Plain

Abstract

The future of deltas worldwide will be determined by a balance of sedimentation, subsidence and sea-level rise, in addition to human-caused perturbations. The Ganges-Brahmaputra delta is perceived to be at great risk to degradation and submergence under multiple sea-level rise (SLR) scenarios. The lower delta plain in southwest Bangladesh is isolated from fluvial processes, yet still receives up to 1 cm of sediment accretion annually via the tidal channel network. In the pristine Sundarbans mangrove forest sedimentation has kept pace with relative SLR. Adjacent landscapes have experienced locally inhibited sediment delivery caused by embankment construction in the 1960s. In the ensuing five decades the land has continued to subside and compact, and in the absence of sediment accretion has experienced a net elevation loss of more than 1 m. The effects of this elevation offset were felt in 2009 when the embankments of several large islands breached during Cyclone Aila and land remained tidally inundated for two years. Despite sustained human suffering during this time, the newly reconnected landscape rebounded with tens of centimeters of tidally deposited sediment, accounting for decades-worth of normal sedimentation, but only partly restoring the elevation lost over the previous five decades. This study demonstrates that the evolutions of these two adjacent landscapes lie on very different trajectories, a finding that has immediate and profound implications for the human population on Polder 32 and other embanked landscapes in the tidal delta plain.