The response of deltas to sea-level rise: Natural mechanisms and management options to adapt to high-end scenarios

The response of deltas to sea level rise (SLR) has mostly been studied from a perspective of human impacts like global warming and impoundment, or from a perspective of natural changes associated with glacial cycles. Here we synthesize the response of deltas to SLR by integrating research looking at past and future evolution to improve the potential to manage deltas to adapt to high rates of SLR. We hypothesize that fluvial-dominated deltas can be managed to survive high rates of SLR (>1 cm year(-1)) that characterized the post-glacial period and will likely characterize coming centuries due to global warming. There are three known mechanisms for deltas to cope with SLR that are self-reinforcing as the rates increase, tending to enhance the efficiency of the deltaic sedimentary trap: (a) an increase in the frequency of delta lobe switching with accelerated SLR leading to the formation of new lobes in shallow areas (b) an increase in the frequency and magnitude of flood events in the delta plain as a consequence of an increased crevassing through the river natural levees, leading to enhanced sediment deposition (c) an increase in the frequency and magnitude of overwash events in the delta fringe enhancing the ability of sandy beaches to adapt to SLR. The current view by much of the management community is that coastal protection is the best strategy for future SLR up to 2-5 m, and beyond 5 m that retreat would be the best (or the only) strategy. However, for the case of deltas a more functional adaptation strategy based on restoration can be envisioned provided that natural processes and ecosystem functions can be managed to increase system's resilience. The central element of this alternative strategy is the idea of "rising grounds" (vertical aggradation), instead of "rising dikes", but a combination of both can also be foreseen and may be needed in many cases. This means that "rising dikes" will be only feasible if "rising grounds" is also implemented; if not, retreat will be the only long-term alternative. We propose that "rising grounds" is the best adaptation strategy inmost deltas for high-end scenarios of SLR, though in some cases the option of retreating may be necessary in combination with structural and functional measures. (C) 2013 Elsevier B.V. All rights reserved.