A SIMPLE MORPHODYNAMIC MODEL OF COASTAL BARRIER RESPONSE TO RISING SEA LEVEL

Low-lying coastal barriers face an uncertain future over the next century, with many projections suggesting end-of-century rates of sea-level rise as high as 1 cm/yr. Where there are beaches (or anywhere the shore is not made out of bedrock), rising sea levels do more than merely flood the existing coast. Higher seas will enable waves and currents to dramatically redistribute the sediment of the coast, resulting in a retreat of the shoreline significantly greater than that expected for simple inundation alone.

Although many models of coastal change have been developed, the majority are either highly calibrated and intended to operate at the temporal scales of engineering projects (< ~5 years), offering little possibility of forecasting never-seen behaviors such as barrier drowning, or long-term geologic models, which typically assume that the coast maintains an ‘equilibrium’ configuration that moves with sea level. We aim at bridging the gap between these approaches by constructing a simple model that focuses on dynamical coupling of two primary barrier components: the marine domain represented by the active shoreface, which is constantly affected by transport and reworking by waves, and the backbarrier system, where the infrequent process of overwash controls landward mass fluxes. The model demonstrates that coastal barriers can respond to an accelerated sea-level rise in complex, less predictable manners than suggested by existing conceptual and long-term numerical models. Model behaviors under constant sea-level rise reveal two potential modes of barrier failure: ‘height drowning’, which occurs when overwash fluxes are insufficient to maintain the landward migration rate required to keep in pace with sea-level rise; and ‘width drowning’, which occurs when the shoreface response is insufficient to maintain the barrier geometry during landward migration. We also identify a mode of discontinuous barrier retreat, where barriers can experience punctuated intervals of rapid rollover and shoreline stability, even with constant rates of sea-level rise. We explore the sensitivity of these modes to external and internal variables, including sea-level rise rate, maximum overwash rate, shoreface response rate, and inland topography.