Barrier Island Response to Sea-Level Rise and Storm Inundation: Initial Modeling of Alongshore Processes

Questions about the fates of barrier islands as rates of sea-level rise increase motivate considerable recent research on barrier island processes. State-of-the-art models (conceptual, analytical, and numerical) typically consider cross-shore sections, addressing island retreat rates and sediment budgets as functions of lithology, morphology, and sea-level forcing. As a complement to these fundamental inquiries, we have developed a numerical model that treats stratigraphy and cross-shore morphology in less detail that existing models, but adds alongshore heterogeneities and alongshore sediment transport.

When a landfalling hurricane inundates a barrier island, washing some of the island far landward, gaps in barriers result. As an island chain recovers, spits grow from the remaining island segments, which become crescent shaped. If these spits merge, the island chain recovers alongshore continuity. However, this recovery process causes a landward shift of the island location, in addition to that imposed by sea-level rise. Thus, recovery from inundating storms increases the rate that coarse sediment (sand or gravel) must be produced by an eroding shoreface for the islands to persist. During the recovery process, the landward retreat rates of crescentic island segments can be locally very high. If the shoreface seaward of these segments cannot be transformed into mobile sediment rapidly enough, or does not contain enough coarse sediment, any remaining subaerial sediment will become disconnected from the shoreface, and model assumptions become invalid. Model results indicate that the rate of sea-level rise beyond which typical barrier-island and shoreface morphology cannot persist depends on large-scale coastal morphology and substrate properties (broadly consistent with other modeling efforts), and on the frequency and severity of inundating storm landfalls.