Rollover, drowning, and discontinuous retreat: Distinct modes of barrier response to sea‐level rise arising from a simple morphodynamic model

Abstract

We construct a simple morphodynamic model to investigate the long-term dynamic evolution of a coastal barrier system experiencing sea-level rise. Using a simplified barrier geometry, the model includes a dynamic shoreface profile that can be out of equilibrium and explicitly treats barrier sediment overwash as a flux. With barrier behavior primarily controlled by the maximum potential overwash flux and the rate of shoreface response, the modeled barrier system demonstrates four primary behaviors: height drowning, width drowning, constant landward retreat, and a periodic retreat. Height drowning occurs when overwash fluxes are insufficient to maintain the landward migration rate required to keep pace with sea-level rise. On the other hand, width drowning occurs when the shoreface response rate is insufficient to maintain the barrier geometry during overwash-driven landward migration. During periodic barrier retreat, the barrier experiences oscillating periods of rapid overwash followed by periods of relative stability as the shoreface resteepens. This periodic retreat, which occurs even with a constant sea-level rise rate, arises when overwash rates and shoreface response rates are large and of similar magnitude. We explore the occurrence of these behaviors across a wide range of parameter values and find that in addition to the maximum overwash flux and the shoreface response rate, barrier response can be particularly sensitive to the sea-level rise rate and back-barrier lagoon slope. Overall, our findings contrast with previous research which has primarily associated complex barrier behavior with changes in external forcing such as sea-level rise rate, sediment supply, or back-barrier geometry.

Second-Pass Assessment of Potential Exposure to Shoreline Change in New South Wales, Australia, Using a Sediment Compartments Framework

“…Our model does not support continuous (rollover) or discontinuous (drowning, overstepping) dynamic barrier behaviours [64,65], and is therefore only applicable to simulating shoreline recession on relatively steep, moderate to high energy wave-dominated coasts, where the existing beach and dune morphology is well-developed. The model is not suitable for simulating shoreline change on low-lying barrier island coasts, where dynamic barrier behaviour controls shoreline change [64][65][66].…”

“…While the washover parameter (V O ) could be used to simulate the effect of washover deposition or dune growth in slowing the rate of shoreline recession, we neglect its use due to lack of data or previous examples. Nonetheless, a more rigorous dynamic barrier model [64,65,78] could be implemented within the framework to address these processes in more detail. We emphasise again that the model as applied here is intended to provide a risk-averse mid-resolution forecast of potential shoreline change.…”

Mechanics and rates of tidal inlet migration: Modeling and application to natural examples

“…The fate of modern barrier islands on coasts with rapid and accelerating sea level rise depends greatly on the retreat rate barrier islands can achieve. Many models of barrier island response to sea level rise consider landward retreat to depend on overwash fluxes: storm-induced episodic sediment supplied from the nearshore environment that overtops barriers and is deposited on backbarrier marshes [Stolper et al, 2005;Lorenzo-Trueba and Ashton, 2014]. Given that 50% of barrier complexes show tidal inlet fill sequences [Moslow and Tye, 1985], inlet migration, through flood-tidal delta deposition, is likely also an important sediment mover that allows barrier islands to transgress.…”

Plan-view evolution of wave-dominated deltas

“…A diversion of 25% of the current Mississippi River flux [Kim et al, 2009b] would be fluvially-dominated. However, as sea-level rise continues to threaten the existence of barrier islands [Lorenzo-Trueba and Ashton, 2014], these sites might in the future become increasingly exposed to waves. The Brazos River delta, Texas, USA, diverted in 1929, is an example where exposure to waves led to the growth of wave-sculpted shorelines [Rodriguez et al, 2000].…”