Dune Dynamics Drive Discontinuous Barrier Retreat

Abstract

Barrier islands and spits tend to migrate landward in response to sea‐level rise through the storm‐driven process of overwash, but overwash flux depends on the height of the frontal dunes. Here, we explore this fundamental linkage between dune dynamics and barrier migration using the new model Barrier3D. Our experiments demonstrate that discontinuous barrier retreat is a prevalent behavior that can arise directly from the bistability of foredune height, occurring most likely when the storm return period and characteristic time scale of dune growth are of similar magnitudes. Under conditions of greater storm intensity, discontinuous retreat becomes the dominant behavior of barriers that were previously stable. Alternatively, higher rates of sea‐level rise decrease the overall likelihood of discontinuous retreat in favor of continuous transgression. We find that internal dune dynamics, while previously neglected in exploratory barrier modeling, are an essential component of barrier evolution on time scales relevant to coastal management.

Storm Driven Migration of the Napatree Barrier, Rhode Island, USA

“…This leads to episodic retreat of the barrier, followed by periods of relative stability in barrier positions when the foredune recovers sufficiently to limit overwash. These results, both shown here at Napatree, reinforce the relationship between dune recovery and storm frequency reported in recent modelling studies [21].…”

“…Higher rates of sealevel rise over the last 40 years [39] do not coincide with any measurable shoreline retreat, and even correspond with a period of progradation (within the positional uncertainty) between 1975 and 2014. The responses of barriers under various sea-level rise scenarios have been the focus of recent modelling efforts, and the dynamics of the foredune have been shown in models to impact barrier retreat-namely, when the dunes are low, barriers are more susceptible to overwash, leading to migration of the barrier [21]. This leads to episodic retreat of the barrier, followed by periods of relative stability in barrier positions when the foredune recovers sufficiently to limit overwash.…”

“…The objective of this study is to document historical changes in shoreline position and morphology of a barrier spit between 1883 and 2014 and examine the roles of storm frequency and magnitude on barrier migration. The erosion and subsequent recovery of foredunes is critical to understanding the style and rate of barrier retreat [21]. Increased storm frequency and sea-level rise are likely consequences of climate change [19], creating a feedback loop of increased frequency of overwash events and reduced periods of barrier recovery.…”

Exploring the Impacts of Shrub‐Overwash Feedbacks in Coastal Barrier Systems With an Ecological‐Morphological Model

“…To assess the ways in which shrubs alter barrier morphology and the rate and style of barrier retreat, we run simulations with and without shrubs across broad ranges of two key input parameters: RSLR and characteristic dune growth rate ($$\bar{r}$$). Dune growth rate is an important factor in controlling shrub expansion and mortality and in determining the vulnerability of a barrier to storm impacts (Durán Vinent & Moore, 2015; Goldstein & Moore, 2016; Reeves et al., 2021). Varying dune growth rates is also akin to varying storm frequency – that is, faster growth rates reduce the time needed for a dune to ecover before the next storm, and vice versa.…”

An Integrated, Probabilistic Modeling Approach to Assess the Evolution of Barrier-Island Systems Over the Twenty-First Century

“…Some of those studies are focused on the evolution of barrier-islands over decadal time scales under episodic events such as storm surges and extreme weather [e.g., 2D MCO model by Rosati et al (2010)]. Some other studies are focused on the evolution of barrier-islands on geological time scales [e.g., GEOMBEST model by Stolper et al (2005), a simple morphodynamic model to assess barrier island evolution by Lorenzo-Trueba and Ashton (2014), the Barrier 3D model by Reeves et al (2021), and the CoastMorpho2D model by Mariotti (2021)]. However, almost all these studies only provide deterministic projections of future barrier-island evolution.…”