Complexities in barrier island response to sea level rise: Insights from numerical model experiments, North Carolina Outer Banks

Abstract

[1] Using a morphological-behavior model to conduct sensitivity experiments, we investigate the sea level rise response of a complex coastal environment to changes in a variety of factors. Experiments reveal that substrate composition, followed in rank order by substrate slope, sea level rise rate, and sediment supply rate, are the most important factors in determining barrier island response to sea level rise. We find that geomorphic threshold crossing, defined as a change in state (e.g., from landward migrating to drowning) that is irreversible over decadal to millennial time scales, is most likely to occur in muddy coastal systems where the combination of substrate composition, depthdependent limitations on shoreface response rates, and substrate erodibility may prevent sand from being liberated rapidly enough, or in sufficient quantity, to maintain a subaerial barrier. Analyses indicate that factors affecting sediment availability such as low substrate sand proportions and high sediment loss rates cause a barrier to migrate landward along a trajectory having a lower slope than average barrier island slope, thereby defining an "effective" barrier island slope. Other factors being equal, such barriers will tend to be smaller and associated with a more deeply incised shoreface, thereby requiring less migration per sea level rise increment to liberate sufficient sand to maintain subaerial exposure than larger, less incised barriers. As a result, the evolution of larger/less incised barriers is more likely to be limited by shoreface erosion rates or substrate erodibility making them more prone to disintegration related to increasing sea level rise rates than smaller/more incised barriers. Thus, the small/deeply incised North Carolina barriers are likely to persist in the near term (although their long-term fate is less certain because of the low substrate slopes that will soon be encountered). In aggregate, results point to the importance of system history (e.g., previous slopes, sediment budgets, etc.) in determining migration trajectories and therefore how a barrier island will respond to sea level rise. Although simple analytical calculations may predict barrier response in simplified coastal environments (e.g., constant slope, constant sea level rise rate, etc.), our model experiments demonstrate that morphological-behavior modeling is necessary to provide critical insights regarding changes that may occur in environments having complex geometries, especially when multiple parameters change simultaneously.

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

“…In that case, the assumption of an encroachment response is no longer valid, and the model will likely over-estimate potential shoreline recession. When considering the well-developed dune morphology of most NSW beaches, relative to low-relief barrier island coasts e.g., [64], and following manual review of the regional-scale model predictions, we are confident that this limitation does not affect the model scenarios that are considered in our exposure assessment (i.e., present-1% exceedance, 2050-1% exceedance, and 2100-50% exceedance forecasts). In some settings, however, low-exceedance (e.g., 1%) and long-term (e.g., 2100) forecasts may over-estimate potential shoreline change ( Figure 11A).…”

“…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.…”

A millennium-long management perspective for promoting the geological-social resiliency of barrier islands: a preliminary proposal for Fire Island, NY

“…This well-documented history when conjoined with the results of numerical modeling experiments implies that barrier islands can migrate landward (high resiliency), disintegrate over time (low resiliency), or drown in place (lowest resiliency) as sea level rises and sand supplies diminish (LorenzoTrueba and Ashton 2013; Moore et al 2009). It seems clear that the former outcome should be favored by conservation policies and not the latter two Bterminal^states.…”

“…However, we do understand their general evolutionary trajectory. For example, geologic histories of barrier islands and computer modeling results (Lorenzo-Trueba and Ashton 2013; Moore et al 2009;Masetti et al 2008) indicate that many barrier islands over the forthcoming centuries and millennia will respond to rising sea level by migrating landward. If resilience thinking were deeply understood, one would hope that most inhabitants of barrier islands would opt to adapt to rather than oppose this dynamic process.…”