Jennifer L. Miselis

Predictions of barrier island berm evolution in a time‐varying storm climatology

Low-lying barrier islands are ubiquitous features of the worlds coastlines, and the processes responsible for their formation, maintenance, and destruction are related to the evolution of smaller, superimposed features including sand dunes, beach berms, and sandbars. The barrier island and its superimposed features interact with oceanographic forces (e.g., overwash) and exchange sediment with each other and other parts of the barrier island system. These interactions are modulated by changes in storminess. An opportunity to study these interactions resulted from the placement and subsequent evolution of a 2 m high sand berm constructed along the northern Chandeleur Islands, LA. We show that observed berm length evolution is well predicted by a model that was fit to the observations by estimating two parameters describing the rate of berm length change. The model evaluates the probability and duration of berm overwash to predict episodic berm erosion. A constant berm length change rate is also predicted that persists even when there is no overwash. The analysis is extended to a 16 year time series that includes both intraannual and interannual variability of overwash events. This analysis predicts that as many as 10 or as few as 1 day of overwash conditions would be expected each year. And an increase in berm elevation from 2 m to 3.5 m above mean sea level would reduce the expected frequency of overwash events from 4 to just 0.5 event-days per year. This approach can be applied to understanding barrier island and berm evolution at other locations using past and future storm climatologies.

Natural and Human-Induced Variability in Barrier-Island Response to Sea Level Rise: Barrier Island Response to Sea Level Rise

Storm-driven sediment fluxes onto and behind barrier islands help coastal barrier systems keep pace with sea level rise (SLR). Understanding what controls cross-shore sediment flux magnitudes is critical for making accurate forecasts of barrier response to increased SLR rates. Here, using an existing morphodynamic model for barrier island evolution, observations are used to constrain model parameters and explore potential variability in future barrier behavior. Using modeled drowning outcomes as a proxy for vulnerability to SLR, 0%, 28%, and 100% of the barrier is vulnerable to SLR rates of 4, 7, and 10 mm/yr, respectively. When only overwash fluxes are increased in the model, drowning vulnerability increases for the same rates of SLR, suggesting that future increases in storminess may increase island vulnerability particularly where sediment resources are limited. Developed sites are more vulnerable to SLR, indicating that anthropogenic changes to overwash fluxes and estuary depths could profoundly affect future barrier response to SLR. Plain Language Summary Barrier islands, thin strings of islands offshore of mainland coasts, are the first line of defense for protecting estuaries and mainland population centers from storms. They are also important for tourism that drives many coastal economies. Sand movement to the top of and across barrier islands is how they keep pace with sea level rise (SLR), so restrictions to those processes may make barrier islands more vulnerable to SLR effects. In our study, we used observations from New Jersey, USA, as inputs to a model that forecasts barrier island changes in response to SLR. This is particularly important for New Jersey, which is expected to experience rates of relative SLR that are higher than average. We found that 28% of the barrier island was vulnerable to a moderate rate of SLR and 100% of the barrier island was vulnerable to a high rate of SLR. Furthermore, we found that barrier island vulnerability increased in heavily populated locations relative to less populated locations. This suggests that human changes to coastal systems likely impact the lifespan of barrier islands. If some barrier islands degrade faster than others, their ability to protect mainland coasts and sustain coastal communities and economies could be compromised.