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Climate Change

Barrier Island Stability Rooted in Their Plant Life

Article:

Vinent, O.D. and Moore, L.J., 2014.  Barrier island bistability induced by biophysical interactions. Nature Climate Change. doi:10.1038/nclimate2474

 

Background:

Barrier islands are coastally located, elongated chains of sand, separated from the mainland by a semi-enclosed body of water called a lagoon (Fig 1). These coastal islands protect the mainland during strong coastal storms, which leaves them particularly susceptible to wave erosion. Barrier islands are fairly common along the Atlantic coast of the United States, with notable examples being the Outer Banks, North Carolina and portions of the southern coast of Long Island, NY (Fig 1).

Figure 1. Long Island, NY Barrier Islands. A series of barrier islands can be seen along the southern shore of Long Island, NY.  Barrier islands are separated from the mainland of Long Island by lagoons.

Figure 1. Long Island, NY Barrier Islands.
A series of barrier islands can be seen along the southern shore of Long Island, NY. Barrier islands are separated from the mainland of Long Island by lagoons.

The stability of barrier islands in response to climate change is a very important topic due to the ability for barrier islands to protect the mainland. Additionally, some barrier islands have been highly developed with numerous homes and businesses. More frequent and intense coastal storms and accelerating sea level rise all pose a threat to the stability of barrier islands, though exactly how these islands will respond to these challenges requires further studying.

Previous studies of the effects of a rising sea level on barrier islands show two distinct responses: Landward migration of the barrier or drowning of the barrier in place. The responses are closely tied to barrier island elevation, which is dictated by the height of the dune. Dunes are built by wind blown sand. The growth of vegetation efficiently traps additional wind blown sand, which allows dunes to grow in height over short spans of time. Barrier islands with tall, well-developed dunes tend to exhibit landward migration, whereas low lying barrier islands tend to be drowned in place. It is clear that the response of the barrier is closely tied to the dynamics of dunes.

In this study, the authors investigated the stability of the Virginia Barrier Islands. This consisted of mathematically modeling the response of barrier islands to rising sea level, dune formation, storm events and dune erosion. The goal of these models is to quantify all the processes that affect the erosion or build up of barrier islands, such that simulations can show how the barrier islands evolve. The results should provide insight regarding how barrier islands will respond to future climate change.

 

Results:

The model simulations unveil two stable barrier island forms: a high stable island or a low stable island (Fig 2). This bistability is closely tied to how barrier islands rebuild following a powerful, erosive storm. During powerful storms, waves and storm surge can wash over barrier islands, destroying the dunes. When a barrier island’s dune and vegetation is wiped out by a strong storm, the island is at a much lower elevation, making it more prone to future washover during lower intensity storms. Vegetation does not tolerate saltwater inundation, resulting in a time lag of vegetation recovery. The dune is unable to rebuild without substantial vegetation, because wind blown sand is more efficiently trapped by vegetation. In less stormy periods, the island slowly rebuilds and reaches a height at which portions of the island are less affected by saltwater, supporting the growth of vegetation and allowing for a rapid period of dune building. If large storm events are frequent enough, the barrier islands never pass the vegetation recovery stage, and the island remains in a low stable island form. Some barrier islands have very high and well-developed dunes, such that overwash during large storms is unlikely to occur. These islands may lose some of their dunes, but still remain vegetated, allowing for rapid post storm recovery. These barrier islands tend to stay in the high stable island form.

Figure 2.  Low and high barrier islands.   a,b. Low elevation barrier islands lacking dune vegetation.  c,d. High elevation barrier islands exhibiting tall dunes with dense vegetation.

Figure 2. Low and high barrier islands.
a,b. Low elevation barrier islands lacking dune vegetation. c,d. High elevation barrier islands exhibiting tall dunes with dense vegetation.

A barrier in the high elevation form can transition to the low elevation form from a particular strong storm event, or from more frequent moderate storms. Once in the low elevation form, a previously high elevation barrier can become “stuck” in the low form if it lacks vegetation. With rising sea level, or more frequent intense storms, barriers islands may remain trapped in the low form for much longer periods of time.

The predicted bistability of barrier islands that resulted from the modeling simulations explains the bimodal (or two peaked) distribution of barrier island elevation that occurs in the Virginia Beach barrier islands (Fig 3f). The modeling simulations also suggest that lower barrier islands more rapidly migrate landward. This result has been observed in the Virginia Beach barrier islands (Fig 3g). The lower islands lose more of their shoreface on an annual basis, and thus are forced into a state of retreat (landward migration).

Figure 3.   f. The distribution of Virginia barrier islands height, showing a low height and high height peak.  g. The correlation of barrier island height to the rate of shoreline retreat, as indicated by the red squares.  Green triangles represent barrier island width.  h. Resulting modeling simulations show the distribution of barrier heights, with a low height and high height peak.  Red squares represent the shoreline rate of retreat as a function of height.

Figure 3.  f. The distribution of Virginia barrier islands height, showing a low height and high height peak. g. The correlation of barrier island height to the rate of shoreline retreat, as indicated by the red squares. Green triangles represent barrier island width. h. Resulting modeling simulations show the distribution of barrier heights, with a low height and high height peak. Red squares represent the shoreline rate of retreat as a function of height.

Barriers trapped in the low elevation form and exposed to rising sea level, more frequent intense storms, and other geological factors such as reduced sediment supply may face the same fate as the Chandeleur Islands of the Mississippi Delta: a shift towards instability. The Chandeleur Islands have been in a state of disintegration due to very strong storms.

 

Significance:

Vacationers, surfers, fishermen, kayakers, boaters, birders, and many, many more people flock to barrier islands during the warm summer months. Many barrier islands are densely populated residential communities with thriving businesses. Other barrier islands are designated state and national parks, serving as a sanctuary for protected and endangered wildlife. The importance and high value placed on barrier islands is conflicted by their vulnerability to coastal storms and rising sea level. This study contributes to the growing body of knowledge regarding barrier island dynamics. Considering the economic and ecological importance of barrier islands, it is important to understand how barrier islands will respond to rising sea level and more frequent, intense storms as predicted with climate change.

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