Shoreline Evolution and Coastal Resiliency at Two Military Installations:
Investigating the Potential for the Loss of Protecting Barriers
Jeff Donnelly, Andrew Ashton, Kwok-Fai Cheung, Volker Roeber
This project evaluates the impacts of sea-level rise (SLR), in conjunction with storms, on the barrier islands fronting Eglin Air Force Base (EAFB), Florida and at Marine Corps Base Camp Lejeune (CL), North Carolina, using prescribed SLR scenarios. Both installations face open oceans and include coastal barriers that dynamically respond to rising seas and currently protect significant backbarrier infrastructure. However, the geologic history, geometry and environmental setting of each system are different; in particular, the extent of back-barrier lagoon as well as the wave and storm exposure varies between the two sites.
Coastal barriers (barrier islands and spits) act as a buffer, protecting estuarine ecosystems and the upland from direct assault by the ocean, particularly during large storms. Under the conditions of moderate SLR experienced over the last several millennia, barriers formed and migrated landward through overwash processes, allowing barriers to maintain themselves. However, the dramatically increased rates of projected SLR significantly exceed those seen in the last 6,000 years, raising a concern that barriers may be unable to survive intact. The loss of the protective barrier beach would leave backbarrier areas exposed to the open ocean and to more frequent and severe storm-induced flooding. Barriers provide protection to a number of U.S. military installations, including the two we focus on in this study.
Key questions addressed in our study are:
1) How will protective barriers respond to 0.5, 1.0, 1.5, and 2.0 m of SLR over the coming century? For these SLR scenarios, what is the potential that barriers will no longer be able to keep up with sea level and will drown completely? How do the constraints of local geographic and geologic settings affect the potential for drowning?
2) How do storms, and variability in storm activity, work in concert with SLR to exacerbate barrier loss or impact barrier morphology?
The project features an integrated field and modeling approach to understand the impacts of SLR on the barriers fronting EAFB and CL. This includes using sedimentary approaches to evaluate the long-term storm history of each region. The two sites were selected as they are both subject to relatively frequent impacts from tropical cyclones, yet have different tidal ranges and wave climates. CL has the added exposure to longer duration, but less intense nor’easter storm events.
In order to address the questions outlined above, we have worked to determine the natural oceanographic and sediment transport processes that control barrier morphology and migration (and potentially drowning). We have developed a numerical model of barrier evolution over the centennial timescale appropriate for the SLR scenarios considered. Numerical modeling approaches were also used to generate a synthetic suite of 10,000 storms for each site. These storm parameters were fed into the National Oceanic and Atmospheric Administration (NOAA) Sea, Lake and Overland Surge from Hurricanes (SLOSH) model to generate storm and storm-surge return intervals for both locations. High-resolution inundation modeling was run for select storms at both sites and includes consideration of a nor’easter event at CL.
Under modern climate conditions, the 1/100 year event at EAFB corresponds to a ~3m surge event. In contrast, the return interval for a 2m surge event at CL is 240 years. In both cases, barrier drowning has a profound impact on back-barrier surge levels, even for relatively modest increases in sea level. This is also true for Nor’easter events at CL. Under modern conditions nor’easter events seem unlikely to overtop the barrier but instead remove sediment from the beachface and transport it offshore, most of which is available for transport onshore during calmer conditions.
The geomorphic model demonstrates the importance of overwash in maintaining intact barriers as sea level rises. Given the uncertainty in quantifying future overwash rates, the model is able to provide envelopes of different shoreline change behaviors for different potential overwash fluxes. Estimates of modern overwash flux rates at both sites are below the quantity required to maintain barrier geometries over centennial timescales.
Long-term storm histories for each region show a consistent picture of Holocene storm history for the Gulf of Mexico and for the US Atlantic coast when integrated with other regional studies. Our data show that storminess across both basins has been highly non-stationary over the past thousands of years, revealing a geologic precedent for significant changes in storm climate. Periods of enhanced storm activity in the past have had significant impacts on barrier morphology.
Our program highlights several critical issues that should be specifically evaluated or taken into consideration as part of any vulnerability assessment for installations fronted by or which rely on barrier beach systems. These include:
- Determining the geometry and geology of the entire system with a reasonably high level of precision and detail.
- Analysing storm recurrence intervals for a given region.
- Understanding of across-barrier sediment transport.
- Running a detailed suite of inundation models with a large ensemble of storm events that are plausible for a specific location or installation.
- Evaluating the historical record in a given area to understand the long-term risk of storm impacts.