Dunes: The Coasts’ First Line of Defense

On October 8, 2016, after causing significant damage in the Caribbean and brushing the coasts of Florida and Georgia, Hurricane Matthew made landfall in McClellanville, South Carolina as a Category 1 storm with sustained winds of 74 to 95 mph. This was the first hurricane to make landfall in South Carolina in 12 years. Hurricane Matthew caused significant inundation from storm surge and river flooding, as well as hurricane force winds along the South Carolina coast. According to the National Hurricane Center (NHC), Charleston Harbor reported the fourth highest storm tide on record (and the second highest since Hurricane Hugo in 1989) of 2.83 m MLLW (mean lower low water) and a maximum storm surge of 1.89 m (NWS, 2016).

Sand dune dynamics, morphology, and evolution are well documented by the scientific community (c.f., Hesp and Walker, 2013). The erosion potential of the dune system is determined by storm surge, wave height and storm duration. Vegetation increases the dune’s ability to resist erosion. Under storm conditions, the dune may stay in place, leaving a scarp and protecting the onshore structures (cover art). Alternatively, the dune may fail from repeated undercutting, or the extraction of sediment from the dune toe to the beach face and nearshore that causes the dune collapse. Wave overtopping of the dunes may also occur during storm conditions, which pushes sediment landward. Coastal residents and visitors have been slower to recognize the benefits of dunes as natural protection against wave and tidal events. We have ascertained that they conceptually support healthy dune systems but they value their ocean views more than the protection these landforms may provide.

The damaged dunes, akin to the ones in the cover of this issue, were left as evidence of the storm surge caused by Hurricane Matthew at Isle of Palms, South Carolina. Dunes are the first line of defense against storm surge along the South Carolina coast where new, hardened shoreline protection structures are prohibited. The dunes along the beaches of the city of Isle of Palms (IOP) protected the infrastructure closest to the ocean, preventing major anthropogenic damage. In some areas, the primary dunes, located closest to the water were obliterated by storm surge and associated waves, even though these dunes are in an active exchange of sediment with the beach under normal conditions. A post-storm report commissioned by Isle of Palms estimated that the 7-mile beach barrier island lost approximately 126,780 cubic yards of sand from the dunes and upper beach (CSE, 2016).

On October 4, 2016, four days before Hurricane Matthew made landfall in South Carolina, then Governor Haley declared a State of Emergency. The declaration of a State of Emergency not only started the evacuation efforts of the coastal counties, but also gave State agencies the power to sign specific orders to help communities to prepare for and recover from the storm. Prior to the hurricane, members of the SC Department of Health and Environmental Control, Office of Coastal Resource Management (SCDHEC OCRM) signed five emergency orders that granted permission to local governments and property owners along the immediate beachfront to protect structures from storm surge. These Emergency Orders permitted minor beach re-nourishment, sand scraping, installation of sandbags and the replacement of beach sand. After the post-storm assessment was completed, IOP executed one of the five SCDHEC OCRM Emergency Orders and scraped the beach to construct an artificial (or mechanical) dune. The scraping took place between October 12–21, 2016. The portions of the beach that were scraped had severe dune damage, but more importantly were found along the western part of the island where the setbacks are narrower and therefore the private properties are located closer to the beach. These mechanical dunes provided short-term protection from the ‘king tides’ or extreme spring tides that were expected the week following the hurricane, however, did not consider the long-term (negative) implications to the beach-dune recovery process.

Coastal dune management, in many scenarios, implies the use of strategies and techniques that do not necessarily work or are effective...