(Please click on photos to enlarge)
Hunting Island is a barrier island along the southern coast of South Carolina. Due to coastal development, it is rare to see a barrier island in such a natural state! The island is a real treasure, but its days are numbered, Hunting Island is eroding away into the sea. Barrier islands are highly dynamic systems, constantly forming and disappearing as currents, waves, and storms erode away and re-deposit sand.
Barrier islands are elongate strips of sand that form parallel to the coastline, often occurring in chains separated by narrow inlets. Lowlands separating the raised sand dunes of the barrier islands from the mainland flood and turn into narrow, protected, low-energy bays, where tidal flats, salt marshes, and lagoons form. Barrier islands play a critical role in protecting the coastline from storms. However, that defense is lost when the heaviest development occurs on the barrier island itself, as so often is the case along the Atlantic and Gulf coasts where barrier islands occur.
The park map of Hunting Island above shows, moving from the coast towards the mainland: beach – dune sequence – salt marsh and tidal creeks. On the southern end of the island, you can see where a new barrier island sequence is forming on the seaward side of the barrier island itself! The sea has filled into the topographic low between dune ridges, creating a lagoon. The spit of land between the lagoon and the sea was breached by recent hurricanes, forming an inlet cutting off the bottom corner of the island, creating a new barrier island called Little Hunting Island. Eventually the spit will be breached again, forming more islands.
While Hunting Island has largely been free to follow its natural processes, some evidence of human interference to slow longshore drift can be observed. Waves do not strike the coastline head-on, but rather at an oblique angle. This results in a net movement of sediment in a prevailing direction parallel to the coastline.
At Hunting Island the prevailing longshore drift direction is to the north, so the island is eroding away to the south and depositing to the north. So does that mean the island is growing to the north? Normally it would, but strong seaward currents from St. Helena Sound along the northern end of the island flush the sediment migrating north from longshore drift directly out to sea, depositing it miles away into the Atlantic Ocean. Currents moving towards the shore are not strong enough to transport this sediment back to the island, resulting in erosion of both ends of the island, and its eventual disappearance. The location of the Hunting Island Lighthouse on the north side of the island has moved south twice since its construction in 1859 due to this erosion, with the original site of the lighthouse being more than a mile north of the current northern extent of the island!
While barrier islands are ephemeral features, this is not ideal for development, and most barrier islands have extensive engineering structures and beach replenishment programs in order to restrict their movement. As is typically the case, these measures are more of a delay than a solution, and often have unintended consequences. By retaining sediment in one area other areas are starved, and by continuing to develop on areas that would be underwater without artificial controls, the inevitable flooding and beach loss are delayed and worsened.
Groins and jetties are two common structures used to control sediment transport along the coast. Groins are barriers constructed perpendicular to the coastline to restrict longshore drift. Sand piles up against updrift side of the groin, while the downdrift side is starved of sand and erodes away. In order to counteract this beach loss on the downdrift side, another groin is emplaced, resulting in an inwardly curved coastline and eventual beach loss at the termination of the groin sequence. A groin sequence is present along the beaches of Hunting Island, and can be seen below. Jetties are similar to groins, but emplaced adjacent to tidal inlets to stabilize the inlet and keep it from migrating.
The temporary nature of barrier islands is quite evident at the southern end of Hunting Island, where natural processes have not been interfered with. The lagoon was breached by Hurricanes Matthew (2016) and Irma (2017), cutting off the southern tip and creating Little Hunting Island. When we visited in June of 2022, the breach was actually partially filled in with sand, turning Little Hunting Island back into a spit. The breach will likely be reestablished with hurricane season coming soon, turning it back into an island. Breaches are not created by overwash during the storm event, but by backwash after the storm. The water level in the lagoon rises during the storm, eventually overtopping its banks and flooding over at the path of least resistance, breaching the spit and creating an island.
One of the most striking features of erosion on the island, and something I had never witnessed before at that scale, can be seen in an area on the southern portion of the island called Boneyard Beach. As the beach has eroded away and the sea has crept further and further up the shore, the saltwater has choked the life out of the trees behind the beach. These trees, with their weakened dead roots, are then toppled and strewn about, creating a “boneyard” of these dead and denuded giants.