Barrier Island Ecology p.2

In addition, we find that some of the animal material making up the sand derives from inland water-based oysters. This supports evidence that the seas have moved. When sea levels were lower, oyster beds would have been on the seaward side of today’s barrier islands, even though they were on the inland side of ancient islands, dunes, and beaches. Remnants of those beds still can be found in the ocean and in our sand today.

Why barrier islands move

As one of the fastest eroding barrier islands on the east coast, Hunting Island is an example of the dynamic forces at work on barrier islands. The most permanent feature of any sea and shoreline is the presence of waves. Waves are created in three ways: wind, earthquakes, and the gravitational pull of the sun and moon. Waves are not just water moving; they are primarily energy moving. This can be illustrated by a bather in the ocean who ducks under a wave to keep from being swept to shore. Beneath the surface of the water, suspended seaweed, grasses, and sand are not rushed to shore with the wave, but instead they make an orbital motion that the bather’s body would have felt also. If these higher than normal tides coincide with a hurricane or even a minor storm, significant change can take place on the barrier islands. Port Royal Sound has the highest tides in the Lowcountry—8-8.5 feet.

As a wave approaches the shore, it touches bottom and when its height is about four fifths of the depth below it, it begins to break. The higher the wave, the more energy it expends on the beach. A steep high wave will create a sudden backwash that sucks the sand off the beach. A flatter, tumbling wave will spread itself out over a greater area on the beach and the sand suspended in the wave is more likely to stay on the beach. In the winter, the beach will seem to be smaller, because the larger waves of the fall and winter storm seasons pull a lot of sand off the beach. But usually the spring waves are not as powerful, and they tend to bring sand back up on the beach, giving us a good wide beach for summer activities. In addition to waves, there are tides that are created by the pull of the sun and the moon on the earth’s seas. The moon has the greatest effect because it is closer. There are two high tides and two low tides every day. Every 27.55 days, or approximately every month, the moon comes its closest to the earth. This is called its “perigree.” Tides rise the highest at this time. Twice every 29.53 days, the sun, moon, and earth align in what is called “syzygy” (siz-i-gee). The tides created by syzygy are called “spring tides” because they seem to spring up. (They occur year round, not just in the spring.) Once every year and a half, the perigree and syzygy will exactly coincide. This is called “proxigee.” At this time, the tides rise faster and the currents run stronger.  If these higher than normal tides coincide with a hurricane or even a minor storm, significant change can take place on the barrier islands. Port Royal Sound has the highest tides in the Lowcountry—8-8.5 feet.

In addition to waves and tides, the beach is affected by longshore and littoral currents. Bathers are often carried down the beach by these currents. Currents are created by waves approaching the beach obliquely—at an angle. When large amounts of sand are moved from one section of the beach to another, it is usually the work of the longshore currents. The result is the creation of new sand bars and movement of sand behind barrier islands into the salt marshes. The ideal angle to transport sand is 30 degrees. Sand captured by the currents moves until the current’s energy is depleted by inlets, submarine canyons, bars, and rock outcrops. Humans attempt to control the movement of sand with structures such as sand fences, jetties, and groins. Sand fences allow some sand to sift through, while reducing the wind velocity and trapping most sand on the windward side.

During hurricanes and storms, a lot of sand is moved in and around a barrier island. Waves find the easiest route to move the sand, including places where paths have been opened through the dunes and through inlets or channels. Sometimes new inlets will be cut that connect the sea to a lagoon behind the island, creating a new island. New sandbars may emerge at the head of channels. Sand is also washed behind barrier islands adding material to the salt marshes and estuaries. You often can see many sandbars at both ends of Hunting Island; they are identified by waves breaking far outside the beach.   

Wind is another important factor in moving islands. Something as tiny as a blade of grass can stop a piece of blowing sand and increase the height of a dune and eventually create an island. A log on the beach will soon accumulate windswept sand. Over time, new dunes or islands will form from the reshaping of the old barrier island. During the reshaping, sand is moved from one area to another in a natural process. Houses built near the shore may be swept away, and sometimes entire towns are wiped out during storms as the beaches are reshaped.

Because Hunting Island is so susceptible to movement, the lighthouse had to be rebuilt a mile and a quarter from its original position. Now the area where it stood has become ocean. At one time Cabin Road on the south side of Hunting Island extended well beyond its current location and had many more cabins, but it too was relocated by the sea. Asphalt remnants of the road remain in places.

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