Causes of erosion for Hunting Island Updated February 28, 2005
There are many causes of coastal erosion ranging from storms to maintenance of harbors. Yet, despite all the possibilities, most coastlines erode an average of less than 3 feet per year. Hunting Island, however, has eroded at over 20 feet per year for the past century. So, why does Hunting Island erode much faster than most beaches? The short answer is because it is surrounded by inlets. Huge sand bars form off the mouths of South Carolina inlets, and these bars control the waves approaching the shore. Waves do most of the work of moving sand along the beach. When waves are irregular, they can focus erosion in one area and cause sand to build up somewhere else. Three inlets affect Hunting Island: Fripp Inlet at the south end; Johnson Creek at the north end; and St Helena Sound which, in some ways, surrounds the entire island. Sand bars associated with these inlets extend offshore for miles. They block incoming waves in some places or, in other places, cause waves to change direction before they strike the shore. The net effect of inlets, sand bars, and irregular waves at Hunting Island is to move sand from the middle of the island toward the ends. The middle erodes and the inlets collect the sand. Of course, it is along the middle of Hunting Island where some critical recreational facilities exist and where the historic lighthouse is located. The accompanying photos taken earlier in February at low tide show these accumulations of sand at Johnson Creek and Fripp Inlet. If all the sand trapped in these sand bars was spread on the beach, Hunting Island would be about 1000 ft wider. Erosion, focused along the center of Hunting Island, will continue until the shoreline develops a crescent shape. Unfortunately, by the time that happens, the lighthouse will be in the ocean and much of the park's facilities will be lost. That's why PRT is planning a project to stabilize the beach and reduce the sand losses to the inlets.
CSE engineers and technicians continued preliminary design work in February. Borings were obtained along the beach ranging from 10 to 30 feet long, to test the character of the underlying sediments. This is an important step for determining the types and dimensions of materials that can be used to build groins. Beaches are often underlain by soft sediments. The areas of Hunting Island where sand retaining structures may be installed, were found to consist of uniform fine sand with little variation in the upper 15 feet. However, some softer material was encountered below this level. CSE has arranged for a testing lab to estimate the strength of the deeper soils. The design goal is to minimize the amount of material in the groins while achieving necessary structural stability. Project costs are proportional to the size of the groins and volume of sand placed on the beach. Each month, PRT will provide an update of the project in this space. For questions or comments, contact dsimms@scprt.com or cse@coastalscience.com.
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