Beach Erosion and Sand Transport at Hunting Island, South Carolina, USA

Abstract

Hunting Island has experienced major shoreline erosion over the past 140 years. The beach has lost sand at the rate of approximately 130,000 m³/year. Over the period 1920-1971, the shoreline retreated 5-7 m/year. Beach renourishment has essentially stabilized the shoreline since 1968. Extensive shoals of the St. Helena Sound ebb tidal delta shield the island from northeast waves. Farther south, the Hunting Island Platform impedes waves approaching from east through southeast. Sediments occurring on the platform and beach are dominantly medium-to-fine sand. Computer simulation of longshore sand transport using the WAVENRG model predicts a gross longshore drift of about 100,000 m³/year and a net longshore drift to the north along Hunting Island of about 12,000 m³/year. Flood tidal currents dominate along the Hunting Island nearshore. These currents flow westward toward the coast then northward into St. Helena Sound. It is concluded that sand is being suspended in the nearshore zone by waves, causing beach erosion, and transported northward into St. Helena Sound by longshore and flood tidal currents. Most of this sand is then swept seaward through ebb-dominated tidal channels to be deposited on the outer Hunting Island Platform. Shoaling waves transport the sand landward to a depocenter along the inner margin of the outer platform. Sand that is able to bypass this depocenter is delivered to the funnel-shaped inner platform. As the inner platform is largely flood-dominated, the sand entrained by waves is transported by flood tidal currents northward to St. Helena Sound. The Hunting Island beach is effectively isolated from any offshore source of replenishing sand. Similar sand transport systems can be expected to exist in regions characterized by similar morphodynamic attributes.