WHAT MECHANISMS TRIGGER COASTAL FLOW SLIDES?

Some beaches regularly experience a rapid decrease in volume due to massive movements of sand from the dry beach into deeper water. These events visually resemble landslides, but are mostly submerged and generally located along steep river or tidal channels. In some cases, these events can remove thousands of cubic yards (cy) of beach sand in a few hours.

On several occasions in recent years, a slide has formed at Seabrook Island (SC). As of June 2020, there have now been five events observed since July 2016. Surveys of a January 2017 event show the slide displaced ~35,000 cy into deep water (35–55 ft depths) along North Edisto River Inlet. This volume is comparable to hillside-scale landslides observed in mountainous regions like the Blue Ridge.

The Seabrook slide is consistently located along a marginal flood channel of North Edisto River Inlet, just below a quarrystone revetment protecting upland properties. In this particular location, erosion of the dry beach could cause undermining of the revetment. Historical charts suggest a small inlet was located along this portion of the beach as recently as ~1920. Reviews of available NOAA and NWS data also suggest exceptional (ie – near-record daily total) rainfall events and spring tide levels may coincide with observed slide events.

This study analyzes available meteorological, water level, geotechnical, and historical shoreline data to identify mechanisms affecting repeat slide events at Seabrook Island (SC). A combination of excessive rainfall, spring tidal currents, and sediment characteristics all appear to affect these events. Because of the unpredictability of these events, and the dynamic nature of the inlet channel adjacent to this portion of the island, it is difficult to observe events in situ and identify specific mechanisms triggering slides. Providing an excess of beach sand may help maintain a shallower shoreface slope and mitigate future slides.