THE AVAILABILITY OF OFFSHORE SAND REOURCES FOR EROSION MITIGATION ON FIGURE EIGHT ISLAND, NC

A major concern of local government and state management officials is the controversial issue of beach nourishment. Coastal communities along the flanks of Cape Fear are facing critical shortages of sand resources for erosion mitigation. Large scale mining of mainland sites and estuarine channels for beachfill is unlikely due to severe environmental restrictions limiting their use. Chronic long-term erosion and recent storm impacts have been instrumental factors leading to a regional assessment of offshore sand resources.

Figure Eight Island, an 8km long private residential community located in northern New Hanover County, is confronting many of the same problems and critical erosion issues that public oriented communities are facing. During the past decade erosion has increased due to the combined effects of bordering unstable inlets, and the impacts of recent hurricanes. The island has taken the initial critical step in formulating a long-range management plan by assessing the availability of offshore sand resources.

Bathymetric and sidescan sonar data indicate that a series of shore-normal ripple scour depressions (RSD) extend across the gently sloping shoreface. Fine, shelly quartz sand is the dominant surface sediment, with coarse shell gravels restricted to the floors of the RSD. The shoreface sediment package consists of variably thick sequences of fine sand with varying amounts of mud interbedded with muddy shell hash and gravels. Thickness of the sequence ranges from <1cm in the vicinity of hardbottoms to 2.5m in mud filled paleo-channels. The majority of the shoreface is blanketed by a 30-100cm thick, mobile sequence of modern sediment that overlies a widespread Oligocene siltstone unit. The data indicate a lack of sand in the offshore and preclude targeting the shoreface as a potential borrow site.

The lack of shoreface sand resources strengthens the need for developing a sound inlet sand management strategy for long-term maintenance of the oceanfront beach. Rich Inlet that forms the northern boundary of the island is an exemplary site for implementation due to its relative stability, and the large volume of sediment retained in the ebb delta.