Carolina Environmental Diversity Explorations

The northern Outer Banks · By Dirk Frankenberg and Blair Tormey

Groins at Cape Hatteras

Figure 19. These groins trap sand near the lighthouse but encourage erosion elsewhere. (Photograph by Blair Tormey. More about the photograph)

When the lighthouse was threatened by erosion in the early 1960s, the federal government responded with a series of efforts to stem the shoreline’s retreat. In 1966, the National Park Service undertook a $300,000 beach replenishment project that pumped sand from the tidal deltas of Buxton Inlet. But the sand was too fine-grained and quickly eroded again.

In 1969, the U.S. Navy subsidized the construction of three 500-foot groins to trap sand in front of the lighthouse. There are drawbacks to the construction of groins, though. Once a sediment trap is created in one location, the downstream beaches become sites of erosion, as the longshore current is able to carry more sediment after depositing its load upstream of the groin. Although the groins at Cape Hatteras were quite effective in trapping sand in front of the lighthouse, the erosion rate down drift of the groins increased by a factor of nine, which caused the beach south of the groins to retreat several hundred meters.

Figure 19 shows one of the groins at Cape Hatteras photographed in 1998. Notice how the groin is very effective in trapping sand on the upstream side (the left in this photograph) but produces pronounced erosion downstream (to the right). The erosion is marked by the lowered level of the beach surface, and the proximity of the shoreline on the southern side of the groin.

Definitions

beach replenishment n.
The addition of new material (sand, sediment, etc) to a beach naturally, or through the action of longshore drift, or artificially through the dumping of large amounts of material.
inlet n.
A recess, such as a bay or cove, along a coast; a stream or bay leading inland, as from the ocean; an estuary; a narrow passage of water, as between two islands; a drainage passage.
longshore current n.
The current located in the surf zone and running parallel to the shore as a result of waves breaking at an angle to the shore.
jetty n.
A protective structure of stone or concrete that extends from the shoreline into the water to prevent a beach from washing away.