Spit Growth and Downdrift Erosion: Results of Longshore Transport Modeling and Morphologic Analysis at the Cape Lookout Cuspate Foreland

Abstract This study examined geomorphologic changes, littoral processes and sediment budget along the west side of the Cape Lookout, North Carolina, cuspate foreland, where a prograding spit at right angles to an eroding barrier island forms prominent features. Progradation of the spit is of interest geologically because it occurs within a transgressive, sediment-starved coastal setting in which adjacent barrier island limbs are eroding and in which immediate sediment sources and transport mechanisms are not obvious. Results indicate that as the spit extends to the north, it also widens to the west through swash bar attachment, leaving a well-defined record of dune ridges that mark former shoreline positions. Although northerly longshore transport occurs only under SW wave conditions, the average rate, ∼0.18 × 106 m3/y, is considered high given the sheltered position of the spit within the cuspate foreland. Results of modeling and development patterns of a series of ridge and runnel systems along the spit indicate that sediments are derived from the eastern limb of the cape or, under certain conditions, from sediments stored in Cape Lookout Shoal by longshore currents. Because the spit forms the distal end of a terminal littoral cell, it has an important effect on the regional sediment budget. Shackleford Banks, oriented at right angles and located immediately downdrift of Power Squadron Spit, is sheltered by the spit at its eastern end but suffers high erosion rates in its central section from lack of sediment input from the updrift direction. This result suggests that change in the input sediment budget can be a major factor in controlling coastal geomorphologic change under uniform physical conditions.