Storm-driven variability of the beach-nearshore profile at Duck, North Carolina, USA, 1981–1991

Abstract

The US Army Corps of Engineers' Field Research Facility (FRF) at Duck, North Carolina, has collected approximately biweekly beach-nearshore profile data to 8-m depth and associated wave data since 1981. Sediment budget analysis was used to examine the medium-scale (years to a decade) variability of the beach-nearshore profile from 1981 to 1991. Significant changes occurred during four groups of energetic storm events during February/March of 1983, 1987, 1989 and December 1989. Each group was comprised of at least two storms within a period of less than 39 days both with H mo > 4 m. During each storm group, offshore sediment movement caused a distinct outer bar to migrate offshore and grow in size resulting in an abrupt increase in the volume of sediment on the upper shoreface. The net profile changes were much larger than the changes due to single storms and the cumulative effect of the storms can be considered as one ‘event’. During these events, the first storm appears to have a destabilizing effect on the profile which has insufficient time to recover before the second (and subsequent) storm (s). As a result, several storms in quick succession are able to have a large impact on the morphology. The intervening periods between the groups of storm events (termed fairweather conditions) lasted up to 4 years. They are characterized by slow, but steady sediment redistribution (averaging 33 m 3 m −1 year −1) from the upper shoreface (> 5 m depth) toward the shore, while the total sediment volume was effectively constant. The onshore feed of sediment was not significantly affected by individual storms during the fairweather conditions. These two processes of (1) morphologic change during groups of storm events and (2) the steady onshore feed of sediments from the shoreface during fairweather conditions appear to play an important role on medium- and long-term profile evolution at least at Duck.