STORM-DRIVEN VARIABILITY OF THE BEACH-NEARSHORE PROFILE AT DUCK, NORTH-CAROLINA, USA, 1981-1991

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. Sedimen t budget analysis was used to examine the medium-scale (years to a dec ade) variability of the beach-nearshore profile from 1981 to 1991. Sig nificant changes occurred during four groups of energetic storm events during February/March of 1983, 1987, 1989 and December 1989. Each gro up was comprised of at least two storms within a period of less than 3 9 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 s ize resulting in an abrupt increase in the volume of sediment on the u pper shoreface. The net profile changes were much larger than the chan ges due to single storms and the cumulative effect of the storms can b e considered as one 'event'. During these events, the first storm appe ars to have a destabilizing effect on the profile which has insufficie nt time to recover before the second (and subsequent) storm(s). As a r esult, several storms in quick succession are able to have a large imp act on the morphology. The intervening periods between the groups of s torm events (termed fairweather conditions) lasted up to 4 years. They are characterized by slow, but steady sediment redistribution (averag ing 33 m(3) m(-1) year(-1)) from the upper shoreface (>5 m depth) towa rd the shore, while the total sediment volume was effectively constant . The onshore feed of sediment was not significantly affected by indiv idual storms during the fairweather conditions. These two processes of (1) morphologic change during groups of storm events and (2) the stea dy onshore feed of sediments from the shoreface during fairweather con ditions appear to play an important role on medium- and long-term prof ile evolution at least at Duck. (C) 1998 Elsevier Science B.V. All rig hts reserved.