Storm sedimentation on the surf zone and inner continental shelf, Duck, North Carolina

Nearshore storm sedimentation on the surf zone and inner continental shelf has previously been documented by beach profiles and cores, but these methods usually provide only pre- and post-storm measurements. By connecting these discrete measurements with continuous sonar altimetry, seabed elevation changes during storms were used to interpret the stratigraphic signature of modern nearshore storm deposits and assess the seabed elevation variability documented by fairweather profiles.; Time series of seabed elevation and co-located measures of wave and current characteristics at 3 water depths (5.5, 8, and 13 m) were collected offshore of Duck, North Carolina. Detailed analyses of seabed elevation changes were conducted for hurricanes and northeaster storms during 1994–1997. Maximum values of net seabed accretion occurred at locations within the outer surf zone, but maximum values of net seabed erosion occurred at locations offshore of the surf zone. At outer surf zone and inner shelf locations, northeaster storms were more likely to cause net accretion than either no net change or net deposition, but hurricanes were as likely to cause net erosion as net deposition.; For a northeaster storm that occurred during the October 1997 SandyDuck experiment, sonar altimeter measurements of seabed elevation were used to establish the chronology of storm sediments collected with diver-operated boxcores. Downcore depths to basal erosion contacts in post-storm cores corresponded remarkably well with erosion maxima measured by sonar altimeters during storm events. Rapid deposition of sediments occurred in the few hours preceding and initial 4–20 hours following maximum wave heights, when gradients in wave height, mean currents, and associated bed shear stresses were relatively large. Nearshore storm deposits consisted of up to 20 cm of parallel to sub-parallel laminated sediments, with occasional ripple cross-stratification and lag deposits composed of gravel and shell fragments.; Continuous sonar altimeter measurements during storm events were compared with nearby pre- and post-storm beach profile data. Sonar altimeters at 5.5, 8, and 13 m depths measured a range of seabed elevations of approximately 40 cm. Smaller ranges of seabed elevations were measured by profiles at 5.5 and 8 m depths, because fairweather beach profiles only document net seabed elevation changes resulting from storm events.