Across-shelf benthic transports on the inner shelf of the Middle Atlantic Bight during the “Halloween storm” of 1991

Abstract

Instrumented bottom boundary layer tripods were deployed on the inner shelf at depths of 13 m and 8 m off the U.S. Army Corps of Engineers Field Research Facility at Duck, North Carolina, USA over a two week period that included the severe and prolonged “Halloween storm” of late October 1991. The storm persisted for five days and generated waves with heights and periods of up to 6 m and 22 s. Although the instrumentation was destroyed, current profile and suspended sediment concentration profile data were recovered from the 13 m site. Wind-driven mean along-shelf currents at 1.24 m above the bed attained speeds of nearly 0.5 m s −1; across-shelf flows, primarily seaward-directed, had speeds varying from 0.05 to 0.15 m s −1. These seaward flows intensified in association with groups of high waves. Total, mean current, and skin-friction bed shear stresses were increased, relative to moderate energy values, by more than an order of magnitude. Notably, the highest shear stresses occurred in association with high, long period swell during the late phase of the storm after winds had turned offshore creating shoreward mean flows near the bed. Suspended sediment concentrations exceeded 1 g 1 −1 throughout the lower meter of the water column and at elevations well above the top of the wave boundary layer. Net across-shelf suspended sediment fluxes were strongly dominated by mean flows. Cospectral analyses show that important, but secondary, roles were also played by infragravity oscillations and by wave orbital velocities.