SEDIMENT MOBILITY DURING STORMS ON THE MID-CONTINENTAL SHELF: ONSLOW BAY, NC

Onslow Bay is a sediment starved system subject to frequent impact by northeasters and hurricanes. These high energy events modify the morphology of hardbottoms and redistribute sediment. This study examines the impact of small storm events on sediment mobility at one location on the mid-continental shelf where the water depth is 28 m.

A self-recording instrument package was secured to a 2 m tall frame anchored on a sand apron approximately 30 m from a marine hardbottom (33° 59'N, 77° 21'W). Instrumentation included a downward looking Pulse-Coherent Acoustic Doppler Profiler (PC-ADP) at 150 cm above the seabed, two optical backscatter sensors (OBS) at 30 cm and 80 cm above the bottom, and an upward looking Acoustic Doppler Current Profiler (ADCP) that profiled the overlying water column. The PC-ADP also serves as a bottom altimeter to monitor changes in seabed height. Flow velocity, turbidity, and seabed altimetry were used to assess flow characteristics leading to sediment disturbance. Pre- and post-storm boxcores were collected and used to document the impact of storm-reworking on the sediment record.

During a small northeaster in September 2000, 30 knot winds sustained over two days generated 3 m waves and increased mean water column velocities to 30 cm s^-1. The magnitude of near-bottom currents increased to 15 cm s^-1. During the storm, turbidity pulses coincided with up to 6 cm of seabed erosion. Maximum sustained turbidities occurred as the storm waned, two days after storm onset. Both pre- and post-storm cores consisted of laminae that were highly bioturbated in the upper 10 cm. Such mobilization events can restructure the substrate, redistribute sediments and infauna, and impact productivity of the nearby hardbottom ecosystem.