OBSERVATIONS OF NEAR-BED CONDITIONS AND SEDIMENT RESPONSE DURING THE PASSAGE OF HURRICANE ISABEL ON THE INNER CONTINENTAL SHELF OF ONSLOW BAY, NC

Hurricane Isabel impacted Onslow Bay in southeastern North Carolina during the week of 9/15/2003 passing 120 miles east of Wrightsville Beach, NC on 9/18. Long period swells with wave heights and periods greater than 1.5 m and 16 s, respectively, first arrived from the southeast on 9/15. Excessive wind and wave activity persisted through 9/19. During this major storm event, a bottom mounted upward looking Acoustic Doppler Current Profiler (ADCP) located five nautical miles east of Wrightsville Beach, NC on the inner shelf of Onslow Bay (34° 06’ N 77° 45’ W) recorded hourly flow velocity profiles while simultaneously collecting surface wave data at four hour intervals. Acoustic backscatter measures were also obtained and used as a proxy for suspended sediment concentration. Between 9/15 and 9/19 near-bottom mean current velocities at the study site attained speeds of 9 cm s^-1 at 1.22 m above the bed. These currents were highly variable in direction prior to 9/17. Bottom wave orbital velocities ranged from 20 to 50 cm s^-1 during storm approach and were associated with long period swells (>14 s) from the southeast. Combined wave-current critical shear velocities calculated using the Styles & Glenn (2002) benthic boundary layer model ranged from 5 to 8 cm s^-1. These conditions exceeded critical values required to mobilize the fine sands present at the study site. An elevated acoustic backscatter signal obtained during this period and fine grained sand (d_m=0.21 mm) collected in a sediment tube mounted 0.23 m above the bottom indicate the suspension of bottom sediments. Tropical storm force winds began to directly impact the study area late on 9/17 and persisted for 34 hours. Near-bottom mean currents increased to 26 cm s^-1 and both orbital velocities and acoustic backscatter signal remained elevated. Subtidal flows, generated in response to the enhanced local wind field, rapidly increased from 8 to 20 cm s^-1. Currents during this period were directed from northeast to southwest and favor the transport of sediment in the onshore direction. Our data indicate that during these types of events, bottom stresses induced by wave orbital action mobilize the sediment, while subtidal currents generated by sustained, high energy winds favor transport of suspended sediment in the across and along-shelf directions.