HYDROGRAPHY AND BOTTOM BOUNDARY LAYER DYNAMICS: INFLUENCE ON INNER SHELF SEDIMENT MOBILITY, LONG BAY, NORTH CAROLINA, USA

Storm-driven processes produced by atmospheric and meteorological forcing dictate sediment transport events in the bottom boundary layer on the inner continental shelf. This study examined the hydrography and bottom boundary layer dynamics of two typical storm events affecting coastal North Carolina, a hurricane and a November event that included two small consecutive extratropical storms during the autumn of 2005. Two upward-looking 1200-kHz Acoustic Doppler Current Profilers (ADCP) were deployed at two separate locations on the inner continental shelf of northern Long Bay, North Carolina at water depths of less than 15 m. Both instruments profiled the overlying water column in 0.35 m bins beginning at a height of 1.35 m above the bottom (mab). Simultaneous measurements of wind speed and direction, wave and current parameters, and acoustic backscatter were coupled with output from a bottom boundary layer (bbl) model to describe the hydrography and boundary layer conditions during each event. The bbl model also was used to generate current and suspended sediment concentration profiles and to quantify sediment transport in the boundary layer during each storm. Both study sites exhibited similar temporal trends in response to changing physical forcing mechanisms, but wave heights during the November event were higher than waves associated with the hurricane. Both near-bottom mean and subtidal currents, however, were of greater magnitude during the hurricane. Suspended sediment transport during the November event exceeded transport associated with the hurricane by 20-70 percent, and substantial spatial variations in sediment transport existed throughout both events. For both storms, along-shelf sediment transport exceeded across-shelf transport and was closely associated with the direction of subtidal currents. Given the substantial variations in sediment type across the bay, complex shoreline, and local bathymetry, the sediment transport rates reported here are very site specific. However, the general hydrography associated with the two storms is representative of conditions across northern Long Bay.