A comparison of measured and modeled suspended sediment concentration profiles during different types of meteorological events on the inner-shelf of Long Bay, South Carolina

An instrumented quadrapod was deployed on the inner-shelf of Long Bay, South Carolina at a depth of approximately 7 meters from July 2008 - December 2009. A multi-frequency Acoustic Backscatter Sensor (ABS) and a pulse-coherent Acoustic Doppler Current Profiler (PC-ADCP) were used to collect co-located long-term measurements of suspended sediment concentrations and high-resolution velocity profiles throughout the bottom boundary layer. To invert the acoustic backscatter voltage collected by the ABS, a semi-empirical backscattering theory (Thorne and Hanes, 2002) was first verified with sediment chamber measurements using local sediments. The verified theory was then employed to determine sediment grain size and suspended sediment concentrations. Additionally, to complement the field observations, two different 1-D bottom boundary layer models (Styles and Glenn, 2000 and Wiberg and Smith, 1983) were applied to estimate the bed shear stress, velocity and suspended sediment concentration profiles. Although both models are based on boundary layer theory (Smith,1977; Grant and Madsen, 1979), and require as inputs of wave and current velocities as well as characteristics of sea-bed sediment, they are distinct in some practical applications such as eddy-viscosity profile, ripple dimension estimation, model gridding, etc. Recorded waves, currents, and acoustic backscatter data revealed that nineteen sediment transport events occurred during the eighteen month study period, which resulted in an order of magnitude range of suspended sediment concentrations. Comparisons of sediment concentration profiles between the two 1-D models and the field measurements were conducted during several different types of sediment transport events. The events that were used for the comparisons included two types of frequently occurring frontal systems, as well a direct hit from Tropical Storm Hannah. During the selected events near-bottom orbital velocities ranged from 20 - 60 cm s-1 and currents ranged from 5 - 15 cm s-1, while the range of in suspended sediment concentrations and transport was greater than an order of magnitude.