DETERMINING THE CAUSES AND THRESHOLD OF SEDIMENT RESUSPENSION IN THE LAFAYETTE RIVER

To better understand the effects of sediment resuspension on the initiation of harmful algal blooms (HAB), the frequency, duration, and extent of resuspension events were characterized for a 3 week period from June 12, 2017 to July 5, 2017 in the Lafayette River, a tidal estuary in the southern Chesapeake Bay. Time-series data from acoustic instrumentation were used in combination with estuary-bed grain size distributions, porosity measurements, and meteorological data to determine the causes and threshold of sediment resuspension. An acoustic Doppler current profiler measured the speed and direction of tidal currents as well as the intensity of acoustic backscatter (a proxy for suspended sediment concentration) in 10-cm bins through the water column. An integrated pressure sensor measured tidal stage and wave height, and was used in conjunction with the velocity data to estimate wave-orbital velocity. These data were then extrapolated to other areas of the river and contextualized with previous conditions.

We define a resuspension event as a period of time during which the backscatter intensity exceeds 65 dB. Based on this definition, 8 different resuspension events were observed during the research period, seven of which occurred in association with increased wave orbital velocity of above 0.15 m/s. Peaks in the current speed, representing a tidal cycle at its maximum velocity, were also found to coincide with resuspension events. All but one resuspension event occurred when winds were from the SW; winds from this direction occurred most frequently.

The study revealed that prolonged stretches of consistent direction and constantly strong winds can generate wave-orbital velocities above 0.15 m/s. When these conditions are coincident with the maximum tidally induced current speeds, the optimal conditions for sediment resuspension exist. It was revealed that the synchronization of the wave orbital velocity and maximum current speed is more effective at resuspending sediment than either factor alone. Since no HAB occurred during the research period, it is still unknown for certain what effect this resuspension has on the initiation of blooms, still, this conclusion gives further insight into the dynamics of the river and can be applied to future research done on the initiation of HABs.