IN-SITU MEASUREMENTS OF PARTICLE SETTLING VELOCITY ON THE NORTHERN CALIFORNIA CONTINENTAL-SHELF

As part of the Sediment TRansport Events on Shelves and Slopes (STRESS) program, a remote optical settling box was deployed on the northern California continental shelf. The device operates by isolating a volume of sediment-laden fluid from the environment and then monitoring its sedimentation behavior with a transmissometer. Results show a bimodal distribution of suspended sediment during low-energy periods on the shelf that reflects the size distribution of bottom sediments. The coarse mode sinks at 0.026 cm s-1 (22 m day-1) and the fine mode settles at 0.0025 cm s-1 (2 m day-1). Between one-quarter and two-thirds of the total mass resides in the coarse mode. Roughly one-quarter is in the fine mode. The remainder sinks too slowly (<0.0015 cm s-1 or <1.3 m day-1) to be resolved during the 18-h measurement cycles. Greatest uncertainty in assigning mass to the various settling velocity classes comes from sensitivity to ill-constrained particle geometry of the conversion from light attenuation to mass. The device failed during higher energy periods, probably due to penetration of fluid into the box. Complete isolation of the fluid from the environment would improve the performance of settling boxes in energetic settings.