VIMS SEA CAROUSEL - A FIELD INSTRUMENT FOR STUDYING SEDIMENT TRANSPORT

Preliminary results from field experiments using a newly developed annular sea-bed flume, VIMS Sea Carousel, are presented to illustrate a method of in situ determination of critical threshold shear stress necessary for sediment entrainment. This flume has been deployed in the lower Chesapeake Bay and on the inner shelf of the Middle Atlantic Bight. We applied a series of controlled bed shear stresses by changing the stress sequentially, to examine the responses of nearly undisturbed natural sediment beds. An optical backscatter sensor mounted on the flume indicated the occurrences of resuspension of bed material. We identified clear threshold values of bed shear stress for resuspension. Generally, the sandy inner shelf sediment had a higher critical bed shear stress (0.22 Pa) for resuspension than that for silty sediments (0.1-0.19 Pa). Data from partially cohesive sediments in the lower Chesapeake Bay indicate significant seasonal differences in critical bed shear stress, 0.19 Pa at the beginning of summer and 0.11 Pa at the end of summer. These differences may be attributed to bioturbation, the seasonal difference in the microflora living on the grains, or spatial heterogeneity of bed.