Evaluation of Sediment Dynamics and the Mobility of Heavy Minerals on a Linear Sand Shoal

Two instrumented tripods were deployed in summer and fall, 1987, at Smith Island Shoal, a coast-parallel sand ridge near the 10-meter depth contour just north of the Chesapeake Bay entrance on the Virginia inner shelf. Previous studies of the shoal have noted a high abundance of heavy minerals (up to 16 percent by weight) contained within the surficial sediments (fineto very-fine sands) on its flanks, as compared to much lower concentrations elsewhere. Measurements of near-bottom current, wave-orbital velocity, wave height and optically-sensed suspended sediment concentration within a meter of the sea bed show that the flank areas of the shoal frequently experience wave-induced, bottom orbital velocities well in excess of the critical level needed to produce grain motion. During periods of moderate wave activity (Hmo = 0.8- 1.0 m, T = 8.5-9.5 s) bottom orbital velocities were "groupy" in their distribution, reaching maximum values as high as 60 cm/s. Pronounced resuspension of bed material occurred within a meter of the bottom and was strongly modulated at wave-group periods of 100 to 200 seconds. Samples of suspended material show approximately the same mineral composition as that of the local bed material but with some indication of height-dependent mineral sorting. Waveinduced resuspension of heavy minerals is thus indicated during all but fairweather conditions, with near-bottom currents generally slow and erratic. This suggests that the potential for selective mineral sorting is restricted to grains moving in suspension. /// A Smith Island Shoals, barre prélittorale parallèele àla côte délimitée par la courbe de - 10 m et située juste au Nord de l'entrée de la Baie de Chesapeake sur la plateforme de Virginie, on a déployé des tripodes instrumentés. Les études antérieures de ces hauts fonds avaient souligne l'abondance de minéraux lourds Gusqu' à16% du poids) contenus dans les sédiments de surface (sables fins très fins) des flancs de la barre, comparativement aux concentrations beaucoup plus faibles partout ailleurs. Des mesures de la vitesse du courant au fond, de la vitesse orbitale de la houle, de sa hauteur et une estimation optique des concentrations de sédiments en suspension ont été faites dans le premier mètre situé au dessus du fond. Ces mesures montrent que les flancs des hauts fonds sont fréquemment l'objet de vitesses orbitales, induites par la houle bien au dessus du seuil critique nécessaire au mouvement des particules. Pendant les perio