Abstract
Fourteen 27-hr tidal-current stations were occupied in the entrance to Chesapeake Bay, Virginia, in a 100-km2 area of subtidal sand banks where the average water depth is 6.5 m. At ebb strength, surface current speed ranged from 132 to 64 cm/sec; near-bottom speed ranged from 79 to 24 cm/sec. At flood strength, surface current speed ranged from 104 to 57 cm/sec; near-bottom speed ranged from 65 to 35 cm/sec. Near the bottom at more than half the stations, flood flow dominates over ebb flow in both peak speed and duration.
Velocity data from bottom to surface were fitted by a parabolic velocity defect law from which shear stress at the bed was estimated. At ebb strength, this stress ranged from 185 to 13 dynes/cm2. At flood strength, this stress ranged from 80 to 4 dynes/cm2. Peak flows at all stations are competent to move bed sediment. An index of net sediment transport rate at each station was obtained from a calculation of competent flow power, corrected to mean tidal range, and integrated over an entire observation period. With two exceptions, net transport of bed sediment is ebb directed.
Decrease in net sediment transport rate along a path of net motion requires deposition of sediment. Locations of deduced deposition correspond to shoals. Prominent indentations, or sinuses, in the margins of shoals, are strongly dominated by net sediment transport toward the closed end of a channel.
Zig-zag shoals extend from the north cape three-fourths of the distance to the south across the entrance. This line of shoals is comprised of oppositely opening sinuses, or parabolas, which are with distance to the south alternately ebb dominated and flood dominated. The shoals represent spit-building action in the presence of strong reversing tidal currents which are mutually evasive. Other shoals in the entrance are due to subaqueous levee building marginal to main channels. Still other shoals in the entrance are constructed when a flood-dominated channel intersects the ebb-directed path of net sediment transport; the interdicted sediment then is flushed landward to form a secondary shoal when spreading of the flood current occurs.
