Bottom sediments and depositional rates near Thimble Shoal Channel, lower Chesapeake Bay, Virginia
Thimble Shoal Channel is a major navigational waterway in lower Chesapeake Bay, Virginia. It is 19.3 km long, 13.7 m deep, and 305 m wide. On an annual basis, the volume of maintenance dredging of the channel averages 400,000 ± 130,000 m 3 . Averaged over the area of the channel, this amounts to a depositional rate of 67 ± 22 mm/yr, which is at least 50 times the depositional rate averaged for Chesapeake Bay as a whole. Comparison of depth changes between 1854 and 1978 in a 205-km 2 study area that encloses the channel, reveals that the average net depositional rate, excluding the channel, is 3.7 mm/yr. In marginal belts parallel to the channel on the north and on the south, the net depositional rates are 9.5 and 6.5 mm/yr, respectively. A computation of depositional rate for the ebb plume of the James River yields a figure of 32 mm/yr as an upper limit for the rate for the center of the area studied. The chart comparisons show that Tail of the Horseshoe, a sediment shoal to the north of Thimble Shoal Channel, has experienced net aggradation on its southern flank as if the shoal were migrating southward toward the channel, a direction of motion exhibited by other shoals of the area. This motion would also contribute to in-filling of the channel. Estimates from a model which considers the distribution of sediment types indicate that filling by settling from suspension and by lateral migration of sediment as bed load into the channel over its northern rim are approximately equal in magnitude.