Threshold of sediment movement by open ocean waves: observations

Abstract

Between 8 and 25 March, 1973, a sequence of bottom photographs, sediment samples, bottom current measurements, and bottom pressure measurements was recorded continuously at several locations on the Washington continental shelf. Concurrently, wave conditions were recorded at 35-m depth on the summit of Cobb Seamount 465 km west of the shelf experiment. During the sampling period the bottom turbidity at 167 m on the shelf varied significantly in response to the magnitude of wave activity, and these data are used to evaluate the threshold wave conditions necessary to iniatiate the observed sediment movement. The bottom oscillatory velocity, orbital diameter of wave motion, and the mean sediment size occurring at the shelf location are compared. For unconsolidated silt-sized sediment the expression ρU 2 1 10 (ρ 8 − ρ)gD = 0.13 d 0 D 1 2 where ϱ 8 and ϱ are the sediment and fluid densities, d 0 is the horizontal excursion of water particles at the sea floor, D is the grain diameter, and U 1 10 the higher 10% of bottom oscillatory velocities, describes the threshold of grain motion as observed.