Currents in a small tidal-flat channel

Abstract

Near-bottom currents observed on a tidal flat are compared with those observed 50 m away inside a shallow (0.25 to 0.40 m deep) channel. For water depths between 0.5 and 2.5 m (when both current meters are submerged), current speeds 0.13 m above the bed on the flat are about 30% greater than those observed 0.13 m above the bed in the channel, and are approximately equal to those observed 0.58 m above the channel bed (0.26 m above the flat elevation). Flow directions on the flat are similar to those in the channel. For flows parallel to the channel axis, the ratio of speeds 0.13 m above the bed on the flat to those 0.13 m above the bed in the channel decreases from about 1.4 to about 1.1 with increasing water depth, consistent with conservation of mass. For flows directed across the channel axis, the ratio of speeds increases from about 1.3 to about 2.2 with increasing water depth. The corresponding ratio of the vertical velocity variances (a proxy for turbulence) decreases from about 1.5 to about 0.2, suggesting that the turbulence near the bed of the channel is greater than that near the bed of the flat for water depths greater than about 1.0 m. In contrast, for along-channel flows, the channel and flat turbulence levels are similar. Drag coefficients estimated with the vertical velocity variance or with a cross-shore momentum balance are approximately 70% larger in the channel than over the visually smoother flat.