Currents in a small channel on a sandy tidal flat

Channels affect drainage and bed stresses on tidal flats. Here, near-bottom currents observed on a sandy tidal flat are compared with those observed 35 m away inside a shallow (≈0.3 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 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. Drag coefficients estimated with the vertical velocity variance are approximately 70% larger in the channel than over the visually smoother flat, consistent with prior studies suggesting that channels may increase tidal-flat roughness. For flows directed along the channel axis (in the cross-flat direction), the ratio of speeds (1.2) is similar to the ratio predicted by a cross-flat momentum (along-channel) balance.