SAND WAVES IN TIDAL CHANNELS

The sandy bottoms of tidal channels are dynamic. Shear stress on the seabed creates periodic undulations, or bedforms. The majority of our knowledge regarding bedforms is based on data from flumes and rivers having essentially steady, unidirectional flow. These assumptions do not accurately describe tidal settings where flow is unsteady and bidirectional. This study demonstrates that, in addition to flow regime and sedimentologic conditions, the bedform geometry is affected by sediment availability, shoaling waves, and dredging practices.

Data were collected at two sites to determine the parameters controlling the geographic extent, morphology, and stability of the sand waves (bedforms > 6 m long). Sequential mapping of a sand wave field at Moriches Inlet, NY showed that the bedforms at this site are moribund. They are 39 cm high, a height which theoretically requires a velocity in excess of 80 cm/s. Maximum measured velocities during the study were 60 cm/s. It is hypothesized that the sand waves develop during moderately large storms when wind or a storm surge may increase the tidal range and ensuing tidal currents. A simple hydrodynamic model indicates that a current velocity of 80 cm/s requires a tidal range of 1.6 m, an increase of ~1.0 m compared to the typical spring tidal range.

At the Humboldt Entrance Channel, CA sand waves are 15 m long and 35 cm high compared to theoretical dimensions of more than 75 m long and 70 cm high. The sand waves are located in depths 7-15 m, where the mean grain size is 0.2-0.9 mm, and the peak current velocities range between 0.4 and 1.0 m/s. Within the channel there was no correlation between mean grain size, water depth, or flow velocity and sand wave size. Therefore, it is hypothesized that other factors such as sediment availability, wave energy, and dredging activity ultimately limit the size of the sand waves. The peak spring tidal current in the center of the channel exceeds 1.4 m/s. The strong current velocities lead to a winnowing of the finer sand and may produce a channel lag. The lag limits the available sediment and impedes sand wave development. Sand wave crests may also be denuded by wave-current interaction during large swell events.