| Shear stresses on the bottom of sandy tidal channels create periodic undulations called bedforms. In turn, these features may impart the dominant source of friction onto the tidal flow. The majority of our knowledge regarding bedforms is based on flume and river studies where the flow is steady and unidirectional. These assumptions do not apply to tidal settings where flow is unsteady and bidirectional. Data collected at two sites tested the hypothesis that, in addition to the flow and sedimentologic regime, sediment availability, wave processes, and dredging practices control the morphology and stability of the bedforms.; Sequential mapping at Moriches Inlet, NY, showed that bedforms at this site are 39 cm high and moribund. Theoretically, bedforms of this height should only form when flow velocities reach 0.8 m/s. However, maximum measured velocities during the study were only 0.6 m/s. It is hypothesized that the bedforms become active during storms when strong winds or storm-induced surges increase the tidal range and the ensuing tidal currents. A two-dimensional, depth-integrated hydraulic model indicates that a current velocity of 80 cm/s will occur when the tidal range exceeds 1.6 m, ∼1.0 m greater than the typical spring tidal range. Regression analysis of hydraulic parameters measured in the field confirms this.; At the Humboldt Entrance Channel, CA bedforms are 15 m long and 35 cm high, smaller than theoretical dimensions expected of more than 75 m long and 70 cm high based on water depth and current velocity. Bedforms are located in water depths of 7-15 m, where the 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 no correlation exists between grain size, depth, or flow velocity and bedform size. It is hypothesized that factors such as sediment availability, shoaling waves, and dredging activity ultimately limit the size of the bedforms. Currents in the channel thalwag exceed 1.4 m/s, producing a channel lag that reduces sand availability for bedform development. Bedform crests are also denuded by wave-current interaction when orbital velocities enhance the flood tidal current. |
