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Distinctive ebb and flood oriented bed-form fields are present on the opposite flanks of a tidal ridge within Willapa Bay, Washington. The ridge is approximately 3 km long, arcuate in shape, asymmetric in profile, and rises a maximum of 13 m above the channel floor. Repeated high-resolution profiling, diving observations, measurement of bed-form geometry, velocity profiles, and oriented cores define the formational processes and the depositional features of this migrating ridge. Large-scale bed forms migrate along and up both ebb and flood flanks of the ridge during each respective tidal flow. Sinuous- to straight-crested sand waves occur on the steeper flood-dominated side; dunes and sand waves occur on the ebb flank. As the bed forms migrate up the ebb flank of the tidal ridge, they become lower and exhibit the following sequence in response to decreasing flow velocity: lunate dunes^rarrcatenary sand waves^rarrsinuous- to straight-crested sand waves. The largest bed forms, 3 m high, occur between depths of 9 to 12 m within the lunate dune field. Bed forms along the ridge crest change orientation with each tidal cycle. Bed forms on the ridge flanks reverse completely during spring tides, but during neap tides only the crests reverse. Oriented cores show unidirectional medium-scale cross-strata with reactivation surfaces on the flood and the ebb flanks of the ridge. Cores taken in troughs of some ebb-oriented dunes, however, show a veneer of ebb cross-strata overlying flood-oriented structures. Cores on the ridge crest have bimodal small-scale cross-strata. The dominant ebb current transports sediment over the ridge crest where it is incorporated into flood bed forms on the reversing tide. Erosion on the ebb flank and deposition on the flood flank has moved the ridge 15 to 30 m/year for the last 20 years. End_of_Article - Last_Page 509 |
