WHOI Investigates Tidal Flats
The study is part of a project funded by the Office of Naval Research to investigate morphological evolution of tidal flats with large tidal ranges and significant fine grained sediment input. The WHOI group is one of several from multiple institutions working concurrently on the Skagit tidal flats, each focused on different aspects of the complex system.
This particular field deployment was designed to study tidal trapping of fine grained sediment by the baroclinic pressure gradient between the Skagit River outflow and the saltier Skagit Bay. The sediment trapping process is similar to that for turbidity maxima found in many deeper estuaries, but on the tidal flats the process is complicated by variability between 5 m of water at high tide and exposed tidal flats at low tide. Sediment deposition in the convergence zone could alter local bed morphology, but the process likely depends on river discharge, sediment supply, and tidal amplitude.
Instruments were deployed for the month of June to measure near-bottom currents, turbulence, salinity, temperature, and suspended sediment. Strong horizontal and vertical gradients in density and suspended sediment required sampling at high frequency and vertical resolution. Nortek Vectors and Aquadopp Profilers were among the instruments used in the deployment. Cabled and fixed-head Vectors provided high-frequency velocity observations that will be used to link turbulence levels to the stratification and resuspension of sediment. The small velocity cells and low-profile design of the Aquadopp Profilers were ideal for measuring the velocity structure over the shallow water column. Analysis has just begun, but thus far the data look promising.