The study was 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 was one of several 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 probably depends on river discharge, sediment supply and tidal amplitude.
Instruments were deployed for a month during summer 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.
Ideal for shallow water column
Cabled and fixed-head Nortek Vectors provided high-frequency velocity observations 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.
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