Hydraulically controlled front trapping on a tidal flat

Abstract

[1] We report observations of baroclinic surface fronts in Skagit Bay. The fronts formed repeatedly in very shallow (<0.7 m deep) water along the edge of a tidal channel shortly after the incoming tide flooded the adjacent tidal flat and eventually departed from channel edge as the flood tide progressed. The fronts were surface convergence zones, trapping GPS drifters and other flotsam. Across-front salinity (temperature) jumps were a few practical salinity units (a few degrees) with freshwater on the channel side. Across-front transects of velocity (from mobile and fixed acoustic Doppler current profilers) and density (from mobile and fixed conductivity-temperature-depth profilers) are presented. Composite Froude numbers indicate that supercritical flow over the flats initially held fronts near the channel edge. As flood tide progressed, depth-averaged flow speeds decreased, while internal wave speeds increased, owing to increasing depths and density gradients. Flow over the flats eventually became subcritical, at which point fronts departed from the channel edge. The front then formed the leading edge of a shallow (around 0.3 m thick) freshwater plume which propagated across the flats against the incoming tidal flow. Water was then exchanged between the channel and flats with a channel edge composite Froude number near 0.5, consistent with mixing-induced reduction in channel-flats exchange. The across-channel velocity shear was correlated with the across-channel wind, consistent with a simple eddy viscosity model. Strong winds prevented plume propagation.