Hydrodynamics and scalar transport in subtidal channels through intertidal mudflats

Subtidal channels drain through intertidal mudflats and marshes and provide important conduits for scalar transport in the intertidal zone. Although the bathymetric shift between channels and mudflat shoals is relatively small, strong density gradients and shallow, variable flow depths make it significant. A combination of field observations and numerical modeling indicates that hydrodynamic processes common to much deeper flows are also important in shallow intertidal settings. Density gradients and bathymetry impact intertidal flow dynamics over spatial scales ranging from turbulent eddies to circulation patterns and net fluxes, and the hydrodynamics are closely coupled to transport of salt and sediment that are critical for intertidal biology and bed morphology.; During each tidal inundation of the mudflats, a sharp salinity front develops between relatively fresh water in tributary channels and saltier bay water. Tidal straining of the salinity gradient generates very strong periodic stratification that at times significantly suppresses turbulent motions and vertical mixing of momentum and scalars. The salinity front also creates substantial baroclinic forcing both longitudinally across the intertidal zone and laterally between subtidal channels and mudflat shoals. Differential advection of the front between channels and shoals produces lateral density gradients that drive secondary circulation and alter along-channel momentum and salinity budgets. Sharp salinity fronts persist throughout the wet season because conditions in the intertidal zone essentially reset and salinity gradients regenerate with each inundation. Through each inundation the salinity front disperses longitudinally due to vertical and lateral shear mechanisms. Periodic stratification enhances vertical shear while frictional effects between channel and shoal create lateral shear dispersion. The channel/shoal bathymetry is important not only for longitudinal transport, but also impacts vertical profiles of turbulence and mixing. Skewed velocity shear across channels generates turbulence at the channel banks and adds mixing energy mid water column; the mixing depends on the flow angle and longitudinal density forcing. Finally, hydrodynamics resulting from the salinity front and channel/shoal bathymetry affect sediment transport across the mudflat, including tidal phasing of suspended sediment concentrations and seasonal patterns of particle size.