September 19, 2024
Journal Article
Mixing of the Connecticut River plume during ambient flood tides: Spatial heterogeneity and contributions of bottom-generated and interfacial mixing
Abstract
The Connecticut River plume is influenced by energetic ambient tides in the receiving waters of Long Island Sound. The objectives of this modeling study are 1) characterizing the spatial heterogeneity of turbulent buoyancy fluxes within the tidal plume, 2) partitioning turbulent buoyancy fluxes into bottom-generated and interfacial shear contributions and rating their relative importance, and 3) quantifying contributions to plume-integrated mixing. The plume formed during ambient flood tides under low river discharge, spring tides, and no winds is analyzed to highlight conditions favoring bottom-generated tidal mixing. Turbulent buoyancy fluxes and the corresponding depth-integrated fluxes through the plume (Bd), are characterized by a high degree of spatial heterogeneity spanning several orders of magnitude. At max ambient flood, strong mixing (Bd~10-5-10-4 m3/s3) occurs near the mouth, in the plume turning region, parts of the downstream end, and along a shallow coastal band. Low to moderate mixing levels (Bd~10-8-10-6 m3/s3) occupy about half of the plume. Buoyancy fluxes are partitioned with a standard method based on the depth of the shear stress minimum between plume-generated and bottom-generated shear maxima (d?? method). Other tested partitioning methods are based on parameterizations for open channel flow and stratified shear flow. All methods indicate a significant plume mixing role for bottom-generated shear, but interfacial mixing is a bigger contributor overall than bottom-generated mixing. All methods are viable, but the d?? method is preferred in this study since it incorporates model shear stress profiles rather than parameterizations. Plume-integrated mixing peaks at max ambient flood and two-thirds of the mixing occurs in concentrated intense mixing areas. A parameter space formed with the ambient tidal Froude number and plume thickness to depth ratio rates bottom-generated tidal mixing to interfacial mixing. The Connecticut plume is in the moderate tidal mixing category. Many tidally modulated plumes around the world likely are moderately to dominantly mixed by bottom-generated tidal mixing.Published: September 19, 2024