Mixing of the Connecticut River Plume During Ambient Flood Tides: Spatial Heterogeneity and Contributions of Bottom-Generated and Interfacial Mixing

The Connecticut River plume is influenced by energetic ambient tides in the Long Island Sound receiving waters. The objectives of this modeling study are (a) characterizing the spatial heterogeneity of turbulent buoyancy fluxes, (b) partitioning turbulent buoyancy fluxes into bottom-generated and interfacial shear contributions, and (c) quantifying contributions to plume-integrated mixing within the tidal plume. The plume formed during ambient flood tides under low river discharge, spring tides, and no winds is analyzed. Turbulent buoyancy fluxes (B) and depth-integrated B through the plume (Bd) are characterized by pronounced spatial heterogeneity. Strong mixing (Bd similar to 10-5-10-4 m3/s3) occurs near the mouth, in the nearfield plume turning region, over shoals, and nearshore shallow areas. Low to moderate mixing (Bd similar to 10-8-10-6 m3/s3) occupies half the plume. Buoyancy fluxes are first partitioned based on the depth of the shear stress minimum between plume-generated and bottom-generated shear maxima. Four other tested partitioning methods are based on open channel flow and stratified shear flow parameterizations. Interfacial and bottom-generated shear contribute to different areas of intense and moderate mixing. All methods indicate a significant plume mixing role for bottom-generated mixing, but interfacial mixing is a bigger contributor. Plume-integrated total and interfacial mixing peak at max ambient flood and the timing of peak bottom-generated mixing varies among partitioning methods. Two-thirds of the mixing occurs in concentrated intense mixing areas. A parameter space with the ambient tidal Froude number and plume thickness to depth ratio as axes indicates many tidally modulated plumes are moderately to dominantly influenced by bottom-generated tidal mixing. A plume of freshwater flows from the Connecticut River into Long Island Sound. Strong tidal currents influence the plume. How freshwater river plumes mix is an important area of research. This study explores where mixing occurs in the plume and which processes contribute to the mixing. A realistic computer simulation of the Connecticut River plume is analyzed. The strongest mixing areas are near the river mouth, in an area beyond the mouth where the plume turns, and over shallow waters. Mixing created by faster plume speeds relative to the surrounding waters is the most important contributor. Mixing driven by tidal currents interacting with the stationary bottom is a significant secondary contributor. The importance of this tidal contribution increases for faster tidal flow and plumes reaching closer to the bottom. Turbulent buoyancy fluxes span several orders of magnitude over the river plume Plume-generated interfacial shear is the most important mixing contributor overall Bottom-generated mixing importance increases with tidal currents and plume thickness