Hydraulics and mixing in a laterally divergent channel of a highly stratified estuary

• Published in: Journal of geophysical research. Oceans 2017-06, Vol.122 (6), p.4743-4760
• Main Authors: Geyer, W. Rockwell, Ralston, David K., Holleman, Rusty C.
• Format: Article
• Language: English
• Subjects: Aspect ratio; Boundary layer; Brackishwater environment; Conservation; Echo sounding; Echoes; Echosounding; Entrainment; Estuaries; Estuarine dynamics; Estuarine mixing; estuary; Flow geometry; Flow stability; Fluid dynamics; Fluid flow; Froude number; Geophysics; gradient Richardson number; Hydraulics; Instability; internal hydraulics; Kelvin-Helmholtz instability; Kinetic energy; Loads (forces); Mixed layer; mixing; Parameterization; Pycnocline; Rapid flow; Richardson number; Rivers; Scaling; Seafloor spreading; Shear; Sounding; Supercritical flow; Tides; Turbulence; Turbulence intensity; Turbulent flow; Turbulent kinetic energy
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1002/2016JC012455

Estuarine mixing is often intensified in regions where topographic forcing leads to hydraulic transitions. Observations in the salt‐wedge estuary of the Connecticut River indicate that intense mixing occurs during the ebb tide in regions of supercritical flow that is accelerated by lateral expansion of the channel. The zones of mixing are readily identifiable based on echo‐sounding images of large‐amplitude shear instabilities. The gradient Richardson number (Ri) averaged across the mixing layer decreases to a value very close to 0.25 during most of the active mixing phase. The along‐estuary variation in internal Froude number and interface elevation are roughly consistent with a steady, inviscid, two‐layer hydraulic representation, and the fit is improved when a parameterization for interfacial stress is included. The analysis indicates that the mixing results from lateral straining of the shear layer, and that the rapid development of instabilities maintains the overall flow near the mixing threshold value of Ri = 0.25, even with continuous, active mixing. The entrainment coefficient can be estimated from salt conservation within the interfacial layer, based on the finding that the mixing maintains Ri = 0.25. This approach leads to a scaling estimate for the interfacial mixing coefficient based on the lateral spreading rate and the aspect ratio of the flow, yielding estimates of turbulent dissipation within the pycnocline that are consistent with estimates based on turbulence‐resolving measurements. Key Points Intense mixing occurs in highly stratified estuaries as a result of laterally divergent, supercritical flow during ebb Mixing zones have nearly continuous shear instability and nearly constant gradient Richardson number close to the critical value of 0.25 Turbulence intensity, quantified by dissipation of turbulent kinetic energy, can be estimated based on the hydraulic state and flow geometry