Hydraulic Control of a Highly Stratified Estuarine Front*

Abstract

Observations at the mouth of the Fraser River (British Columbia, Canada) indicate an abrupt frontal transition between unstratified river outflow and a highly stratified river plume with differences in salinity greater than 25 psu across a few meters in the vertical direction and several hundred meters in the horizontal direction. The front roughly follows a natural break in the bathymetry, crossing the channel at an angle of approximately 45°, and is essentially stationary for a period of approximately 3.5 h centered on the low tide following the larger of two daily ebbs. The location of the front is coincident with observations of significantly supercritical internal Froude numbers at the front, based on velocities in the along-flow direction. This observation contradicts the one-dimensional theory, which indicates that the Froude number should be 1. However, because the front is oriented obliquely to the outflow, a coordinate system can be selected that is normal to the front and for which a critical Froude number of 1 is obtained. This indicates that a Froude angle, similar in application to a Mach angle for transonic flows, can be used to determine critical conditions when the front is oblique to the principal flow direction.

Role of mixing in the structure and evolution of a buoyant discharge plume

“…The nature of this lift-off, and the role of mixing in the near field are two key components of understanding near field structure. Observations of river plumes have found that lift-off typically occurs uniformly across the cross section [e.g., Wright and Coleman, 1971], consistent with two-layer hydraulic theory [i.e., Armi and Farmer, 1986;Farmer and Armi, 1986], or at an angle to the cross section, consistent with a two-dimensional (2-D) extension of hydraulic theory [MacDonald and Geyer, 2005]. Landward of the lift-off location, the discharging water mass is essentially unmodified, but substantial mixing occurs seaward of the lift-off in both the Mississippi [Wright and Coleman, 1971] and Fraser River [MacDonald and Geyer, 2004] outflows.…”

“…This will happen when either the lateral encroachment of the plume by colder ambient waters at depth overcomes vertical mixing processes and closes in across the entire plume, or when the local bathymetry triggers the formation of a bottom front. Liftoff in the latter case can be well described by classic one dimensional two-layer hydraulic theory [e.g., Armi and Farmer, 1986;Farmer and Armi, 1986], or more recent two-dimensional extensions of hydraulic theory [MacDonald and Geyer, 2005], which are beyond the scope of the present study. Atkinson [1993] and others have predicted lift off points for a uniformly sloping bottom.…”

Oblique Internal Hydraulic Jumps at a Stratified Estuary Mouth

“…Klymak and Gregg (2001) found counterrotating eddies in the lee of a sill and narrows in Knight Inlet that caused significant biases in their along-channel estimates of volume flux. MacDonald and Geyer (2005) found first-order, cross-channel variability in the Fraser River ebb plume liftoff zone as well, and a 3D simulation of the Strait of Gibraltar highlighted sharp horizontal flow and density gradients oriented obliquely to the channel (Sánchez-Garrido et al 2011).…”

“…Froude number-based front angle control was developed by Garvine (1981) to model the position of a persistent buoyant river outflow front in the presence of an ambient cross current as well as by MacDonald and Geyer (2005) to characterize the angle of an arrested bottom front of the Fraser River liftoff zone. Correspondence between observations and the remarkably simple theory demonstrates its utility in characterizing these internal hydraulic phenomena.…”

“…However, these results were again for barotropic (single layer) jumps. The analogous approach in stratified flows, which treat oblique internal hydraulic jumps as arrested, long, interfacial waves, has not received as much attention, though the general concept has been used with some success to describe observed obliquity of ebb plume fronts at the surface (Garvine 1981(Garvine , 1982 and at the bottom (MacDonald and Geyer 2005). There are notably few observations of oblique internal jumps in marine environments.…”