Turbulent energy production and entrainment at a highly stratified estuarine front

Abstract

[1] Rates of turbulent kinetic energy (TKE) production and buoyancy flux in the region immediately seaward ($1 km) of a highly stratified estuarine front at the mouth of the Fraser River (British Columbia, Canada) are calculated using a control volume approach. The calculations are based on field data obtained from shipboard instrumentation, specifically velocity data from a ship mounted acoustic Doppler current profiler (ADCP), and salinity data from a towed conductivity-temperature-depth (CTD) unit. The results allow for the calculation of vertical velocities in the water column, and the total vertical transport of salt and momentum. The vertical turbulent transport quantities (u 0 w 0 , S 0 w 0 ) can then be estimated as the difference between the total transport and the advective transport. Estimated production is on the order of 10 À3 m 2 s À3 , yielding a value of e(nN 2 ) À1 on the order of 10 4 . This rate of TKE production is at the upper limit of reported values for ocean and coastal environments. Flux Richardson numbers in this highly energetic system generally range from 0.15 to 0.2, with most mixing occurring at gradient Richardson numbers slightly less than 1 = 4 . These values compare favorably with other values in the literature that are associated with turbulence observations from regimes characterized by scales several orders of magnitude smaller than are present in the Fraser River.

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

“…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.…”

“…The generic structure described here is likely to be consistent for a wide variety of plumes, from small scale industrial discharge plumes and tidal creeks to larger scale river plumes, particularly those that enter the ocean at supercritical speeds, such as the Mississippi [Wright and Coleman, 1971] or the Fraser [MacDonald and Geyer, 2004] Rivers. However, it is also likely that certain physical mechanisms may be more dominant at different scales, as discussed further in section 5.5.…”

“…A control volume approach [MacDonald and Geyer, 2004] was used in the estimation of the turbulent transport quantities. This approach relies on conservation of two quantities: volume and heat.…”

“…Measurements in the Fraser River estuary and elsewhere [MacDonald and Geyer, 2004] suggest that buoyancy flux in a stratified shear environment should scale as B = Figure 8. Schematic of the control volume approach, from along plume and across plume vantage points.…”

“…Similarly, much can be drawn from the essentially separate bodies of literature that exist for the two types of plumes, with industrial plumes historically treated in the engineering literature [e.g., Jirka et al, 1981;Adams and Stolzenbach, 1977], while geophysical plumes have typically been discussed within the oceanographic literature [e.g., Wright and Coleman, 1971;MacDonald and Geyer, 2004;Lentz, 2004]. In order to further such comparison and understanding, this paper evaluates observational data from the near field of an industrial thermal plume.…”