Hydrology [H]

H33D

CC:716A Wednesday 1400h

CC:716A Wednesday 1400h

Turbidity Currents and Oceanic Overflows: Observations, Modeling, and Parameterization of Gravity Currents II

H33D-01 INVITED

On the causes of instability along the shifting plunge line of a sediment-laden density inflow, Lillooet Lake, Canada

Previous field work at Lillooet Lake, British Columbia, has revealed a transitory, pulsing, nature to the position of the plunge line, where regions of the scallop-shaped plunge line advance and retreat with a period of c. 4 minutes. Field measurements with acoustic Doppler current profilers have shown how this shifting plunge line generates discontinuous density underflows that may progress down the delta slope into the lake, or become buoyant as interflows when the lake is temperature stratified. Here, we present new observations on these plunge-line dynamics and an analysis that suggests the pulsing behavior may be explained as a turbulent Rayleigh-Taylor (RT) instability, which together with the momentum of the flow as it enters the lake, produces a shift in the position of the lobes along the plunge line and hence pulsing underflows. This paper will present new details of these underflows and a simple analysis that examines their period as RT fingers that penetrate into the ambient lake water.

H33D-02

Generation of Internal Waves by a Supercritical Stratified Plume

The generation of internal waves by a propagating river plume is studied in the framework of a fully-nonlinear non-hydrostatic numerical model. The vertical fluid stratification, parameters of tide, river discharge and the bottom topography were taken close to those observed near the Columbia River mouth. It was found that in the beginning of the ebb tidal phase the river water intruding into the sea behaves as a surface jet stream. It collides with the stagnant shelf waters and sinks down in the area of the outer plume boundary forming a head of the gravity current. In supercritical conditions which are normally realized at the first stage of the ebb tidal phase, internal waves are arrested in the head of the gravity current because their phase speed is smaller than the velocity of the plume. They are released and radiate from the plume when the speed of the decelerating front becomes smaller than the internal wave phase speed. This mechanism of the wave generation is sensitive to the stratification of the ambient shelf waters. It was found that dramatic decay of the buoyancy frequency profile from the surface to the bottom provides the most favourable conditions for the efficient disintegration of the head of the gravity current into a packet of internal waves and their fast separation from the plume. In the case when the fluid stratification on the shelf is close to monotonous, the disintegration of the head of the gravity current into a packet of solitary internal waves is not expected. NERC project NE/E01030X/1.

H33D-03

Mixed Region Collapse in Stratified Ambients

Compared with the substantial body of theoretical, experimental and computational work examining the evolution of gravity currents in uniform ambient fluids, relatively little is known about intrusions in stratified environments. Most lock-release laboratory experiments have focused upon one of two symmetric circumstances: in one the ambient is a two-layer fluid with equal-depth upper and lower layers; in the other, the ambient is uniformly stratified. In both these cases, the intrusion density is the average ambient fluid density and is observed to propagate long distances at mid-depth with approximately constant speed. If symmetry is broken, experiments reveal qualitatively different behaviour. The intrusion propagates a much shorter distance and large-amplitude internal waves are excited. These studies are extended to examine asymmetric intrusions in uniformly stratified fluids resulting both from a full-depth lock-release experiment and also from a localized mixed patch in rectilinear and axisymmetric geometries.

H33D-04 INVITED

Turbulent Mixing and Dispersion in a Laboratory Gravity Current

Turbulent mixing and dispersion occurs when a wall-bounded stably-stratified gravity current generates
sufficient shear to destabilized the layer. The instability of such a flow is described by the Richardson number,
Ri_{g} = N^{2}/S^{2}, where N is the local Brunt-Vaisala frequency and S is the local mean shear rate. The Kelvin-
Helmholtz instability that occurs for low Ri_{g} < 0.25 causes vigorous vertical mixing. Such instabilities are
important in the mixing and entrainment of the gravity currents occurring in oceanic overflows. We present
high-resolution velocity and density measurements of the development of a stably-stratified gravity current on a
smooth plane, inclined at 10° with respect to horizontal. The velocity and density fields are measured
simultaneously using particle image velocimetry and planar laser induced fluorescence. As opposed to many
measurements of gravity currents where the current itself is laminar, our gravity current is turbulent with
Reynolds number Re ≈ 5000. From the measured velocity/density fields we compute the energy
dissipation ε ≈ 1 cm^{2}/ s^{3}, the mean shear rate S ≈ 1.5 s^{-1}, and
the average N ≈ 0.4 s^{-1}. We then compute the vertical turbulent transport of momentum and
density by directly evaluating the vertical Reynolds stress, Re_{T} = ‹ u' w' › and the buoyancy flux
‹ ρ' w' › where u', w', and ρ' are horizontal and vertical velocity fluctuations and
density fluctuations, respectively. By relating ‹ u' w' › and ‹ ρ' w' › to mean
gradients, we obtain Prandtl mixing lengths ℓ_{v} ≈ ℓ_{ρ} ≈ 0.6 cm ≈ ℓ_{s} =
ε^{1/2}/S [1]. The apparent equivalence of the mixing lengths suggests an extrapolation to ocean
overflow conditions, e.g., the Mediterranean overflow [2], for which we predict turbulent eddy viscosity and
diffusivity coefficients of about 600 cm^{2}/s .
1. P. Odier, J. Chen, M.K. Rivera, and R.E. Ecke, *Mixing in stratified gravity currents: Prandtl mixing length*,
arXiv:0901.4836
2. J. Price et al, *Mediterranean outflow mixing dynamics *, Science **259**, 1277 (1993).

H33D-05 **[WITHDRAWN]**

Velocity Profile Normalization of Field-Measured Turbidity Currents

Multiple occurrences of turbidity currents were observed in moored-ADCP measurements in Monterey
(2002/03) and Hueneme (2007/08) submarine canyons, California. These turbidity currents, almost all of which
were supercritical (densimetric Froude number greater than unity), lasted for hours and obtained a maximum
speed of greater than 200 cm/s. The layer-averaged velocity of the turbidity currents varied from 100+ cm/s at
the onset of the turbidity currents to 20+ cm/s toward the end of the events. The thickness of the turbidity
currents tended to increase from 10 to 40 m over an event. Empirical functions, obtained from laboratory
experiments whose spatial and time scales are two to three orders of magnitude smaller than the field
measurements [e.g. Altinakar, Graf, and Hopfinger, 1996, Flow structure in turbidity currents, *Journal of
Hydraulic Research*, 34(5):713-718], were found to represent the field data fairly well. However, the best
similarity collapse of the turbidity current velocity profiles was obtained when the streamwise velocity was
normalized by the layer-averaged velocity and the elevation was normalized by the turbidity current thickness.
This normalization scheme can be generalized to the same empirical function y = exp (-α x^{m}) for the
jet region above the velocity maximum.

H33D-06

Turbulent Flow Characteristics near in the Bottom Boundary Layer of Experimental Density and Turbidity Currents

A series of experimental saline gravity currents and turbidity currents were analyzed to quantify the distribution
of shear stress and turbulent kinetic energy in the bottom boundary layer. Profiles of Reynolds stress and
turbulent kinetic energy were obtained from measurements of turbulent fluctuations of velocity components,
made using a high-resolution acoustic Doppler velocimeter. We focused on the bottom 5 cm of the 20 cm deep
gravity currents in order to characterize the bottom boundary layer. For flows that had similar velocity profiles,
the sediment-laden flows had Reynolds stresses in the bottom boundary layer that were approximately 35%
higher than the equivalent saline flows. The shear stress can also be expressed as a drag coefficients, so that
C_{D} = 1.3 × 10^{-3} for saline gravity currents and C_{D} = 1.8 × 10^{-3} for the turbidity
currents, in general agreement with some previous experimental studies. The Reynolds stress distribution
mirrors the velocity gradient evolution as highest stresses occur where the gradient is largest and lowest
stresses are found at the velocity maximum where the gradient is almost zero. At the base of the bottom
boundary layer, we found that the Reynolds shear stress can be related to the gravity current's turbulent kinetic
energy (TKE), by τ = 0.2 ×TKE. This result had previously been found in non-stratified boundary
layers of channel flows. This relationship could allow bottom stresses to be estimated from single-component
velocity measurements in the bottom boundary layer of a turbidity current.