Hydrology [H]

 CC:Hall E  Wednesday  0800h

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

Presiding:  M Wells, University of Toronto; J Peakall, University of Leeds; B Sutherland, University of Alberta


Modeling of the Wyville Thomson Ridge Overflow

* Stashchuk, N (nstashhuk@plymouth.ac.uk), University of Plymouth, Drake Circus, Plymouth, PL4 8AA, United Kingdom
Vlasenko, V (vvlasenko@plymouth.ac.uk), University of Plymouth, Drake Circus, Plymouth, PL4 8AA, United Kingdom
Sherwin, T J (toby.sherwin@sams.ac.uk), Scottish Association for Marine Science, Dunstaffnage Marine Laboratory, Oban, PA37 1QA, United Kingdom

The part of the Arctic water which moves through the Faroe Shetland Channel outflow the Wyville Thompson Ridge (WTR) in the direction of the Ymir Trough (YT) and the Cirolana Deep (CD). The region to the south of WTR was twice surveyed in April with CTD measurements. In seven days between surveys water temperature at the bottom of both CD and YT fell considerably, from 4.46 to 3.03°C in the deep and from 3.93 to 2.54°C in the trough. The outflow of cold and dense water from the WTR into CD was numerically simulated using MITgcm model. The pathway of overflow, temperature, salinity, velocity profiles and transport estimates are compared to those observed in YT and CD. The result of the three-dimensional simulations show a favorable agreement with the measurements. The numerical experiment with passive tracer revealed an explosive detrainment regime of deep water mixing. It was found also that a cyclonic and anticyclonic eddies are formed in CD area as a result of cold water overflow. NERC-project NE/F012403/1

H31B-02 [Moved to H33D]

Turbulent Mixing in Stratified Free Shear Layers

* Mashayek, A (amashaye@atmosp.physics.utoronto.ca), Department of Physics, University of Toronto, 60 St. George St., Toronto, ON M5S 1A7, Canada
Peltier, W (peltier@atmosp.physics.utoronto.ca), Department of Physics, University of Toronto, 60 St. George St., Toronto, ON M5S 1A7, Canada

We study the processes through which three dimensional turbulence develops in stratified free shear layers and the impact of various secondary instabilities on the efficiency of the mixing process. The efficiency of irreversible mixing, as compared to reversible "stirring", is determined by the extent to which, relative to the increase in kinetic energy, background potential energy increases in the course of flow evolution. Although previous analyses by Peltier and Caulfield (eg ARFM, 2004) have demonstrated that such analyses deliver close agreement with experimental results in which this efficiency is found to be of order 0.2, the previous numerical analyses have been performed under rather restrictive conditions in which, for example, the sub- harmonic pairing interaction was suppressed. One goal in this further extension of these previous analyses is to determine the extent to which the efficiency of irreversible mixing may be impacted when the flow is able to access this additional mode of secondary instability. Upon saturation of the primary two dimensional KH billow, a family of secondary instabilities inevitably develops. When the numerical simulations are restricted to a domain in which the streamwise extent allows only one wavelength of the fundamental KH billow to evolve, stability analysis reveals the possibility of two secondary instabilities. In an unstratified fluid, saturation of the billows is followed by the appearance of streamwise vortex streaks which originate through a 'hyperbolic' instability localized in the vorticity braids between the adjacent vortex cores. In a density stratified fluid the vortex streaks that are precursory to turbulent collapse arise as a consequence of a convective instability that is focused in the "eyelids" of the billows where the originally stable density gradient is inverted. If the domain of the study is extended from one wavelength of the fundamental KH billows to several wavelengths, detailed Floquet analysis of the linear stability of the evolving two dimensional nonlinear wave predicts the appearance of transverse secondary instabilities. In this circumstance, vortex pairing is found to be the fastest growing transverse mode. The possibility of the amalgamation of more than two vortices in a single merger event is not negligible, however, in fact the Floquet analysis provides growth rates for every possible interaction in which n vortices merge to form m < n vortices. Our interest is in circumstances in which such interactions compete with the convective mechanism through which streamwise vortex streaks are formed and the impact that this competition has upon the efficiency of turbulent mixing. The results depend upon the Reynold's , Richardson and Prandtl numbers and we will provide a preliminary discussion of these dependencies, the goal being to determine whether these analyses of turbulent mixing processes might require a revision of the manner in which such mixing is parameterized in geophysical flows.


Flow of turbidity current viewed from failures of telecommunication cables

* Hsu, S (hsu@ncu.edu.tw), National Central University, Taiwan, 300, Chung-Da Road, Chung-Li, 32001, Taiwan
Sibuet, J (jcsibuet@ifremer.fr), National Taiwan Ocean University, Taiwan, 2, Pei-nien Road, Keelung, Taiwan

Submarine landslides or slumps may generate turbidity currents consisting of dilute mixture of sediment and water. Large and fast-moving turbidity currents can incise and erode continental margins and cause damage to artificial structures such as telecommunication cables on the seafloor. In this study, we show submarine landslides and turbidity currents associated with the 2006 Pingtung earthquake off SW Taiwan. Furthermore, eleven submarine cables across the Kaoping canyon and Manila trench were broken in sequence from 1500 m to 4000 m deep. We have established a full-scale calculation of the turbidity current velocities along the Kaoping canyon channel from the middle continental slope to the adjacent deep ocean. Our results show that turbidity current velocities vary downstream at steps of 20 m/s, 3.7 m/s and 5.7 m/s which demonstrates a positive relationship between turbidity current velocities and bathymetric slopes. As evidenced by the violent cable breaks happened in the 2006 Pingtung earthquakes, the destructive power of turbidity current to underwater facilities is clearly demonstrated and, in many cases, underestimated.


Fluid flow in submarine channels with compound bends

* Keevil, G M (gkeevil@see.leeds.ac.uk), University of Leeds, School of Earth and Environment University of Leeds, Leeds, LS2 9JT, United Kingdom
McCaffrey, W D (mccaffrey@see.leeds.ac.uk), University of Leeds, School of Earth and Environment University of Leeds, Leeds, LS2 9JT, United Kingdom
Peakall, J (j.peakall@see.leeds.ac.uk), University of Leeds, School of Earth and Environment University of Leeds, Leeds, LS2 9JT, United Kingdom

Submarine channels are the primary conduit for the transport of clastic sediment into the deep oceans and as such have a vital role in the distribution of sediment. To understand the mechanisms of sediment distribution it is necessary to investigate the fluid dynamics that operate within submarine channels. It is the fluid flow which governs both the gross sediment deposition and the detailed facies distribution. Several recent laboratory and numerical studies have focused on the structure of fluid velocity and sediment concentration with single bends. This work has suggested that coherent flow structures are common within submarine channels, but they differ significantly from those found within fluvial channels. The results from two series of experiments are presented in which saline gravity currents flowed through a fixed form channel model comprising 10 bend pairs, where the channel model was contained within an elongate flume tank. The series of experiments comprised two subsets: in the first subset the channel overspill was allowed to escape laterally, modeling the likely flow process within aggradational channel systems. The second subset modeled the processes likely within an erosive channel contained within a bounding topography; the overspill fluid was not able to escape laterally, but was forced to flow longitudinally. The results from both series of experiments reveal significant channel overspill and the presence of secondary flow cells best developed at bend apexes. The downstream velocity distribution showed significant differences between the two series of experiments: where the laterally unconstrained flow exhibited a normal density current velocity distribution, the laterally constrained flow exhibited a highly altered velocity distribution. In both constrained and unconstrained cases the within channel discharge was shown to decrease downstream with distance until equilibrium was reached at which point the discharge remained constant. This work demonstrates that compound bends significantly adjust or tune the flow within submarine channels. This tuning acts to equilibrate the magnitude of the flow with the geometric capacity of the channel. As a consequence of the flow tuning, channel overspill was shown to be most significant within the first few bends. Additionally the degree of lateral confinement was shown to exhibit significant control on the downstream velocity distribution. This work suggests that the distribution of overbank sediments will be highly complex and controlled by the interaction of both discharge and channel geometry.