Abstract (poster presentation), American Geophysical Union Fall Meeting, San Francisco, 10-14 December 2007
Numerical Simulation of Recent Turbidity Currents in the Monterey Canyon System, Offshore California SNORRE HEIMSUND1, JINGPING XU2 & WOJCIECH NEMEC3 1
Complex Flow Design, P.O. Box 1248, NO-7462 Trondheim, Norway;
[email protected] USGS, 345 Middlefield Road, m.s. 999, Menlo Park, CA 9402, USA;
[email protected] 3 Dept. of Earth Sci., Bergen Univ., NO-5007 Bergen, Norway;
[email protected] 2
The method of computational fluid dynamics (CFD) has been used, in the form of a 3D numerical model (Flow-3D™), to perform a full-scale simulation of turbidity currents measured in December 2002 by three moorings in the Soquel and Monterey canyons. The model was verified by simulation of laboratory flows, and was upscaled to the Monterey Canyon system on the basis of high-resolution bathymetric data and flow measurements. The measured velocity profiles were sufficient to assess the flow thickness, initial velocity and duration in the canyon head zone. A computational grid with a highest feasible resolution was used, and both bathymetry and hydrostatic pressure were accounted for. The volumetric sediment concentration and exact grain-size composition of the flows were unknown, and thus a range of values for the initial concentration and bed roughness were assumed and assessed on a trial-and-error basis. The simulations reveal the behavior of a turbidity current along its descent path, including its local hydraulic characteristics (the 3D field of velocity, sediment concentration, shear stress, strain rate, and dynamic viscosity, as well as the magnitude of velocity and turbulent shear). The results confirm that the velocity structure of turbidity current is highly sensitive to variation in seafloor topography. The December 17th flow in the Soquel Canyon appears to have lost capacity by dilution over a relatively short distance and shown significant velocity fluctuations, which is attributed to the rugged topography of the canyon floor. A major loss of momentum occurred when the flow plunged at high angle into the Monterey Canyon, crashing against its bend’s southern wall. The December 20th flow in the Monterey Canyon, in contrast, developed a considerably longer body and strongly accelerated towards the canyon’s sharp second bend before crashing against its western wall. The mooring data show a downcanyon decline of velocity and suggest gradual waning, but the flow in reality appears to have had a new waxing phase. The CFD simulations allow the potential behavior of future flows to be predicted and, through a longer series of runs, the zones of erosion and deposition in the canyon system to be delineated.
