UBC Theses and Dissertations
A laboratory study of slope flow induced by a surface salt flux Hardenberg, Bon J. van
The salt expulsion caused by the freezing of seawater and the drainage of brine from the ice creates a convectively mixed layer, which extends to the bottom in shallow coastal regions. This buoyancy flux at the surface was simulated in laboratory experiments by percolating salt water through a porous membrane into a tank. Shadowgraph images show that a down-slope flow is induced when the bottom of the tank is set at an angle. Velocity maxima in the slope flow, measured from the movement of injected dye ranged from 0.09 to 0.66 cm/s. Fluid densities were determined using thermistors and small-volume conductivity micro-cells developed for this purpose. For bottom slope angles between 2.2° and 5.5°, and at computed salt fluxes between 1.82★10⁻⁵ and 1.63★10⁻⁶ g/cm²/s, the salinity profiles showed slope flow depths between 7 and 17 mm with a rise in salinity of 0.24 to 0.92 ppt above those in the mixed layer. Entrainment at a density interface without shear, using this experimental arrangement, agreed closely with predicted results by Bo Pedersen. Using the entrainment model for a turbulent gravity current, entrainment factors computed from the data of the slope flow experiments were up to two orders of magnitude larger than those predicted for flows in a quiescent environment. This is contrary to visual evidence of the experiments or to Arctic field data, which indicate low rates of entrainment. This suggests that a different model is required to explain the interaction between such flows and the turbulent environment.
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