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Hydraulic analysis of outflow winds in Howe Sound, British Columbia Finnigan, Timothy D.


Previous studies (Jackson, 1993) suggest an outflow wind, which flows below an inversion in a well defined layer through Howe Sound, may exhibit hydraulic behaviour. Strong outflow winds in Howe Sound are simulated in the laboratory using a single fluid layer in a small scale one-dimensional physical model. Model results are presented and compared with observations recorded in Howe Sound during a severe outflow wind event in December, 1992. Field observations affirm the findings of the physical modelling with both indicating the presence and location of controls and hydraulic jumps in the wind layer. Hydraulic behaviour is found to change as the synoptic pressure gradient and the flow rate increase. An additional comparison is made with output from the computer model, Hydmod of Jackson and Steyn (1994b). Numerical simulations, configured for the conditions present in Howe Sound during the December, 1992 event, indicate channel hydraulics (and thus spatial wind speed variation) closely resembling the physical model and field results. Outflow winds are studied in more detail through a series of experiments conducted with a three-dimensional physical model which is geometrically and kinematically similar to the prototype, Howe Sound. The results reveal the structure of the wind layer over a wide range of possible field conditions. Hydraulic features, which do not behave in a traditionally one-dimensional manner, are identified. The 3D model results, although more detailed, verify the findings of the 1D modelling in general. Together the results provide a predictive tool for determining hazardous zones of extreme wind during an outflow event.

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