PHYSICAL MODELING OF AN OUTFLOW EVENT IN HOWE SOUND, BRITISH-COLUMBIA

Outflow winds occur when differing air masses are separated by a coast al mountain barrier. In extreme cases the cross-barrier pressure gradi ent and the high degree of stratification (often approaching a distinc t layered structure) result in channel winds which exhibit hydraulic f eatures. We present a study of outflow winds in Howe Sound, British Co lumbia. A field investigation, aimed specifically at locating and quan tifying hydraulic effects, was undertaken during the winter months of 1992/1993. Microbarographs positioned in the region recorded pressure changes at discrete locations in the streamwise direction. The pressur es obtained during a severe outflow wind event, which occurred from De cember 27, 1992 to January 1, 1993, show a highly variable lower-layer depth suggestive of hydraulic control. Experiments were conducted wit h a three-dimensional physical model that is geometrically and kinemat ically similar to Howe Sound. Synoptic conditions recorded during the outflow wind event in Howe Sound in December 1992 were used to determi ne appropriate model flow forcing. The expanse of supercritical flow a rea was observed to be relatively sensitive to changes in along-channe l pressure gradient and downstream depth, when compared to changes in discharge. Channel sinuosity and local topography appeared to force cr itical conditions at specific locations. For example, a channel bend c ombined with headlands was observed to force a situation where subcrit ical and supercritical streams flow side by side. Flow separation, res ulting in lateral sheer discontinuities, produced similar conditions. These effects are discussed and put into context with field observatio ns. Field and model results show good agreement.