The Influence of the Spring-neap Tidal Cycle on Currents and Density in Burrard Inlet, British Columbia, Canada

Abstract

Tidal mixing through two constrictions at the entrance to a fjord-type estuary in British Colombia, Canada, is studied from a mooring programme in the spring of 1995 and from ship-born ADCP measurements in the spring of 1997. Time series from the moorings show that during spring tides, currents at depth are as high as 1·5ms−1. Small floods bring outside water through the narrows relatively unmixed which sinks down to depth. In contrast, large floods result in intrusions of highly mixed water up to 1kgm−3lower in density. During neap tides currents at depth are generally small, density decreases slowly and turbulent vertical diffusion is believed to be the dominant process at depth. Analysis of vertical diffusion near the bottom at one station during neap tides produces estimates of the vertical eddy diffusivity which are somewhat higher than those from similar studies in other B.C. fjords. Results from the ADCP study reveal the presence of strong vertical velocities downstream of the narrows during large floods, possibly associated with hydraulic jumps. The flow through the narrows at such times is indeed supercritical with respect to the first baroclinic mode.