The influence of the spring-neap tidal cycle on currents and density in Burrard Inlet (Vancouver harbour), British Columbia, Canada

The physical oceanography of Burrard Inlet/Vancouver Harbour in British Columbia, Canada, is studied. Particular focus is given to tidal mixing through First and Second Narrows, two constrictions that separate the western harbour from the coastal waters outside and the eastern harbour from the western harbour, respectively. The mixing at these two narrows is believed to be a significant controlling factor on the density field within Burrard Inlet and also within Indian Arm, the northern extension of this estuary. During the 1995 spring, current meters which also measure temperature and conductivity were moored at 6 m and 16 m above the bottom at three locations: outside First Narrows, inside First Narrows and inside Second Narrows. During spring tides, currents at depth are as high as 1.5 m/s; small floods bring outside water through the narrows relatively unmixed which sinks down to depth; large floods result in intrusions of highly mixed water up to 1 kg m⁻³ lower in density. During neap tides currents at depth are generally small, density decreases slowly and 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. Harmonic analyses of pressure time series from two of the current meters and tide gauge records are used to estimate the total energy loss from the barotropic tide over First and Second Narrows. Comparison with previous studies is in good agreement; however, there appears to be some seasonal variation. Conditions in the harbour were investigated by CTD and ADCP during the 1997 spring. CTD casts suggest that internal-tide resonance within the harbour during neap tides may be a contributing factor to the weak currents at depth during such periods. ADCP transects through Second Narrows reveal the presence of strong vertical velocities, possibly associated with hydraulic jumps, downstream of the narrows during large floods. The flow through the narrows is indeed supercritical with respect to the first baroclinic mode on large floods. Estimates of the dissipation rate of turbulent kinetic energy from current meter casts in the most turbulent regions give ∈ of 1-50 cm² s⁻³ which is as large or larger than any previous estimates in the ocean.

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