Estuarine and tidal currents in the Broughton Archipelago

Abstract

Current meter observations, Conductivity-Temperature-Depth (CTD) profiles, and river discharges are combined with two numerical models to understand better the estuarine and tidal circulation in the Broughton Archipelago, a complex region of islands, channels, and fiords that has become a primary location for salmon farms in British Columbia. Though tidal currents are strong in many sub-regions, the primary transport mechanisms in the archipelago are the estuarine flows resulting from river and glacial runoff, and the near-surface currents that arise from strong winds. The harmonic finite element model, TIDE3D, is shown to reproduce the barotropic tidal currents with reasonable accuracy, but unlike other regions of the British Columbia coast where an extensive archive of historical observations has permitted a diagnostic calculation of average seasonal flows, sparse and noisy CTD observations did not allow a similar computation here. In order to simulate these background flows, the prognostic finite volume model, ELCIRC, was initialized with a smoothed version of these same historical temperature and salinity fields andforced with tides and river discharge. Though the near-surface flows were reproduced with acceptable accuracy, the estuarine and tidal flows at depth were found to be much too weak as a result of numerical dissipation arising from the Eulerian-Lagrangian time stepping. Nevertheless, ELCIRC did confirm current observations suggesting that the bottom estuarine flow in Knight Inlet actually comes from Queen Charlotte Strait via the "back-door" of Fife Sound and Tribune Passage. Consistent with anecdotal evidence, ELCIRC also showed that the surface estuarine flow coming down Knight Inlet bifurcates with part going down Tribune Channel and Fife Sound and part continuing down Knight Inlet. The relevance of these background flows.for aquaculture issues, such as oxygen renewal and the transport of sea lice and viruses, is discussed. © Canadian Meteorological and Oceanographic Society.