Subtidal circulation in a deep‐silled fjord: Douglas Channel, British Columbia

Abstract

Douglas Channel, a deep fjord on the west coast of British Columbia, Canada, is the main waterway in the fjord system that connects the town of Kitimat to Queen Charlotte Sound and Hecate Strait. A 200 m depth sill divides Douglas Channel into an outer and an inner basin. This study examines the low‐frequency (from seasonal to meteorological bands) circulation in Douglas Channel from data collected at three moorings deployed during 2013–2015. The deep flows are dominated by a yearly renewal that takes place from May/June to early September. A dense bottom layer with a thickness of 100 m that cascades through the system at the speed of 0.1–0.2 m s−1, which is consistent with gravity currents. Estuarine flow dominates the circulation above the sill depth, and the observed landward net volume flux suggests that it is necessary to include the entire complex channel network to fully understand the estuarine circulation in the system. The influence of the wind forcing on the subtidal circulation is not only at the surface, but also at middepth. The along‐channel wind dominates the surface current velocity fluctuations and the sea level response to the wind produces a velocity signal at 100–120 m in the counter‐wind direction. Overall, the circulation in the seasonal and the meteorological bands is a mix of estuarine flow, direct wind‐driven flow, and the barotropic and baroclinic responses to changes to the surface pressure gradient caused by the wind stress.

Coupling of Estuarine Circulations in a Network of Fjords

“…The net volume transport in the Douglas Channel is up‐fjord and about 19,000 m 3 /s. This value is consistent with the observation‐based estimates of net up‐fjord transport (Wan et al, ). The net volume transport in the Devastation Channel is inward into the triple junction and about 18,900 m 3 /s, indicating that the inward transport in the subsurface layer is larger than the outward transport in the surface layer.…”

“…Wind‐driven currents often dominate the near‐surface flow in fjords (e.g., Alberni Inlet, Farmer & Osborn, ; Knight Inlet, Baker & Pond, ). Recent observations (Wan et al, ) show that indeed wind can reverse the down‐fjord surface estuarine flow during strong wind events in the Douglas Channel.…”

“…Pickard () analyzed historical hydrographic surveys in strongly stratified fjords with high freshwater input near surface zones and found that the surface freshwater layer is thin and confined to the upper 1–10 m with a sharp pycnocline at the base. Specifically, the observed thickness of the surface layer at Douglas Channel and Gardner Canal is about 3–7 and 4.5–7 m, respectively (Pickard, ; similar values were also obtained by Webster, , and Wan et al, ). The propagation of the thin surface freshwater layer was studied in laboratory experiments by Ellison and Turner (), and the results suggest that after an initial mixing near the river mouth, the thin river plume will not mix much further with the water below and rather flow out driven by pressure as a baroclinic wave.…”

“…Wan et al () analyzed ADCP observations made in Douglas Channel and revealed that estuarine flow dominates the circulation above the sill level. However, the ADCP observations show a net up‐fjord flow in the Channel, which is in contradiction to the expected net down‐fjord flow in a typical estuarine circulation.…”

Surface Drift and Dispersion in a Multiply Connected Fjord System

“…This strong stratification can result in increased wind influence on surface currents (Thomson, ). Wan et al () investigated the near‐surface currents and found that 60%–70% of the subtidal variance in the currents could be explained by a linear regression against wind speed. The wind‐driven flow ( 5% of the wind speed) is superimposed on an estuarine surface outflow of 14–23 cm s .…”