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Physical modelling of tidal resonance in a submarine canyon Le Souëf, Kate Elizabeth

Abstract

The Gully, Nova Scotia (44 degrees N) is unique amongst studied submarine canyons poleward of 30 degrees due to the dominance of the diurnal (K₁) tidal frequency, which is subinertial at these latitudes. Length scales suggest the diurnal frequency may be resonant in the Gully. A physical model of the Gully was constructed in a tank and tidal currents were observed using a rotating table. Resonance curves were fit to measurements in the laboratory canyon for a range of stratifications, background rotation rates and forcing amplitudes. Resonant frequency increased with increasing stratification and was not affected by changing background rotation rates, as expected. Dense water was observed upwelling onto the continental shelf on either side of the laboratory canyon and travelled at least one canyon width along the shelf. Most of this upwelled water was pulled back into the canyon on the second half of the tidal cycle. Friction values measured in the laboratory were much higher than expected, possibly due to upwelled water surging onto the shelf on each tidal cycle, similar to a tidal bore. By scaling observations from the laboratory to the ocean and assuming friction in the ocean is also affected by water travelling onto the shelf, a resonance curve for the Gully was created. Resonance curves explain why the diurnal frequency dominates over the semi-diurnal (M₂) frequency throughout the year at the Gully, even if stratification at the shelf break varies.

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