Observations of intratidal variability of flows over a sill/contraction combination in a Chilean fjord

Underway velocity measurements were carried out for the first time in a Chi lean fjord using an acoustic Doppler velocimeter with the purpose of elucid ating the intratidal variability of flows through a pass, Paso Galvarino. T he pass included a sill, where the bottom sloped by roughly 30%, and a coas tline contraction of -90%. The relatively small dimensions of the pass allo wed for rapid sampling of the flow evolution throughout the tidal cycle. Th e backscattered sound signal from the velocimeter and from an echo sounder were used to describe the vertical excursions of the pycnocline throughout the domain and to identify regions of enhanced vertical mixing within the p ass. The spatial variability of the flow in the pass was consistent with un iform two-layer flow. At the narrowest section of the contraction the pycno cline dropped sharply around both maximum flood and maximum ebb, while the flow accelerated downstream relative to the tidal flow. The slope of the py cnocline changed sign from flood to ebb, which was atypical of other fjord observations but could be explained by the transitions from subcritical to supercritical flow. These transitions switched location at either side of t he narrowest section of the contraction. Leeward of this section, increased sound backscatter suggested intensified turbulence that extended over a gr eater area during ebb than during flood because the distance between the po int of pycnocline drop and the end of the pass was longer during ebb. Enhan ced vertical mixing within the pass was reflected in the tidally averaged f ields by a three-layer flow that consisted of near-surface and near-bottom flow converging toward the pass and flow around the pycnocline diverging aw ay from the pass.