Flow and Hydraulics near the Sill of Hood Canal, a Strongly Sheared, Continuously Stratified Fjord

Abstract

Hood Canal, a long fjord in Washington State, has strong tides but limited deep-water renewal landward of a complex constriction. Tide-resolving hydrographic and velocity observations at the constriction, with a depth-cycling towed body, varied markedly during three consecutive years, partly because of stratification variations. To determine whether hydraulic control is generally important and to interpret observations of lee waves, blocking, and other features, hydraulic criticality is estimated over full tidal cycles for channel wide internal wave modes 1, 2, and 3, at five cross-channel sections, using mode speeds from the extended TaylorGoldstein equation. These modes were strongly supercritical during most of ebb and flood on the gentle seaward sill face and for part of flood at the base of the steep landward side. Examining local criticality along the thalweg found repeated changes between mode 1 being critical and supercritical approaching the sill crest during flood, unsurprising given local minima and maxima in the cross-sectional area, with the sill crest near a maximum. Density crossing the sill sometimes resembled an overflow with an internal hydraulic control at the sill, followed by a hydraulic jump or lee wave. Long-wave speeds, however, suggest cross waves, particularly along the shallower gentler side, where flow downstream of a large-amplitude wave was uniformly supercritical. Supercritical approaching the sill, peak ebb was critical to mode 1 and supercritical to modes 2 and 3 at the base while forming a sluggish dome of dense water over the sill. Full interpretation exceeds observations and existing theory.

Transport and Dynamics of the Panay Sill Overflow in the Philippine Seas*

“…Gregg and Pratt (2010) recently did this for the flow in the Hood Canal by solving the extended TaylorGoldstein (eT-G) equation (Pratt et al 2000), which takes the cross-sectional topography into consideration but assumes that the vertically varying velocity and density is uniform in the cross-channel direction. This approach would be more problematic at the Panay Sill where rotationally induced, cross-channel gradients are stronger than for the Hood Canal.…”

Internal hydraulic control in the Little Belt, Denmark – observations of flow configurations and water mass formation

“…On a regional scale, internal hydraulic control has also attracted attention, for example in connection with estuarine circulation and bottom water renewal in semi-enclosed shelf seas and biological and environmental effects thereof (e.g. Nielsen, 2001;Roman et al, 2005;Hietala et al, 2007;Gregg and Pratt, 2010).…”

“…First, the con-ditions under which hydraulic control is occurring in nature are often very complex, including, primarily, irregular topography or stratification, large breadth-to-length and breadthto-depth ratios, friction, mixing, and the rotation of the earth (e.g. Farmer and Denton, 1985;Nielsen, 2001;Gregg and Pratt, 2010). In a great deal of existing theoretical or laboratory studies that have led to much of the existing knowledge on this subject some or all of these conditions have often been avoided in order to make the problem tractable.…”

“…On the other hand, a large number of observational studies have suggested the existence of hydraulic control in various locations without being able to observe the phenomenon directly (e.g. Armi and Farmer, 1988;Pratt et al, 2000;Fristedt et al, 2005;Girton et al, 2006;Gregg and Pratt, 2010). In some of these cases pronounced asymmetry and intense mixing around the assumed point of control are absent, suggesting that friction plays an important role in keeping the flow subcritical.…”

“…the Denmark Strait or the Strait of Gibraltar, lead to the formation of water masses that may influence the circulation and can be traced far away from their locations of origin (Price et al, 1993;Dickson and Brown, 1994;Price and Baringer, 1994;Girton and Sanford, 2003). On a small or regional scale it is of interest to find out to what extent localized physical processes, especially energetic and efficient processes such as hydraulic control, are able to contribute to the formation of water masses and drive other processes (Gargett et al, 2003;Gregg and Pratt, 2010). Previously we have suggested that hydraulic control in the Little Belt could be accounting for enhanced primary production and oxygen depletion near the bottom, observed in wide areas in either direction away from the narrow part (Lund-Hansen et al, 2008).…”