Internal hydraulic flows on the continental shelf: High drag states over asmall bank

Observations of currents, hydrography, and turbulence provide unambiguous e vidence for hydraulic control of flow over an isolated three-dimensional to pographic feature on Oregon's continental shelf. The flow becomes critical at the crest of the bank, forming a strong supercritical downslope flow in the lower layer. Farther downstream, internal hydraulic jumps form as the b ottom flow becomes subcritical. As a consequence, turbulence is greatly enh anced in the bottom boundary layer, in the sheared interface above the swif tly flowing bottom current, and in the internal hydraulic jump. The dissipa tion rate of turbulent energy is consistent with the mean energy removal ra te for a hydraulic jump in an idealized two-layer flow. This enhanced turbu lence constitutes a "high drag" state of the flow in which the form drag in troduced by the topography exerts significant influence on the flow around it and mixing is increased 10(2) - 10(3) times the background values.