Intratidal to Interseasonal Variability of Oblique, Internal Hydraulic Jumps at a Stratified Estuary Mouth

Interfacial disturbances generated at sharp, lateral constrictions cannot propagate directly against internally supercritical flow, but instead accumulate into internal hydraulic jumps oriented obliquely to the flow. Observational results at the Columbia River mouth (Oregon/Washington) via remote sensing and in-situ platforms highlight the complexity of these across-estuary phenomena. The jumps were identified by locally enhanced backscatter intensity in shore based X-band marine radar imagery, and corroborated by the presence of sharp gradients in both the surface current field (via along-track interferometric synthetic aperture radar) and the pycnocline depth (via winched CTD profiler transects). Specifically, the observed jump angles are consistent with those estimated via two-layer hydraulics. The tidally resolved occurrence and horizontal evolution of the jumps were monitored via the marine radar during the spring freshet, as well as during the annual low discharge in September. The jumps appeared exclusively during ebb; one recurred regularly, independent of tidal amplitude and river discharge, while the other was less consistent during the strongest ebbs. The jump angles varied over the course of each ebb, but were typically highly oblique to the flow. The intertidal consistency of this variability was remarkable, with some interseasonal dependence.