Dynamics of Tidal Intrusion Fronts

Tidal intrusion fronts are surface convergences that occur at the mouths of estuaries during the flood tide, separating incoming salt water from lower-salinity, stratified estuarine water. These features occur in all types of estuaries and may be of particular significance in mixing, dispersion and gas exchange; however, until now they have not received the attention they deserve. The Undersea Remote Sensing (USRS) program undertook an intensive, multi-platform investigation of the tidal intrusion front at the mouth of the James River estuary, including in situ sensing with autonomous vehicles and shipboard towed arrays, remote sensing from land-based radar, autonomous aircraft and satellites as well as novel underwater remote sensing with acoustic sensors. Now-cast modeling in combination with real-time radar imagery guided the surveys, leading to optimal measurements of the rapidly time-evolving, three-dimensional structure of the tidal intrusion front. The study has revealed some notable findings. First, the front is not formed locally, but it originates as one or more lateral flow separation fronts that formed during the ebb and are advected into the main channel at the change of tide. The internal hydraulic regime is supercritical through most of the flood tide, with the front being maintained in a stationary position with a Froude angle ( arcsin ( 1/Froude number) ) of roughly 45 degrees. The surface convergence produces greatly enhanced surface roughness, breaking waves and intense bubble entrainment into the subducting water at the front, irrespective of the ambient sea conditions. The gradient Richardson number tends to hover near 0.25 throughout the frontal region, indicating the marginal instability regime and vigorous buoyancy flux.