HIGH-RESOLUTION MEASUREMENTS MADE ACROSS A TIDAL INTRUSION FRONT

High spatial resolution current and hydrographic measurements have bee n made across a tidal intrusion front in the James River estuary, Virg inia, during the first few hours of flood. The current measurements we re made with a surface-towed acoustic Doppler current profiler which w as used in both profile and side-scan configurations. The profile data show a 30-50 cm s(-1) inflow of water from the Chesapeake Bay, a time -evolving across-front density gradient, and downward velocity beneath the front of about 15 cm s(-1). The side-scan observations directly m easure the across-front change in current and provide a view of the th ree-dimensional character of the frontal interface. The nearsurface ho rizontal strain rate, a critical parameter for understanding the modul ation of surface waves across the front, is estimated to be about 0.04 s(-1). Using Phillips' [1984] theory as a guide, fronts with such lar ge strain rates as these should be discernible in imagery from the ERS 1 C band synthetic aperture radar for wind speeds of less than or equ al to 4 m s(-1).