Structure of an Axial Convergence Zone from Acoustic Doppler Current Profiler Measurements

An acoustic doppler current profiler was used to measure the current distribution in a convergence zone located in the Piscataqua River—a well-mixed estuary in New Hampshire, U.S.A. The convergence zone was distributed along a line parallel to the channel axis and could be located visually by accumulated debris. Observations of transverse circulation during the flood tide along a transect crossing the front are presented. Results show surface currents directed towards the centre of the channel, sinking in the middle, and divergence towards the sides of the channel at the bottom. Conductivity and temperature measurements indicate accompanying higher densities in the centre of the channel. A simple mathematical model is developed, applied and compared with the data. Major features of the frontal circulation are reproduced indicating that transverse circulation is driven primarily by the cross-channel density gradients.