Acoustic Observations of Bubble Plumes at a Tidal Intrusion Front in the James River Estuary

During flood tides the mouth of the James River hosts a complex tidal intrusion front. The front has sharp gradients in density and currents, which cause surface wave breaking and bubble injection. The bubbles are subsequently entrained at the front, where strong downwelling occurs. Here we present ship-based measurements of broadband acoustic backscattering (50-420 kHz) with an emphasis on characterizing the size and evolution of bubble plumes entrained at the James River tidal intrusion front. Full water-column sampling using high-frequency broadband acoustic scattering techniques can be used to not only visualize processes such as mixing, but also to infer dominant sources of backscattering related to the hydrodynamics. Acoustic backscattering measurements are supported by concurrent ship-board ADCP measurements of horizontal velocities and a towed CTD array. Surface wave spectra, optical wave breaking observations, sustained downwelling velocities, and near-surface turbulence measurements are derived from Lagrangian drifters. Observed bubble plumes penetrate to approximately one-half of the water depth and extend over 100 m from the front. Separation of the bubble plume from the surface downstream of the front suggests the dissolution of bubbles whose buoyancy-driven rise velocities are insufficient to overcome the initial injection depth at the front.