Measuring currents in a coastal inlet by advection of turbulent eddies in airborne optical imagery

Water current vector fields in a coastal inlet are retrieved from temporal sequences of optical imagery collected from an aircraft, the velocity being calculated by tracking modulations in the radiance from the surface. The modulations are shown to be consistent with an advecting passive tracer in turbulent flow, leading to the physical mechanism for this signature being turbulent eddies in the flow. The intensity modulations are hypothesized to be light scattered from variations in the sediment load being transported by these turbulent eddies, although other imaging mechanisms such as surface roughness variations may be occurring as well. Supporting evidence is the prevalence of the signature in sediment-laden, nearshore flows along an exposed coast and through a tidal inlet and eddy-like variations in the echo intensity of acoustic Doppler current profilers (ADCPs). Also, the 3-D frequency-wave number spectrum of mapped image sequence data exhibits a classic shape that is consistent with surface turbulence generation by boils and subsequent mixing and advection by eddies in shallow channels. The velocity vector is retrieved by the geometry obtained in the least squares fit of a 2-D planar surface representing advective variance in the 3-D spectrum. These retrievals agree within 15 cm/s root-mean-square (RMS) with concurrent ADCP measurements and within 2 cm/s RMS of measurements of the Doppler shift of surface gravity waves that also are present in the data. The method provides an important new capability for measuring currents over large areas of the littorals, specifically including locations where waves are not present and visible variations in radiance from the water often are not strong.