Estuarine flows measured from an aircraft

Abstract

Knowledge of the circulation in rivers and estuaries is important for a number of routine as well as emergency operations. However, spatial-temporal characteristics of currents are difficult to measure using presently available instrumentation, particularly in the case of tides in complex morphology. This paper provides initial results for a new technique using a small aircraft, thereby enabling potential for rapid, repeated, high-resolution maps of near-surface currents over a large area. A sequence of EO images is mapped to a common geodetic coordinate frame on the surface. A dense grid of 3D space-time data cubes is assembled, and the 3D frequency-wavenumber spectrum of emitted light is calculated in each. An apparent low-frequency 2D planar spectral surface is found in most of the spectral cubes. This appears to be caused by variations in emitted light scattered from the near-surface sediment load. This planar surface represents advection of this scalar variable by the local mean flow vector, as per G.I. Taylor's "frozen flow" law for turbulent eddies embedded in the flow. The orientation of this surface in the spectral cube provides an accurate measurement of the speed and direction of the mean flow in each data cube, and the "thickness" of this surface potentially is a measure of the turbulence level. Maps of current vector retrievals are realistic, and specific numerical values agree with sparsely available ADCP data, typically within 10% rms normalized speed.