Space-Time Imaging of Shoaling Waves and Surf

A fundamental barrier to consequential evaluation of modem, very capable shoaling wave and surf models has been the inability to provide high-quality ocean data with which to test model results. This paper describes a development intended to satisfy this need by providing space-time visible images of the nearshore from which three parameters crucial to such evaluations are simultaneously retrieved. These fields are the wave spectrum, bathymetry and currents. A panchromatic digital framing camera has been mounted on a small aircraft and used to collect time series of images of waves as they shoal and break. The camera system is controlled by a computer-driven turret which provides accurate location and pointing angles so that the images can be mapped to the mean water level on a common geodetic reference surface. This effectively separates space and time variations associated with the waves. The resulting time series imagery can be mapped and displayed much like a movie taken from a sky hook. These data are used with algorithms to retrieve the ocean parameters of interest, specifically the wave spectrum, water depth and currents. The 3-D frequency-wavenumber spectrum is calculated in sub-regions of the nominal 2 km scene, and the theoretical dispersion relation for linear gravity waves is fit to the spectrum, with the local water depth and current as free parameters.