Estimating breaker height and type in the surf zone using SAR

Abstract

In synthetic aperture radar (SAR) images of coastal areas, bright streaks, or smears, can very often be observed in regions that appear to contain breaking waves. In particular, such smears are routinely seen within the surf zone. It is of great interest to determine whether these manifestations of breaking waves within the SAR imagery can be used to characterize any aspects of the surf zone. Standard SAR imaging theory can be used to explain the bright smears by relating the increase in brightness to the increase in turbulence within the breaking region, and by relating the smearing to the fact that the water in the breaking region is moving with a range of velocities which cause varying azimuth shifts in the SAR imaging process. This implies that the smear widths can be used to estimate the range of velocities occurring within the breaking region. What is needed therefore is to be able to relate these velocity ranges to surf parameters of interest, in particular to the height of the breaking wave. This paper summarizes an effort to develop an algorithm that will allow the breaking wave height to be estimated using SAR imagery. A model was developed that relates the range of velocities within the breaking region to parameters that could be estimated using SAR imagery. Procedures were then developed to estimate the model parameters from the SAR imagery. Results of applying these procedures are shown for airborne SAR images collected over a region of known water depth in order to calibrate parameters within the model and to estimate the sensitivity of the algorithm in terms of resulting errors in water depth at breaking. They indicate that it may be possible to estimate breaking wave height using SAR images.