Remote Sensing of Radiation Stress Gradients From Optical Imagery

Radiation stress, the excess momentum due to the presence of waves, is a function of wave energy. Thus, as waves break in the surf zone, gradients in radiation stress are created with this excess momentum providing a force that is typically of order 10² dynes/cm², several orders of magnitude greater than wind forcing. These gradients drive all nearshore currents and subsequent sediment transport. Currently, there is no method for direct measurement of radiation stress gradients; all estimations are based on simple differences between pairs of spatially separated sensors. In addition, in-situ sampling is difficult in the hostile environment of the nearshore surf zone. On the other hand, the distinct optical signature of wave breaking has lead to interest in a possible remote sensing technique for direct estimation of radiation stress gradients. For a beach with plane parallel contours, it can easily be shown that the diagonal element of the radiation stress gradient can be given by: {{∂ S_xy} / {∂ x}} = <ɛ> {{sin α} / {c}}, where <ɛ> is the dissipation of the wave field due to breaking, sin α is the local angle of incidence, and c is the wave celerity. Remote optical measurements of wave direction (Lippmann and Holman, 1991) and wave phase speed (Stockdon and Holman, 2000) have already been shown to be viable techniques. In addition, Lippmann and Holman (1989) and others have suggested a relationship between modeled wave dissipation and patterns of image intensity from ten-minute time-exposure images. However, no comparisons were made in that work with actual in-situ energy flux measurements. Our current work will focus on better development and testing of this relationship, based on in-situ wave energy measurements collected during the 1997 Sandy Duck field experiment. Emphasis will be on a direct comparison of dissipation measurements computed by finite difference from adjacent sensors and the spatial mean optical intensity between the sensors.