Effect of hydrodynamics and bathymetry on video estimates of nearshore sandbar position

Remote sensing of wave breaker patterns, clearly visible as high-intensity bands in time exposure video images, has become a powerful tool to obtain l arge-scale (kilometers) and long-term (years) time series of nearshore sand bar position. However, intensity-based bar crest positions x(i) differ from directly measured positions x(b) by a time-varying distance Deltax, which is of O(10 m) and depends on the offshore wave height Ho, the water level e ta (0), and the bathymetry itself. The effect of these parameters on Deltax was investigated from simultaneous video observations and bathymetric surv eys, obtained in the double-barred system at Egmond aan Zee, Netherlands, a nd from wave model predictions, assuming that the roller energy represents image intensity. When the wave field over a bar was predicted to be nonsatu rated, x(i) was observed and predicted to move offshore as either eta (0) d ecreased or Ho increased. Under saturated conditions, Deltax only responded to changes in eta (0). Additional model investigations showed that an incr ease in outer bar crest depth, similar to that observed during interannual bar behavior, significantly reduced the Deltax variability at the outer bar and increased the Deltax variability at the inner bar. Implications of our observational and model findings for studying sandbar position from video imagery are outlined.