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

Abstract

Remote sensing of wave breaker patterns, clearly visible as high-intensity bands in time exposure video images, has become a powerful tool to obtain large-scale (kilometers) and long-term (years) time series of nearshore sandbar position. However, intensity-based bar crest positions xi differ from directly measured positions xb by a time-varying distance Δx, which is of O(10 m) and depends on the offshore wave height H0, the water level η0, and the bathymetry itself. The effect of these parameters on Δx was investigated from simultaneous video observations and bathymetric surveys, obtained in the double-barred system at Egmond aan Zee, Netherlands, and from wave model predictions, assuming that the roller energy represents image intensity. When the wave field over a bar was predicted to be nonsaturated, xi was observed and predicted to move offshore as either η0 decreased or H0 increased. Under saturated conditions, Δx only responded to changes in η0. Additional model investigations showed that an increase in outer bar crest depth, similar to that observed during interannual bar behavior, significantly reduced the Δx variability at the outer bar and increased the Δx variability at the inner bar. Implications of our observational and model findings for studying sandbar position from video imagery are outlined. Copyright 2001 by the American Geophysical Union.