Synoptic imaging of nearshore bathymetric patterns

A new technique for near real-time optical imaging of surf zone morphology, synoptic imaging, implemented at Scripps Beach, La Jolla, California, permits simultaneous measurements of the time-varying position of individual linear and lunate megaripples, fields of bed forms, rocks, cobble patches, beach steps and sandbars, and troughs. A varying threshold filter for brightness is applied to video frames sampled at 1 Hz to remove pixels corresponding to surface foam and suspended bubbles or sediment. Remaining fragments of the video frames are averaged over time intervals generally ranging from 5 to 10 min to produce images of the surf zone sand bed. To facilitate accurate measurement of the position of sand bed features, these images are enhanced to increase feature visibility, transformed to plan view, and corrected for refraction using surveyed bathymetry and tide level. Contrast in the images results from differences in water depth, shading variations from tilted bed surfaces, and changes in bed material. The surveyed positions of megaripples and sandbars correspond to the positions of persistent features in the images. A ray path model quantifying sand bed contrast as a function of water depth, angle of line-of-sight, and light attenuation coefficient in water predicts that for the Scripps Beach field site, bed forms with height as small as 0.03-0.2 m can be detected in water depths up to 2.5 m, in accord with observations, and that visibility can be improved by changing the line of sight toward overhead viewing. Bed visibility is reduced by persistent surface foam and water turbidity generated by waves with period <8 s and height exceeding 1.5 m. Example images showing transitions between linear and lunate megaripples, location of megaripple crestlines, and the interaction between sandbars, bed form fields, and cobble patches illustrate the potential of synoptic imaging.