Surf zone megaripples

New measurements of megaripple formation, orientation and migration in the surf zone at Scripps Beach, La Jolla, California, during 1999 and 2000, are used to test existing models for bedform orientation and to test a new A new technique for near real-time optical imaging of surf zone morphology, synoptic imaging, implemented at Scripps Beach, permits simultaneous measurements of the time-varying position of individual linear and Innate megaripples, fields of bedforms, rocks, cobble patches, beach steps and sand bars 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 minutes to produce images of the surf zone sand bed.; Two existing models for bedform orientation are tested against measurements of megaripple crestline orientation from Scripps Beach. Generally poor agreement was found between predictions and measurements of crestline orientation. The gross bedform normal transport model predicted crestline normal orientation within 20° of measured orientation for 38% of records (correlation coefficient, R = 0.11; not statistically significant at 95% confidence interval), with increased agreement coinciding with longitudinal and oblique bedforms. The defect model predicted crestline normal orientation within 20° of measurements for 22% of records when the ratio between defect and transport event time scales is assumed >>1 and 14% of the records when this ratio <<1. The role of transient adjustment in flow and crestline orientation and the effect of bedforms on flow and transport directions are found to partially explain the departure of megaripple migration from mean sediment flux direction.; on the hypothesis that megaripples form and persist unless the bed passes through the swash zone or flow conditions change too rapidly, correctly predicts the bed state (presence or absence of megaripples) for 74% of measurements over one year at Scripps Beach. (Abstract shortened by UMI.)...