Surf zone megaripple orientation: Measurements and models

Abstract

Two existing models for bed form orientation are tested against measurements of megaripple crest line orientation from Scripps Beach. Optical time‐averaged images of the surf zone sand bed at 10 min intervals are processed with an automatic crest line tracing algorithm. Flow measurements concurrent with images of the sand bed collected during 3–7 April and 17–23 July 1999 are used as input to the gross bed form normal transport model (bed forms align to maximize gross transport across crests) predicting steady state crest orientation and the defect model (bed forms reorient at a rate proportional to the differential velocity of crest line ends, or defects) predicting a time‐dependent orientation. Generally poor agreement was found between predictions and measurements of crest line orientation. The gross bed form normal transport model predicted crest line normal orientation within 20° of measured orientation for 38% of records, with increased agreement coinciding with longitudinal and oblique bed forms. The defect model predicted crest line normal orientation within 20° of measurements for 22% of records when the ratio between defect and transport event timescales is assumed ≫1 and 14% of the records for the ratio ≪1. Although migration and mean sediment transport direction are assumed to coincide in both models, RMS deviation in bed form migration direction from the calculated 30 min mean sediment transport direction was found to be 38°. Additionally, RMS deviation of megaripple migration direction from crest line normal orientation was 39°.