Tidally modulated occurrence of megaripples in a saturated surf zone

Abstract

An abstracted model for saturated surf zone megaripple occurrence, based on the hypothesis that megaripples form and persist unless flow conditions change too rapidly or the bed passes through the swash zone, correctly predicts the bed state (presence or absence of megaripples) for 73% of measurements over 1 year at Scripps Beach, more than seven standard deviations above the agreement between a random model and the measurements. Transitions between bed states are predicted within 1.5 hours for 54% of measurements, compared with 6% agreement for a random model. The model is forced with a record of water depth over the beach, H (from offshore tide measurements and inferred changes in bed elevation), assuming that changes in flow conditions and depth are determined by tide level alone in a saturated surf zone. Model parameter values are estimated independent of model results: megaripple formation time, T = 2.5 hours; maximum RMS depth change over time T, Δ H RMS * = 0.1 m; and minimum depth (averaged over time T ) below which megaripples are smoothed, H sw = 0.3 m. Allowing model parameters to vary increases agreement between measurements and model predictions to a maximum 82% for parameter values T = 3 hours, Δ H RMS * = 0.05 m, and H sw = 0.1 m. Neither bed state nor bed state transitions are correlated with offshore wave conditions.