WAVE-FORMED RIPPLES IN NEARSHORE SANDS

Abstract

Ripples are generated and modified by wind-generated waves and their profiles are controlled by the nature of the near-bottom wave motion and by the size of the bed material. Wave-formed ripples develop under a definable set of conditions called the ripple regime. The ripple regime is bounded by those conditions that initiate grain motion, low-wave intensity, and by those that cause the disappearance of ripples, onset of sheet flow. Sheet flow occurs when intense wave motion causes several grain layers to be in motion. Three distinct ripple types occur in nearshbre areas of fine sand - relict ripples, vortex ripples, and transition ripples. Vortex and transition ripples lie within the active ripple regime, whereas relict ripples do not. Ripples in fine sand were studied in the field at La Jolla, California, where profiles were obtained using a newly developed high-resolution sonar capable of vertical resolution of the order of one millimeter. Simultaneous profile and wave-pressure measurements permit correlation of the ripple profiles with individual waves and with the wave spectrum. The sonar, with its rapid scan capability (^ one meter per second), gives instantaneous measurement of the actively changing bed features in nearshore waters. The combination of bottom scans and wavepressure measurements extends previous wave-ripple studies to include all of the nearshore ripple regime. The relation between the wave and ripple data from this study is best shown by plotting ripple steepness n/A against the wave form of the Shields relative stress criterion 0. Vortex ripples (n /\ ^_ 0.15) occur for 0 values less than 40 but greater than the minimum value which is determined by the onset of grain motion.