EXPERIMENTAL STUDY OF WAVE RIPPLE DEFECTS AND THEIR ROLE IN ADJUSTMENT OF RIPPLE SPACING

Laboratory flume experiments were conducted to investigate the response of wave ripples in fine sand to discreet changes in oscillatory flow conditions. A wide range of initial and final wave conditions was used to simulate a broad spectrum of changes in oscillatory flows that occur in natural environments. Deviations from parallel, evenly spaced ripples, called defects, appear to facilitate the adjustment of ripples that are out of equilibrium with the flow. Previous studies have identified several broad types of defects, but have typically oversimplified their role in adjusting spacings within wave-ripple fields. This study shows that previously described defects, such as terminating crests, bifurcating crests, and secondary crests, originate and evolve in a variety of different manners over time, and are thus understood based on these characteristics. Our experiments reveal a plethora of additional defects and processes that include changes in sinuosity, edge dislocations similar to those in growing crystals, oval to circular depressions or “cups”, and more complicated configurations involving multiple crests. In some cases, defect types and interactions are spatially segregated into loosely defined domains that can govern the nature and behavior of defects in adjacent domains. Spatial differences in flow conditions along the length of the flume may have led to this variation in defect abundance and behavior, a condition likely to be the general rule in modern environments.