Ripples with secondary crests as a possible indicator of palaeo-wave direction: a laboratory experiment

Abstract

The geometry of ripples having single or double secondary crests in each trough was examined through a wave flume experiment. The results showed that ripples with a secondary crest (s) exhibit asymmetrical profiles. In the case of ripples with a single secondary crest, the crest appears on the onshore side of each trough. In the case of double secondary crests, the distance from an onshore main ripple crest to one secondary crest on the offshore-dipping slope is larger than the corresponding distance between the other secondary crest and an offshore-adjacent main crest, and also the former secondary crest is located higher than the latter. Such geometric asymmetry becomes significant depending on the degree of velocity asymmetry, which is inherent in wave-induced oscillatory flow. The present experiment suggests that the geometrical characteristics of this type of ripples in the geological record enable us to infer the direction of ancient wave propagation.