Flows over crescentic bars under control of wave energy

Abstract

Crescentic bars with wavelengths of 250-450 m were observed on a micro-tidal, open coast (Anmok Beach, Korea) to unravel the flow patterns over them under various wave conditions. The horn and bay environments of the crescents were distinguished to measure respective near-bottom flow conditions in the nearshore 3-8 m deep. Offshore waves were measured at a station about 18 m deep. The measurement duration spanned approximately one winter month and two stormy spells of several days each in summer and another winter comprising three different field campaigns. In addition, bar morphologies were measured with single- and multi-beam echo-sounders before and after each hydrodynamic measurement. Winter-storm waves yielded discernable bar movements offshore accompanied by a merging of small bays into a larger one. During the field campaigns, the bars moved alongshore by far less than a half of their wavelength. Hydrodynamic measurements show a suite of the flow patterns including onshore and offshore uniform flows and rip cell circulations. The flow patterns depend exclusively on the offshore significant wave height (H-s), or a non-dimensional parameter Hp=Hstan theta/hc, where h(c) is the water depth of the bar crest and tan theta the slope gradient. Therefore, the specific ranges of H-p could be given to individual flow patterns measured. These results are used to illustrate a coupled behavior of the bars and flows in a beach-state cycle forced by varying wave energies.