MULTIPLE BAR FORMATION BY BREAKER-INDUCED VORTICES: A LABORATORY APPROACH

Da Ping Zhang, Tsuguo Sunamura

Abstract


Using a two-dimensional wave flume and a two-videocamera system, mechanism and process of multiple bar formation were investigated in the light of breaker-induced vortices. A fixed-bed experiment revealed that (1) the vortex formed at the wave break point can be classified into three: oblique, A-type horizontal, and B-type horizontal vortices; (2) the vortex formed in the surf zone is always oblique; and (3) the location of a vortex reaching bottom can be described by wave properties and the bottom slope, and its water depth can be expressed by breaker height alone. A movable-bed experiment indicated that bars are initiated by the vortices reaching bottom in the surf zone; the number of bars formed coincides with the number of such vortices. Suspension of the bottom sediment due to vortex action and the mean-drift-velocity pattern can play an important role in multiple bar formation; mechanism for break-point bar formation is "convergence", while that for inner bar formation is "congestion". Two major modes for the development of multiple bars were found: simultaneous and successive: these depend on the interaction of vortex action and topography in the surf zone. Multiple-bar features such as trough spacing and crest depth could be explained from vortex features found through the fixed-bed experiment.

Keywords


bar; bar formation; laboratory approach; breaker induced vortices; vortex

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