SEDIMENT SUSPENSION DUE TO LARGE SCALE EDDIES IN THE SURF ZONE

Kazuo Nadaoka, Seizo Ueno, Tatsuyuki Igarashi

Abstract


Laboratory experiments using a fiber-optic LDV system and a small pressure transducer have been made to reveal detailed characteristics of the velocity field in the surf zone and its relationship to the sediment suspension with special reference to the three-dimensional large scale eddies referred to as "obliquely descending eddies", the existence of which was recently revealed by Nadaoka (1986). A conditional sampling technique has been used to find that the obliquely descending eddies bring highly intermittent intensive turbulence to the bottom with the large onshoreward momentum at the upper layer of the water and thus essentially characterize the turbulent flow field in the surf zone. Visual observation and concentration measurements, especially a coherence analysis of two data sets of concentration close to the bottom, have shown that the sediment suspension is mostly governed by such large scale eddies in a wide extent of the surf zone; i.e., the eddies hit the bottom and then lift up the sediment into suspension, yielding the spot-like sediment cloud in accordance with the three-dimensional eddy structure.

Keywords


surf zone; large scale eddy; sediment suspension

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