Laboratory Measurement of Suspended Sediment Concentration in Surf Zone with CT Concentration Meter

Abstract

Organized eddies generated by wave breaking have an important role on sediment transport in surf zones. In order to clarify the characteristics of intermittent sediment clouds generated by vertically-oriented eddies, the Computed Tomography (CT) technique was applied for a laboratory experiment to capture instantaneous two-dimensional distributions of suspended sediment concentration in a near-bottom horizontal plane. The Extended Bayesian Method with a smoothing filter as the prior information model was applied to obtain the optimum solution of the distribution. A specification test showed that concentration distribution was reproduced fairly well. Successive images of concentration distribution due to wave breaking were obtained and movements of sediment clouds were investigated. Concentration at the center of the measuring area was simultaneously measured for verification by a conventional optical forward-transmission-type sedimentmeter. It was found that horizontal distribution of suspended sediment concentration in a surf zone is quite inhomogeneous and the high concentration regions are formed as they indicate separated sediment clouds. Distinctive patterns of sediment clouds were observed in a relatively small number of trials. The length scale of the clouds was found to be in the order of the water depth. The primary cause of sediment cloud formation can be considered sediment pickup by intermittent eddies generated by wave breaking.