Laboratory modelling of vertical sediment mixing in the surf zone

ABSTRACT

A comprehensive set of laboratory experiments were conducted to investigate the spatial distributions of mixing depth in the surf zone. A wooden sandbox of 0.15-m-deep and 1.0-m-long is fabricated and placed in the middle of the bed slope. In the experiments, five different sediment diameters were used and the same diameters of fluorescent sand tracers were sprinkled on the sand bed surface to investigate the mixing depth. After a specific duration of regular wave generation  , core samples were taken at six different cross-shore locationsand analyzed the mixing depth. Moreover, the flow velocity field in the surf zone was obtained by using a Large Eddy Simulation model and discussed the relationship with spatial distributions of mixing depth. The results reveal that the mixing depth has cross-shore spatial variation and the maximum mixing depth occurred at the impinging point for all tested sediment diameters. The spatial distributions for each diameter were repeatable; however, the depth decreases in a logarithmic manner from smaller to larger grain size. Furthermore, the spatial distribution of mixing depth is correlated with the bottom root mean square vertical velocity and proposed a new simple mathematical relationship for mixing depth that shows good accuracy with experimental data.

KEYWORDS:

• Mixing depth
• fluorescent sand tracer
• spatial distribution
• laboratory experiment
• large eddy simulation

Acknowledgments

The authors would like to thank Professor Yoshiyuki Nakamura and Dr Hiroto Higa of Yokohama National University for providing fruitful comments and suggestions to improve the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research study was partially funded by the Great Britain Sasakawa Foundation (GBSF) grant no. 5696.