Sediment particle movements observed using tracers under accretive wave conditions in the nearshore zone

ABSTRACT

Field observations were conducted during relatively mild wave conditions at a dissipative barred sandy beach to investigate the cross-shore sediment dynamics from the inner surf zone to the offshore side of the outer bar. Fluorescent sand tracers of five different colors were placed on the seabed at different cross-shore locations, and sand core samples were extracted respectively 11 and 20 days after the placement of tracers. Moreover, nine cross-shore bathymetric/topographic beach surveys were conducted, and the water level, wave, and wind conditions were observed. The objective of this research was to investigate the mechanisms of the on- and offshore-directed sediment transport during the inner- and outer-bar accretion events. The analyzed results revealed that at the inner surf zone, the sediment was dominantly transported in the onshore direction. The sediment was also transported offshore-ward due to the undertow; however, this occurred until the offshore end of the trough region. Overall, the outer bar shape affected the on-offshore sediment transport rate. The sediment movements in the onshore region and offshore region of the outer bar were not related; however, the outer bar was formed with the sediments from both the inner- and outer-bar regions.

KEYWORDS:

• Cross-shore sediment transport
• barred beach
• fluorescent sand tracer
• core sampling
• field observation

Acknowledgments

The authors would like to thank the Marine Information Division, Port and Airport Research Institute and Kashima Port and Airport Construction Office, for allowing the use of wave data from the Port of Kashima. Moreover, the authors gratefully acknowledge the Kochi Core Center, Japan (14B051), for the use of their facilities. In addition, the authors would like to thank Prof. Yoshiyuki Nakamura and Dr. Hiroto Higa of Yokohama National University for their helpful comments.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This research was funded through Japan Society for the Promotion of Science [15KK0202]; [26709034].