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Natural Hazards and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/nhess-2020-97
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/nhess-2020-97
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  14 May 2020

14 May 2020

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A revised version of this preprint is currently under review for the journal NHESS.

Dynamicity of multi-channel rip currents induced by rhythmic sandbars

Yao Zhang1, Xiao Hong2, GuoDong Xu1, Xunan Liu1, Xinping Chen1, Yuxi Sun1, Bin Wang1, and Chi Zhang3 Yao Zhang et al.
  • 1National Marine Hazard Mitigation Service, Ministry of Natural Resources, Beijing, China, 100000
  • 2South China Sea Prediction Center, Guangzhou, China, 510000
  • 3Hohai University, Nanjing, China, 210098

Abstract. In response to frequent fatal beach drownings, China's first operational attempt on the rip current hazard investigation was made by the National Marine Hazard Mitigation Service (NMHMS). A great number of recreational beaches were found developing rip currents interlaced with rhythmic sandbars, varying by season and location evidenced by satellite images and morphodynamic calculation. Considering insufficient understanding of the multi-channel rip system, case analysis and numerical study were conducted to explore its dynamicity and circulation characteristics under various wave climates in present work. The strength of rip currents was generally proportional to wave height and channel width under certain limits. Increasing wave height was not always a promotion and could even weaken the rip current due to the strong wave-current shear. Interesting pump and feed interactions between adjacent rip currents in the multi-channel system were observed. The rip current might be totally absent in narrow channels when the majority of water flows through neighboring broader pathways. The rip current was highly sensitive to the incident wave angle. Alongshore currents prevailed over the rip current when the wave angle reached 11 degrees to shore normal, which was not favorable to the existence of channeled sandbars. Vortices appeared around the edge of the bar owing to nonuniform wave breaking over rapid-varying bathymetry. The setup water was created shoreward by the sandbar array and substantially increased as the wave deviated from the normal incidence. The water surface depression in the rip channel was not observed as the wave angle increased, which fundamentally explained why the rip current could not persist when the incident wave became slightly oblique. In future, incident wave angle should be further incorporated into empirical formulas or probabilistic models to predict the rip current for expected improvement in accuracy.

Yao Zhang et al.

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Yao Zhang et al.

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Latest update: 23 Sep 2020
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Short summary
Rip currents and surf zone drowning records are reported at many recreational beaches in China, which show seasonal and spatial variations. Case studies and numerical simulations were conducted to explore the dynamicity of multi-channel rip currents generated by rhythmic sandbars. The scientific findings provides fundamental advance in the understanding how the rip current responds to morphodynamic and hydrodynamic changes, and could be applied to better predict the rip current hazard.
Rip currents and surf zone drowning records are reported at many recreational beaches in China,...
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