Entrained air in bore-driven swash on an impermeable rough slope

Han Jing Dai, Gustaaf Adriaan Kikkert, Bo Tao Chen, Dubravka Pokrajac

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The aim of the present investigation is to clarify the role and evaluate the importance of air entrainment
in the swash zone by carrying out a set of detailed laboratory experiments. Experiments involved
generating a single, highly repeatable, large-scale, bore-driven swash event on a sand-rough impermeable
beach with slope 1:10. Measurements that yield the characteristics of the entrained air, including the void
fractions, bubble size and bed-parallel bubble velocity, and the hydrodynamics are obtained at five crossshore
locations in the swash zone using an optical probe and a combined PIV/LIF system. The results
show that two distinct bubble clouds enter the swash zone, the first is the result of local entrainment at
the wave tip and the second are the remaining bubbles of the air that is entrained at the second plunge
point of wave breaking before the wave starts climbing the beach. The void fractions are up to 0.20 and
bubble size up to 20mm which are similar to air entrainment after wave breaking in the surf zone and
deep water. Local entrainment of air continues in the swash zone, but void fractions decrease rapidly
with distance up the slope and no air is present after flow reversal. Energy dissipation in the swash zone
attributable to entrained air is at least of order 1% of the total energy dissipation. This is smaller than in
the surf zone or deep water because of the smaller volume of entrained air and the greater total energy
dissipation in the swash zone.
Original languageEnglish
Pages (from-to)26-43
Number of pages18
JournalCoastal Engineering
Early online date27 Dec 2016
Publication statusPublished - Mar 2017

Bibliographical note

The authors gratefully acknowledge financial support from the Hong Kong Research Grant Commission under grant number 613711 and the TUYF Charitable Trust under grant number TUYF12EG06.


  • bores
  • entrained air
  • experimental measurements
  • hydrodynamics
  • swash zone


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