Experimental results on the flow characteristics of bottom boundary layer induced by a solitary wave propagating over a horizontal bottom are presented. Particle-trajectory flow visualization technique and high-speed particle image velocimetry (HSPIV) were used to elucidate detailed velocity fields underneath solitary waves with the ratios of wave height to water depth from 0.130 to 0.386. The results show that the velocity profiles can be classified into two classes with respect to the passage of the solitary wave-crest at the measuring section: the pre-passing and post-passing phases. For the pre-passing phase, the velocity distributions can be deduced to a unique similarity profile with the use of unsteady free stream velocity and time-dependent boundary layer thickness as the characteristic velocity and length scales. On the other hand, the similarity profile for the flow reversal, acting like an unsteady wall jet, is obtained from the velocity distributions during the post-passing phase. The velocity deficit between the unsteady free stream velocity and the maximum negative velocity as well as the (time-dependent) thickness of reversal flow were identified as the characteristic velocity and length scales, respectively.

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