Study on threshold motion of sediment and bedload transport by tsunami waves

• The flow of tsunami wave uprush and backwash belongs to the rapid unsteady flow.
• Threshold motion by tsunami wave is derived based on time-mean near bottom velocity for total duration.
• The drag force and its duration are necessary and sufficient for threshold motion in rapid unsteady flow.
• Some commonly used or recent bedload formulas are not suitable for tsunami flow.
• An appropriate criterion of “sediment transport impulse” is conjectured.

Tsunami wave produces high bed shear stresses and can mobilize a mount of sediment movements over large areas. In this study, laboratory experiments were performed in a wave flume to investigate sediment transport by tsunami wave attacking sand beaches. The flow field of tsunami wave was calculated by the numerical model OpenFOAM®. The numerical results showed that flow characteristic of simulated tsunami wave uprush and backwash belongs to the rapid unsteady flow. The drag force and its duration are necessary and sufficient to trigger particle entrainment. It is recommended that time-mean near bottom velocity averaging over the force duration rather than the maximum velocity is relevant to better description of the incipient motion phenomenon in rapid unsteady tsunami flow. The experimental results show that some of the commonly used or recent bedload formulas for steady flow or wave are not suitable for the rapid unsteady or non-uniform tsunami flow. According to the experimental data presented in this paper, the bedload transport formulas for the tsunami uprush and backwash processes are proposed. An appropriate criterion of “sediment transport impulse”, which is the product of magnitude and duration, is conjectured and could well explain the sandy beach deposition and erosion caused by tsunami wave.