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Tidal bores are sudden discontinuities in water depth, and can be classified as undular and breaking bores when tidal waves propagate upstream in a river mouth and on beaches. The impact of different types of bores on sediment transport is not clear, hence specially designed experiments were conducted to clarify the mechanism of sediment initiation. In this study, a laser Doppler anemometer, a force sensor and ultrasonic displacement meters accompanied by video recordings were used to investigate simultaneously the incipient motion of sediment under tidal bores. No sediment motion was observed during the initial steady flows. It was observed that only a few particles changed their initial position during the undular bore, however a group of particles were seen moving upstream during the breaking bore. The inception of sediment motion was closely related to the passage of the breaking roller, i.e., the forces that caused the inception of particle motion were not only due to velocity but also to the sudden discontinuity in free water surface. According to the experimental results, the upward vertical force due to swelling in free water surface before the roller toe was the main force in destabilizing the particles. Later, a large upstream longitudinal force was found to be the dominant cause promoting upstream particle motion during the breaking roller passage. Furthermore, phases of both horizontal and vertical forces play an important role in tidal bore propagation. It is interesting to note that by using the measured force, the observed phenomenon can be well explained by the Shield threshold criterion.

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