Abstract 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. Here, 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 obser...