The influence of flow acceleration on stone stability

By carrying out experiments in a flume containing a local contraction, the stone stability in an accelerated flow is investigated. In the contraction the stability of two different stone sizes, subject to different velocity-acceleration combinations, is analysed. If the hypothesis is correct, than for some velocity-acceleration combinations movement occurs while for the same velocity combined with a lower acceleration no movement occurs. The shear stress occurring in the accelerated flow is determined using the shear velocity. According to the classical Shields method the shear velocity is responsible for the movement of the stones. Movement is detected for lower shear velocities then expected. According to the hypothesis this is a result of the extra generated force on the stones due to acceleration.