Initiation of Motion of Coral Gravels Under Asymmetric Oscillating Flow

This study investigates the characteristics of incipient motion of coral gravels of various sizes and shapes under different wave-associated oscillating flow conditions. We conducted experiments in an oscillatory flow tunnel using natural coral gravel sediment sampled from the beaches of Okinawa Island. The impact of sediment grain size, shape, and density on incipient motion under oscillating flow with various skewness conditions was analyzed using Particle Image Velocimetry, with a focus on fluid shear stresses and pressure gradient forces. The results indicate that sediment movement is dominantly driven by the pressure gradient force under the velocity profiles with large skewness, by shear stresses under those with low skewness, and by the combined influence of both forces under those with intermediate skewness. For coral gravels with relatively large grain size, a relative importance of the pressure gradient force were enhanced. Considering both the effects of shear stresses and pressure gradients, along with sediment properties and bed configuration, the proposing combined parameter shows potential to give more reliable threshold condition of incipient motion of arbitrary sediments including coral gravels.