Incipient Motion Criteria of the Uniform Gravel Bed under Falling Spheres and Natural Stones Impact in Open Channel Flows

The investigation on the incipient motion of the sediment transport is crucial to control the stability of the bed and banks of a natural alluvial channel to response the prevention of the hazards to life and infrastructure worldwide. Previous studies on the shields parameter and its influencing factors have been well documented. Although those researches have tended to focus on the incipient motion criteria under flow condition, grain properties, channel slope, relative depth, less attention has been paid to the sediment moving induced by the inflow from debris flow that frequently occurs in mountain river due to a heavy rainfall. This phenomenon could not only add additional forces to the sediment transport in the bed of the river but also affect and change the direction and the magnitude of the sediment motion. In this study, the incipient motion of uniform sediments under falling spheres and natural stones were evaluated in the flume channel with 9 m long, 0.6 m wide and 0.6 m deep. The slope of the channel was set from 0.8% to 2.3% with 0.031 m³/s to 0.137 m³/s water discharge. The sorted median grain size d₅₀ (2.03 mm, 6.53 mm, and 14.20 mm) of the bed material were used. The spheres diameters were chosen to be 4 cm, 6 cm, 8 cm, and 10 cm, whereas the nominal diameters of natural stones were 7.23 cm, 8.31 cm, and 9.31 cm. Effects of the falling sphere and stones were investigated for the condition where sediment nearly close to critical incipient motion condition. Based on the results obtained, the motions of sediment particles under the impacts of falling sphere were interpreted using the Shield curve. The size, weight, and shape of the falling spheres and the natural stones have significant effects on the movement condition of the sediment bed. In addition, the relationship between drag force and the length movement of spheres and stones were plotted. The saltant trajectories and the frequency of the maximum rolling distance, saltation length and saltation height of the sediment bed were investigated.

Keywords: Incipient motion, transport, critical shear stress, drag force.