Effects of particle shapes and sizes on vertical sortings in numerical experiments of slope failure in the water

Hazumu Tadokoro, Shoji Fukuoka

Tuesday 30 june 2015

12:15 - 12:30h at Oceania (level 0)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling

Parallel session: 5B. Sediment - River

Gravel-bed rivers are composed of particles of various shapes and sizes. The shapes and sizes of particles in the gravel-bed river have great influences on sediment transport and river bed variation. Nevertheless, we have supposed spheres as representative particles in the analysis of bed variation in gravel-bed rivers. Effects of particle size and shapes on sediment transport have not been clarified, because it is difficult to observe particle motions in gravel-bed river during floods. Little study has been done to consider effects of particle shapes on sediment transport and bed variation. Particles shapes have to be considered properly in thinking about sediment transport and bed variation in gravel-bed rivers. Vertical sortings of sediment particles play an important role on a sediment transport phenomena. The effects of particle shapes and sizes on vertical sorting were investigated by comparing motions of spheres and gravels in numerical experiment of slope failures in the water. Particles used in numerical experiment were made to simulate the motion of gravels with different shapes and sizes. The numerical experimental results demonstrated that the angle of repose of gravel particles was larger than that of spheres due to engagement effects of gravel particles. Moreover, sphere particles failures progressed smoothly with time. On the other hand, gravel particles collapsed by small amount at lower parts by large engagement effects among particles, but upper particles fell down in a relatively long time. As a result of resistance among particles, the travel distance of spheres was larger than that of gravels. In the case of both spheres and gravels, large particles moved as a body parallel to the channel bed, but small particles tended to drop in the lower part of the channel. Consequently, the vertical sorting made particles of the surface layer larger. The surface layer particles of spheres became larger than those of gravels. Moreover, the porosity of gravels in the moving layer was larger than that of spheres. The results found in these numerical slope failure experiments provided very important knowledge for the sediment transport mechanism of gravel-bed rivers.