Experimental mechanics of sand stratification

The specific objectives of this study of heterogeneous sand mixtures are to: 1) define particle segregation mechanics in terms of the mechanical interaction between moving particles in the bed layer at the interface with the stationary particles on the bed; 2) report on several laboratory experiments where lamination and stratification of sand deposits were obtained in a large laboratory flume under steady flow and a continuous sediment supply. The interaction of moving sand particles at the interface with stationary bed material is paramount to the understanding of sand stratification. The shearing motion of a mixture of coarse and fines and particles of identical mass density is considered. The percentage of grain kinetic energy lost is large when small rolling particles impact larger stationary particles. Conversely, the kinetic energy of large rolling particles is almost unchanged after impacting small stationary particles. Consequently, the grain kinetic energy of the fine fractions of a mixture is rapidly depleted and the fine particles first cover the bed surface to form thin elongated fine-grained lenses. Coarse sand particles maintain their ability to roll on top of the fine-grained bed surface. As the kinetic energy of coarser particles is gradually depleted, they deposit on top of the fine-grained lenses, thus resulting in particle segregation. Under continuous settling, the fine-grained lenses find themselves buried under coarser particles and repetitive segregation produces the distinct features of laminated deposits. Large scale stratification experiments demonstrate that thick stratified deposits can be obtained under steady flow and a continuous supply of heterogeneous sand particles. Plane-bed deltas are characterized by a fine-grained laminated topset and a coarse-grained foreset. Fine-grained lenses indicate particle segregation at the interface between the stationary surface and the moving bed layer.