THE ROLE OF RELATIVE SUBMERGENCE ON CLUSTER FORMATION

The objective of this research is to investigate the formation of cluster microstructures in mountain gravel streams. An emphasis is placed on the effects of large clast particles on the formation and evolution of cluster microforms. Specifically, two cases are examined in a laboratory flume, first when the relative submergence of the clast particles is less than 3 and second when the relative submergence exceeds 3. It is documented in the literature that for relative submergence greater than 3 the dimensionless critical shear stress is independent of the submergence ratio. In this case, the incipient motion criterion developed by Shields is applicable and particles transport can be described by the conventional sediment transport equations. For low relative submergence (viz. less than 3) the critical shear stress becomes strongly dependent of the Froude number due to the significant presence of surface waves. Preliminary results in the laboratory for the first case of experiments show that the relative submergence affects the formation of clusters. It is found that the required magnitude of the shear stress for clusters to form is about 2 times greater than the magnitude of the shear stress determined by Papanicolaou et al. and Strom et al. for relative submergence greater than 3. Future work will provide information about the effects of clusters on sediment transport rates. Specifically, the effects of clusters on fine particles transport will be monitored.