Grain sorting processes in bed load transport: a theoretical and experimental study for gravel bed rivers

Field and laboratory investigations indicate that gravel-bed rivers with poorly-sorted grain size distribution and low lateral confinement, such as in the case of multithread, braided and transitional rivers but also wandering single-thread rivers, can simultaneously present active channel variations, both in the planimetric and altimetric directions, together with planimetric and vertical sorting. Actually channel adjustments present a complex interaction with sorting processes: transport and depositional processes of heterogeneous sediment mixtures induce the formation of various sorting patterns, among which free migrating patches such assorting waves also known as bedload sheets. Such sorting waves are mainly observed by the alternation of bands of finer and coarser material in the streamwise direction. This PhD research investigates the sorting processes of a bimodal grain size distribution both in the lateral and in the vertical direction, with particular focus on the study of bedload sheets. This work also aims at investigating the effects of lateral confinement on river bed morphodynamics and in particular on grain sorting processes; and besides, given that grain sorting plays a crucial role in affecting longitudinal river profile, as well as sediment patterns and bed forms, being indeed the connection between bed topography and bed sediment composition close, we would like to provide simple theoretical analyses and tools able to predict morphodynamic bed river variations driven by sorting. Different methodologies are applied to successfully achieve this purpose. Firstly, a deep investigation of the state of knowledge of mixed-size sediment morphodynamics, sorting processes, sorted sediment patterns and bedload sheets is carried out. Then, a 1D linear stability analysis addresses the problem of sorting waves’ formation by investigating the stability of a uniform shallow-water flow above an erodible bed composed by a bimodal sediment mixture. Results are presented by illustrating the arising instabilities dominated by sorting. Although the monodimensional shallow-water framework, the simple flow model adopted allows for the formulation of an algebraic eigenvalue problem that can be solved analytically, allowing for a deep insight into the mechanisms that drive instabilities. Then, small scaleflumeexperimentsarecarriedoutbyreproducingthreedifferentflowconfinement configurations and involving a bimodal mixture of natural sediments. All the configurations show large fluctuations of parameters characterising bed system in termsofbedsedimentcomposition,slope,activechannelwidthandsedimenttransport rate. The analysis of such parameters allows defining feedbacks among morphological variables. Results of such investigations shed light on the role of lateral confinement on sorting processes and morphodynamics of barebed rivers.