Research on the morphodynamics of bedrock rivers has primarily focused on bedrock incision, and little is known about the alluvial morphodynamics of rivers with exposed bedrock surfaces. More specifically, there is a lack of information on the morphodynamics of low slope bedrock reaches due to the recent recognition of such systems. Here, we present the results of laboratory experiments specifically designed to gain novel insight into flow resistances, flow hydrodynamics, and sediment transport processes in equilibrium partially exposed bedrock reaches transporting nonuniform sand as bed material in low slope areas. The experiments show that (1) downstream of a stable alluvial‐bedrock transition flow depth decreases in the streamwise direction, (2) bedform amplitude may decrease in the streamwise direction, and (3) stable patterns of downstream fining may form. Given the bedrock geometry, the water surface elevation at downstream boundary and the characteristics of the bedform regime in an alluvial channel subject to the same flow rate and sediment supply at equilibrium control bedform characteristics and sediment sorting patterns in the bedrock reach. When this distance is significantly smaller than the alluvial equilibrium flow depth or when the alluvial equilibrium bedform regime is close to the dune‐antidune transition, bedforms in the bedrock reach are closer to the dune‐antidune transition than at alluvial equilibrium with a consequent reduction in bedform amplitude. If the distance between the water level at the downstream boundary and the bedrock surface is close to the alluvial equilibrium flow depth and the alluvial equilibrium bedforms are well in the dune regime, a stable pattern of downstream fining can be expected. The comparisons between experimental and modeled sediment transport rates and equilibrium grain size distributions of the sediment further show that surface‐based bedload transport models derived for alluvial systems reasonably predict equilibrium sediment transport rates and bed surface size distributions in bedrock reaches if the presence of exposed bedrock is accounted for in terms of alluvial cover fraction.

Turbidity current and coastal storm deposits are commonly characterized by a basal sandy massive (structureless) unit overlying an erosional surface and underlying a parallel or cross-laminated unit. Similar sequences have been recently identified in fluvial settings as well. Notwithstanding field, laboratory and numerical studies, the mechanisms for emplacement of these massive basal units are still under debate. It is well accepted that the sequence considered here can be deposited by waning-energy flows, and that the parallel-laminated units are deposited under transport conditions corresponding to upper plane bed at the dune-antidune transition. Thus, transport conditions that are more intense than those at the dune-antidune transition should deposit massive units. This study presents experimental, open-channel flow results showing that sandy massive units can be the result of gradual deposition from a thick bedload layer of colliding grains called sheet flow layer. When this layer forms with relatively coarse sand, the non-dimensional bed shear stress associated with skin friction, the Shields number, is larger than a threshold value approximately equal to 0Á4. For values of the Shields number smaller than 0Á4 the sheet flow layer disappeared, sediment was transported by a standard bedload layer one or two grain diameters thick, and the bed configuration was characterized by downstream migrating antidunes and washed out dunes. Parallel laminae were found in deposits emplaced with standard bedload transport demonstrating that the same dilute flow can gradually deposit the basal and the parallel-laminated unit in presence of traction at the depositional boundary. Further, the experiments suggested that two different types of upper plane bed conditions can be defined, one associated with standard bedload transport at the dune-antidune transition, and the other associated with bedload transport in sheet flow mode at the transition between upstream and downstream migrating antidunes.

Recent studies reveal that low-slope bedrock reaches (bedrock surface slope milder thañ 5 m/km) are more common than previously thought and can be found in engineered rivers and densely populated deltas. Here we present a novel formulation of alluvial morphodynamics of low-slope bedrock rivers transporting nonuniform bed material that accounts for the nonuniformity of the sediment size and the presence of small scale bedforms such as dunes and can thus be of aid to solve management/ restoration problems in low-slope bedrock rivers. The formulation is implemented in a one-dimensional morphodynamic model. Numerical results are compared with laboratory experiments on equilibrium bedrock reaches downstream of stable alluvial-bedrock transitions. The differences between experimental and numerical results are comparable with those obtained in the alluvial case. Model applications simulate (1) bedrock reaches with a stable bedrock-alluvial transitions, (2) an alluvial-bedrock transition subject to sea level rise, and (3) steep bedrock reaches. Upstream of a stable bedrock-alluvial transition the flow decelerates in the streamwise direction with the formation of a stable pattern of downstream coarsening of bed surface sediment. In response to sea level rise, alluvial-bedrock transitions migrate downstream and bedrock-alluvial transitions migrate upstream. Opposite migration directions are expected in the case of sea level fall. When applied to steep channels, the model predicts gradual alluviation, but it fails to reproduce runaway alluviation.