Formulations for the threshold conditions between vegetated and barebed channel are derived • Threshold conditions are mainly related to species-dependent characteristics • Submerged plants are more vulnerable to removal than emergent as relative emergence reduces the threshold value

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This paper uses flume experiments to investigate the influence of lateral width confinement on channel morphology, sediment sorting, and bedload transport. Three runs of about 60 hr were carried out under constant feeding rate equal to 0.6 l/s and 8 g/s, with a bimodal mixture of natural sediments, a fixed flume slope of 3%, and width imposed by lateral walls from 0.12 to 0.50 m in order to model three different flow confinement configurations. Despite the three runs transporting at the same rate on average, they presented different gravel-bed river morphologies and different degrees of bed complexity. The three runs also presented differences in bedload transport rate fluctuations associated with different magnitude and mechanisms of bed storage and release of sediments. The intermediate width configuration (Run 2) was found to have a minimum storage and release of sediments compared with both the wider configuration (Run 1) and the narrower configuration (Run 3). Runs 1 and 3 were characterized by a significant storage and release of sediments resulting in a highly fluctuating bedload transport rate; however, while Run 1 (braiding morphology) stored and released sediments through lateral deposits and bed variations with planimetric sorting, Run 3 (single-thread channel) stored and released sediments through variations in bed elevation accompanied by vertical sorting. We extrapolated this concept to the full set of gravel-bed river morphologies, thus speculating about the channel morphology for optimal sediment transfer.

Aquatic vegetation is ubiquitous in lowland rivers, and it is typically present in the shape of spatial self-organized patches of biomass. In this work, we mathematically define the threshold conditions for the incipient formation of self-organized vegetated patterns in the shape of central or multiple row patches. The analysis is carried out through a linear stability analysis whereby the 2D eco-hydrodynamic model is linearized and the growth rate of small-scale perturbations is evaluated considering a basic state represented by an initially uniformly vegetated and straight channel having a certain aspect ratio and Froude number. Results illustrate that, for given vegetation properties, instability arises when both the Froude number and the aspect ratio are higher than a given threshold; in this case, self-organization occurs, and spatial patterns of patches will develop according to the wavelength associated to the maximum growth rate. Moreover, instability and self-organization take place when the undisturbed vegetation density is lower than upper bound; this suggests that densely vegetated channels, as in the case of rivers populated by invasive species, will not experience the formation of any spatial patterns.