Understanding and predicting the propagation, deposition and re-suspension of suspended particulate matter (SPM) in river networks is important for managing water resources, ecological habitat, pollution, navigation, hydropower generation, reservoir sedimentation, etc. Observational data are scarce and costly, and there is little feedback on the efficiency of numerical simulation tools for compensating the lack of data on a river scale of several hundreds of kilometres.

The interplay between streamwise flow, curvature-induced secondary flow, sediment transport and bed morphology leads to the formation of a typical bar-pool bed morphology in open-channel bends. The associated scour at the outer bank and deposition at the inner bank may endanger the outer bank's stability or reduce the navigable width of the channel. Previous preliminary laboratory experiments in a sharply curved flume with a fixed horizontal bed have shown that a bubble screen located near the outer bank can generate an additional secondary flow located between the outer bank and the curvature-induced secondary flow and with a sense of rotation opposite to the latter. This bubble-induced secondary flow redistributes velocities and bed shear stresses. The reported study investigates the implications of a bubble screen on the flow and the morphology in configurations with mobile bed. Velocity measurements show that the bubble-induced secondary flow shifts the curvature-induced secondary flow in inwards direction and reduces its strength. The bubble screen considerably reduces morphological gradients. Maximum bend scour is reduced by about 50% and occurs further away from the outer bank where it does not endanger the bank stability anymore. The location of maximum scour coincides with the junction of the curvature-induced and bubble-induced secondary flows. At this same location, the maximum streamwise velocities and maximum vertical velocities impinging on the bed also occur, which indicates their importance with respect to the formation of bend scour. The bubble screen also substantially reduced deposition at the inner bank. These preliminary experiments show the potential of a bubble screen to influence and modify the bed morphology.

Weirs or run-of-river dams can disrupt bedload transfer with negative ecological effects downstream due to sediment starvation. The way and the degree to which bedload is trapped is nevertheless not straightforward and few studies have examined this topic. This study focuses on a 13km-long reservoir of the Rhône River, France, created by a diversion dam equipped with bottom gates. Our main objective was to determine the degree of alteration of the bedload transfer downstream and to identify to which extent the implementation of Ecomorphogenic Flow (EmF), defined as environmental flow whose objective is specifically to increase bedload transfer through the reservoir to promote downstream habitat diversity, could increase bed mobility. The results show that the potential for morphological adjustments in the reservoir was already low before dam completion (1968) in response to a substantial decrease in coarse sediment supply, but that this potential was progressively reduced due to the impoundment. However, the bedload transfer continuity has been at least partially maintained since dam completion. According to numerical simulations, only particles smaller than medium gravels (14 mm) could be exported downstream of the dam for relatively rare discharge (50-years return-interval flood). Implementation of EmF could neatly improve the bedload transfer since it would allow to strongly increase the competence: for a 2-years and a 50-years return-interval floods, the maximum particle size exportable downstream is respectively 9 and 4 times larger than for normal functioning of the reservoir operating.

Purpose Suspended particulate matter (SPM) transport through rivers is a major vector of nutrients and pollutants to continental shelf areas. To develop efficient sediment management strategies, there is a need to obtain quantitative information on SPM sources. For many years, the geochemical properties of SPM have been commonly used as tracers to identify sediment sources. In large watersheds, with numerous sources, the expected alteration of tracers during their transport requires that their reactivity be taken into account. Materials and methods To overcome this issue, we tested the use of major and trace element signatures in the residual fraction of SPM, using two different extraction methods. This original fingerprinting approach was applied to the Upper Rhône River basin (~20,000 km 2 ) in order to assess the respective SPM contributions of its main five tributaries (Arve, Ain, Fier, Guiers, and Bourbre Rivers) for contrasted hydrological conditions (base flow periods, flood events and dam flushing). By incorporating element concentrations previously corrected from particle size distribution in a mixing model coupled to Monte Carlo simulations, we estimated the associated uncertainties of the SPM contributions from each tributary. The relative SPM contributions obtained using this fingerprinting approach were compared with those calculated with a 1-D hydro-sedimentary model. Results and discussionThe use of element concentrations, such as Zn, P, Cu, Pb, Mn, or Sr, in the total fraction of SPM as conservative fingerprinting properties was not suitable, since they are mainly bound (> 50%) to reactive carrier phases. By using Ba, Ni, Fe, Mg, Cu, Sr, and V concentrations in the SPM residual fraction, the apportionment modeling of SPM sources was successfully assessed. The fingerprinting approach showed that, in base flow conditions, SPM originated mainly from the Arve River. During dam flushing event, the fingerprinting approach consistently estimated that re-suspended sediments came from the Arve River, while the 1-D hydro-sedimentary model estimated a proportion of re-suspended sediment originating within the Rhône River. Conclusions This original fingerprinting approach highlighted the relevance of using geochemical properties in the non-reactive fraction of SPM in order to obtain reliable information on spatial sources of SPM in a large river basin. This methodology opens up promising perspectives to better track SPM sources in highly reactive environments such as estuaries/delta or in historical sediment archives.

The ability of a bubble screen to redistribute the flow field and bed morphology in shallow rivers and open channels has been investigated in laboratory experiments. Rising air bubbles generated by a pressurized porous tube situated on the bed induced secondary flow perpendicular to the porous tube. The secondary flow redistributed the longitudinal velocity, which caused also morphological redistribution under mobile-bed conditions. The strength and size of the bubble-induced secondary flow were independent of the base flow velocity and increased with water depth. The size of the secondary flow cell ranged from 3× (immobile bed) to 7× (mobile bed) the water depth. Similar sizes of bubble-induced secondary flow cells have been reported in literature for water depths ranging from 0.1 to 5 m, indicating that the laboratory experiments are relevant for natural rivers and open channels. A mutually strengthening interplay occurred between the bubble screen, the bubble-induced secondary flow, and the morphology. The bubble-induced secondary flow considerably increased the rising velocity of the air bubbles, which on its turn strengthened the secondary flow. The morphological redistribution increased the flow depth in the region covered by the secondary flow cell, which on its turn increased the size and strength of the secondary flow cell, and its effect on the morphological redistribution. This coupled hydrodynamic-morphologic behavior explains the larger size and strength of the secondary flow over a mobile bed than over a flat immobile bed. The results demonstrate the potential of the bubble screen as a technique to modify the morphology in a variety of applications in shallow rivers and open channels.