Active geomorphological interventions, such as reprofiling of river bars, are often used to increase bar dynamics and prevent vegetation encroachment. River restoration management should be planned based on the knowledge of what processes will follow the intervention and on the anticipation of the consequences. However, in many cases, the associated physical processes are not clearly identified whereas their consequences on bar morphodynamics are still not fully understood. This study aims to bring new insights into the biomorphodynamics evolution of the riverbed after restoration works by using a 2D biomorphodynamic model developed in the TELEMAC-MASCARET system. It seeks to compare and evaluate the performance of five bar reprofiling scenarios in which the bar elevation is lowered to just below the water level at specified design discharges. The study area is located in the channelized and regulated alpine gravel-bed Isère River (France). Bar dynamics and early stages of vegetation establishment are analyzed for the first 2 years after each restoration scenario. The results indicate that plant colonization would occur in all cases. Overall, maximizing the reduction of bar height is the most effective way to improve the bar dynamics and limit future vegetation encroachment.

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This study investigates the effects of vegetation patch patterns on the morphological evolution of alluvial river channels at the reach-scale. For this, a new two-dimensional numerical biomorphodynamic model has been developed using the Telemac-Mascaret system. Considering the newest development in the topic, the effects of vegetation on bedload transport are included by extending Einstein's parameters for the sediment transport formula. The model was subsequently validated by published laboratory experiments reproducing alternate bar dynamics with different vegetation establishment scenarios. The validated model was then used to study the influence of vegetation patch patterns on the channel morphological evolution considering the two most observed ones: (a) the filled pattern with plants well distributed within the patch, and (b) the stripe pattern with plants established only along the patch edges. 14 scenarios were simulated in total, including sensitivity analyses on the coefficients of vegetation characteristics. The results indicate that the morphological responses of an alternate bar system to the stripe pattern consist of channel widening, steeper slope and reduced water depth, with increased sediment transport rates. The effects of the filled pattern are similar but weaker. The results also show that with the stripe pattern, the alternate bars tend to migrate toward the centerline forming center bars. Besides, the scroll bars forming downstream are shorter, corresponding to less visible chute channels, compared to the filled pattern. Despite much less vegetation coverage, the stripe pattern decreases the bar elongation rates in a way similar to the filled pattern.@en

Bars are large sediment deposits alternating with deeper areas that arise from alluvial river bed instability and forcing. The present study aimed at investigating the combined influence of flow and longitudinal width variations on the co-evolution between bar pattern and sediment sorting in a sandy-gravel river reach. To this goal, a fully non-linear 2D numerical model was developed to reproduce the morphodynamic behavior of bars in a reach of the Loire River consisting in a typical channel expansion/contraction. Numerical results showed that varying water discharge promoted a competition between low and high bar modes: i.e., from alternate to multiple bar patterns. Low bar modes were associated with coarse sediment over bar tops and fine sediment in pools, and this sorting pattern was inverted for higher bar modes. Surface sediment was coarser and the degree of sediment sorting was greater after periods of low than high flow. Due to high sediment mobility, sediment sorting did not significantly modify bar morphodynamics.

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Rivers typically present heterogeneous bed material, but the effects of sediment nonuniformity on river bar characteristics are still unclear. This work investigates the impact of sediment size heterogeneity on alternate bars with a morphodynamic numerical model. The model is first used to reproduce a laboratory experiment showing alternate bar formation with nonuniform bed material. Subsequently, the influence of sediment size heterogeneity on alternate bars is investigated distinguishing hybrid from free bars, definition based on the presence/absence of morphodynamic forcing, considering the results of nine scenarios. In four of them, a transverse obstacle is used to generate forcing. The computations are carried out with the Telemac-Mascaret system solving the two-dimensional shallow-water equations with a finite element approach, accounting for horizontal and vertical sediment sorting processes. The results show that sediment heterogeneity affects free migrating and hybrid bars in a different way. The difference lies in the presence/absence of a migration front, so that distinct relations between bed topography, bed shear stress, and sediment sorting are obtained. Sediment sorting and associated planform redistribution of bed roughness only slightly modify free migrating bar morphodynamics, whereas hybrid bars are greatly impacted, with decreased amplitude and increased wavelength. Increased sediment size heterogeneity increases the degree of sediment sorting, while the sorting pattern remains the same for both free and hybrid bars. Moreover, it produces averagely higher, longer, and faster free bars, while in the case of hybrid bars their wavelength is increased but no general trend can be determined for their amplitude.

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