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Sediment supply plays an essential role in river morphology. However, the specific impact of sediment supply on river morphology is not apparent. According to the hydrograph boundary layer (HBL) concept, upstream riverbed changes caused by the imbalance between sediment supply and the capacity can propagate only a limited length and have a negligible effect on the riverbed beyond such a short length. We performed a two-dimensional morphodynamic calculation to test the concept of HBL, which was proposed under a one-dimensional simulation, meaning that the concept of HBL is still valid for plane changes in river morphology. We employed an unsteady flow with equilibrium or constant sediment supply in a straight, modeled gravel-bedded channel with an unerodible bank to simulate alternate bar morphodynamics. The results show that regardless of the sediment supply condition, the alternate bar features formed downstream of the HBL are considerably similar. This suggests that sediment disturbance at the upstream end has a negligible effect on the mobile-bed dynamic processes, including alternate bar formation and development downstream of the HBL.

Climate change is expected to alter the distribution of flow discharge in rivers worldwide. We study the impact of climate‐driven flow changes on the shape of riverbed, and specifically on alternate bars, large deposits of gravel/sand that often form in rivers. We consider the illustrative example of the Alpine Rhine River, showing two nearby reaches with similar hydro‐morphological characteristics, but different channel width. Hydrological projections are obtained from literature, while the evolution of alternate bars is predicted through a novel, semi‐analytical model. Results show a remarkably different behavior of the two reaches: the upstream one, being wide enough for a full development of alternate bars, is resistant to flow alterations; the downstream reach, whose width is close to threshold conditions, is highly susceptible to future changes, showing a strong tendency to increase bar prominence. These findings reflect a general tendency of near‐threshold geomorphic systems to be vulnerable to anthropic stressors. Plain Language Summary The worldwide alteration of the river flow induced by climate change is likely to significantly impact the bed morphology of embanked rivers, which is often characterized by the presence of alternate bars, namely repetitive sequences of large sediment deposits and scour zones. Bar formation is both a major issue for river management (due to local erosion at instream structures and increase of flood risk), and an important resource for sustaining biodiversity, because bar morphology templates rich habitats for river fauna and vegetation. We analyze the effect of climate change on river bars by considering existing state‐of‐the‐art projections of future flow discharge, and by implementing a mathematical model suitable to perform long‐term simulations, while keeping the essential ingredients to reproduce bar dynamics. Model results reveal a very different adaptation of the riverbed to climate change: relatively wide reaches are expected to maintain the current alternate bar characteristics, while reaches whose width is close to a critical threshold value are likely to experience a remarkable alteration in the next decades, which provides a noteworthy example of how near‐threshold geomorphic systems may be highly sensitive to climate change. Key Points Climate‐driven changes of hydrological regime can have a significant impact on bar morphology The formation of alternate bars in channelized rivers is promoted by a decrease of frequency of high flows Sensitivity of bar morphology to climatic stressors depends on how far the river width is from the key morphodynamic threshold value

Alternate bars are highly mobile features that play a critical role in river morphodynamics at the reach scale. Previous studies have highlighted discharge, slope, sediment size, and initial channel width as key factors in their development, but the sensitivity of initial channel width under varying unsteady flow conditions remains less understood. This study employs numerical simulations to investigate how channel width affects alternate bar formation under unsteady hydrographs, assuming a constant slope and uniform sediment. The hydrographs consist of four stages: rising limb, peak flow, falling limb, and low flow. Two groups of peak discharge are considered: (i) peak discharge sufficient to generate alternate bars and (ii) higher peak discharge that fails to generate alternate bars. The results reveal contrasting controls across these two groups. In the first group, the Shields number governs bar dynamics, as both wide and narrow channels with similar Shields numbers exhibit comparable trends in bar development despite differing half of width-to-depth ratios. In the second group, half of width-to-depth ratio becomes the dominant factor influencing bar formation. Moreover, when half of width-to-depth ratios are similar, the range of vorticity and the intensity of secondary flows further modulate bar dynamics.

Sediment supply is widely held to be one of the primary controls on bar topography in alluvial channels, yet quantitative linkages between sediment supply and bar topography are not well developed. We explore the conditions under which alternate bars form and how they respond to the elimination of sediment supply in two linked laboratory experiments. The first set of experiments was conducted in a 28 m long, 0.86 m wide flume channel using a unimodal sand‐gravel mix. The second set of experiments was conducted at field scale in a 55 m long, 2.74 m wide channel using a unimodal gravel mixture. In both experiments, alternate bars and patchy surface grain‐size distributions developed under steady flow and sediment supply conditions. The cessation of the sediment supply induced a reduction in the surface grain‐size heterogeneity and the bars were eliminated. In both flumes, mean boundary shear stress had declined, but were capable of moving sediments after the bars disappeared, albeit at relatively small rates compared to when the bars were present. In the smaller flume, the previously stationary bars migrated out of the flume and were not replaced with new bars. A nearly featureless bed formed with limited surface grain‐size heterogeneity, a slightly coarsened surface and a slightly reduced slope. In the larger flume, the formation of alternate bars was induced by an imposed upstream flow constriction and as such, the bars did not migrate. Termination of sediment supply led to progressive erosion of bed topography and loss of the bars, coarsening of the bed surface, loss of bed texture patchiness and significant slope reduction. The original alternate bar topography redeveloped when the sediment supply was restored once sufficient deposition had occurred to reconstruct the original channel slope. This shows that the bar loss was reversible by establishing the previous conditions and highlights the importance of sediment supply for bar formation. The role of sediment supply in bar formation and stability is not often recognized in stream restoration. Our results suggest that the loss of sediment supply can significantly affect alternate bar topography and that considerable volumes of sediment may be needed restore channel bars. Key Points Reducing sediment supply reduces surface grain‐size heterogeneity Alternate bars disappear without an upstream sediment supply Significant gravel volumes need to be added to restore bars

Migrating alternate bars form in alluvial channels as a result of morphodynamic instability. Extensive literature can be found on their origin and short‐term development, but their long‐term evolution has been poorly studied so far. In particular, it is not clear whether migrating bars eventually reach a (dynamic) equilibrium, as in previous studies bars were observed to elongate with time. We studied the long‐term evolution of alternate bars by performing two independent long‐duration laboratory experiments and some numerical tests with a physics‐based depth‐averaged model. In a straight flume with constant water flow and sediment recirculation, migrating bars followed a cyclic variation. They became gradually longer and higher for a while, then quickly much shorter and lower. In one case, all migrating bars simultaneously vanished almost completely only to reform soon after. At the same time, steady bars, two to three times as long, progressively developed from upstream, gradually suppressing the migrating bars. We also observed simultaneous vanishing of migrating bars in an annular flume experiment, this time at intervals of 6–8 d. Numerical simulations of long alluvial channels with constant flow rate and fixed banks show periodic vanishing of a few migrating bars at a time, occurring at regular spacing. Under constant flow rates, migrating bars appear as a transition phenomenon of alluvial channels having a cyclic character. These observations, however, might hold only for certain morphodynamics conditions, which should be further investigated. Key Points Study of long‐term development of alternate bars Migrating bars regularly vanish and reform Steady bars suppress migrating bars

Alternate bars migrate downstream during floods due to coupled erosion and deposition on both sides of alluvial river channels. During low discharge periods, vegetation can grow on the tops of these bars, reducing migration rates and increasing bar wavelengths and bar heights. We explore two specific effects of above-ground vegetation on flow and transport. First, above-ground roots and groundcover can reduce bedload transport rates due to near-bed roughness, an effect not explored in most previous studies. Second, vegetation bodies (i.e. the above-ground trunk, stem, branches, and leaves) generate hydraulic drag. We model vegetation influences on alternate bar evolution using previously proposed equations which consider both vegetation body and above-ground root effects. We investigated three scenarios: vegetation body effects only, above-ground root effects only, and the full vegetation system (i.e., body and above-ground roots together). We find that vegetation body and root effects both reduce the bar migration rate and increase the bar wavelength. Reduced flow velocities over the bars due to vegetation body effects tend to enhance velocities and localized erosion on the opposite side of the channel, which in turn increases relative bar heights. Bar morphology and migration rate are most sensitive to vegetation growth rates at lower flood discharges where bar-top vegetation persists from year to year and older vegetation has stronger impacts on flow and transport. Higher peak floods tend to remove and reset vegetation growth, resulting in little sensitivity to growth rate.

We investigate the effects of alternate bar morphology on the hyporheic flow in gravel bed rivers. Our goal is to investigate the relations between residence time distribution of a conservative tracer and the parameters controlling bed form morphology. We assume homogeneous, isotropic or anisotropic hydraulic properties of the streambed sediment and constant flow regime in equilibrium with the bed form, which is considered fixed because its formation timescale is much longer than that of the subsurface flow. Under these assumptions, we solve the in‐stream and hyporheic flow fields analytically in a three‐dimensional domain. We approximate the former with the shallow water equations and model the latter as a Darcian flow. The two systems are linked through the hydraulic head distribution, which is predicted at the streambed by the surface model and applied as a boundary condition to the hyporheic flow model. We solve the solute transport equation in the hyporheic zone for a conservative tracer by means of particle tracking. Our model predicts that the mean value and variance of the hyporheic residence time depend on the alternate bar amplitude at equilibrium. This result is found to be applicable also to discharges that are lower (70% in our simulations) than the formative and submerge the bars entirely. Moreover, our analysis shows that 95% of the hyporheic flow is confined in a near‐bed layer, whose depth is about the width of the channel and shallows from low to steep gradient streams. This causes the hyporheic mean residence time to reach a threshold when the alluvial depth is deeper than the channel width. Our results also show that as the stream slope increases, the streamlines compact near to the streambed, thereby reducing the mean residence time and its variance. Finally, we observe that the hyporheic residence time of pulse injections of passive solutes is lognormally distributed, with the mean value depending in a simple manner on the amplitude of the alternate bars.

Predicting when riparian vegetation establishes on river bars is crucial to estimate how morphology and ecology of gravel bed rivers respond to anthropogenic or climatic changes. However, the characteristic timescale required for vegetation establishment on gravel bars remains poorly investigated. The Windows of Opportunity (WoO) concept provides an ideal framework for estimating such timescale by analysing time series of disturbance periods following seed dispersal. Here we present the results of a study conducted in a 41 km long reach of the Alpine Rhine river characterised by the presence of alternate gravel bars, which show differences in morphological activity and vegetated areas. We mapped the time evolution of vegetation cover between 1996 and 2017 by using aerial images and related vegetation occurrence to bar topography. Observations show that vegetation mainly colonised bar surfaces between 2005 and 2008, enduring on bars that showed less morphological activity. Only few patches, which were removed by downstream bar migration, were observed on bars that experienced significant morphological changes. This allowed us to identify conditions that were more favourable to vegetation recruitment and growth. To explain the vegetation pattern observed along the whole reach, we developed a simple modelling framework based on the WoO concept including the effects of flow variability on seed dispersal and seedling recruitment and survival. Model applications demonstrate that vegetation successfully establishes on bare areas if plants can withstand hydrodynamic disturbances for about 85 days after seed dispersal. We also identified timing and location of successful recruitment events and discussed how they are linked to bar morphology, seed dispersal, and riverbed morphodynamics. This study provides a first attempt to quantify the WoO in a gravel bed river with alternate bars, representing a step towards the development of quantitative tools for predicting river morphological trajectories.

Rivers experience a wide range of discharges. It is nowadays acknowledged that is not realistic to assume that the morphology of a river is influenced by only a single formative discharge. Rather, it is the full range of flows that are able to move sediments and erode banks that affect the fluvial morphology. Thus, the channel morphology emerges from the interactions between different competent discharges. A goal that has still not been completely achieved in geomorphology is the understanding of the role of discharge variability on river morphological processes. In this paper, we present the results of an experimental investigation concerning the impact of the sequencing of two competent discharges on a self‐forming pseudomeandering pattern. The inception of the pattern, the bar dynamics, and the bend erosion are investigated. A comparison of the experiments performed with steady and unsteady discharges has indicated the key role of the discharge variability in promoting and sustaining the pseudomeandering channel. These experimental findings shed light on some important morphological processes (bar deformation, low‐flow channel incision, and triggering of the bend inception) that are affected by discharge variations to a great extent, in agreement with some field studies and conceptual models.

. We adopt a multidisciplinary approach toward the quantitative assessment of juvenile fish habitats in Alpine rivers using analytical modeling. The study focuses on braided and single-thread channel configurations together with their associated hydrodynamic patterns. A distinct difference between flows in these channels is the number and spatial arrangement of recirculation zones. These are due to the separation of flow from the river banks and result in a higher retention of flow in braided channels. Braided channels were also shown to provide more favourable shelter and nursing conditions for fish larvae and juveniles by mitigating high velocities during floods, by maintaining relatively shallow areas of flow, and by significant adjustments in the thermal regime. A historical analysis revealed a significant reduction of braided reaches along Alpine rivers that have most likely led to a significant degradation of the fish fauna.