Frédéric Liébault

Gabriel Melun

Guillaume Piton

Margot Chapuis

Paul Passy

Sandrine Tacon

Keywords: Fluvial geomorphology Fluvial metamorphosis, sediment wave, Extreme flood, Southern French Alps, storm Alex Fluvial metamorphosis, sediment wave, extreme flood, Southern French Alps, Storm Alex

Documenting and interpreting channel responses to catastrophic floods help understanding how rapid fluvial metamorphosis can propagate in a catchment under sediment cascading effects. The recent example of the October 2020 Storm Alex in SE France (~ 500 mm of rain in 24 h) provides a unique opportunity to investigate major geomorphic responses along 70 km of two confined alpine valleys (Vésubie and Roya) and to link them to sediment wave initiation and propagation. GIS-based analysis of remote sensing data (high-resolution ortho-imagery and airborne LiDAR data) acquired before and after the flood allowed combining channel changes with sediment erosion and deposition along a 35-km reach of the Vésubie, including the most impacted portions of the valley. In the Roya, the analysis was restricted to 2D morphological changes reconstructed with the sequence of ortho-imagery. Archives of aerial imagery were also used to integrate the storm impact in the historical trajectory of the rivers. The reconstruction of geomorphic responses shows a quasi-continuous fluvial metamorphosis along the investigated stream networks, with dramatic active channel widening and aggradation, having no antecedent analogs during the last 70 years in both valleys. The different glacial imprints between the two valleys is considered a key factor explaining the exacerbated channel response in the Vésubie, where a braided channel emerged along a 35-km river length. Many evidences strongly support that the fuelling effect of alluvial storage is a key element of the sediment cascade at the origin of the braided channel formation. This regional case study allows us to discuss the critical role of floodplain and terrace erosion in the formation of the post-flood braided channel, and to compare the geomorphic impact of the storm with similar reported cases in the literature.

Introduction

The geomorphic effect of floods is a basic concern of fluvial geomorphology, insofar as processes of sediment transfer and redistribution during these events can produce major and persistent morphological adjustments of river channels. Of particular interest is the geomorphic imprint of events with a magnitude far above extreme values of the contemporary flood regime (return periods > 100 yrs). These have been often referred to as catastrophic floods, with some demonstrative examples related to dam failures [START_REF] Pitlick | Response and recovery of a subalpine stream following a catastrophic flood[END_REF], rapid glacial melting during volcanic eruptions [START_REF] Magilligan | Geomorphic effectiveness, sandur development, and the pattern of landscape response during jökulhlaups: SkeiOararsandur, southeastern Iceland[END_REF], glacial-lake outburst floods [START_REF] Cenderelli | Flow hydraulics and geomorphic effects of glacial-lake outburst floods in the Mount Everest region, Nepal[END_REF], and tropical convective or cyclone-driven rainfall cells [START_REF] Croke | The use of multi temporal LiDAR to assess basin-scale erosion and deposition following the catastrophic January 2011 Lockyer flood, SE Queensland, Australia[END_REF][START_REF] Milan | Morphodynamics of bedrock-influenced dryland rivers during extreme floods: Insights from the Kruger National Park, South Africa[END_REF][START_REF] Heritage | Quantifying and contextualising cyclone-driven, extreme flood magnitudes in bedrock-influenced dryland rivers[END_REF]. It is often recognized that catastrophic floods generate a major transformation of alluvial landforms along important river lengths like the sudden interruption of multi-secular gradual floodplain accretion by the stripping effect of a flood related to channel widening [START_REF] Nanson | Episodes of vertical accretion and catastrophic stripping: a model of disequilibrium flood-plain development[END_REF]. Examples of flood-induced fluvial metamorphosis (sensu [START_REF] Schumm | River metamorphosis[END_REF], like channel pattern shifting from single-thread to braiding, have also been reported [START_REF] Arnaud-Fassetta | Hydro-geomorphic hazards and impact of manmade structures during the catastrophic flood of June 2000 in the Upper Guil catchment (Queyras, Southern French Alps)[END_REF][START_REF] Rinaldi | An integrated approach for investigating geomorphic response to extreme events: methodological framework and application to the October 2011 flood in the Magra River catchment, Italy[END_REF]. However, the variability of morphological responses to floods for a given recurrence interval hinders determination of a critical return period associated with major geomorphic impacts [START_REF] Magilligan | Geomorphic and sedimentological controls on the effectiveness of an extreme flood[END_REF]). The maximum unit stream power of a flood offers a better predictor that takes into account local slope and valley floor confinement [START_REF] Baker | Flood power[END_REF], a threshold value of 300 W m -2 having been proposed by [START_REF] Magilligan | Geomorphic and sedimentological controls on the effectiveness of an extreme flood[END_REF] for a catastrophic flood to occur, but many other influential factors should be considered. A recent work addressing geomorphic effectiveness of debris floods in steepland streams has shown that the exceedance of the critical shear stress required to mobilize the coarse fraction of the bed material can be used to determine the magnitude of the channel response, this criterion being used to differentiate damaging and catastrophic debris floods [START_REF] Church | What Is a Debris Flood?[END_REF]. This emphasizes the importance of considering not only hydraulic forces generated by floods, but also channel resistance to disturbance [START_REF] Brunsden | A critical assessment of the sensitivity concept in geomorphology[END_REF][START_REF] Milan | Modelling differential geomorphic effectiveness in neighbouring upland catchments: implications for sediment and flood risk management in a wetter world[END_REF]. The geomorphic effectiveness of floods also depends on the climatic setting influencing the flashiness of the flow regime and the timescale of recovery [START_REF] Wolman | Relative scales of time and effectiveness of climate in watershed geomorphology[END_REF], the recent flood history or memory of the fluvial system [START_REF] Kochel | Geomorphic impact of large floods: review and new perspectives on magnitude and frequency[END_REF], the duration of the flood [START_REF] Costa | Geomorphically effective floods. Natural and Anthropogenic Influences in Fluvial Geomorphology[END_REF], and the sediment supply conditions [START_REF] Beschta | Channel changes following storm-induced hillslope erosion in the Upper Kowai Basin, Torlesse Range, New Zealand[END_REF].

The effect of sediment supply on geomorphic responses to extreme floods is particularly critical in mountain rivers and streams, due to their proximity to hillslope sediment sources. Hillslope-channel coupling controls the efficiency with which energy is transmitted through the fluvial system, explaining why sediment connectivity of upland catchments is recognized as a fundamental factor of their geomorphic sensitivity to disturbances [START_REF] Brunsden | A critical assessment of the sensitivity concept in geomorphology[END_REF][START_REF] Harvey | Coupling between hillslopes and channels in upland fluvial systems: implications for landscape sensitivity, illustrated from the Howgill Fells, northwest England[END_REF][START_REF] Fryirs | Dis)Connectivity in catchment sediment cascades: a fresh look at the sediment delivery problem[END_REF]. Some recent major flood events in small mountain streams of the French Pyrenees [START_REF] Blanpied | La connectivité sédimentaire dans la haute chaîne pyrénéenne par l'analyse de la crue de juin 2013: le rôle des formations superficielles[END_REF] and Italian Alps [START_REF] Pellegrini | The morphological response of the Tegnas alpine catchment (Northeast Italy) to a Large Infrequent Disturbance[END_REF][START_REF] Scorpio | Storm characteristics dictate sediment dynamics and geomorphic changes in mountain channels: A case study in the Italian Alps[END_REF] illustrate how sediment connectivity can be used to reconstruct the sediment cascade associated with extreme events. The initiation and propagation of sediment waves is a classic geomorphic response to extreme floods in upland environments, with active channel widening and aggradation as the typical morphological imprints of the sediment wave [START_REF] Beschta | Channel changes following storm-induced hillslope erosion in the Upper Kowai Basin, Torlesse Range, New Zealand[END_REF][START_REF] Pitlick | Response and recovery of a subalpine stream following a catastrophic flood[END_REF][START_REF] Madej | Channel response to sediment wave propagation and movement, Redwood Creek, California, USA[END_REF][START_REF] Nelson | Channel response to an extreme flood and sediment pulse in a mixed bedrock and gravel-bed river[END_REF]. In the case of the June 1957 catastrophic flood in the Guil valley of the Eastern French Alps, it has been early recognized that the remobilization of alluvial storages through local channel widening played a critical role in the downstream propagation of the sediment wave associated with this event [START_REF] Tricart | Mécanismes normaux et phénomènes catastrophiques dans l'évolution des versants du bassin du Guil (Hautes-Alpes, France)[END_REF]. This was supported by the lithological composition of alluvial deposits generated by the flood, showing relatively short displacements of individual grains during the flood. This early study echoes a more recent review of extreme sediment transport events, emphasizing the key role of sediment storage for "fuelling rather than buffering high sediment transport rates" [START_REF] Korup | Earth's portfolio of extreme sediment transport events[END_REF]. Fluvial terraces were also identified as the major sediment source during an extreme flood in a small upland catchment of Northern England [START_REF] Marutani | Influence of storm-related sediment storage on the sediment delivery from tributary catchments in the Upper Waipaoa River, New Zealand[END_REF].

From a more practical perspective related to the prevention against flooding hazards in upland environments, it is crucial to better characterize controlling factors of the channel response to extreme floods. Of particular importance is the width ratio, computed as the ratio between the post-flood and the pre-flood active channel width. This widely-used normalized index is crucial for management strategies based on a functional river corridor width ensuring at the same time a protection against channel shifting during floods, and the preservation of the ecological quality of the river [START_REF] Piégay | A review of techniques available for delimiting the erodible river corridor: a sustainable approach to managing bank erosion[END_REF]. The occurrence of many extreme flood events in upland environments of Europe during the last two decades gave rise to many case studies dedicated to width ratios. It was notably shown that the widening rate at the reach scale depends not only on the specific stream power of the flood, but also on local factors such as the initial channel width or the channel confinement in the valley floor [START_REF] Krapesch | Scale orientated analysis of river width changes due to extreme flood hazards[END_REF][START_REF] Rinaldi | An integrated approach for investigating geomorphic response to extreme events: methodological framework and application to the October 2011 flood in the Magra River catchment, Italy[END_REF][START_REF] Surian | Channel response to extreme floods: Insights on controlling factors from six mountain rivers in northern Apennines, Italy[END_REF]2016;[START_REF] Righini | Geomorphic response to an extreme flood in two Mediterranean rivers (northeastern Sardinia, Italy): analysis of controlling factors[END_REF]Lucia et al., 2018;[START_REF] Scorpio | Basin-scale analysis of the geomorphic effectiveness of flash floods: a study in the northern Apennines (Italy)[END_REF][START_REF] Ruiz-Villanueva | Impacts of a large flood along a mountain river basin: the importance of channel widening and estimating the large wood budget in the upper Emme River (Switzerland)[END_REF][START_REF] Scorpio | Storm characteristics dictate sediment dynamics and geomorphic changes in mountain channels: A case study in the Italian Alps[END_REF][START_REF] Brenna | What drives major channel widening in mountain rivers during floods? The role of debris floods during a high-magnitude event[END_REF].

In early October 2020, a catastrophic flood occurred in several confined alpine valleys of SE France. This flood was generated by a Mediterranean episode subsequent to a storm coming from North Atlantic, known as the Storm Alex [START_REF] Carrega | Une catastrophe hors norme d'origine météorologique le 2 octobre 2020 dans les montagnes des Alpes-Maritimes[END_REF]. Geomorphic responses to the storm have been particularly dramatic in the Vésubie and the Roya valleys, where valley floors have been buried under massive gravel depositions. This major geomorphic event offers a unique opportunity to investigate how a sediment disaster may occur in alpine valleys under the effect of extreme meteorological forcings. A detailed analysis of the sediment cascade and associated geomorphic response was done using airborne remote sensing data, with several specific objectives: [START_REF] Milan | Modelling differential geomorphic effectiveness in neighbouring upland catchments: implications for sediment and flood risk management in a wetter world[END_REF] documenting the morphological imprint of the storm and contextualizing it within the recent historical trajectories of the rivers; [START_REF] Piton | Why do we build check dams in Alpine streams? An historical perspective from the French experience[END_REF] analyzing the spatial variability of reach-scale channel responses; (3) comparing the morphological responses of the two rivers; (4) and interpreting sediment cascading effects using available information on the geomorphic response.

Physical setting

Vésubie and Roya valleys

The Vésubie and Roya are two neighboring alpine gravel-bed rivers of SE France, draining the southeastern flank of the Argentera-Mercantour mountain range, in the hinterland of the city of Nice, with respective drainage areas of 392 and 671 km² (Fig. 1). The Vésubie is a left-bank tributary to the Var River that joins the Mediterranean Sea close to Nice, while the Roya flows directly to the sea at Ventimiglia (Italy). A 36-km study reach has been considered for the Roya, from the upper valley down to the Italian border (Fig. 1). Channel slope is decreasing from 10% to 1% along this reach. The 35-km study reach considered for the Vésubie catchment is divided into three sub-reaches (Fig. 1): [START_REF] Milan | Modelling differential geomorphic effectiveness in neighbouring upland catchments: implications for sediment and flood risk management in a wetter world[END_REF] the Boréon between the confluence with the Vallon de Salèse down to the confluence with the Vésubie (length of 6.8 km, mean channel slope of 6.7%); [START_REF] Piton | Why do we build check dams in Alpine streams? An historical perspective from the French experience[END_REF] the Upper Vésubie (also known locally as Madone de Fenestre) from the confluence with the Vallon de Prals down to the confluence with the Boréon (length of 8.9 km, mean channel slope of 8.7%); and (3) the Vésubie from the confluence with the Boréon down to the upstream end of the narrow gorges present in the lower valley (length of 19.5 km, mean channel slope of 3%). Predominant channel morphology for these rivers before Storm Alex was a single-thread pattern with only few exposed gravel bars. Although the two catchments share several features related to their common physical setting, they also have some specificities (Tab. 1). Geology of the two catchments comprised both crystalline and sedimentary rocks, but the proportion of crystalline rocks is much higher in the Vésubie, where the upper sub-catchments are composed quasi-exclusively of geological formations from the granitic basement of the Southwestern Alps (Fig. 2). Crystalline rocks are also present in the Upper Roya, but only marginally in its northwestern part. The major part of the Roya catchment as well as the middle and lower parts of the Vésubie catchment belongs to the Mesozoic and Cenozoic sedimentary domains, with dominant rock types like limestones, sandstones, and marls.

Upper catchments were glaciated during Pleistocene cold periods, and massive glacial deposits are present along many upper valleys. Imprints from cold periods are important to consider since they can have a significant effect on present-day sediment sources and stores of fluvial systems.

Reconstructions of maximum glacier extensions during the Würm show that the main valley of the Vésubie down to Lantosque was glaciated, while most of the Roya valley stayed unglaciated [START_REF] Julian | Les glaciations des Alpes Maritimes: essai de mise au point[END_REF]. The spatial extent of alluvial deposits (fluvio-glacial terraces and recent floodplain) is reflecting that, these being much more extended in the Vésubie, notably in the upper and middle portions of the valley (Fig. 2). Massive fluvio-glacial deposits found in the Vésubie between Roquebillière and Saint-Martin-Vésubie have been interpreted as a consequence of the valley obstruction by the Gordolasque glacier during the last glacial retreat [START_REF] Julian | Les glaciations des Alpes Maritimes: essai de mise au point[END_REF]. Fluvio-glacial deposits are extremely limited in the Roya valley, where the river flows through narrow proglacial gorges along almost all of its course.

Confined proglacial gorges are also present in the Vésubie, but they are mainly developed along the lower river course.

Both catchments are characterized by widespread forest stands, covering around 60% of drainage areas (Tab. 1). If other vegetation types are considered (open forest, shrub, and herbaceous covers), total vegetated proportions of the catchments reach 94% and 89% for the Roya and Vésubie, respectively. This not only shows a relatively small present-day agricultural pressure in both catchments, restricted to gentle slope areas near villages, but also a limited spatial extension of active erosion landforms on hillslopes, representing less than 6% and 11% of drainage areas for the Roya and Vésubie, respectively. It is also interesting to see that surfaces of RTM (Restauration des Terrains en Montagne) State Forests are relatively limited (Fig. 1). They represent only 1% and 6% of catchment areas for the Roya and Vésubie, respectively. These correspond to degraded lands purchased by the State in the late 19 th century to implement erosion-control works [START_REF] Liébault | Land-use change, sediment production and channel response in upland regions[END_REF][START_REF] Piton | Why do we build check dams in Alpine streams? An historical perspective from the French experience[END_REF], and they provide a first indication of past erosion conditions on hillslopes.

October 2020 Storm Alex

Alex is the name given by Météo France (in partnership with national weather services of Spain, Portugal and Belgium) to a meteorological storm generated by the rapid formation of a cyclonic depression in North Atlantic, at around 600 km from Brittany (NE France), the 1 st October 2020. This depression reached Brittany on October 2 nd , and generated a southern flux of warm and humid air mass coming from the Mediterranean Sea, that induced an extreme rainfall episode in the Alpes-Maritimes (SE edge of France). The 24h Météo France rainfall map computed from radar data shows two rainfall hotspots of 600 and 500 mm in the upper catchments of the Roya and Vésubie rivers, respectively (Fig. 3). Rain gauges from EDF located at Saint-Martin-Vésubie and at Les Mesches (Upper Roya) provided values of 502 and 633 mm in 24h, respectively. The return period of these rainfall amounts estimated from a statistical analysis is >1000 yrs [START_REF] Cerema | RETEX technique Alex: inondations des 2 et 3 octobre 2020, consensus hydrologique[END_REF]. This Mediterranean episode generated catastrophic floods in the two catchments. There is a lack of data on peak discharges, because gauging stations were destroyed during the flood, but field surveys of hydraulic sections using flood marks combined with hydrological modelling provided a range of possible peak discharges, with values comprised between 700 and 1100 m 3 /s for the Vésubie at its mouth, and between 1100 and 1800 m 3 /s for the Roya at the Italian border [START_REF] Payrastre | The 2 October 2020 Alex storm in south-eastern France: a contribution of the scientific community to the flood peak discharges estimation[END_REF]. Return periods associated with these peak discharges are comprised between 100 and 500 years. The relatively long duration of the floods, 18 to 24 hours [START_REF] Cerema | RETEX technique Alex: inondations des 2 et 3 octobre 2020, consensus hydrologique[END_REF], likely contributed to their dramatic morphological effects. Reconstructed peak discharges of Storm Alex for both rivers are not only much higher than annual maximum daily peak discharges obtained from relatively short hydrological time series of gauging stations, but also much higher than all the estimated peak discharges of known historical floods of the 20 th century (Fig. 4). However, information is missing for comparing the magnitude of the Storm Alex flood with older historical floods from the second half of the 19 th century, whose impacts were sufficiently important to have left traces in the historical archives. Like Storm Alex, those old damaging large floods systematically occurred during autumn : November 1856, October 1882, October 1892, and November 1906 (Fig. 4).

Material and methods

Remote sensing data

The documentation of the Storm Alex consequences remarkably benefited from an express IGN aerial survey that was undertaken only 3 days after the disaster to evaluate damages and impacted areas, following a request from local authorities in charge of natural hazard management (DDTM and ONF-RTM of the Alpes-Maritimes). More than 2000 aerial images combined with airborne LiDAR data were captured between the 5 th and 7 th October 2020. These remote sensing data were rapidly shared in a dedicated web site (https://www.geoportail.gouv.fr/donnees/ortho-express-alex), allowing an early exploitation for the geomorphic study.

The channel response to Storm Alex and its integration into the recent trajectory of the rivers have been investigated through a classical exploitation of available remote sensing data (Tab. In complement to ortho-imagery, two airborne near-infrared LiDAR surveys were available in the Vésubie for undertaking a topographic differencing analysis of the Storm Alex. Unfortunately, only the post-Alex LiDAR survey is available for the Roya, making impossible to duplicate the analysis for both rivers. Post-Alex surveys were not only used for topographic differencing, but also for morphological mapping of the modern valley floors and post-flood active channels for both rivers (see next section). Near-infrared LiDAR data are restricted to terrestrial portions of the surveyed areas and cannot provide bathymetric topography. This implies some limitations for topographic differencing (see section 3.4), but not for morphological mapping. LiDAR DEMs were also used for the manual extraction of the modern valley floor, defined here as the spatial extent of historical alluvial deposits. This can be seen as the channel shifting space of the last two centuries. In the European Alpine context, those deposits are generally composed of the presentday forested floodplain and the low alluvial terrace of the Little Ice Age (LIA), which is often occupied by softwood forest stands [START_REF] Bravard | La métamorphose des rivières des Alpes françaises à la fin du Moyen-Age et à l'époque Moderne[END_REF][START_REF] Liébault | Causes of 20th century channel narrowing in mountain and piedmont rivers of Southeastern France[END_REF][START_REF] Comiti | How natural are Alpine mountain rivers? Evidence from the Italian Alps[END_REF]. Old terraces from Pleistocene cold periods and alluvial fans from headwater catchments were not included in the modern valley floor. A threshold of 10 m was considered for excluding old terraces; this means that any mainstem fluvial deposit with a relative elevation of less than 10 m with respect to the mean elevation of the active channel was considered as part of the modern valley floor. This is the rule adopted for the valley bottom mapping in the Rhône basin and its Mediterranean margin [START_REF] Alber | Spatial disaggregation and aggregation procedures for characterizing fluvial features at the network-scale: application to the Rhône basin (France)[END_REF]. Our own checking of this relative elevation threshold in the Roya and Vésubie valleys, that are part of the Mediterranean margin of the Rhône basin, confirms its efficiency for separating the modern valley floor from Quaternary terraces indicated in the geological maps. Because alluvial fans and old terraces were strongly eroded during Storm Alex, it was necessary to extract the modern valley floor before and after the storm, using the two available LiDAR DEMs. For the Roya, only the poststorm valley floor was extracted. One example of modern valley floor mapping is provided in Figure 6. the elevation from the 2018 LiDAR survey). This pre-processing step allowed reducing the systematic error of elevation from 16 cm to -1.1 cm in the lower Vésubie, from 7.5 cm to -0.16 cm in the upper Vésubie, from 4.9 cm to -0.8 cm in the Boréon, and from 8.9 cm to 0.8 cm in tributaries.

Erosion and deposition volumes were computed with the QGIS raster analysis toolbox using the DEM of Difference (DoD) obtained by subtracting the two co-registered LiDAR surveys. An area of interest was manually delimited along the study reaches to restrict the volume computation to erosion and deposition induced by fluvial processes along mainstems. This area is composed of the post-Alex active channel, vegetated islands of the floodplain, and active margins of the active channel (Fig. 8).

Active margins can be easily detected from the post-Alex DoD; they correspond to sediment deposition zones of the floodplain, and lateral sediment recruitment zones from remobilization of alluvial terraces, alluvial fans, and footslopes. Sediment erosion and deposition volumes along selected tributaries were also extracted inside a mask manually delimited along the stream network and its active margins, including all the active connected hillslope sediment sources visually detected from topographic differencing. This analysis was conducted on 35 tributaries that have been considered as significant sediment suppliers to the mainstem according to geomorphic activity detected on alluvial fans by topographic differencing (Fig. 9). Geomorphic activity was considered as significant when widespread erosion and deposition zones are detected in the DoD. Tributaries of the upper Boréon valley (upstream from the upper end of the Boréon study reach) were not considered in the analysis because a major reservoir dam has efficiently trapped sediment transport from this part of the catchment, as attested by the incomplete filling of the reservoir during Storm Alex. In few tributaries (n=7), catchments were partially covered by the October 2020 LiDAR survey, so topographic differencing of uncovered surfaces was obtained with a 2021 LiDAR survey from IGN.

Only unclassified raw LiDAR point clouds were available for this survey, so an automatic extraction of ground points was done with R using the lidR package [START_REF] Roussel | lidR : An R package for analysis of Airborne Laser Scanning (ALS) data[END_REF], and classification errors were manually corrected with CloudCompare. This procedure gives DEMs of good quality, except for the Venanson catchment, where a low ground point density was obtained due to a dense forest cover. Results from topographic differencing for this catchment must then be considered with caution.

Gross erosion and deposition volumes were computed after a DoD thresholding, considering a spatially uniform level of detection (LoD) equal to 0.13 cm. This value was obtained from root mean square errors (RMSE) of LiDAR elevations computed for the 2018 and 2020 surveys, using a set of 128 ground control points measured with a dGPS in stable areas by the private company in charge of the 2018 survey. The LoD was computed using a 95% confidence interval (LoD = 1.96(RMSE2018²+RMSE2020²) 0.5 ) [START_REF] Lane | Estimation of erosion and deposition volumes in a large, gravel-bed, braided river using synoptic remote sensing[END_REF]. Approaches based on a spatially distributed error were not considered here, since they have been mostly developed for improving change detection of areas experiencing a small morphological activity (Wheaton et al., 2010;Milan et al., 2011). After thresholding, the LoD was subtracted from all DoD cells for the computation of gross erosion and deposition volumes, following recommendations from [START_REF] Carley | Significant decadal channel change 58-67 years post-dam accounting for uncertainty in topographic change detection between contour maps and point cloud models[END_REF]. Thresholding of the DoD was not considered for the net sediment budget computation, as thresholding can be a source of bias and error in net change analysis [START_REF] Lane | Estimation of erosion and deposition volumes in a large, gravel-bed, braided river using synoptic remote sensing[END_REF][START_REF] Anderson | Uncertainty in quantitative analyses of topographic change: error propagation and the role of thresholding[END_REF]. Uncertainty bounds of sediment volumes were computed following the procedure presented by [START_REF] Anderson | Uncertainty in quantitative analyses of topographic change: error propagation and the role of thresholding[END_REF], using the following equation:

𝜎 𝑣 = 𝑛𝐿 2 √ 𝜎 𝑟𝑚𝑠 2 𝑛 + 𝜎 𝑠𝑐 2 𝑛 𝜋𝑎 2 5𝐿 2 + 𝜎 𝑠𝑦𝑠 2 (1)
with σv the volume uncertainty (m 3 ), n the number of DoD cells in the analysis, L the cell size (m), σrms the uncorrelated random error (m), computed here as σrms = (RMSE2018 2 +RMSE2020²) 0.5 , σsc the spatially correlated random error (m), πa² the circular area over which errors are correlated (m²), and σsys the systematic error, which was considered here as equal to the residual systematic error after coregistration. The spatially correlated random error and the πa² term were obtained from a semivariogram analysis performed using the ArcGIS kriging tool, following the procedure described in [START_REF] Anderson | Uncertainty in quantitative analyses of topographic change: error propagation and the role of thresholding[END_REF].

Results

Active channel responses

Ortho-images taken before and after Storm Alex show that the active channel surface of the Vésubie has increased by a factor 4 after the storm; from 68 ha in 2017 to 274 ha in 2020 (Fig. 10). This active channel expansion is far greater than the surface increase attributable to the November 1997 flood, based on a threshold separating alluvial and gorges reaches (see supplementary material), a downstream decrease of this index is observed for the Vésubie (Fig. 13), with a first decreasing step at a river distance of around 16 km, and a sharp decline downstream the last gorges. This cannot be attributed to an increase of the valley floor width, which fluctuates around a constant mean of ~130 m in the lower 13 km of the study reach, excluding gorges sections (Fig. 12). The Roya does not show any clear longitudinal pattern. A more chaotic signal is observed for this river, each alluvial cell of the valley floor (defined as alluvial units comprised between gorges) showing a specific response. The width ratio, obtained by considering the ratio between post-event and pre-event active channel widths, is classically used to characterize the flood impact on channel widening. Longitudinal gradients of width ratio for the two rivers show very different patterns (Fig. 14). In the case of the Vésubie, the higher widening rates are observed in the middle part of the study reach, and a regular downstream decrease is observed from a river distance of around 11 km. Once again, a sharp decline is observed downstream the last gorges. The Roya is characterized by a regular downstream decrease of the width ratio up to a river distance of 25 km. From this point, the widening rate stays at a low level down to the lower end of the study reach. Alex first confirm a negative correlation with the initial channel width, as previously reported by several case studies addressing geomorphic responses to major floods in temperate mountain environments (Fig. 16). Roya data points are relatively well aligned with the compiled scatterplot, but those from the Vésubie are clearly plotting as an upper envelope, indicating significantly higher widening rates for a given initial width. Vésubie, compared to reported width ratios in temperate upland environments (data from [START_REF] Arnaud-Fassetta | La part respective des facteurs hydroclimatiques et anthropiques dans l'évolution récente (1956-2000) de la bande active du Haut-Guil, Queyras, Alpes françaises du Sud[END_REF][START_REF] Krapesch | Scale orientated analysis of river width changes due to extreme flood hazards[END_REF][START_REF] Rinaldi | An integrated approach for investigating geomorphic response to extreme events: methodological framework and application to the October 2011 flood in the Magra River catchment, Italy[END_REF][START_REF] Surian | Channel response to extreme floods: Insights on controlling factors from six mountain rivers in northern Apennines, Italy[END_REF][START_REF] Righini | Geomorphic response to an extreme flood in two Mediterranean rivers (northeastern Sardinia, Italy): analysis of controlling factors[END_REF][START_REF] Ruiz-Villanueva | Impacts of a large flood along a mountain river basin: the importance of channel widening and estimating the large wood budget in the upper Emme River (Switzerland)[END_REF][START_REF] Scorpio | Basin-scale analysis of the geomorphic effectiveness of flash floods: a study in the northern Apennines (Italy)[END_REF][START_REF] Scorpio | Storm characteristics dictate sediment dynamics and geomorphic changes in mountain channels: A case study in the Italian Alps[END_REF][START_REF] Brenna | What drives major channel widening in mountain rivers during floods? The role of debris floods during a high-magnitude event[END_REF] (see Tab. S1, supplementary material); the best fit was computed for the compiled dataset only

Several potential predictors of width ratios were tested using the Vésubie dataset only, since information is missing for the computation of these predictors in the Roya. The first tested predictor is a proxy of hydraulic forcing. As previously mentioned, gauging stations of both rivers were destroyed during the storm, consequently no measured peak discharges could be used to calculate the stream power on both rivers. However, it has been possible to compute a stream power index (SPI) :

𝑆𝑃𝐼 = 𝐴 𝑑 ⋅𝑠 𝑤 𝑝𝑟𝑒 (2)
with Ad the drainage area (km²), s the channel slope before the flood (m m -1 ), and wpre the active channel width before the flood (m). This simple index implies a peak discharge linearly proportional to the drainage area, and does not consider rainfall distributions that clearly show an upstream concentration of high-intensity rainfall cells (Fig. 3). Consequently, this proxy is smoothing the actual spatial variability of the stream power that was most probably higher in the upstream reaches.

However, if alluvial reaches are considered, a significant positive non-linear relation was obtained with the width ratios (wr = 11.53 SPI 0.62 , with wr the width ratio, R²=0.40, p<0.0001) (Fig. 17A). The effect of valley confinement was also tested by considering the index of confinement before the storm (IC = wpre/wvf, with wvf the modern valley floor width before the storm). A significant negative nonlinear relation was obtained by considering together stream reaches classified as alluvial and gorges (wr = 1.27 IC -0.77 , R²=0.59, p<0.0001) (Fig. 17B). Width ratios were also cross-compared with bedlevel changes of the active channel (Fig. 17C). This comparison shows a positive linear correlation by

Alex storm sediment budget for the Vésubie

Topographic differencing using LiDAR data collected before and after Storm Alex along the 35 km study reach of the Vésubie catchment allows estimating a total sediment erosion of 4.13 Mm 3 ± 11 000 m 3 (mean unit erosion rate in the area of interest of 1.27 m 3 m -2 ), and a total sediment deposition of 3.49 Mm 3 ± 16 000 m 3 (mean unit deposition rate in the area of interest of 0.83 m 3 m -2 ) (Fig. 18). The cumulative sediment supply from tributaries is estimated at 1.51 Mm 3 ± 25 600 m 3 (Fig. 18, Table S2).

The net sediment budget shows 2.06 Mm 3 of sediment yield at the exit of the study reach (± 50 000 m 3 ). This value must not be considered as bed material yield, since it certainly incorporates an important fraction of fine sediments that has been transported in suspension during the flood.

The cumulative sediment transport along the three study reaches clearly shows a rapid downstream increase of sediment transport in the two upper valleys, under the cumulative effect of sediment supply from local erosion and tributaries (Fig. 19). A decrease of sediment transport is observed downstream of the Boréon-Upper Vésubie confluence, due to a positive net sediment budget in the valley floor and a low supply from tributaries, but this short buffering zone is rapidly interrupted downstream of the Storm Alex induced dramatic active channel widening along almost all the investigated stream networks of the Vésubie and Roya. In the Vésubie, the active channel widening is associated with a strong aggradation (Figure 17C). These are typical morphological adjustments related to extreme floods under conditions of very high sediment supply (e.g. [START_REF] Arnaud-Fassetta | Hydro-geomorphic hazards and impact of manmade structures during the catastrophic flood of June 2000 in the Upper Guil catchment (Queyras, Southern French Alps)[END_REF][START_REF] Rinaldi | An integrated approach for investigating geomorphic response to extreme events: methodological framework and application to the October 2011 flood in the Magra River catchment, Italy[END_REF]. However, what is really exceptional is the magnitude with which those adjustments occur, notably in the Vésubie. The comparison in the Vésubie between Storm Alex and the last noticeable flood of 1997 (return period estimated at about 100-yrs for the upper catchment) shows an order of magnitude difference in terms of active channel expansion. This dramatic intensity of channel widening is in good agreement with estimated return periods of hydro-meteorological forcings, largely exceeding 100 years. Similar flood-induced values of channel widening after catastrophic floods are generally found in streams with much smaller catchment-size than the Vésubie (Table S1). The only reported case of mean channel widening equivalent to the Vésubie (width ratio of 4.6) for a catchmentsize of several hundreds of km² is the August 2005 flood in the Trisanna River of the Austrian Alps, with a mean width ratio of 4.1 [START_REF] Krapesch | Scale orientated analysis of river width changes due to extreme flood hazards[END_REF]. The estimated return period of rainfall forcings of this event (≥ 1000 yrs) is also similar to Storm Alex. The mean width ratio for the June 1957 flood in the Guil, which can be viewed as the last analog of Storm Alex in terms of geomorphic response in the French Alps, is only 2.3 (Arnaud-Fassetta and Fort, 2004).

The documentation of channel aggradation in the Vésubie also provides some exceptional values, with a maximum around 10 m in proximal reaches, and with values exceeding 4 m along reaches of several km in length (Figure 13). These values largely exceed the few reported maximum flood-induced aggradation values in alpine environments, that most often do not exceed 2 m on long distances [START_REF] Scorpio | Basin-scale analysis of the geomorphic effectiveness of flash floods: a study in the northern Apennines (Italy)[END_REF][START_REF] Scorpio | Storm characteristics dictate sediment dynamics and geomorphic changes in mountain channels: A case study in the Italian Alps[END_REF]. Unfortunately, most of the recently reported case studies of extreme floods in alpine environments are restricted to 2D morphological effects, simply because data are generally missing for documenting bed-level changes. One exception is the October 2018 Vaia storm in Northeast Italy (200 yrs return period, [START_REF] Brenna | Sediment-water flows in mountain catchments: insights into transport mechanisms as responses to high-magnitude hydrological events[END_REF], where topographic differencing from sequential LiDAR surveys has been used to document post-flood erosion and deposition along valley floors of the Tegnas Torrent [START_REF] Pellegrini | The morphological response of the Tegnas alpine catchment (Northeast Italy) to a Large Infrequent Disturbance[END_REF] and Rio Cordon [START_REF] Rainato | Hydrological, geomorphic and sedimentological responses of an alpine basin to a severe weather event (Vaia storm)[END_REF].

Results from the Tegnas case study have shown contrasted bed-level changes, with an alternating sequence of aggradation (maximum values < 4 m) and incision along a 6-km reach. In the Rio Cordon, the dominant channel response was channel incision related to the breaking of the coarse surface layer.

To find magnitudes of rapid channel aggradation equivalent to Storm Alex (>5-10 m), it is necessary to look at rivers that have been impacted by catchment-scale deforestation [START_REF] Marutani | Influence of storm-related sediment storage on the sediment delivery from tributary catchments in the Upper Waipaoa River, New Zealand[END_REF][START_REF] Gomez | Downstream fining in a rapidly aggrading gravel bed river[END_REF][START_REF] Gomez | Gully erosion and sediment production: Te Weraroa Stream, New Zealand[END_REF][START_REF] Liébault | Land-use change, sediment production and channel response in upland regions[END_REF] or by sediment waves coming from landslides [START_REF] Korup | Landslide-induced river channel avulsions in mountain catchments of southwest New Zealand[END_REF] or debris-flow torrents [START_REF] Liébault | Long profile responses of alpine braided rivers in SE France[END_REF].

The oldest available aerial photographs show that in the relatively wide alluvial plain of the Vésubie valley, the dominant channel morphology of the late 1940s was a single-thread pattern with few active gravel bars. This situation is not very common for alpine gravel-bed rivers of that time, most of them showing much wider active channels than today, with dominant fluvial patterns like braiding or wandering [START_REF] Bravard | La métamorphose des rivières des Alpes françaises à la fin du Moyen-Age et à l'époque Moderne[END_REF][START_REF] Liébault | Causes of 20th century channel narrowing in mountain and piedmont rivers of Southeastern France[END_REF][START_REF] Comiti | How natural are Alpine mountain rivers? Evidence from the Italian Alps[END_REF][START_REF] Nardi | Spatio-temporal patterns of channel changes in response to a major flood event: the case of the Magra River (central-northern Italy)[END_REF]. Wide active channels of the mid-20 th century have been interpreted as a legacy from high sediment supply conditions that prevailed during the LIA, under the cumulative effect of more erosive climatic forcings and human-induced deforestation on hillslopes. Most of these wide active channels rapidly transformed into fluvial terraces 1950s onward, due to sediment supply decrease and human pressures (e.g. gravel mining, dams). However, it has been recently shown that extreme floods may recreate active channels as wide as they were in the 1950s [START_REF] Nardi | Spatio-temporal patterns of channel changes in response to a major flood event: the case of the Magra River (central-northern Italy)[END_REF]. The historical trajectory of the Vésubie deviates from this narrative (this is also the case of the Roya, but narrow gorges in this valley prevents comparison with other alpine rivers). Active channel evolution of the last 70 years in the Vésubie rather show the permanence of a narrow single-thread channel, with marginal morphological activity. This cannot be attributed to a low hydrological activity, since several large floods occurred during this period, mainly during the 1950s and 1990s (Fig. 4). This means that the Vésubie was adjusted for a long time to a low sediment supply from its catchment before Storm Alex hits. Major sediment sources on hillslopes are limited to 4-5 permanent and active sediment production zones of small headwater catchments located in the Upper Vésubie (Madone de Fenestre)

and Boréon valleys, where sediment supply to the mainstem by debris flows can be considered as relatively regular. This persistent low level of geomorphic activity on headwaters is also suggested by the limited spatial extent of RTM State Forests in the catchment (Fig. 1). These zones cover only 6%

of the Vésubie catchment, whereas in other catchments of the Southern French Prealps, they can cover around 20% [START_REF] Liébault | Tributaries and the management of main-stem geomorphology[END_REF]. This could be due to a lower past agricultural pressure than in other alpine valleys, related to harsh topographic conditions (e.g. narrow and steep valleys) or specific historical facts, but this needs to be confirmed by a detailed investigation of past land-use conditions.

However, it is possible that braided channels were much more developed in some reaches of the Vésubie during the LIA, as attested by some pictures of the late 19 th century (Fig. 20). A cycle of channel recovery likely related to a major flood that occurred in October 1882 (Fig. 4), and that was potentially at the origin of the formation of a braided channel, is clearly visible in Figure 20. Thorough investigations of old maps and cadasters from the 19 th century could be of great interest for determining if braiding of the Vésubie was continuously developed along the alluvial plain at that time, notably under the effect of successive large floods that occurred during the second half of the 19 th century, or if it was only a transient and localized alluvial landform. It could be also interesting to undertake a specific study of past sediment supply conditions in these catchments, with respect to the history of human pressures (land-use changes, torrent-control works), in order to better understand their specific morphological trajectories. The fate of post-Alex braided or wandering channels should theoretically be a rapid recovery, with downstream progressing channel narrowing and incision, and subsequent formation of fluvial terraces, like reported by many case studies of channel recovery after sediment wave propagation [START_REF] James | Incision and morphologic evolution of an alluvial channel recovering from hydraulic mining sediment[END_REF][START_REF] Pitlick | Response and recovery of a subalpine stream following a catastrophic flood[END_REF][START_REF] James | Secular sediment waves, channel bed waves, and legacy sediment[END_REF]. It is indeed hard to imagine that Storm Alex will initiate a long-term trajectory of active channel widening and aggradation, because the storm did not generated any major destabilization of hillslopes that could persistently sustain a high sediment supply during usual rainfall events, and most of the historical channel shifting space in the valley floor is already occupied by the active channel (76 %). The regime of low sediment supply from hillslopes that prevailed before the storm will likely be restored during the next decades. Channels should therefore relatively rapidly recover a morphology adjusted to this regular regime, unless an increase of flood magnitude and frequency would occur under the effect of climate change. Although recent regional studies have shown a substantial increasing trend of extreme autumn rainfall events in the Southwestern Alps since the late 1950s (Blanchet et al., 2021a), under the effect of increasing Mediterranean circulations (Blanchet et al., 2021b), statistical analysis of flood discharge series in French Mediterranean rivers has revealed that there is no general tendency towards more severe floods in this region [START_REF] Tramblay | Detection and attribution of flood trends in Mediterranean basins[END_REF]. This is in good agreement with detections of flood-rich and flood-poor periods across

Europe for the 1960-2010 period, showing a decrease of the frequency of stations with flood-rich periods for the Mediterranean region [START_REF] Lun | Detecting Flood-Rich and Flood-Poor Periods in Annual Peak Discharges Across Europe[END_REF]. Inverse trends of flood hydrology and extreme rainfall in this region has been interpreted as a consequence of a decreasing soil moisture [START_REF] Tramblay | Detection and attribution of flood trends in Mediterranean basins[END_REF]. All these elements suggest that future trends in hydro-meteorological forcings of channel recovery for the Vésubie and Roya valleys remain quite uncertain.

However, if a downstream progressing channel incision occurs, it will fuel a high sediment supply to distal reaches, and those reaches will need more time to recover. The "legacy sediment" (sensu James, 2020) related to the formation of massive alluvial deposits in the valley floor needs also to be considered. These alluvial stores may have a long-term impact on the sediment supply of the rivers, and subsequently on the recovered channel morphology, that could be different from the pre-storm conditions. It is possible that the recovered active channel width will be higher than that before the storm during decades. This will also depend on the controlling effects of extensive flood-protection works that have been done after the storm (dredging, bank protection works, earthworks in the valley floor), and it will be quite difficult to untangle natural processes of channel recovery from those related to massive protective operations in the valley floor.

Fluvial metamorphosis and sediment wave

Although flood-induced fluvial metamorphosis has already been reported in alpine environments, case studies showing the formation of braided channels over several tens of kilometers, like for the Vésubie, are extremely rare. Very few analogs exist in the recent history of the French mountains, for instance since the beginning of the 20 th century. The June 1957 flood that occurred in the Guil and Ubaye alpine valleys is one documented example of flood-induced braided channel "mega-formation" [START_REF] Arnaud-Fassetta | Hydro-geomorphic hazards and impact of manmade structures during the catastrophic flood of June 2000 in the Upper Guil catchment (Queyras, Southern French Alps)[END_REF][START_REF] Brousse | Evolution hydrogéomorphologique de la bande active de l'Ubaye (Alpes françaises du Sud) de 1956 à 2004 : contribution à la gestion des crues[END_REF]. A similar phenomenon likely occurred during the October 1940 flood in the Têt and Tech valleys of the Eastern French Pyrenees (at least 840 mm of rain in 24 hours, [START_REF] Soutadé | Les inondations d'octobre 1940 dans les Pyrénées Orientales d'après les rapports adressés à M. Pardé par les instituteurs[END_REF]), but data are missing for characterizing the pre-flood channel morphology of these rivers. Aerial photographs taken in 1941 however reveal the presence of wide braided channels along many reaches of both valleys (https://remonterletemps.ign.fr/). Case studies of flood-induced braiding formation outside France have been also reported, but they document localized braiding formation affecting short river lengths [START_REF] Nardi | Spatio-temporal patterns of channel changes in response to a major flood event: the case of the Magra River (central-northern Italy)[END_REF][START_REF] Scorpio | Basin-scale analysis of the geomorphic effectiveness of flash floods: a study in the northern Apennines (Italy)[END_REF] or relatively small mountain streams [START_REF] Brenna | What drives major channel widening in mountain rivers during floods? The role of debris floods during a high-magnitude event[END_REF]. It is likely that a major fluvial metamorphosis occurred after the August 2005 catastrophic flood in Austrian Tyrol, as suggested by the very high mean width ratio reported by [START_REF] Krapesch | Scale orientated analysis of river width changes due to extreme flood hazards[END_REF] for the 34-km surveyed reach of the Trisanna River, but no information was provided about braiding formation in this case study.

Storm Alex provides some elements for the identification of key processes associated with the rapid mega-formation of braided channels after extreme floods. Of particular importance are the sediment cascading effects that could explain the emergence of a continuous braided pattern along 35 km, like observed in the Vésubie valley. Although sediment supply from tributaries has been very high, it is not really possible to explain the longitudinal extent of the fluvial metamorphosis by a prograding sediment wave generated in upper valleys, e.g. from upstream of the Boréon -Upper Vésubie confluence. Many evidences rather suggest that the fueling effect of sediment transport by alluvial storage is likely the key mechanism by which the sediment wave was regenerated step by step along the reach (Fig. 18 and19). The sharp decline of many geomorphic indicators (width ratio, post-flood confinement index, channel aggradation) downstream the gorges of the Vésubie suggests an attenuation of the sediment wave in the gorges, which must be considered as segments where the fuelling effect by erosion of the pre-existing alluvial storages could not occur. Some important stripping effects of surficial deposits in the margins of the active channel have been documented by topographic differencing, showing that fluvial erosion of alluvial fans, fluvio-glacial terraces, and colluvium deposits at the foot of hillslopes strongly contributed to the regeneration of the sediment wave along all the investigated stream reaches. This fuelling effect from lateral stripping of active channel margins is estimated at 4.1 Mm 3 , and it is almost three times more than sediment supply from tributaries (1.5 Mm 3 ). Although this figure is very high, it certainly corresponds to a lower bound estimate of the actual fuelling effect from the valley floor, because an early phase of massive erosion of fluvial terraces and floodplains likely occurred before channel aggradation. This was for example the case during a catastrophic flood that occurred in October 2011 in the Northern Apennines, where floodplain stripping before the formation of boulder bars was clearly seen from video documentation [START_REF] Rinaldi | An integrated approach for investigating geomorphic response to extreme events: methodological framework and application to the October 2011 flood in the Magra River catchment, Italy[END_REF]. This early phase of massive erosion cannot be detected by topographic differencing. However, ortho-images show in many places the stripping of riparian forests (and also buildings) that were present on alluvial surfaces that became the post-flood active channel (Fig. 21).

Although some ortho-images clearly show that some riparian forests have been buried by overbank gravel deposition, i.e. without preliminary erosion of the terrace, this situation is far from being dominant in the valley, where most of the trees has been removed before gravel deposition. All these elements strongly support the idea that the fuelling effect of alluvial storage is a key element for understanding not only sediment transport processes during extreme floods [START_REF] Tricart | Mécanismes normaux et phénomènes catastrophiques dans l'évolution des versants du bassin du Guil (Hautes-Alpes, France)[END_REF][START_REF] Korup | Earth's portfolio of extreme sediment transport events[END_REF], but also associated geomorphic responses.

calculation shows that a difference of only 20% (e.g. fine fraction of 40% for eroded volumes, vs. 20% for deposited volumes) reduces the sediment yield to only 380 000 m 3 . Whatever the fine fraction of the sediment yield, it is clear that the suspended load of the Vésubie during Storm Alex has been extremely important, as suggested by the big turbidity plume that has been captured by satellite imagery in the Mediterranean Sea at the mouth of the Var River [START_REF] Piton | Large in-stream wood yield during an extreme flood (Storm Alex, October 2020, Roya Valley, France): Estimating the supply, transport, and deposition using GIS[END_REF].

Implications for channel widening during catastrophic floods in alpine valleys

Recent literature on extreme-flood-related morphological adjustments in the Alps initially explores the link between stream power and channel widening in Austrian catchments [START_REF] Krapesch | Scale orientated analysis of river width changes due to extreme flood hazards[END_REF], relying on [START_REF] Magilligan | Geomorphic and sedimentological controls on the effectiveness of an extreme flood[END_REF] proposition of a 300 W m -2 threshold of specific stream power for major morphological adjustments to occur. [START_REF] Surian | Channel response to extreme floods: Insights on controlling factors from six mountain rivers in northern Apennines, Italy[END_REF] and [START_REF] Righini | Geomorphic response to an extreme flood in two Mediterranean rivers (northeastern Sardinia, Italy): analysis of controlling factors[END_REF] also stressed a significant effect of stream power on channel widening, with better results when the specific stream power is computed using the pre-flood channel width. This was later confirmed in two small catchments of Southwestern Germany (Lucia et al., 2018), where the authors were able to develop an explicit relationship between width ratio, specific stream power and confinement index, explaining 67% of the variability. Contemporary work in the Swiss pre-Alps [START_REF] Ruiz-Villanueva | Impacts of a large flood along a mountain river basin: the importance of channel widening and estimating the large wood budget in the upper Emme River (Switzerland)[END_REF] explores a wide range of controlling factors on channel widening, including for the first time not only hydraulic parameters, but also factors specifically linked to the triggering rainfall event. They highlight that models explain between 20% and 50% of channel widening variability, with the most significant variables being total maximum precipitation, stream power index and peak discharge.

Recent works from [START_REF] Scorpio | Basin-scale analysis of the geomorphic effectiveness of flash floods: a study in the northern Apennines (Italy)[END_REF][START_REF] Scorpio | Storm characteristics dictate sediment dynamics and geomorphic changes in mountain channels: A case study in the Italian Alps[END_REF] in Italian catchments (Northern Apennines and South Tyrol) stressed the decisive role of valley confinement on channel adjustments, as well as flood power through both hydrological and hydraulic parameters. A preliminary analysis based on a simple stream power index computed for the Vésubie stream network shows encouraging results for the prediction of width ratios, provided that stream reaches classified as gorges are excluded (Fig. 17A). Further investigations are needed to integrate results from the hydrological modelling of Storm Alex for the computation of a distributed map of specific stream power along the stream network of both catchments [START_REF] Payrastre | The 2 October 2020 Alex storm in south-eastern France: a contribution of the scientific community to the flood peak discharges estimation[END_REF]. However, it clearly appears that Storm Alex channel adjustments are not only controlled by hydraulic forces, but also by sediment supply. The positive correlation that we obtained between width ratios and bed-level changes strongly supports a strong sediment forcing on channel widening (Fig. 17C).

Knowledge about flood-induced active channel widening is crucial for determining a "safety space" or "freedom space" [START_REF] Biron | Freedom Space for Rivers: A Sustainable Management Approach to Enhance River Resilience[END_REF] for managing flooding hazard in alpine valleys. The documentation of Storm Alex active channel expansions provides a new dataset of width ratios associated with catastrophic floods having extreme return period in alpine environments. This dataset shows a 40% higher mean width ratio for the Vésubie as compared to the Roya, while hydrometeorological forcings were more intense in the Roya catchment. This highlights the critical role of local conditions on channel response, the wider modern valley floor and residual alluvial storages of the Vésubie having clearly been favorable for active channel expansion. The limiting effect of channel confinement on width ratios, statistically confirmed for the Vésubie, has been also reported by several case studies [START_REF] Surian | Channel response to extreme floods: Insights on controlling factors from six mountain rivers in northern Apennines, Italy[END_REF][START_REF] Righini | Geomorphic response to an extreme flood in two Mediterranean rivers (northeastern Sardinia, Italy): analysis of controlling factors[END_REF]Lucia et al., 2018;[START_REF] Scorpio | Basin-scale analysis of the geomorphic effectiveness of flash floods: a study in the northern Apennines (Italy)[END_REF][START_REF] Scorpio | Storm characteristics dictate sediment dynamics and geomorphic changes in mountain channels: A case study in the Italian Alps[END_REF]. All these studies found that the confinement index is a better predictor of channel widening than stream power, and our better correlation with the index confinement than with the stream power index agree with this result. The strong controlling effect of confinement during Storm Alex is also illustrated by the fact that along most of the upper confined reaches, active channel widening occurred over almost the entire modern valley floor, or even over a greater width than that of the pre-flood valley floor by eroding hillslope toe. This has strong implications for flood risk management in confined alpine valleys, where the entire valley floor should be considered as a potential zone of active channel expansion, as already shown during an extreme flood that occurred in the Apennines in October 2011 [START_REF] Rinaldi | An integrated approach for investigating geomorphic response to extreme events: methodological framework and application to the October 2011 flood in the Magra River catchment, Italy[END_REF][START_REF] Surian | Channel response to extreme floods: Insights on controlling factors from six mountain rivers in northern Apennines, Italy[END_REF].

Storm Alex also confirms a much higher geomorphic sensitivity for reaches having a narrow active channel before the flood, as previously reported by many case studies (Fig. 16). This means that narrow single-thread channels located in wide floodplains of alpine valleys should be considered as particularly sensitive zones for active channel expansion, even though the available space in the nearby floodplain may be of interest for development and urbanization. The compilation of data on width ratios during extreme floods is helpful for determining an upper envelope of maximum potential active channel widening simply based on the initial width, and data points from the Vésubie clearly help to determine this upper envelope. It is interesting to see that maximum width ratios observed in the Vésubie are not outside the scatterplot of documented extreme floods in European upland rivers and streams (Fig. 16). This means that this scatterplot likely integrate enough variability for predicting the most extreme flood-induced geomorphic responses of temperate upland environments.

Conclusion

The reconstruction of geomorphic responses of two confined alpine valleys of the Southeastern French

Figure 1 .

 1 Figure 1. General relief map of Vésubie and Roya river basins in the Southern French Alps, with localization of the study reaches. RTM State Forests correspond to land purchased by the State in the late 19 th century for erosion-control works. White dots correspond to main villages.

Figure 2 .

 2 Figure 2. Geology and glacial imprints of the Roya and Vésubie catchments; geological formations classified from 1/50 000 digital geological maps from BRGM (BD Charm-50). Maximum glacier extensions during the Würm (white dotted lines) adapted from Julian (1997). Black square brackets correspond to upper and lower limits of study reaches.

Figure 3 .

 3 Figure 3. 24h rainfall map produced from the fusion of radar and rain gauge data for the time interval comprised between 02/10/2020 06:00 UTC and 03/10/2020 06:00 UTC (Météo France ANTILOPE rainfall database reanalyzed by EDF). Catchment divides appear in red, the stream network in thin black lines, study reaches in thick black lines.

Figure 4 .

 4 Figure 4. Estimated peak discharges of Storm Alex in the recent flood history of the Vésubie (A) and Roya (B). Periods of hydrological recording from gauging stations appear in grey (data from https://hydro.eaufrance.fr/). Estimated peak discharges of ungauged large floods (triangles) and years of known large floods from historical archives (black squares) have been obtained from ONF-RTM-INRAE (2022 a and b).

  photographs available from the dedicated IGN website (https://remonterletemps.ign.fr/). Many other

3. 2 .

 2 Active channel and modern valley floor mappingExtraction of active channel was done manually by an expert-based photointerpretation mapping using QGIS. A single operator did this task for the seven dates of the Vésubie to avoid as much as possible an operator bias in active channel mapping. It was not possible to follow this rule for the Roya, where extraction was done by two operators. However, common mapping guidelines were shared between operators to avoid major interpretation differences. Areas considered as part of the active channel include low-flow channels, unvegetated gravel bars, and gravel bars occupied by sparse vegetation (Fig.5A). Wooded islands have been excluded when their size exceeds about 150 m², for practical reason (saving time during manual digitizing of active channels). Areas in the margins of the floodplain showing clear flooding marks, like overbank accumulations of fine sediments and/or large woods, were also excluded (Fig.5B). These mapping rules are justified by the most commonly used definition of active channel as a temporary storage of the coarse sediment load of a river that is frequently mobilized (or has been recently mobilized) by flow events[START_REF] Williams | The sedimentology of a braided river[END_REF][START_REF] Liébault | Causes of 20th century channel narrowing in mountain and piedmont rivers of Southeastern France[END_REF][START_REF] Haschenburger | Floodplain stages in the braided Ngaruroro River, New Zealand[END_REF]. The combined used of ortho-images and LiDAR Digital Elevation Model (DEM) was particularly useful for post-Alex active channel extraction, because in some places, it was uneasy to map the limit between alluvial bars and destabilized banks or talus slopes based on ortho-images alone. This was much easier by looking at the post-Alex LiDAR derived slope raster (Fig.5C and D). The ante-Alex slope raster obtained from the 2018 LiDAR survey (MNS LIDAR25cm©SIGMNCA obtained courtesy of the Métropole de Nice) was also very useful for improving extraction along narrow river reaches where the active channel was masked by riparian vegetation (Fig.5 E and F). Those instances were mostly encountered in the upper reaches of the Vésubie catchment before Storm Alex (Madone de Fenestre and Boréon valleys). It was not possible to use this approach for the Roya, where no LiDAR surveys are available before the storm. It is therefore possible that the active channel width of the Roya before the storm has been under-estimated along reaches where the channel was masked by alluvial forest.

Figure 5 .

 5 Figure 5. Examples of active channel GIS manual extraction (in blue for every panels): (A) post-Alex active channel extraction in the Boréon valley, showing inclusion of low-flow channels and unvegetated gravel bars, and exclusion of wooded islands; (B) post-Alex active channel extraction of the Vésubie near Roquebillière,

Figure 6 .

 6 Figure 6. Example of modern valley floor manual extraction in the Vésubie valley obtained from the 2018 LiDAR DEM: (A) extent of the valley floor in red, with localization of the cross-section (in white) shown in panel B; (B) cross-section extracted from the 2018 LiDAR DEM, showing the 10-m threshold of relative elevation above the active channel used for excluding old alluvial terraces

Figure 7 .

 7 Figure 7. General principle of the GIS spatial segmentation procedure implemented with QGIS plugins for the computation of active channel width

Figure 8 .

 8 Figure 8. Example of area of interest used for extracting post-Alex erosion and deposition volumes along mainstems; this area is not limited to the post-Alex active channel, as it also includes active parts of the valley floor which can be easily detected from the post-Alex DoD; in this example, lateral sediment supply from bank erosion of an alluvial fan on the right bank, and of an old alluvial terrace on the left bank, are clearly visible; background: hillshade view of the 2020 LiDAR DEM

  which has been estimated at only 21 ha (from 94 ha in 1995 to 115 ha in 1999). The mean active channel width increased from 19 m to 79 m after the storm, and from 27 m to 33 m after the 1997 flood (Fig.11A). The 2D morphological effect of Storm Alex is therefore 10 times greater than that of the last 100-yr flood in the valley. Archives of aerial imagery clearly reveal the exceptional nature of this geomorphic impact for the last 70 years, the mean active channel width of the Vésubie having stayed below ~30 m since at least 1948 (Fig.11A). The singular geomorphic signature of the storm is also illustrated by the expansion of the modern valley floor, which has increased form 317 ha in 2018 to 363 ha in 2020, under the effect of lateral erosion of colluvial deposits and fluvio-glacial terraces.Almost all of the post-Alex modern valley floor has been occupied by the new active channel (76%); this channel confinement index has never exceeded 36% during the last 70 years.The active channel surface of the Roya increases by a factor 2.3 after the storm; from 59 ha in 2017 to 137 ha in 2020 (Fig.10). The mean active channel width increased from 15 m to 37 m (Fig.11B). It was not possible to make a comparison with precedent extreme floods for the Roya, because the last major historical flood occurred in November 1926, before the first set of aerial images. However, like for the Vésubie, the post-Alex geomorphic response of the Roya does not have any antecedent analog during the last 70 yrs. The post-Alex channel confinement index is 75%; this value is equivalent to the Vésubie, and it reflects a similar relative geomorphic resetting of the valley floor. The absence of a LiDAR survey before the event prevented us to map the modern valley floor before the flood, but the very hard bedrock outcrops constituting the limits of the Roya valley floor let us think that the valley floor expansion observed in the Vésubie did not occur in the Roya, or only very occasionally.

Figure 10 .

 10 Figure 10. Time evolution of the active channel surface of the Roya and Vésubie rivers; surfaces occupied by modern valley floors give the available space for active channel development in the two valleys

Figure 12 .

 12 Figure 12. Longitudinal gradients of post-Alex active channel expansion in the Vésubie (A) and Roya (B) study reaches; vertical lines correspond to major tributary confluences

Figure 13 .

 13 Figure 13. Longitudinal gradients of the post-Alex index of confinement (IC) for the Vésubie (A) and Roya (B); the index of confinement corresponds to the ratio between post-Alex active channel width and modern valley floor width; data points correspond to mean values computed for 500 m intervals; error bars correspond to ± one standard deviation; points were classified as gorges or alluvial units according to a scaling law of valley floor width vs. drainage area obtained for the Vésubie (see supplementary material)

Figure 14 .

 14 Figure 14. Longitudinal gradients of the Storm Alex width ratio for the Vésubie (A) and Roya (B); data points correspond to mean values computed for 500 m intervals; error bars correspond to ± one standard deviation; points were classified as gorges or alluvial units according to a a scaling law of valley floor width vs. drainage area obtained for the Vésubie (see supplementary material)

Figure 15 .

 15 Figure 15. Bed-level changes of the Vésubie after Storm Alex obtained from topographic differencing of LiDAR DEMs; aggradation in red and incision in blue; vertical dotted lines correspond to main tributary confluences; gorges are represented in grey; the back arrow corresponds to a major nickpoint in the long profile, with a local slope increase

Figure 16 .

 16 Figure 16. Width ratios of Storm Alex as a function of the pre-flood active channel width for the Roya and

  considering together stream reaches classified as alluvial and gorges (wr = 0.72Δz + 2.94, R²=0.30, p<0.001, with Δz the bed-level change). Despite a strong data scattering, it clearly appears that the rate of active channel expansion is partly controlled by the magnitude of net sediment accumulation at reach scale.

Figure 17 .

 17 Figure 17. Width ratio of the Vésubie and Boréon channels during Storm Alex as a function of the stream power index (A), the index of confinement before the flood (B), and bed-level change of the active channel during the flood (C).

  confluence with the Venanson (river distance of 17 250 m), where sediment transport increases again under the effect of local erosion and tributary inputs down to the exit of the study reach. Sediment supply from local erosion largely exceeds that from tributaries along the three study reaches. The most active zones of sediment recruitment from local erosion are the upper valleys and the reach of the Vésubie located downstream of the long-profile nickpoint (Fig.17), where local channel incision has been observed (Fig.15).

Figure 18 .

 18 Figure 18. Cumulative volumes of sediment erosion and deposition in main stem valley floors, and cumulative sediment supply from tributaries obtained from LiDAR-based topographic differencing in the Vésubie catchment during Storm Alex; uncertainty bounds of sediment volumes are included in the thickness of lines

Figure 20 .

 20 Figure 20. Sequence of pictures of the Vésubie channel in the upstream vicinity of Roquebillière, showing a cycle of channel recovery following a major flood that occurred in October 1882, that was potentially at the origin of the braided channel visible in the 1888 photo (© Archives Départementales des Alpes-Maritimes, collection Eaux et Forêts, commission des reboisements des Alpes-Maritimes ; 1888 photo : 23FI 0838; 1899 photo : 23FI 0874). The picture of 2020, taken just after Storm Alex, shows the reformation of a braided channel in this area (© Vincent Koulinski).

  Alps following Storm Alex (October 2020) shows a quasi-continuous fluvial metamorphosis along the investigated stream networks, with dramatic active channel widening and aggradation, having no antecedent analogs during the last 70 years in both valleys. The different glacial imprints between the two valleys is considered a key factor explaining the exacerbated channel response in the Vésubie, where a braided channel emerged along a 35 km river length. Many evidences strongly support the idea that the fuelling effect of alluvial storage is a key element of the sediment cascade at the origin of the braided channel formation. While hydro-meteorological forcings were more intense in the Roya, a much higher active channel widening was observed in the Vésubie, its wider modern valley floor having clearly be favorable for active channel expansion. The strong controlling effect of confinement is illustrated by the fact that along most of the upper confined reaches, active channel widening occurred over the entire modern valley floor, or even over a greater width than that of the pre-flood valley floor by eroding hillslope toe. This has strong implications for flood risk management in confined alpine valleys, where the entire valley floor should be considered as a potential zone of active channel expansion during extreme events.

Table 2 .

 2 Remote sensing data used for 2D morphological mapping and topographic differencing

	Dates	Types	Resolution	Dates of aerial	Sources	Study sites
			(m)	campaigns
	1948	ortho-images	0.50	23/08/1948	BD-ORTHO-Historique IGN	Roya, Vésubie
		(black and white)
	1995	aerial photos		25-26/06/1995	IGN (1995-FD06/200)	Vésubie
	1999	ortho-images	0.50	04-05/06/1999	BD-ORTHO IGN	Vésubie
		(color)
	2004	ortho-images	0.50	09-26/06/2004	BD-ORTHO IGN	Vésubie

* values in brackets correspond to average densities of 3D point clouds classified as ground points (number of points / m²)

Acknowledgements

This study was supported by the Direction Générale de la Prévention des Risques (DGPR) of the French Ministry of Environment, by the EU Interreg-Alcotra RITA project (grant agreement n.8504), and by the Université Côte d'Azur through the "Crédits Scientifiques Incitatifs 2021 -volet Recherche" funding call. ONF-RTM 06, Météo France, IGN, CEREMA, Métropole de Nice, and Pierre Brigode (Univ. Côte d'Azur) are acknowledged for the support in data sharing and general discussions and exchanges during post-flood surveys of Storm Alex. This work was performed in the framework of the LTER CNRS-INEE-ZABR (Zone Atelier Bassin du Rhône). Special thanks to

Mélissandre who started the active channel mapping of the Vésubie during her "stage de 3 e ". This paper benefited from comments of two anonymous reviewers.

The catchment-scale sediment budget reconstructed for the Vésubie provides a sediment yield estimate of 2.09 Mm 3 . This estimate certainly incorporates an important fraction of suspended load, which remains hard to evaluate. It has not been possible to undertake some grain-size distribution analysis for eroded and deposited volumes included in the sediment budget, but it is likely that a differential of the percentage of fines exists between eroded and deposited volumes, notably for tributaries. One reason for this is that eroded volumes obtained for tributaries are mainly composed of remobilized colluvial deposits with a supposedly high content of fines (e.g. sands coming from weathered granitic rocks, silts and clays coming from marly rocks), while deposited volumes are made of gravel or boulder bars along distal reaches, and debris-flow deposits on alluvial fans. This differential of fine fraction is likely less important in the valley floor, since eroded and deposited volumes are mainly composed of alluvial storages, even if some eroded colluvial deposits have been also incorporated in the valley floor sediment budget. To illustrate the importance of this differential on sediment yield, a simple