# Extreme flood deposits in the Roya Valley during the Holocene: characterisation and recurrence intervals

Raphaël Kerverdo, S. Lafuerza, C. Gorini, E. Fouache, D. Granjeon, N.

Bianchi, N. Haghipour, R. Clementucci, N. Loget, S. Migeon, et al.

► To cite this version:

Raphaël Kerverdo, S. Lafuerza, C. Gorini, E. Fouache, D. Granjeon, et al. Extreme flood deposits in the Roya Valley during the Holocene: characterisation and recurrence intervals. Quaternary Science Reviews, 2026, 373, pp.109749. 10.1016/j.quascirev.2025.109749. hal-05421839

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<figure><img src="image_1.png" /></figure>
# Extreme flood deposits in the Roya Valley during the Holocene: characterisation and recurrence intervals

Raphaël Kerverdo<sup>a,∗</sup>,<sup>†</sup> S. Lafuerza<sup>a,∗∗</sup>, C. Gorini<sup>a</sup>, E. Fouache<sup>b</sup>, D. Granjeon<sup>c</sup>, N. Bianchi<sup>d</sup>, N. Haghipour<sup>e,f,h</sup>, R. Clementucci<sup>e</sup>, N. Loget<sup>a,†</sup>, S. Migeon<sup>f,g</sup>, R. Deschamps<sup>c</sup>

## ARTICLE INFO

**Keywords:**

Extreme event

Paleoflood

Holocene

Sedimentology

OSL

Radiocarbon dating

## ABSTRACT

Floods are among the costliest natural disasters, causing extensive damage and fatalities worldwide. Mountainous regions of the Mediterranean Alps are highly vulnerable to extreme hydrological events, whose frequency is increasing in the context of climate change favouring convective storms. In October 2020, the Roya valley was struck by an extreme hydrological event. The amount of rainfall (return period >1000yr) and discharge (>100yr), triggered major geomorphological changes, including substantial channel widening, slope destabilisation and extensive bank erosion.

This study provides unprecedented sedimentological data on torrential dynamics in the Roya Valley, based on ¹⁴C and OSL dating of newly exposed flooding deposits following Storm Alex. Five clusters of preserved fluvial and torrential deposits ranging from the Younger Dryas-Holocene transition to the Little Ice Age are identified. These preservation clusters correspond to periods of wetter condition and anthropogenic influence.

By combining sedimentological analyses with documented pollen and sediments records from lakes in the Mercantour massif, as well as historical archives, this study presents one of the first flood-frequency reconstructions for a Mediterranean Alpine catchment over a multi-millennial timescale. Past flooding events (on centennial to millennial scales) leave distinctive sedimentary signatures, characterised by thick deposits and the mobilisation of coarse particles, before being reworked within the broader autogenic dynamics of the river system. Reconstructing the frequency and occurrence of past extreme flood events provides key insights into the conditions under which such events occur, and offers valuable information for the development of flood-risk scenarios, particularly in mountainous areas that are among the most affected by contemporary climate change.

## 1. Introduction

Climate variability throughout the Holocene has controlled both the frequency and magnitude of extreme hydrological events in mountain catchments, yet the sedimentary archives of such events remain incompletely understood (Baker, 2008; Macklin et al., 2006). In region such as the western Alps, millennial-scale transitions between humid and arid phases modulate flood dynamics (Magny et al., 2011; Wilhelm

et al., 2022; Wirth et al., 2013), but the terrestrial record is often affected by discontinuities (Miall, 2022), incomplete stratigraphic resolution (Plotnick, 1986; Sadler, 1981; Straub et al., 2020), and poor chronological control. Fluvial deposits result from the interplay between external forcings and the internal dynamics of sediment transport systems. Rivers operate as transfer zones (Schumm, 1977) where sediments can be stored, removed and redistributed according to system length and autogenic dynamics (Hajek and Straub, 2017). Consequently, the

<footnote><sup>a</sup> Institut des Sciences de la Terre de Paris (ISTeP), Sorbonne Université, CY Cergy Paris Université, CNRS-INSU, 75005 Paris, France</footnote>

<footnote><sup>b</sup> Laboratoire Médiations, Institut de Géographie, Sorbonne Université, Paris, France</footnote>

<footnote><sup>c</sup> IFP Energies Nouvelles, Rueil-Malmaison, France</footnote>

<footnote><sup>d</sup> Musée départemental des Merveilles, Tende, France</footnote>

<footnote><sup>e</sup> Department of Earth and Planetary Sciences, ETH Zurich, Switzerland</footnote>

<footnote><sup>f</sup> Université Côte d'Azur, CNRS, IRD, Observatoire de la Côte d'Azur, Géoazur, Valbonne, France</footnote>

<footnote><sup>g</sup> Sorbonne Université, 75005 Paris, France</footnote>

<footnote><sup>h</sup> Laboratory of Ion Beam Physics, ETH- Zurich</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>** Corresponding author.</footnote>

<footnote>E-mail addresses: raphael.kerverdo@sorbonne-universite.fr (R. Kerverdo), sara.lafuerza@sorbonne-universite.fr (S. Lafuerza).</footnote>
holocene fluvial record is highly discontinuous (Gibbard and Hughes, 2021; Lewin et al., 2005) and its preservation is strongly governed by basin geometry and climatic determinants (Gibbard and Hughes, 2021).

Lakes, by contrast, provide continuous, long-term sedimentary archives (Sabatier et al., 2022) and can record numerous types of events, including earthquakes, floods, GLOFs, and tsunamis. Each of these events produces characteristic sedimentary deposits, such as turbidites, mass-transport deposits, debris flows, and homogenous layers (Sabatier et al., 2022). Large-magnitude flood deposits are typically associated with density underflows (flow density greater than lake water), i.e., turbidity currents, which are highly erosive and transport coarse particles, producing erosional surfaces and graded turbidites. Underflow velocity correlates with the velocity of the inflowing stream (Hsu et al., 1984). These lacustrine archives provide useful analogues for comparison with fluvial deposits of equivalent age. However, both lake and river records remain sensitive to the spatial distribution of precipitation during individual events. However, the degree of continuity within lake archives varies considerably across timescales. While long Holocene lacustrine sequences may still include hiatuses or condensed intervals when sedimentation rates are low or disturbed, late-Holocene records from several Alpine lakes are continuous, varved, and very well dated (Wilhelm et al., 2022; Wirth et al., 2013). High-altitude lakes, typically draining small catchments, are highly sensitive to moderate hydrological variability and can record frequent low- to medium-magnitude events with exceptional chronological precision. In contrast, valley-floor fluvial systems integrate signals over much larger drainage areas and are therefore preferentially sensitive to rare, high-magnitude floods (e.g., rainfall events >1000 yr RP; discharge >100 yr RP). These contrasted sensitivities highlight the complementarity of these archives and the importance of combining both lacustrine and fluvial records when reconstructing Holocene flood histories.

The Roya Valley (south-eastern France) has been continuously occupied since at least the mid-Holocene (Beaulieu, 1977; Bianchi et al., 2014; de Lumley-Woodyear, 1969; Huet and Martin, 2002), with fluctuations in human activity recorded through archaeological layers and land-use changes (Huet, 2017; Kharbouch, 1996). Despite its long history of settlement, the palaeoflood record remains fragmentary: terraces and alluvial fans preserve episodes of aggradation, but systematic investigations are scarce often disconnected extreme episodes events, and rarely tied to precisely dated climatic shifts.

The catastrophic flood triggered by Storm Alex on October 2, 2020 (Carrega and Michelot, 2021; Liébault et al., 2024; ONF-RTM, ONF-DRN, INRAE-ETNA, 2023; Pons et al., 2024) profoundly modified the fluvial landscape, incising banks and exposing extensive buried sedimentary successions along the main channel and adjacent fans (Kerverdo et al., 2025, Kerverdo et al., submitted). These freshly exposed palaeo-deposits offer a rare opportunity to document extreme flood events over multiple millennia and to assess their recurrence in relation to both natural climatic oscillations and anthropogenic impacts.

In this study, we (i) present the geomorphological framework and stratigraphic setting of five key sectors within the Roya Valley, (ii) apply radiocarbon (¹⁴C) and optically stimulated luminescence (OSL) dating to constrain the timing of extreme-flood deposits, (iii) describe sedimentological facies indicative of flood versus debris-flow processes, and (iv) estimate palaeo-return periods of extreme floods throughout the Holocene. These newly exposed deposits allow us to constrain the timing, magnitudes, and sedimentary signatures of extreme floods throughout the Holocene. They provide the basis for reconstructing long-term flood recurrence and for assessing the preservation potential of extreme-event archives in a mountain fluvial system.

## 2. State of art

Mountain catchments respond rapidly to intense rainfall owing to their steep morphometric parameters and orographic precipitation patterns, which vary markedly in space and time (Rotunno and Houze,

2007). During flash floods, the volumes of water and sediment mobilised can be immense, generating a spectrum of flow types, often associated with the term « hyperconcentrated flows » (Brenna et al., 2020; Church and Jakob, 2020; Costa, 1984; Pierson, 2005a; Pierson and Costa, 1987) whose impacts are often catastrophic (Benito et al., 1998; D.R.E.A.L Midi-Pyrénées, 2013; Huet and Martin, 2002; ONF-RTM, ONF-DRN, INRAE-ETNA, 2023). Yet, the highly erosive nature of these events means instrumental records of peak water levels and discharges remain fragmentary in steep alpine rivers.

Palaeoflood hydrology, as formalised by Kochel and Baker (1986), seeks to reconstruct flood magnitudes and frequencies using palaeostage indicators (PSI), such as slack-water deposits, boulder bars or levees, and high-water marks (HWM). While HWMs permit detailed analysis of recent floods using post-event surveys (Bodoque et al., 2015; Marchi et al., 2010) reconstructing pre-twentieth-century floods is hindered by the scarcity of precise aerial, hydraulic and topographical data. Historical chronicles and archival documents can partly bridge this gap, but only for the most exceptional events (Benito et al., 2022; Wilhelm et al., 2018).

To extend palaeoflood records further back, sedimentary archives such as alluvial-fan and terrace deposits can be examined using Optically Stimulated Luminescence (OSL) and radiocarbon dating (¹⁴C).

Studies of slackwater deposits (Benito et al., 2003; Thorndycraft et al., 2005), floodplain sediments (Benito et al., 2008; Toonen et al., 2015) and dendrochronological data (Díez-Herrero et al., 2013) reveal flood series over millennial timescales. However, in steep channels, erosion preferentially dominates over deposition, resulting in highly discontinuous flood records. Downstream of steep reaches, significant re-deposition can occur (Kerverdo et al., 2025; Liébault et al., 2024; Scorpio et al., 2022), through depositional facies vary markedly between channel cores and margins (Wilhelm et al., 2018).

On October 2, 2020, Storm Alex struck the Roya Valley with an estimated return period (rainfall) exceeding 1000 years, overtopping the 100-year flood levels on both Roya and Vésubie rivers (Carrega and Michelot, 2021; Pons et al., 2024). The event eroded gully heads and banks, but also triggered pronounced downstream aggradation—average bed aggradation of 1–2 m over tens of kilometres and local accumulations approaching 10 m (Liébault et al., 2024; ONF-RTM, ONF-DRN, INRAE-ETNA, 2023).

## 3. Study area

The study area is the northern half of the Roya Valley (Fig. 1), a border region between France and Italy, with the frontier boundary running along the eastern edge of the valley. The Roya originates at the Col de Tende and flows in a general north-south direction. More precisely, from the Col de Tende to Tende, it flows in a north-south direction, then turns slightly towards the town of Tende, flowing almost east-west. From Tende to Breil-sur-Roya, it flows from north-north-east to south-south-east. Finally, from Breil-sur-Roya to Ventimille, its outlet, it takes a new direction through the north-north-west to south-south-west gorges.

The Roya River flows through the Mesozoic cover and the Argentera-Mercantour Massif in its upper reaches (Fig. 1). Five main tributaries on the right bank of the Roya and the riverbanks sections along the Roya itself, were selected for this study (Fig. 1 and Table 1). These tributaries were selected because they were among the most active during Storm Alex (Kerverdo et al., 2025; ONF-RTM, ONF-DRN, INRAE-ETNA, 2023), where the flood incised previously deposited alluvial and torrential terraces (Carrega and Michelot, 2021; ONF-RTM, ONF-DRN, INRAE-ETNA, 2023), allowing us to assess whether similar paleo-Alex events had occurred in the same locations.

From south to north, the following catchments are distinguished (Table 1, Fig. 2):

**Lavina catchment:** Covering an area of 10.3 km², the Lavina stream is 3.1 km long, with an average slope of approximately 60 %. It originates at the Col de Brouis and joins the Roya River at the artificial lake of
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Digital Elevation Model (DEM) and simplified geological map of the Roya Valley (Alpes-Maritimes Department). White circles indicate major towns; coloured symbols denote stratigraphic outcrop locations. Labels A to E corresponds to the Lavina, Caïros, Bieugne, Consciente and Viévola torrents, respectively. The black line shows the Roya River and its main tributaries included in this study.</figcaption></figure>

<table><caption>**Table 1**
Main morphometrics parameters of studied tributaries of the Roya River.</caption><thead><tr><th>Rivers</th><th>Area (km²)</th><th>Perimeter (km)</th><th>Length (km)</th><th>Max. relief (m a.s.l.)</th><th>Min. Relief (m a.s.l.)</th><th>Average Relief (m a.s.l.)</th><th>Average Hillslope (%)</th></tr></thead><tbody><tr><td>Lavina</td><td>10.3</td><td>13</td><td>3.1</td><td>1828</td><td>281</td><td>1547</td><td>60 %</td></tr><tr><td>Caïros</td><td>42.1</td><td>34.4</td><td>10.6</td><td>2681</td><td>386</td><td>2295</td><td>40 %</td></tr><tr><td>Bieugne</td><td>83.6</td><td>44.8</td><td>24.8</td><td>2918</td><td>644</td><td>2274</td><td>20 %</td></tr><tr><td>Consciente</td><td>4.2</td><td>9.8</td><td>3.8</td><td>2199</td><td>903</td><td>1296</td><td>60 %</td></tr><tr><td>Viévola</td><td>6.6</td><td>10.9</td><td>3</td><td>2281</td><td>933</td><td>1348</td><td>50 %</td></tr></tbody></table>
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Longitudinal profile of the Roya River and its major tributaries, derived from LiDAR acquired in June 2021 LiDAR (IGN, 2023). Markers correspond to stratigraphic-logs positions (labels refer to site codes), and the dashed red line indicates the position of the main towns of the Roya Valley. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

Breil-sur-Roya. Geological formations include Lower Triassic deposits (gypsum, sandstone) and undifferentiated recent alluvium.

One alluvial terrace was studied in this catchment in right bank of Lavina Torrent (Table 2). It corresponds to the outcrop called L1K (Fig. 2). It is located approximately 1 km upstream from the outlet, in an area where the river slope decreases from over 8% to around 6%, which explains the presence of an 8000 m² zone of alluvial terraces. The investigated outcrop is approximately 3 m high and 5 m long. Two ¹⁴C dates were obtained, one from the middle and one from the top of the deposit.

**Caïros catchment:** Draining an area of 42.1 km², the Caïros is a former glacial valley (Blanchard, 1949) characterised by Würmian terraces of glacial alluvium on its valley floor and flanked by steep Jurassic carbonate cliffs. It marks the boundary between the Mesozoic formations and the southern edge of the Argentera-Mercantour Massif. The Caïros conflues with the Roya upstream of the Nocé Gorges, below the village of Saorge.

Three outcrops were studied in the Caïros catchment (Table 2, Fig. 2):

* C1M is located a few dozen metres upstream from the confluence of the Caïros River and Roya River, on the right bank of the Roya. The

deposits lie about 0.5 m above the current river level and reach a height of 1 m. They correspond to an alluvial terrace. Three dates were obtained: one OSL and two ¹⁴C.

* C5K is located 850 m upstream from the outlet, on the left bank of the Caïros River. This site presents a 3-m-high and approximately 30-m-long outcrop, subdivided into five stratigraphic logs. The deposits form an alluvial terrace positioned about 1 m above the current streambed.

* C2M is situated 3050 m from the outlet, on the right bank of the Caïros River. The outcrop lies about 1 m above the streambed and measures 3 m in length and 1 m in height.

**Bieugne catchment:** The largest catchment in the Roya basin, covering 83.6 km², it includes the glacial valleys of Minière and Val Castérino, which merge at approximately 1400 m.a.s.l near the artificial Mesches Lakes, where an EDF hydroelectric dam is located. The average slope is lower, at around 20%. The Bieugne stream flows into the Roya River at Saint-Dalmas-de-Tende (Fig. 2).

Two outcrops were investigated (Table 2, Fig. 2):

* B1K is located 700 m upstream from the outlet on the right bank. It reaches 7 m in height and approximately 50 m in length. According

<caption>**Table 2**</caption>

Main characteristics of outcrops studied for each catchment (Fig. 2). The location of each outcrop is indicated by its distance from the current outlet and its altitude relative to the current riverbed. The size of the outcrop, the type of deposit observed and the measurements taken are also indicated.

<table><thead><tr><th>Catchment</th><th>Outcrop</th><th>Location (distance from outlet)</th><th>Elevation Above streambed</th><th>Dimensions (H × L)</th><th>Type of Deposit</th><th>Dating performed</th></tr></thead><tbody><tr><td>Lavina</td><td>L1K</td><td>~1 km upstream, right bank</td><td>~1 m</td><td>~3 m × 5 m</td><td>Torrential terrace</td><td>2 × ¹⁴C</td></tr><tr><td>Caïros</td><td>C1M</td><td>Few dozen metres from confluence (Roya)</td><td>~0.5 m</td><td>1m × ~2 m high</td><td>Alluvial terrace</td><td>1 × ¹⁴C, 1 × OSL</td></tr><tr><td>Caïros</td><td>C5K</td><td>850 m from outlet, left bank</td><td>~1 m</td><td>~3 m × 30 m</td><td>Alluvial terrace</td><td>3 × ¹⁴C</td></tr><tr><td>Caïros</td><td>C2M</td><td>3050 m from outlet, right bank</td><td>~1 m</td><td>~1 m × 3 m</td><td>Alluvial terrace</td><td>2 × ¹⁴C, 1 × OSL</td></tr><tr><td>Bieugne</td><td>B1K</td><td>700 m from outlet, right bank</td><td>~2–3 m</td><td>~7 m × 50 m</td><td>Terminal moraine</td><td>1 × ¹⁴C</td></tr><tr><td>Bieugne</td><td>B1A</td><td>Opposite B1K, left bank</td><td>~0.5 m</td><td>~1m × ~0.5 m high</td><td>Terminal moraine</td><td>1 × ¹⁴C</td></tr><tr><td>Consciente</td><td>Co1K</td><td>800 m from outlet</td><td>~1 m</td><td>~3m × ~3 m</td><td>Anthropogenic + natural terrace</td><td>2 × ¹⁴C</td></tr><tr><td>Consciente</td><td>Co2K</td><td>1000 m from outlet</td><td>~1 m</td><td>~1 m × ~1 m</td><td>Anthropogenic terrace</td><td>1 × ¹⁴C</td></tr><tr><td>Viévola</td><td>V1A</td><td>Vievola fan</td><td>~0.3 m</td><td>~5m × ~2 m high</td><td>Flood &amp; land-use deposits</td><td>4 × ¹⁴C, 1 × OSL</td></tr><tr><td>Viévola</td><td>D3K'</td><td>Apex of fan, margin of Dente torrent</td><td>~1–2 m</td><td>~10 m × ~3 m high, several 100 m</td><td>Stacked paleosols</td><td>1 × ¹⁴C</td></tr></tbody></table>
to the geological map (Gêze and Nesteroff, 1996), the deposits correspond to a terminal moraine situated near the town of Saint-Dalmas-de-Tende. The outcrop was subdivided into two stratigraphic logs. The deposits lie around 2–3 m above the current streambed. One ¹⁴C data was obtained from a sandy horizon.

* B1A is located opposite B1K, on the left bank of the Bieugne torrent. It is smaller than B1K, with a height of approximately 0.5 m, and lies about 0.5 m above the current streambed. One ¹⁴C date was obtained.

**Consciente catchment:** Adjacent to Viévola on its southern border, the Consciente basin spans 4.2 km², with an average slope of 60 %. It has an elongated shape and is composed mainly of Triassic formations (gypsum and cargneules) and basement rocks (granite). The Consciente stream is approximately 3.8 km long.

Two outcrops were studied in the Consciente catchment (Table 2, Fig. 2):

* Co1K and Co2K are located 800 and 1000 m upstream from the outlet. The site corresponds to a former land-use area, where anthropogenic terraces are embedded within a natural terrace partially eroded by Storm Alex. The location corresponds to an old kiln furnace. Three ¹⁴C dates were obtained: two in Co1K (one from charcoal found inside the furnace and one from a horizon within the embedded terrace) and one from Co2K (charcoal at the base of the furnace deposit).

**Viévola catchment:** Covering approximately 6.6 km² with an average slope of 50 %, this basin has a radial drainage pattern fed by multiple torrents. It is located on the right bank of the Roya, marking the transition between the eastern edge of the Argentera-Mercantour Massif and the Mesozoic cover. Viévola was one of the most productive (solid load) catchments during Storm Alex.

Two outcrops were investigated (Table 2, Fig. 2):

* V1M is located on the Vievola fan and corresponds to a section incised during Storm Alex. The outcrop forms a panorama several metres long and approximately 2 m high. One major land-use horizon was identified, along with flood deposits. A total of five dates were obtained: four using ¹⁴C and one using OSL.

* D3K' is located at the apex of the Vievola fan, on the margin of the Dente torrent. It corresponds to a sequence of stacked paleosols, measuring 3 m in height and extending over several hundred metres in length. One ¹⁴C date was obtained.

# 4. Methods

## 4.1. Field stratigraphy and sampling

Following the October 2, 2020 flood, most fluvial terraces along the Roya and its tributaries were scoured and filled. Field campaigns carried out in March 2021 and June 2023 targeted these freshly exposed outcrops for OSL and radiocarbon sampling (Table 2). At each site, the uppermost ~5 cm of weathered surface were removed, the sedimentary section was logged with detailed photographic documentation, and the maximum particle size (MPS) preserved within the deposit was measured. Sediment samples for grain-size analysis were collected at key facies transitions.

Flood layers or sequences (FL) were delineated using multiple palaeostage indicators (PSIs):

- Fine-grained facies (clay/silt laminae) marking high-stand or overbank deposits (Benito et al., 2003; Craig Kochel and Baker, 1986).

- Winnowed pebbles-cobbles lags corresponding to the recession phase (Blair, 1999; Blair and McPherson, 1994).

- Cross-bedded sands and graded laminae indicating waning traction currents (Nemec and Steel, 1984).

We did not rely solely on erosional surfaces, as a single flood in braided reaches may produce up to 2 m of aggradation punctuated by several internal scours (Kerverdo et al., submitted). Instead, the combination of vertical facies successions and PSIs enabled isolation of individual flood events.

## 4.2. Chronological constraints

A multi-method dating strategy was employed (Ballesteros-Cánovas et al., 2024), with a primary focus on radiocarbon (¹⁴C) dating owing to its efficiency and reliability for this sedimentary context. All radiocarbon dating was performed at ETH Zürich on woods and charcoal samples, at Department of Earth and Planetary Sciences.

### 4.2.1. Radiocarbon dating

Radiocarbon dating was carried out on the total organic carbon (TOC) extracted from 18 sedimentary samples (weighted around 3 to 5gr each). To remove carbonates, the samples were fumigated in silver capsules with 37 % hydrochloric acid (HCl), followed by a neutralisation step using NaOH at 65 °C for 48 h to remove residual acidity. Radiocarbon measurements were carried out on gaseous targets using the MICADAS mass spectrometer at the ETH Zurich laboratory.

The measurements were standardised using oxalic acid II (NIST SRM 4990C), and corrections were applied to account for the constant contamination induced by fumigation and the capsules. Radiocarbon ages (expressed in ¹⁴C BP years) were calibrated with OxCal software using the IntCal20 calibration curve. The calibrated ages are presented in calibrated years before the present (cal BP), accompanied by the 2σ uncertainty intervals.

### 4.2.2. OSL dating

Sandy horizons (<250 µm) were sampled at >1 m depth below the surface and above the modern river level using black PVC tubes (2 mm wall, 30 cm length). Each tube was driven ~20 cm into the target horizon, capped and sealed, yielding 200–300 g of material. Laboratory measurements and age determinations were carried out at Curt-Engelhorn-Zentrum Archaeometry gGmbH. OSL results were validated against ¹⁴C ages while acknowledging potential biases linked to partial bleaching and moisture-content uncertainties.

### 4.2.3. Integation of documented pollen records

To position the results within a broader climatic context, a synthesis of published pollen records from Lac Long Supérieur, Tourbière du Gias del Ciari, and Lac des Grenouilles from Kharbouch (1996) is provided in Supplementary Table S2.

## 4.3. Historical floods compilations

A compilation of historical floods in the Roya Valley (Table S1), was assembled using departmental archives, local associations and reports from the French Geological Survey (BRGM). These data provide a framework of flood impacts from the 14th century onwards, allowing comparison with the sedimentological palaeoflood record.

## 4.4. Paleoflood reconstruction

### 4.4.1. Independent dual-approach strategy

Following Benito et al. (2022), two independent approaches were adopted to estimate palaeodischarges associated with the observed deposits. These approaches are conceptually distinct and do not aim to define a minimum–maximum envelopes. They do not rely on any logarithmic or empirical relationship between maximum particle size (MPS) and deposit thickness but instead provide two complementary estimates
of palaeoflow.

### (1) Competence-based approach:

The first approach is based on the maximum particle size (MPS) preserved within each deposit. The diameter of the coarsest transported clasts is used to infer the shear stress required for their mobilisation, which in turn provides a first-order estimate of the flow depth and discharge at the time of deposition (Costa, 1983; Benito et al., 2022).

### (2) Deposit-thickness approach:

The second approach uses the preserved deposit thickness as an indicator of the minimum flow depth. This relies the assumption that the deposits were emplaced rapidly, during a single extreme event, and that

they temporarily preserved the channel geometry at the moment of deposition. This allows a second independent estimation of palaeodischarge based on measured thicknesses of flood-related sediment packages.

Two assumptions accompany both approaches:

#### (i) Flow duration and flow conditions.

The reconstructions focus on the main body of the flood (lasting on the order of a day) which exerts the dominant control on sediment transport, rather than on short-lived rising limbs. The methods therefore use flow conditions averaged over the duration of the event, consistent with standard palaeohydrological practice.

#### (ii) Morphometric parameters.

<caption>**Table 3**</caption>

Lithofacies and sedimentary structures observed on outcrops, and interpreted depositional and environmental processes. Lithofacies codes are based on Miall (1996) modified from Miall (1977).

<table><thead><tr><th>Lithofacies Code</th><th>Lithology</th><th>Sedimentary structures</th><th>Dimensions</th><th>Depositional processes</th><th>Environment interpretation</th></tr></thead><tbody><tr><td>Gmm</td><td>Disorganized, clast-supported to matrix-supported, medium to coarse sand to pebble matrix, clast up to > 1m, angular to sub-angular clasts, very poorly sorted to poorly sorted</td><td>Ungraded, massive, no imbrication or sporadic imbrication, erosional base, a-axes clasts orientation parallel to the flow direction</td><td>0,3m-1m thick interbedded, can pass laterally to stratified sand/silty deposits of 5 cm-10 cm, 2-5 m lateral extent</td><td>En masse deposit, non-newtonian fluid and laminar flow (Hampton, 1972; Lowe, 1982)<br>Debris flow (Costa, 1984; Hampton, 1979; Iverson, 1997; Middleton and Hampton, 1973; Mulder and Cochonat, 1996)</td><td>Deconfined environnement and/or decrease of slope</td></tr><tr><td>GCm</td><td>Cobbles to boulders conglomerate, sub-rounded to rounded, clast-supported, coarse sand to pebbles matrix inter-clasts, maximum particle size reach 1m to 2 m, poorly to moderate sorted</td><td>Crudely horizontal bedding, massive, structureless, ungraded to normal grading, moderate to strong imbrications, imbrication dip angle ranging between 20 and 30°, most imbricated clasts dipping downstream, erosional base</td><td>1 m to 2m thick, 10 m-100 m long</td><td>Bedload, high sediment concentration and high discharges (Allen, 1981; Heinz et al., 2003; Jo et al., 1997; Miall, 1996; Todd, 1989)</td><td>Channel in-filling</td></tr><tr><td>GPc</td><td>Medium to very coarse pebbles with outsizing clasts up to cobbles/boulders, clast-supported, sub-angular to sub-rounded, poorly to moderately sorted</td><td>Crudely horizontal bedding to bedding, ungraded, moderate to strong imbrication, scour-filling</td><td>1-3 m thick and length ranging between 1m and 10 m</td><td>Bedload transport (Miall, 1977, 1996)</td><td>Longitudinal bedform</td></tr><tr><td>Sp</td><td>Fine to coarse sand, well to very well sorted</td><td>Horizontal bedding, tabular cross-stratification, sharp contact at the base, cross-stratification dip >20°</td><td>Cross stratification 20-40 cm thick, several m long</td><td>Bedload, lower-flow regimes, lateral accretion and migration of ripples (Ashley, 1990; Miall, 1996; Venditti, 2007)</td><td>Channel in-filling</td></tr><tr><td>Sr</td><td>Fine to coarse sand, well to very well sorted</td><td>Horizontal bedding, faint to marked ripples and lateral transition to planar laminae, crudely to marked load coast, distinct contact at the top and base</td><td>20-30 cm thick, 10-30 m long, wavelength ~30-50 cm, amplitude ~ 2-5 cm</td><td>Bedload (traction carpet, saltation), lower-flow regime (Ashley, 1990; De Cala et al., 2020; Harms, 1979)</td><td>Unconfined channel</td></tr><tr><td>Sh</td><td>Fine to coarse sand, well to very well sorted</td><td>Horizontal bedding and stratifications, planar laminae,</td><td>20-50 cm thick, 100 m length</td><td>Bedload (traction carpet, saltation) and/or upper-flow regime (Ashley, 1990; De Cala et al., 2020; Harms, 1979)</td><td>Sandy bedforms within the channel</td></tr><tr><td>Fm</td><td>Clay to silt, very poorly sorted with outsizing clast up to cobbles of 10 cm</td><td>Horizontal bedding, massive, plane to undulated base</td><td>20 cm-70 cm</td><td>Suspended transport, low energy, decantation (Miall, 1996)</td><td>Floodplain</td></tr><tr><td>Fh</td><td>Silt to sand matrix, matrix-supported, outsizing calibre of cobbles</td><td>Horizontal bedding, no stratification, presence of weak to moderate cobbles imbrications, roots traces</td><td>1 m long to several metres long and 30-50 cm thick</td><td>Finer part of gravels bars, low water velocity (Legout et al., 2018) obstruction deposits (Vargas-Luna et al., 2015; Wu et al., 2020)</td><td>Top bar</td></tr><tr><td>P</td><td>Clay to sandy matrix, angular to sub angular fine to medium pebbles</td><td>Horizontal bedding, dark colour, rich in organic matter, archaeological assets</td><td>20 cm-70 cm thick, several metres to 50 m length</td><td>Buried old soils or by subsequent sedimentation (Orr and Roberts, 2024; Tabor and Myers, 2015)</td><td>Paleosoil and anthropogenic soil</td></tr><tr><td>C</td><td>Pebbles to cobbles, matrix interclasts is coarse sand to granules, clast-supported, angular, partially to openwork texture, poorly sorted</td><td>Massive, structureless, disorganized. Presence of sliding/rolling fabric, distinct horizontal bedding to absence of bedding</td><td>Thick range from 15 cm to several metres height and decrease from the upslope to the downslope</td><td>Sliding to rolling, rock fall (Blikra and Nemec, 1998; Cruden and Varnes, 1996; Hungr et al., 2001, 2014)</td><td>Colluvium</td></tr></tbody></table>
To estimate the paleo-hydraulic characteristics of the channels, present-day morphometric parameters (channel slope and width) are used as a first-order approximation of Holocene conditions. This assumption is made cautiously, acknowledging that extreme events – such as Storm Alex – can induce significant short-term widening, aggradation and scour in steep Alpine streams. Channel slope and width were extracted from pre-flood and post-flood orthophotos (IGN, 2020) and from digital elevation models (DEMs) acquired before and after Storm Alex (IGN, 2023). Average tributaries width ranges from 5 m (pre-flood) to 30 m (post-flood), while the Roya River varies from 16 m (pre-flood) to 40 m (post-flood). Extracted channel slopes range from 0.04 % to 0.12 % (Kerverdo et al., submitted).

### 4.4.2. Normalisation

All discharge estimates were normalised as percentages of Storm Alex reference discharge (RDSA) for comparative purposes (Borga et al., 2007; Brigode, 2021). This normalisation facilitates direct comparison between palaeoevents and the 2020 flood, and helps identify past events with hydraulic and sedimentological signatures comparable to those of Storm Alex. The mean discharge associated with Storm Alex (RDSA) ranges between approximately 15 and 1800 m³/s across the studied catchments (see Supplementary Table S3 for individual values).

## 5. Results

Most of the recorded lithofacies and stratigraphic logs are located in the lower reaches of the tributaries or near their confluences (Fig. 2). In the following sections, we will describe the dating and the lithofacies of the selected stratigraphics logs.

### 5.1. Radiocarbon and OSL dating

A total of 22 ages (14 flood-related, 8 anthropogenic) were obtained from ten stratigraphic sections (Table 4).

Five ¹⁴C dates were excluded as reworked or stratigraphically inconsistent; the remaining ages, together with the OSL results, define the chronology of flood and anthropogenic layers. Calibrated age ranges

(2σ) and analytical details (water content, dose rates, equivalent doses) are summarised in Table 4, with anthropogenic levels highlighted in green and flood deposits in blue. All retained dates underpin the reconstruction of Holocene flood frequency and human activity in the Roya Valley.

### 5.2. Lithofacies

Ten primary lithofacies are identified within the Holocene alluvial and torrential terraces of the Roya Valley. Each facies is defined by its grain-size, sorting, sedimentary structures (e.g., grading, imbrication, bedding geometry) and inferred depositional process. A full description of codes (Miall, 1977, 1996), sedimentological characteristics, dimensional ranges and environmental interpretations is provided in Table 3 (cf. Figs. 3–8 for facies examples). Briefly:

- Gmm (Gravel massive): massive, poorly sorted sand–pebble matrix with rare imbrication-interpreted as debris-flow deposits in confined channels.

- GCm (Gravel and Cobble): clast-supported cobble–boulder conglomerate with moderate imbrication-channel-infill bedload.

- GPc (Gravel and Pebble): medium-to-very-coarse pebble lag with scour-fill structures-longitudinal bedforms.

- Sp, Sr, Sh, (Sand): sandy facies ranging from planar cross-stratified beds (Sp) through rippled sands (Sr) to horizontally laminated sands (Sh) - traction-transport and dune migration.

- Fm, Fh (Fine): fine-grained overbank or top-bar deposits (clay–silt to silt–sand) formed by suspension settling under low-energy conditions.

- P (Paleosol): organic-rich palaeosols or occupation levels.

- C(Colluvial): matrix-supported angular pebble–cobble layers-colluvial inputs from adjacent slopes.

These facies associations form the basis for subdividing the logs into palaeoflood sequences (see Section VI).

<caption>**Table 4**</caption>

OSL and¹⁴C dating. For OSL: WC is water content in %, DL is Dose Rate (mGy/a), De is Equivalent Dose (Gy) and Age expressed in BP with associated error. For¹⁴C: Radiocarbon's dates sample from stratigraphic logs sample in the Roya valley and compiled from the studies of Marchiel (2021) and Fouache et al. (2022). Green dates correspond to anthropogenic layers, blue layers to flood deposits. See text for explanation, description of samples and calibration.

<table><thead><tr><th colspan="8">OSL dating</th></tr><tr><th>Logs</th><th>Samples</th><th>Localisation</th><th>Depth (m)</th><th>WC(%)</th><th>DL (mGy/a)</th><th>De (Gy)</th><th>Age (Years)</th></tr></thead><tbody><tr><td>C2M</td><td>OSLR1</td><td>43°59'52.54"N 7°30'32.48"E</td><td>0,75</td><td>5,3</td><td>3,76 ± 0,38</td><td>25,6 ± 2,44</td><td>6810 ± 700</td></tr><tr><td>C1M</td><td>OSLR3</td><td>43°59'34.12"N 7°32'46.13"E</td><td>0,75</td><td>18,8</td><td>2,74 ± 0,35</td><td>8,5 ± 1,05</td><td>3110 ± 400</td></tr><tr><td>R1M</td><td>OSLR4</td><td>43°56'23.37"N 7°31'2.66"E</td><td>2</td><td>3</td><td>2,09 ± 0,12</td><td>0,64 ± 0,03</td><td>300 ± 30</td></tr><tr><td>V1M</td><td>OSLR5</td><td>44°6'38.46"N 7°33'50.46"E</td><td>1,4</td><td>10,2</td><td>1,45 ± 0,36</td><td>2,77 ± 0,369</td><td>1920 ± 490</td></tr></tbody><thead><tr><th colspan="8">¹⁴C dating</th></tr><tr><th>Logs</th><th>Samples</th><th>Localisation</th><th>Material dated</th><th>¹⁴C yr BP</th><th>Calibrated yr BP (2σ)</th><th></th><th></th></tr></thead><tbody><tr><td>C1M</td><td>C14R4</td><td>43°59'34.12"N 7°32'46.13"E</td><td>Remaining organic matter</td><td>210 ± 30</td><td>308 - ?*</td><td></td><td></td></tr><tr><td>C2M</td><td>C14R3</td><td>43°59'52.54"N 7°30'32.48"E</td><td>Remaining organic matter</td><td>370 ± 30</td><td>500-316</td><td></td><td></td></tr><tr><td>C2M</td><td>C14R1</td><td>43°59'52.54"N 7°30'32.48"E</td><td>Remaining organic matter</td><td>480 ± 30</td><td>544-495</td><td></td><td></td></tr><tr><td>B1M</td><td>C14R9</td><td>44° 3'14.46"N 7°35'8.68"E</td><td>Remaining organic matter</td><td>110 ± 30</td><td>269-12*</td><td></td><td></td></tr><tr><td>V1M</td><td>C14R5</td><td>44° 6'38.46"N 7°33'50.46"E</td><td>Remaining organic matter</td><td>1500 ± 30</td><td>1467 - 1307</td><td></td><td></td></tr><tr><td>V1M</td><td>C14R6</td><td>44° 6'38.46"N 7°33'50.46"E</td><td>Remaining organic matter</td><td>920 ± 30</td><td>915-741</td><td></td><td></td></tr><tr><td>V1M</td><td>C14R7</td><td>44° 6'38.46"N 7°33'50.46"E</td><td>Remaining organic matter</td><td>760 ± 30</td><td>728-665</td><td></td><td></td></tr><tr><td>V1M</td><td>C14R8</td><td>44° 6'38.46"N 7°33'50.46"E</td><td>Remaining organic matter</td><td>480 ± 30</td><td>544-495</td><td></td><td></td></tr><tr><td>B1K</td><td>P1</td><td>44° 3'12.64"N 7°35'5.51"E</td><td>Remaining organic matter</td><td>3387 ± 72</td><td>3829 - 3459</td><td></td><td></td></tr><tr><td>L1K</td><td>P2</td><td>43°56'21.14"N 7°30'11.40"E</td><td>Remaining organic matter</td><td>8433 ± 94</td><td>9549 - 9138</td><td></td><td></td></tr><tr><td>L1K</td><td>P3</td><td>43°56'21.14"N 7°30'11.40"E</td><td>Remaining organic matter</td><td>10 387 ± 108</td><td>12 621 - 11 834</td><td></td><td></td></tr><tr><td>Co1K</td><td>P6</td><td>44° 6'11.45"N 7°33'54.30"E</td><td>Charcoal</td><td>393 ± 63</td><td>524-311</td><td></td><td></td></tr><tr><td>Co1K</td><td>P9</td><td>44° 6'11.45"N 7°33'54.30"E</td><td>Remaining organic matter</td><td>427 ± 64</td><td>545-314</td><td></td><td></td></tr><tr><td>Co2K</td><td>P10</td><td>44° 5'57.35"N 7°33'42.65"E</td><td>Charcoal</td><td>449 ± 65</td><td>624-316</td><td></td><td></td></tr><tr><td>D3K'</td><td>P12</td><td>44° 6'37.73"N 7°33'21.91"E</td><td>Remaining organic matter</td><td>936 ± 64</td><td>957-726</td><td></td><td></td></tr><tr><td>C5K</td><td>P13</td><td>43°59'37.04"N 7°32'11.39"E</td><td>Remaining organic matter</td><td>3183 ± 72</td><td>3565 - 3224</td><td></td><td></td></tr><tr><td>C5K</td><td>P14</td><td>43°59'37.04"N 7°32'11.39"E</td><td>Remaining organic matter</td><td>3861 ± 70</td><td>4513 - 4085</td><td></td><td></td></tr><tr><td>C5K</td><td>P15</td><td>43°59'37.04"N 7°32'11.39"E</td><td>Remaining organic matter</td><td>1258 ± 64</td><td>1298 - 1006</td><td></td><td></td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>Fig. 3. Panorama and stratigraphic L1K log comprising six interpreted flood sequences with two $^{14}$C ages. A) correspond to the picture of the studied outcrop, B) Interpreted section with the associated log on the right panel. Facies codes: GCm (massive cobble–boulder conglomerate), Sr (through rippled sands), Sh (horizontally laminated sands), Fm (floodplain fines), C (colluvium). Facies codes in Table 3.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 4. Stratigraphic logs of FA1 located Caïros (C2M) catchments. On the left panel, picture of the outcrop and interpreted log on the right panel. C2M comprises two sequences with two $^{14}$C ages and one OSL. Facies codes: GCm (massive cobble–boulder conglomerate), Sh (horizontally laminated sands), Fm (floodplain fines). Facies codes in Table 3.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 5.** A) Panorama C5K on the Caïros River, showing five synthetic logs (i–v). Seven flood sequences were identified -only sequences C FL 4, C FL 5 and C FL 6 in logs ii and v have ¹⁴C age ranges.</figcaption></figure>

## 5.3. Quantitative analyses

### 5.3.1. Bed-thickness and maximum particle sizes (MPS)

The Caïros and Bieugne catchments exhibit the coarsest particles observed. The largest diameter clasts correspond to boulders, with approximately 0.5 m in the Lavina torrent (Gmm facies) and up to 1m in Caïros deposits at C2M (Gmm facies) and C5K (GCm facies), as well as in the Bieugne torrent at B2K (GCm). These coarse deposits are associated with bed thicknesses averaging between 1.5 and 2 m. In several beds-Viévola (V1K), Bieugne (B1M, B1K), Caïros (C1M), and the Roya River (R1M)-maximum particle sizes are represented by cobbles and pebbles. Floodplain deposits are more developed in Viévola (V1K), Caïros (C1M, C2M), and the Lavina torrent (L1K), with an increase in fine-grained facies (Fm, Fh) over time, corresponding to a decrease in the thickness of high-magnitude deposits (Gmm, GCm).

These observations suggest that boulders are a good reliable indicator of major hydro-sedimentary events, due to both their size and the associated bed thickness. In contrast, deposits composed of cobbles to coarse sand in high-gradient environments appear to be less straightforward markers for identifying major floods, like in the Bieugne sections (B1K, B2K).

### 5.3.2. Paleohydraulic discharges

Value of peak palaeodischarge are estimated through the methods outlined in Section IV. All discharge estimates were normalised as percentages of Storm Alex peak discharge (Brigode, 2021). Hereafter, the acronym RDSA is used for the reference range of discharge associated with Storm Alex. These values are estimated at the outlet of the studied catchments for direct comparison (Supplementary Table S3).

The estimated peak discharge based on the MPS approach range between 0.1 and 280m³/s, corresponding to a 0–139 % of RDSA. The

estimations based on the bed thickness range between 0.1 and 184 m³/s and corresponds to 0–77 % of RDSA. Data analysis allows the distinction of three classes of flood intensities (based on the lower envelope, i.e, width = 5m): negligible events (8 layers <1 % RDSA), low-intensity events (1 % < 9 layers<22 % RDSA), and moderate to high events (26 % < 4 critical layers<77 %). The negligible events correspond primarily to small Gmm beds located in Lavina and one Caïros site. The low-intensity events correspond to Sr, Sh and minor Gmm/GPc facies, mainly observed in Lavina, Bieugne and Caïros. The moderate-to-high events correspond to 4 layers (GCm) in Caïros and Lavina, reflecting extreme floods comparable to large historical events. Uncertainties stemming from assumed channel width, slope, bed rugosity or particle size reflecting competence hypothesis are indicated by discharge ranges in supplementary data. The coarsest facies and highest MPS values were consistently found in Caïros and Lavina sections, suggesting high sensitivity of these subcatchments to extreme floods.

## 5.4. Historical floods in the Roya valley

A comprehensive list of 33 documented floods between 1338 AD and 2020 AD is presented in supplementary data, based on departmental archives (Département des Alpes-Maritimes), local association records (ASPB, 2022) and BRGM technical reports (BRGM, 2005). Key events include:

- September 21, 1338: Destruction of Pont Sélés at Breil-sur-Roya, first recorded flood (Botton, 1996).

- 1527: Major valley-wide flood causing widespread damage (ASPB, 2022).

- November 3rd, 1538: Levensa tributary flood inundated ~60 houses in La Brigue (ASPB, 2022).
<figure><img src="image_7.png" /><figcaption>**Fig. 6.** Stratigraphic logs FA2 at (i) C1M (confluence Caïros-Roya), (ii) B1M (Bieugne torrent) and (iii) R1M (Roya River). Four ¹⁴C dates constrain one to two flood sequences per log. Facies codes: GCm (massive cobble-boulder conglomerate), GPc (massive pebbles-cobbles conglomerate), Sp (planar cross-stratified sands), Fm (floodplain fines), C (colluvium). Facies codes in Table 3.</figcaption></figure>

- October 12, 1836: Exceptional flood and debris flows in Breil-sur-Roya and Lavina torrent, high-impact event (ASPB, 2022; Botton, 1996).

- November 21, 1926: Reference flood with estimated peak discharge of 900 m³/s at Breil-sur-Roya (BRGM, 2005).

- November 4, 1994: Semi-major flood destroying the bridge at Saint-Dalmas-de-Tende; peak flow ~576 m³/s (BRGM, 2005).

- October 2, 2020 (Storm Alex): Catastrophic flood with peak discharges of 1100–1800 m³/s, unprecedented in historical records (Pons et al., 2024; ONF-RTM, ONF-DRN, INRAE-ETNA, 2023).

Between 1338 and 1703, only five floods are documented, reflecting limited record-keeping. The opening of the commercial road (Route du Sel) and increased settlement led to more frequent entries in departmental and BRGM archives. This historical dataset provides a critical benchmark for validating this palaeoflood reconstructions, highlighting both the rarity of extreme floods prior to the instrumental era and the marked increase in reported events since the 18th century.

## 6. Depositional environments

Based on the ten lithofacies in Table 3, four recurring facies associations (FAs) are identified based on flow processes and magnitude across all outcrops (Figs. 3–9). These associations reflect distinct process regimes rather than specific locations. Calibrated ages (cal BP) are cited; full analytical details are in Table 4.

**FA1 (Gmm - Sr - Sh - Fm - C).**

**Spatial context:** FA1 (Fig. 3) is defined with the reference outcrop (L1K) on Lavina torrent. Additional other representative sections

include L1K (left bank of Lavina torrent) and C2M (right bank of the Caïros river).

**Reference outcrops:** Lavina torrent (L1K; Fig. 3).

**Facies assemblage:** Gmm - Sr - Fm - Sh - Gmm - Fm - C. Similar FA1 sequences are also observed in the Caïros river (C2M) where a basal Gmm dates to 7510–6110 cal BP (Fig. 4).

**Description (Lavina - Fig. 3):** A ~3 m sequence begins with a thick debris-flow deposit (Gmm), followed by a 20 cm thick Sr, a thin overbank clay-silt layer (Fm), interbedded with planar sands (Sr/Sh) and a beginning of successive debris flows (12 621–11 834 cal BP). Multiple Gmm indicates three major pulses, with colluvial cobble-pebble deposits (C, 9549 - 9138 cal BP) marking the late phase.

**Interpretation:** The sequence reflects repeated high-energy flood events, represented by (Sm) and (Sr) flood deposits, as well as pulses of debris flows indicated by (Gmm). These high-magnitude deposits are interbedded with (Fm) floodplain sediments, corresponding to lower-magnitude events.

**Description (Caïros - Fig. 4):** A 2 m terrace section comprises Gmm-Sh-Fm-Sh-Fm-Sh. The basal debris-flow (Gmm; 7510–6110 cal BP) gives way to coarse sand bedforms (Sm) and overbank fines (Fm) dated 544–316 cal BP.

**Interpretation:** This high magnitude section is characterised by the infilling of (Fm) deposits following an erosive event recorded by (Gmm) and (Sm) deposits.

**Interpretation FA1:** All three sites record repeated debris flow events separated by low energy overbank deposition and soil formation, reflecting episodic torrential pulses through the Holocene.

**FA2 (GCm - GPc - Sp - Fm - Fh - C).**

**Spatial context:** FA2 is defined primarily on the left bank of the Caïros
<figure><img src="image_8.png" /><figcaption>**Fig. 7.** Panorama and logs on the Bieugne fan: A) Panoramic view of logs B1K (left) and B2K. (right). One ¹⁴C dating was carried out on a sandy level. B) Zoom on Sr and Sh succession in B1K. C) Close up of GCm at B2K. Facies codes: GCm (massive cobble-boulder conglomerate), GPc (massive pebbles-cobbles conglomerate), Sr (through rippled sands), Sh (horizontally laminated sands), C (colluvium). Facies codes in Table 3.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 8.** Panorama and stratigraphic log V1M located on the Viévola fan. Five flood sequences are distinguished and four were dated by ¹⁴C (from 1467 – 1307 cal. BP) and one by OSL (2410–1430 cal. BP). On the left panel, colluvium cover the base of the section. Picture on the right corresponds to the fresh section without colluvium. Facies codes: Gmm (gravel massivz conglomerate), GPc (massive pebbles-cobbles conglomerate), Sh (horizontally laminated sands), C (colluvium), P (paleosols). Facies codes in Table 3.</figcaption></figure>

River. The reference section is the C5K panorama (~100 m length; Fig. 5), where five synthetic logs (i–v) were surveyed. Additional representative logs at C1M (200 m upstream of the Roya confluence) and R1M (left bank of the Roya, just before Breil-sur-Roya; Fig. 6) confirm FA2 architecture.

*Reference outcrops:* Caïros river (C5K; Fig. 5).

*Facies assemblage:* Gcm-GPc-Sp-Fm-Fh.

**Description (C5K – Fig. 5):** GCM dominates the basal aggradation in logs i–v, with ages 3565–3224 cal BP (log ii) and 4513–4085 cal BP (Sp in log v). GPc appears as scour-fills overlain by planar cross-stratified sands (Sp). Sp in log v (4513–4085 cal BP) records waning traction currents.

Fm, Fh occur as overbank fines and top-bar layers (e.g. Fh at 1298–1006 cal BP). Individual flood sequences are delimited by Fh layers within GCm beds, yielding two episodes in logs ii–iv and three in log v (GCm-GPc-Sp; GCm; GCm-GPc).

**Interpretation:** High magnitude floods sequences (GCm) marked by intense bedload transport and punctuated by minor floods (GPc, Sp) and related very low magnitude floods (Fh).

**Description (C1M & R1M – Fig. 6):** At C1M (Fig. 6), a 2 m terrace comprises three beds: basal GPc (3110–2710 cal BP), a middle GCm, and upper overbank fines (210–0 cal BP). At R1M, a Sp-Fh-GCm-C succession on the Roya's left bank is capped by colluvium (C). The Sp bed yields an OSL age of 330 – 270 cal BP. Both sites exhibit channel-infill gravels followed by waning-flow sands and overbank silts.

**Interpretation:** FA2 records a series of high-magnitude floods depositing coarse gravels in channel cores, with subsequent waning stages marked by planar sands and overbank fines. This architecture is consistent across C5K, C1M and R1M, from mid-Holocene to Medieval times.

### FA3 (GCm - GPc - Sr - Sh).

**Spatial context:** FA3 is defined with the reference's sections B1K and B2K, located on the left bank of the Bieugne torrent. These outcrops correspond to two stratigraphic logs of 50 m length and 7 m height outcrop (Fig. 7).

*Reference outcrops:* B1K and B2K on Bieugne fan (Fig. 7).

*Facies assemblage:* Gcm-Sr-Sh.

**Description:** Basal massive conglomerate (GCm; 3829–3459 cal BP at B2K) overlain by rippled sands (Sr) and laminated sands (Sh) of several metres thick, indicating bar-scale deposition under waning flows.

**Interpretation:** Deposition within a braided channel, with downstream fining reflecting energy dissipation. Absence of floodplain deposists suggest that B1K and B2K correspond to the active channel of the Bieugne torrent. Coarse fraction corresponding to important sedimentary supply made available by the retreat glacier and subsequently transported by the Bieugne torrent.

### FA4 (Gmm - Fm - P - C).

**Spatial context:** FA4 is defined across two catchments located in the upper Roya Valley. In particular, the Vievola fan and Dente catchment (V1M, D3K, Co1K, Figs. 8 and 9), where at the apex of the Vievola fan, a narrow section reveals a succession of multiple paleosols (Co1K, Fig. 9A and B). On the alluvial fan, a section of 2.5 m high reveals minor floods surging overlying a thick paleosol (V1M, Fig. 8). In Consciente valley, were anthropogenic terraces eroded by Storm Alex reveal a 3 m high section with a kiln furnace (D3K, Fig. 8C and D).

*Reference sites:* Viévola fan (V1M). Similar outcrops occur in the Consciente and Dente catchments (Fig. 8).

*Facies assemblage:* Gmm - Fm - P - C.

- **Description:** A debris-flow base (Gmm; 2410–1430 cal BP) overlain by an organic paleosol (P; 1467–1307 cal BP), then alternating colluvium deposits (C) and multiple fines layers (Fm).

- **Interpretation:** High energy-episod capped by anthropogenic(s) soil (s). The colluvial deposits are explained by the presence of anthropogenic terraces, which promoted the accumulation of reworked material at the foot of the slope (see supplementary data).

**Description (Co1K):** Organic-rich palaeosols (P) with charcoal (524 – 311 cal BP) within kiln structures, underlain by colluvial cobble-pebble layers (C) and overlain by overbank fines (Fm at 545–314 cal BP) (Fig. 9A and B).

**Interpretation:** Medieval to Early Modern human land use (kiln
<figure><img src="image_10.png" /><figcaption>**Fig. 9.** A, B) Lime-kiln remnants (Co1K) stripped by Storm Alex and his interpretation. C, D) Dente torrent paleosols without overlying flood deposits. Three $^{14}$C datings are available over the two sections: two on Co1K and one on P2 (panel D). Facies codes: P (paleosols), facies code is in Table 3.</figcaption></figure>
activity, terrace agriculture) punctuated by low-energy floodplain and slope-wash deposition. The absence of overlying floodbeds on some palaeosols suggests limited preservation or lateral sourcing of later floods.

*Description (D3K)*: The section exposed by Storm Alex, reveals a ~3-m-high sequence composed of four successive paleosols (P1–P4). The second paleosol (P2) yielded a radiocarbon date of 957–726 cal BP. Notably, no flood deposits overlie these paleosols (Fig. 9C and D).

*Interpretation*: Beginning in the Medieval period, this sequence reflects a relatively calm phase, with no preserved flood deposits and minimal evidence of slope-wash processes. The dated horizon, enriched in organic matter, suggests land use and anthropogenic influence during this period.

These four FAs encompass the full range of processes from high-energy debris flows (Gmm, V1M) to low-energy suspension settling and soil formation, illustrating the complex interplay of climatic forcing

and human impact in the Roya Valley during the Holocene.

## 7. Discussion

This study reconstructs, for the first time, the Holocene flood history of the Roya Valley by integrating sedimentological logging, radiocarbon and OSL dating with historical flood records. Our key findings are: (i) the identification of five discrete preservation intervals ("Ro-stages") that structure the Holocene flood archive; and (ii) evidence that both climatic oscillations and human land use have modulated flood frequency and sediment preservation. Below, we place these results in a broader palaeoclimate and geomorphological context, discuss the main limitations, and outline perspectives for future work.

<figure><img src="image_11.png" /><figcaption>**Fig. 10.** Probability density functions of flood deposits dated by carbon-14 (orange) and OSL (green) from the Younger Dryas to present. Marker shapes correspond to the MPS range (as in Fig. 9), with right-axis showing particle size. Lower panel zooms on the last 1600 cal yr BP: green dots for occupation probabilities (palaeosols), blue curve for flood density. Historical floods (see supplementary data) are superimposed. MCA: Medieval Climate Anomaly; Ro: Roya preservation interval. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
## 7.1. Five Ro-stages: a framework for holocene hazard

We define five main periods of enhanced deposit preservation in the Roya valley (Fig. 10):

* Ro1 (12 621–9138 cal BP) and Ro2 (7200–6400 cal BP), consistent with regional studies documenting high sediment yields during deglaciation (Bonneau et al., 2017; Mariotti et al., 2021).

* Ro3 (4513–4085 yrs cal BP) and Ro4 (3829–2710 yrs cal BP) are characterised by metre-scale debris-flow beds and high palaeodischarges (Supplementary Table S3), notably at C5K in the Caïros catchment. These clusters likely relate to the 4.2 ka BP climatic perturbation and subsequent reorganisation of the hydrological regime (Magny et al., 2009).

* Ro5 (2320–12 yrs cal BP) comprises finer overbank sediments and palaeosols associated with the late Holocene, including the Medieval Climate Anomaly (MCA) and Little Ice Age (LIA), and intensified human activity. Within Ro5, two sub-intervals emerge over the ~1600 years (Fig. 10):

* Sub-period 1: ~1000–1300 cal BP (MCA)

* Sub-period 2: ~150–550 cal BP (LIA)

Flood-probability density curves remain low overall (0.01–0.035) due to limited number of dated beds, but show a clear peak during sub-period 2, consistent with wetter LIA conditions (Wilhelm et al., 2022, 2022, 2022). Paleosols-probability show an increase of anthropics layers during the MCA. The historical record of 33 documented floods (e.g. 1527, 1836, 1926, 2020; Table S1) falls entirely within Ro5. From ~3000 cal BP onwards, human land use (FA4) introduced deforestation, terracing and soil formation, further restricting the preservation of flood deposits to specific geomorphic domains (terraces, fans, confined reaches).

The Ro-stages correlate well with regional flood-prone phases: the 4.3–3.8 ka event (Magny et al., 2009), the mid-Holocene lake flood peaks (Magny et al., 2011; Wirth et al., 2013), and documented Southern Alps flood intervals (Benito et al., 2015) (Fig. 10). The synchrony of coarse deposits in the Bieugne and Caïros catchments during Ro3–Ro4 suggests a dominant climatic forcing - most likely enhanced precipitation and storminess - superimposed on local geomorphic controls. While maximum particle size (MPS) can reflect high shear-stress conditions within a flood, and bed thickness may provide qualitative information on event magnitude, in steep Alpine catchments these relations are non-unique. We therefore use MPS and preserved thickness only as complementary indicators within two independent palaeodischarge estimations, without assuming a direct or empirical relationship.

Extreme rainfall episodes tend to occur during periods of climatic change toward wetter conditions, associated with changes in atmospheric circulation and aeorosol forcing (Coumou and Rahmstorf, 2012; IPCC, 2023; Rush et al., 2021). Such events can mobilise a year's worth of sediment within hours (McLeod et al., 2024), after which sediment is progressively reworked. In the Roya Valley, these phases are followed by vertical fining of grain-size distributions and shifts in facies assemblages, which may serve as indicators of past high-energy episodes (Armitage et al., 2011; Whittaker et al., 2011).

For example, the Lavina torrent shows clear upward-fining phases:

* phase 1: Gmm - Sr - Fh,

* phase 2: Sh/Fm - Gmm - Sr - Gmm - Fh - C.

Similar successions occur in the Caïros tributaries (C5K, C1M, C2M). Periods of lower-magnitude floods in these steep catchments are marked by well-developed floodplain deposits and a progressive increase in their thickness.

In contrast, the Viévola and Consciente catchments record long-lived palaeosols and colluvial units with limited evidence for large floods since ~1500 cal BP (V1M, D3'K, Co1K). Conversely, the Bieugne

tributary preserves thick, cobble-rich beds dated to ~100 ± 30 cal BP to present, likely corresponding to more intense flood episodes with high discharges and sediment loads. These contrasts can be explained by the interplay between catchment morphometry, sediment supply and highly localised rainfall patterns.

## 7.2. Natural climate variability and human impact

### 7.2.1. Pollen and sediments records from Mercantour lakes

Pollen and palaeoecological records from the Mercantour massifs (e. g. Vallée des Merveilles, Lac Long Supérieur, Lac des Moutons) document strong climate variability and evolving land use throughout the Holocene (Finsinger, 2001; Finsinger et al., 2021; Kharbouch, 1996, 2000; Ponel et al., 2001). During the Younger Dryas, cold conditions prevailed, followed by the Preboreal–Boreal transition characterised by open environments and a progressive rise of the treeline. From the Older Dryas to the Preboreal, climatic improvement is marked by expansion of Pinus, upward migration of forest limits and peat-bog development, punctuated by short-lived cooling pulses.

During the Boreal–Subboreal transition, landscapes remained semi-open, whereas the Subboreal–Subatlantic shift was associated with a drying trend and coarser littoral deposits at Lac Long Supérieur (Kharbouch, 1996). From ~3000 cal BP onwards, peat and diatom records from Lac des Moutons indicate a decline in Abies and Larix in favour of herbaceous taxa, suggesting renewed climatic stability but also increased human pressure. In the Subatlantic, both wetter conditions and intensified pastoral activity are inferred (Kharbouch, 2000). These records place Ro1–Ro2 within the Preboreal to Subboreal interval, when deglaciation, high sediment supply and expanding vegetation cover coexisted. Ro3–Ro4 coincide with the transition from Subboreal to early Subatlantic conditions and the 4.2 ka event (Brisset et al., 2013; Magny et al., 2009), at a time of enhanced hydrological variability and increasing human presence. Ro5 spans the late Subatlantic, when cooler climate phases (e.g. LIA) and anthropogenic land use jointly shaped the sedimentological response of the catchment.

Additional lacustrine archives in the region show comparable trends. Lake Allos, ~70 km east of the Roya, displays low flood frequency between 7000 and 1700 cal BP, with pronounced peaks at 1200–1850 and 270–800 cal BP, corresponding to late Antiquity and the LIA (Brisset et al., 2017; Le Roy et al., 2015; Magny et al., 2002). Lake Petit (~60 km from the Mediterranean Sea) records a diatom-inferred shift at ~4200 cal BP from stable, low-erosion conditions to enhanced detrital input linked to the 4.2 ka event (Cartier et al., 2018; Magny et al., 2009), followed by sustained pasture erosion and progressive from ~3000 cal BP and progressive acidification from 2400 cal. BP to present (Brisset et al., 2013).

Taken together, these lacustrine archives indicate relative climatic stability between ~8000 and 4200 cal BP, followed by heightened hydro-sedimentary activity during and after the 4.2 ka event. This pattern corroborates our Ro3–Ro4 flood clusters and supports the interpretation of Ro5 as a finer-grained, human-influenced signal associated with late Holocene cooling and increased land use.

### 7.2.2. Regional holocene climate forcing

Despite dating uncertainties, the timing of the Roya flood-preservation stages (particularly Ro3–Ro5) broadly matches known phases of increased flood activity in the European Alps and western Mediterranean. Multi-millennial lake records show that flood frequency tends to rise during cooler, wetter phases, often associated with negative NAO conditions that favour the southward displacement of the westernerlies over eastern Iberia, southern France and southern Italy (Benito et al., 2015; Wirth et al., 2013). Volcanic forcing and changes in total solar irradiance have also been proposed as additional controls on the frequency of extreme events (Starkel, 1999). In this context, the Roya sedimentary archive appears sensitive to the same large-scale palaeoclimatic forcing that governs Holocene flood dynamics across the wider
Alpine region.

Stacked flood compilations from the Alps show distinct peaks at 10 000–8200, 6300–4900, 2700–2400, ~1000 and 500–200 cal BP (Wirth et al., 2013). These intervals partly overlap with our Ro3–Ro5 stages, especially during the Subboreal to early Subatlantic transition. In the Western Mediterranean Alps data, Lake Savine records enhanced flood activity at 5600–4850, 3900–2350, 2000–1500, 1150–1050, 650–50 cal BP (Sabatier et al., 2017), again consistent with phases of increased hydrological variability captured in the Roya catchment.

Marine and downstream continental archives show similar behav-
iour. TERR-alkane sea-surface temperature proxies from the Ligurian
margin document seven high-flood episodes (Jalali et al., 2016) that
coincide with increased sediment flux in the Rhône delta plain (Fanget
et al., 2014; Provansal, 2002). In Southern France more broadly, eight
Holocene flood clusters have been identified at 7800–7150; 4800–4500;
3000–2750; 2300–1750; 1500–1350; 950–800; 600–400 and 250–100
cal BP (Benito et al., 2015), with particularly strong activity between
~4200 and 2400 cal BP in the Pre-Alps (Provansal, 2002; Provansal
et al., 1999) corresponding closely to our Ro3–Ro4 clusters and the
transition from mid-Holocene stability to more disturbed late-Holocene
conditions.

Overall, the Roya flood-preservation stages fit within a regional
picture in which extreme hydro-sedimentary events become more
frequent during phases of cooler, wetter climate and enhanced atmo-
spheric instability. At the same time, individual >100-year floods do not
always mirror broader climatic trends (Starkel, 2002; Wilhelm et al.,
2022), underscoring that the Roya record represents a climatically
forced but locally filtered signal.

7.2.3. *Human impact*

As highlighted in previous sections, evidence of human activity is
preserved in the sedimentary archives of the Viévola and Consciente
catchments. At the apex of the Viévola fan, several palaeosols (V1M,
D3'K) have developed without overlying flood deposits. The upper
paleosol at V1M, dated to 1467-1307 cal BP, is overlain by colluvial
deposits that grade into present-day anthropogenic terraces. In the
Consciente valley (Co1K), charcoal from kiln structures dates furnace
activity to the 15th century, again without subsequent floodbeds.

In the upper Roya Valley ("Vallée des Merveilles", Mont Bégo region), land-use phases are documented at 7450–7150, 6200–4900, 4945–4226, 4250–3700, 3635–3366 and 2581–1737 cal BP (Beaulieu, 1977; Finsinger et al., 2021; Kharbouch, 1996, 2000; Ponel et al., 2001). Human impact in the Roya basin becomes more pronounced from ~3000 cal BP and intensifies after ~1700 cal BP, with increasingly open landscapes due to deforestation, terracing and grazing (Finsinger et al., 2021; Kharbouch, 1996). Similar trends are reported from Lake Ledro and the western Swiss Alps between 3600 and 2700 cal BP (Dapples et al., 2002; Vannière et al., 2013).

The small number of archaeological remains between ~3300 and 2500 cal BP (Bianchi et al., 2014; Lafage, 2024) suggests a temporary decline in human presence, possibly linked to unfavourable climatic conditions. Over the last ~1500 years, however, there is clear evidence for intensified human activity, which, combined with wetter and cooler phases, likely enhanced slope erosion and promoted the frequency of small to moderate floods (Blöschl et al., 2019).

7.3. Factors limiting deposit preservation

Three main factors bias the preservation of extreme-event deposits in
the Roya Valley:

* Spatial skew and autogenic reworking

The spatial distribution of extreme event is highly uneven (Miall, 2014). During Storm Alex, for example, only the right-bank tributaries of the Roya were heavily affected, whereas most left-bank tributaries

experienced limited morphological change (Pons et al., 2024). Catch-
ments morphometry controls both the capacity to store sediment and the
efficiency of subsequent remobilization (Milan, 2012; Lièbault et al.,
2024; Welfringer et al., 2025). Extreme events may deliver large sedi-
ment volumes to the valley floor, which are later reworked, leading to a
"shredded" stratigraphic signal (Jerolmack and Paola, 2010; Paola,
2016) in which discrete deposits are disconnected from a single flood
event.

* Selective incision and tectonic uplift

Long-term uplift and river incision also modulate preservation. Uplift rates in the region remain stable throughout the Holocene, ranging from 0.3 to 1.4 mm yr⁻¹ (Bigot-Cormier, 2002; Bogdanoff et al., 2000; Sternai et al., 2019). Cosmogenic ¹⁰Be-derived denudation rates for the Var catchment average 0.24 ± 0.04 mm yr⁻¹ (Mariotti et al., 2021), while river incision rates for the Bevera and Bendola tributaries of the Roya range from 0.28 ± 0.04 mm yr⁻¹ (28–5 ka) to 0.84 ± 0.51 mm yr⁻¹ (10–5 ka) (Cardinal, 2023). Modelled incision rates for the Roya River itself lie between 0.2 and 0.5 mm yr⁻¹ (Petit et al., 2023).

These values point to continuous incision driven by long-term uplift, consistent with rates observed across the Western Alps and probably linked to lithospheric and mantle processes and/or postglacial isostatic rebound (Nocquet et al., 2016; Sternai et al., 2019). Incision enhances the removal of fine deposits and favours the preservation of coarse, high-magnitude beds. Disentangling the relative roles of tectonic inci- sion versus extreme-event erosion remains challenging.

Along the upstream–downstream gradient, these processes create a clear pattern: in the Lavina torrent, aggradation phases between ~12 600 and 9000 cal BP are followed by progressive terrace incision continuing to the present; further upstream, the coarsest and oldest deposits in the Caïros and Bieugne valleys date between ~7000 and 2700 cal BP; in the Viévola catchment, only younger deposits (last ~1430 cal years) are preserved.

* Facies variability

Even within single event such as Storm Alex facies vary strongly both between and within catchments complicating hydraulic reconstructions. Identifying facies associations is crucial for reconstructing past extreme flood events. In the Roya Valley, right-bank deposits range from massive, structureless beds typical of hyperconcentrated flows (Gmm, GCM; Benvenuti and Martini, 2002) to well-stratified traction-current and overbank facies (Sr, Sh, Fm; Blair, 1999; Blair and McPherson, 1994). This variability reflects transitions between turbulent bedload transport, hyperconcentrated flows (Pierson, 2005a, 2005b) and gravity-driven debris flows triggered by slope failures (Cruden and Varnes, 1996; Hungr et al., 2001; Iverson, 1997).

Hydro-sedimentary regimes are primarily controlled by catchment morphometry (Ilinca, 2021; Wilford et al., 2004) and substrate lithology; for example, schists and marls provide abundant fines that favour debris-flow formation (Moscariello et al., 2002; Remaître, 2006). Distinguishing between localised versus catchment-wide events therefore remains a major source of uncertainty in interpreting extreme-flood deposits.

7.4. Implications and future direction

the Ro-stage framework and associated facies catalogue provide key
boundary conditions for:

* Landscape evolution models that couple long-term uplift with episodic incision during extreme events (e.g. Braun and Willett, 2013).

* Flood-hazard assessments, by supplying millennial-scale constraints on recurrence intervals and potential sediment-volumes.
* Interdisciplinary studies, linking geomorphology, hydrology and archaeology through a shared chronological framework.

To move towards a more predictive understanding, three priorities emerge:

(i) targeted coring of alluvial-plains along the main Roya channel to recover fine-grained flood records that are under-represented in terrace outcrops;

(ii) systematic measurements of incision depths and sediment budgets after well-documented floods; and

(iii) coupled hydro-sedimentary modelling to test the sensitivity of extreme-event responses to channel geometry, slope and substrate properties.

Such efforts would transform our descriptive Ro-stage framework into a more mechanistic, process-based model of extreme-event response under combined climatic and anthropogenic forcing.

## 8. Conclusions

We present an unprecedented, multi-millennial record of Holocene torrent dynamics in the Roya Valley. The exceptional exposures created by Storm Alex allowed the precise calibration of sedimentary facies and the construction of a robust ¹⁴C/OSL chronological framework.

Our sedimentary archives reveal a transition from high-magnitude floods during the early and middle Holocene to finer, lower-energy deposits in the late Holocene, increasingly affected by human land use. These changes coincide with major regional climatic episodes – including the 4.2 ka cooling and the Little Ice Age – and overlap with historically documented floods.

By combining sedimentological evidence, regional palaeoclimate proxies and historical archives, this study provides one of the first continuous reconstructions of flood-frequency for a Mediterranean alpine torrent. Beyond establishing flood chronologies, our facies-dating catalogue offers essential boundary conditions for quantifying and modelling long-term incision rates in catchments subjected to extreme events.

We recommend extending this approach to the Roya mainstem and neighbouring tributaries, through alluvial-plain coring and high-resolution geomorphic surveys, supported by hydraulic modelling to estimate palaeoflood magnitudes and inundation extents. Incorporating such long-term archives into regional hazard assessments will be crucial for anticipating future hydrological extremes under ongoing climate change.

Even centennial- to millennial-scale floods leave diagnostic signatures in steep alpine environments. Embedding palaeoflood magnitudes into land-use planning and infrastructure design can therefore substantially increase resilience within these highly dynamic, high-hazard landscapes.

## Authors contribution

You will find below the contributions of all the authors: Raphaël KERVERDO: Field data acquisition, data processing, conceptualisation (sedimentological, hydraulic, geoarchaeological), implementation, writing and corrections. Sara LAFUERZA: Field data acquisition, support with conceptualisation of sedimentological and hydraulic aspects, proofreading and corrections Christian GORINI: Field data acquisition, support with the conceptualisation of sedimentological aspects, proofreading and corrections." Eric FOUACHE: Field data acquisition, support with the conceptualisation of sedimentological and geoarchaeological aspects, proofreading and corrections. Didier GRANJEON: Assistance with data processing (hydraulic aspects), conceptualisation and discussions on hydraulic processes and climatic links, proofreading and corrections. Nicoletta Bianchi: Field data acquisition, support with the

conceptualisation of geoarchaeological aspects, proofreading and corrections. Negar Haghipour: Radiocarbon dating carried out at ETH Zurich, and interpretation of the resulting data Romano Clementucci: Support with conceptualisation, assistance with interpretation and integration of tectonic data and incision rates. Nicolas Loget: corrections and interpretations relating to geomorphological and tectonic aspects. Sébastien Migeon: Proofreading and corrections. Rémy Deschamps: Proofreading and corrections.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgement

First of all, we'd like to thank Franck Suméra, General Curator for Heritage and Scientific Management of the Alpes-Maritimes, DRAC PACA, Regional service of archeology. Thanks to the Musée des Merveilles team for welcoming us to the museum on several occasions and for the discussions we had about human occupation, particularly Silvia Sandrone and Sylvain Rouah. We would also like to thank Vincenzo Picotti for hosting us at the ETH laboratory in Zurich.

The authors would like to thank the Terre-Mer team of the ISTEP laboratory for its financial support for fieldwork.

I would like to thank Gueorgui RATZOV for the fieldwork in the Caïros valley.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.quascirev.2025.109749.

## Data availability

All data and/or code is contained within the submission.

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