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The exceptional rainfall on Oct. 2nd, 2020 on the Maritime Alps and on the Liguria regions have considerably modified the bed morphology of several rivers in the area. Far beyond the rather localized damages observed on the Tinée or on the Fora di Taggia rivers, the Roya and the Vésubie rivers endured exceptional active width widening (see Melun et al., submit.) that triggered considerable infrastructure damages (approx. 1 billion euros worth). This exceptionally intense storm episode (663 mm in 24h at the Mesches Dam in the Roya catchment ; CEREMA, 2021) reactivated sediment supply from hillslopes that triggered a fluvial metamorphosis: the Roya and the Vésubie, that were single channels, mostly of a step-pool morphology prior to the flood, switched to multithreaded riverbeds, characteristic of an excess of sediment supply compared to transport capacity. Based on sediment fluxes monitoring (RFID sediment tracking and geophone) before and during the flood, this study contributes 1/ to characterise the intensity of sediment transport in the Roya main branch during the 2020/10/02 episode, 2/ to develop a prospective conceptual model of sediment connectivity in the Roya catchment and 3/ to stress few essential guidelines for the reconstruction of these Maritime Alps valley bottoms.; Les précipitations exceptionnelles du 2 octobre 2020 sur les Alpes-Maritimes et la Ligurie ont considérablement modifié la morphologie de plusieurs cours d’eau de la région. Au-delà des dégâts assez localisés observés sur la Tinée ou la Fora di Taggia, la Roya et la Vésubie en particulier ont vu leur bande active s’élargir de manière exceptionnelle (cf. Melun et al., submit.), occasionnant des dégâts considérables aux infrastructures (coût estimé :env. 1 milliard d’euros). Ces deux cours d’eau sont passés d’un chenal unique, souvent en step-pool, à une morphologie à chenaux multiples, caractéristique d’un excès de sédiments provenant des versants, réactivés à l’occasion de cet événement particulièrement intense en termes de précipitations (663 mm en 24h aux Mesches ; CEREMA, 2021). En s’appuyant sur les campagnes de mesures (traçage RFID et sismique acoustique passive) avant et pendant l’événement, nous nous proposons de contribuer 1/ à caractériser l’intensité du transport sédimentaire sur la Roya lors de l’épisode du 02/10/2020, 2/ à développer un modèle prospectif de fonctionnement hydro-sédimentaire du bassin de la Roya et enfin 3/ à souligner quelques principes de gestion qui nous semblent essentiels pour la reconstruction de ces vallées maralpines.

International audience; The Alex storm caused particularly catastrophic floods in the Roya, Tinée and Vésubie valleys(south-eastern France) on 2 October 2020. This event will certainly serve as a reference for floodrisk management in the future and it is important to document it as far as possible, in order topreserve its memory and enable further studies. Different national services and operators havebeen involved to gather and consolidate datasets describing this event. This article presentsa contribution from the scientific community to the reconstruction of the event’s peakdischarges, conducted within the HyMeX research programme. Field cross section surveysmade it possible to obtain 32 peak discharge estimates, contributing to a detailed descriptionof the flood genesis on the main rivers and their small tributarie; La tempête Alex, qui a touché le 2 octobre 2020 les vallées de la Roya, la Tinée, et la Vésubiedans les Alpes-Maritimes, constitue un événement de référence qu’il est important de docu-menter au mieux pour en conserver la mémoire et permettre son étude ultérieure. Lesdifférents services et opérateurs de l’État se sont largement mobilisés pour acquérir et regrou-per des jeux de données décrivant cet événement. Cet article présente une contribution de lacommunauté scientifique à la reconstitution des débits de pointe des crues, conduite dans lecadre du programme de recherche HyMeX. Des relevés de sections d’écoulement ont permisd’obtenir 32 estimations de débits de pointe, contribuant à la connaissance des crues obser-vées sur les cours d’eau principaux et leurs petits affluents.

Ce rapport vise à présenter la méthodologie utilisée pour modéliser des débits générés par la tempête Alex au droit de 105 exutoires situés dans les bassins du Var amont, de la Tinée, de la Vésubie, de la Bévéra et de la Roya. Une comparaison des débits simulés avec ceux estimés au droit de quatre stations hydrométriques de la DREAL PACA (la Tinée à Tournefort, la Vésubie à Utelle, la Bévéra à Sospel et la Roya à Tende) est réalisée. Il s’agit de résultats préliminaires, à consolider avec une analyse de sensibilité et une comparaison avec d’autres modèles plus complexes et potentiellement plus « réalistes ».

International audience

International audience; Recent floods have highlighted the high vulnerability of Hispaniola Island (split by Haiti in the West and Dominican Republic in the East) towards hydro-meteorological hazards. For example, the coastal city of Cayes, in southern Haiti, has been heavily impacted by the torrential rainfall of October 5 to 6, 2018, with numerous damaged buildings. In addition, most urban areas lack a hydro-meteorological reference allowing to anticipate the damages that can be generated by different types of intense events. In this context, it is necessary to work on the definition of thresholds of critical rainfall accumulation that will then be used to anticipate future damages and to forecast such impacts. This work must be adapted to the regional context of scarcity of long precipitation observed series. Thus, our research aims at defining heavy rainfall statistics for two watersheds (one in Haiti (Cayes) and one in the Dominican Republic (Yaque del Sur)) using MSWEP gridded rainfall estimates (Beck et al., 2019), available from 1979 to 2017 with a spatiotemporal resolution of 0.1°/ 3h.The first step of this work is to fit the annual maximum rainfall series of each rainfall station and their corresponding MSWEP grid cell by the Generalized Extreme Values (GEV) distribution in order to evaluate the capacity of MSWEP to compute the IDF curves at the stations. Then, the stochastic storm transposition method developed by Wright et al., (2013) was used with MSWEP to generate rainfall events with durations of 3 to72 h, and derive annual exceedance probabilities of 1/5, 1/50, and 1/100 over the two watersheds studied. The rainfall statistics obtained at the watershed scale will then be compared to the statistics derived from the basin rainfall series obtained by the Thiessen polygons with the observed rainfall series available over the watershed. Such methodology might be used regionally to map heavy rainfall hazard and defined critical rainfall accumulation thresholds.

International audience; The main goal of this paper is to present a hydro-meteorological assessment of the last three major floods observed on the small coastal watersheds of the French Riviera (October 3, 23 November 2015, 2019 and 1 December 2019). The joint analysis of precipitation data from rain gauges and of several quantitative precipitation estimates products (PANTHERE, ANTILOPE and RAINPOL) shows that the two episodes of 3 October 2015 and 1 December 2019 are characterised by very high rainfall intensities, with local accumulations of more than 100 [mm] in less than 3 hours, while the November 2019 episode was less intense and more uniform over the studied catchments. Estimated peak flows are of the same order of magnitude for the 2015 and December 2019 episodes, i.e. between 10 and 20 m3/s/km2 for the headwater catchments (drained surfaces less than 10 km2), and around 5 m3/s/km2 for the downstream catchments (drained surfaces between 10 and 60 km2). These floods are comparable to the major floods observed in recent years on a European scale, without being those that have produced the highest specific flows. This hydro-meteorological assessment shows the necessity and the interest of the instrumentation of the French Riviera coastal catchments and the added value of organising post-flood feedback in order to estimate the precipitation and flows likely to be observed in these regions prone to flash floods. Such studies are also useful to take a step back on the data and tools used for hydro-meteorological forecasting purposes on these catchments.; L’objectif de cet article est de présenter un bilan hydro-météorologique des trois dernières crues majeures observées sur les petits bassins côtiers de la Côte d’Azur (3 octobre 2015, 23 novembre 2019 et 1er décembre 2019). L’analyse conjointe de données issues de pluviomètres et de différents produits de lames d’eau (PANTHERE, ANTILOPE et RAINPOL) montre que les deux épisodes du 3 octobre 2015 et du 1er décembre 2019 sont caractérisés par des intensités très importantes, avec des cumuls localement supérieurs à 100 mm en moins de 3 heures, alors que l’épisode de novembre 2019 a été moins intense et plus uniformément réparti sur l’ensemble du territoire. Les débits de pointe estimés sont du même ordre de grandeur pour les épisodes de 2015 et de décembre 2019, soit entre 10 et 20 m3/s/km² pour les têtes de bassins versants, et autour de 5 m3/s/km² pour l’aval. Ces crues s’avèrent comparables aux autres crues majeures observées ces dernières années à l’échelle européenne, sans pour autant être celles ayant produit les débits spécifiques les plus importants. Ce bilan hydro-météorologique montre la valeur ajoutée d’organisation de retours d’expérience post-crue afin d’estimer les précipitations et débits susceptibles d’être observés dans ces régions sujettes aux crues éclairs.

International audience

Climate change plays a major role in determining future water availability and quality in several parts of the world. With the potential for hydroelectric development on the Churchill River, Labrador, it is important to understand the influence of climatic conditions on river discharge to foresee potential climate change effects on future productivity. Persistent drought events recorded between 1940 to 1960 and 1985 to 2000 underline the significant risk that changing climate may represent for energy supply, infrastructure security, and Canadian economy. Typically, predictive climatic models use historical temperature and precipitation records. In the study region (eastern Canada), climatic data cover less than seventy years and long reconstructions from natural archives are inexistent, which means it is unrealistic to simulate hydro-climatic variabilities and estimate future drought risks. In the boreal climate, tree-ring δ13C and δ18O series can: (1) be directly linked to climatic conditions, individually or in combination, and (2) reflect river discharge, a regional integration of overall climatic conditions. This study aims at improving the knowledge of long-term hydrologic variability in eastern Canada by reconstructing regional discharge from tree-ring δ13C and δ18O series. Two sites were selected, 40 km north and 8 km south of the Churchill River, to produce tree-ring δ13C and δ18O series extending from 1800 to 2009. Individual and combined isotopic results show significant statistical correlation with summer index discharge from three rivers near the Churchill (r = -0.61, -0.59 and -0.67 for δ13Cmean, δ18Omean and δ13C_δ18Ocombined, respectively; n = 41; p < 0.05; δ13C_δ18Ocombined series normalized with a z-score that integrates C and O isotope variations with the same weight). Therefore, summer index discharge has been reconstructed from the combined δ13C and δ18O data using a linear regression model. The resultant discharge series agrees with a discharge reconstruction obtained from the ANATEM method (1880-2009 period), which is based on a reanalysis of geopotential height fields and on similarities between days of reconstruction and observation. The reconstructions significantly correlate together (r = 0.43; n = 130; p < 0.05), as well as with Churchill River data (r = 0.55 and 0.71 for tree ring isotopes and ANATEM, respectively; n = 18; p < 0.05). The discharge reconstructions also show an overall significant long-term decrease from 1880 to 2009, and short-term decreases associated to drought periods (e.g. 1889-1892, 1935-1942, 1945-1948). The part of the isotopic reconstruction extending beyond the ANATEM reconstruction also shows periods of low discharges (e.g. 1807 to 1828 and 1845 to 1861). These results suggest that this part of Canada has experienced an overall decrease in summer discharge over the past 200 years.