Conséquences de la forêt méditerranéenne sur les écoulements de crue - Synthèse des recherches menées en France

1 tableau ; 4 figuresTrois ensembles de bassins expérimentaux permettent d'étudier, en France méditerranéenne, les conséquences de la forêt sur les crues. Les résultats diffèrent de façon considérable d'un bassin à l'autre, montrant la complexité des relations pluies-débits. Il est à noter que l'augmentation des crues extrêmes est loin d'être avérée dans tous les cas. Une réflexion sur le fonctionnement hydrologique des bassins permet de lever la contradiction apparente entre les résultats observés

Hierarchical prediction errors in midbrain and septum during social learning

Social learning is fundamental to human interactions, yet its computational and physiological mechanisms are not well understood. One prominent open question concerns the role of neuromodulatory transmitters. We combined fMRI, computational modelling, and genetics to address this question in two separate samples (N=35, N=47). Participants played a game requiring inference on an advisor's intentions whose motivation to help or mislead changed over time. Our analyses suggest that hierarchically structured belief updates about current advice validity and the adviser's trustworthiness, respectively, depend on different neuromodulatory systems. Low-level prediction errors (PEs) about advice accuracy not only activated regions known to support "theory of mind", but also the dopaminergic midbrain. Furthermore, PE responses in ventral striatum were influenced by the Met/Val polymorphism of the Catechol-O-Methyltransferase (COMT) gene. By contrast, high-level PEs ("expected uncertainty") about the adviser's fidelity activated the cholinergic septum. These findings, replicated in both samples, have important implications: They suggest that social learning rests on hierarchically related PEs encoded by midbrain and septum activity, respectively, in the same manner as other forms of learning under volatility. Furthermore, these hierarchical PEs may be broadcast by dopaminergic and cholinergic projections to induce plasticity specifically in cortical areas known to represent beliefs about others. Copyright The Authors (2017). Published by Oxford University Press

Ravinement en montagne : processus, mesures, modélisation, régionalisation

Cemagref (Agricultural and Environmental Engineering Research Centre) in Grenoble and the Scientific Group "Draix, mountain erosion studies" held a conference in Digne, in the French Alps, from October 15 to 17, 2003, devoted to the theme of gully erosion in mountain areas. The aims of this international seminar were to evaluate the state of the art in research on erosion by water in uplands, to allow scientists to meet and exchange ideas on related topics, and to initiate further collaborations, with particular emphasis on field work in research basins. This meeting, which encompassed 32 oral presentations and 24 posters, was attended by 80 participants from 12 countries. A one-day scientific excursion to the Draix experimental site was dedicated to multidisciplinary researches on mountain erosion and upland floods.Gully erosion represents an important sediment source in river systems and accounts for as much as 70 to 90% of the overall sediment production of a catchment. In mountain areas, the steep slopes enhance gully processes, accelerate sediment transfer from uplands to valley bottoms and generate natural hazards : mudflows, overflowing of heavily loaded floods, silting up of reservoirs, for example. Therefore, there is a need for monitoring and for experimental and modelling studies in order to forecast the effects of climatic or land use changes on sediment dynamics in mountain catchments. In mountain environments, the development of appropriate measurement and observation techniques is quite difficult. Original devices were presented, operating from the small plot scale to the large gully and catchment scales.Several presentations focused on the processes and factors controlling erosion and gullying in mountain areas : these include mechanical breakdown and chemical weathering of rocks, bed load and suspension sediment transport, in which threshold effects have a determining influence. Mass movements are also a substantial source of sediments in steep upland basins. These may occur as superficial earth flows affecting the weathered layer, and as deep-seated landslides or rock-block slides. Several papers examined the susceptibility of the terrain to the mass movements, the conditions for landslide initiation, the relations between mass movements in black marls and rockfalls in the overlying limestone levels, and the formation and propagation of debris flows. The major role of a few paroxystic events in sediment production was pointed out by several authors. Particle detachment, mass movement initiation, and sediment transport are processes subjected to threshold effects. The connectivity in the basin network may vary from one event to the other. The studies on sediment load rates during floods showed that the availability and location of sediment stocks in the channel and gully bottoms play a greater role than the transport capacity of the flow. The role of the vegetation cover in diminishing sediment yields is widely accepted. Therefore, it is still necessary to understand how the aerial parts and roots of vegetation interact with runoff and erosion in order to model the evolution of gully systems subject to climatic or land use changes. These studies show that erosion modelling becomes more complex as one goes from the plot or slope scale to the catchment scale, which necessitates a transport-distance approach. At this scale, the question of connectivity between flow paths becomes crucial and depends on the time scale of the study.The present issue of the journal publishes five papers related to erosion measurement and remote sensing, to erosion dynamics at the catchment or at a regional scale, and interactions between erosion, and vegetation and restoration works.The paper by D. Raclot et al. presents a first attempt of at using low-altitude aerial photographs taken from a drone for erosion surveys in the Draix area. The relatively poor vertical accuracy (70 cm) does not allow yet for the measurement of short-term ablation, but the good quality and resolution of the pictures, and the possibility for of repeated acquisition offer good opportunities for the survey of landform evolution, especially in remote areas of the studied catchment. Two others additional papers (E. Gauché, J. Collinet & P. Zante) present fruitful methodologies for mapping and topographic survey of gully development. Recent progress in GIS and DEM techniques complete the quantitative observation of erosion processes. Addressing scale issues, the paper by J. Collinet and P. Zante presents different techniques in Tunisia covering rainfall simulation on plots in the flat upstream areas, monitoring of gully evolution on the intermediate slopes, sedimentation in reservoirs at the outlets. The authors show the importance of the processes linking these three slope interfaces. In Morocco, E. Gauché found contrasting rhythms of landscape evolution between mountain slopes and alluvial deposits downstream. Lithology and relief play a role, which is reinforced by land use change to a point that an erosion crisis may be identified, mainly due to the decrease of the extent of tilled areas. The paper by F. Rey presents an experimental bioengineering device for trapping sediments in gullies in marls of the French Alps and shows how brush layers 1.2 m thick across a gully may trap 0.1 to 0.3 m3 per year. The paper by M. Cohen and F. Rey discusses the paradox between positive vegetation dynamics at the regional scale and active erosion observed at the local scale and shows that processes vary depending on spatial scales : for instance, gully erosion may co-exist with vegetation spread on interfluves.Temporal and spatial scales as well as connectivity in soil erosion systems appear as key questions for further research on gully erosion, including appropriate measurement techniques, process analysis, modelling, and upscaling

Processes, spatio-temporal factors and measurements of current erosion in the French Southern Alps : a review

International audiencePresent erosion in mountainous areas of Western Europe causes land management problems, particularly for areas located downstream of erosion zones. Except for transalpine roads and ski resorts, economic activities no longer require as much space as they did in the past. Therefore, natural reforestation has provided significant protection for alpine hillslopes during the 20th century. However, extreme ftoods continue to cause severe damage in intra-alpine valleys, as well as in piedmont and surrounding plains, making the study of present water erosion phenomena very important. Many studies have investigated the processes and factors of water erosion on slopes at both the catchment and plot scales. They have focused on rock fragmentation and transportation in different fields, the spatial and temporal explanatory variables , the consequences downstream (ftooding, sedimentation, river bed evolution) and the impact of ftoods. ln the French Alps, present erosion has been studied in a variety of outcrops, with several recent studies conducted in fields such as marls, clayey deposits, molasses and moraines. These kinds of outcrops are found throughout the alpine massif, including an area of special interest on the great Jurassic black marI outcrop where badlands are frequently observable. Geomorphologists and hydrologists have been particularly interested in the strong erosion processes in marIs, seeking to determine the main patterns and the impact of spatial and temporal factors on soil loss quantities. The main climatic factors of rock disaggregation were found to be the freeze-thaw and wet-dry cycles, which destroy rock cohesion, and the splash effect of rain. The principal site variables are vegetation cover, exposure and dip-slope angle. Erosion rates are two or three orders of magnitude higher on bare soils than on pastures; northern aspect slopes suffer two to four times as much soilloss as southern aspect slopes. Finally, the angle forrned by the slope and the dip also deterrnines different behaviours: erosion rates are higher when slope and dip are perpendicular than when they are parallel. The transportation agents are mostly debris ftows and runoff caused by intense precipitation. Annual erosion depth in the marls is generally assumed to be substantial, up to 10 mm. The high value can be explained by the severity of the climatic conditions and the brittleness of the lithology, which results in numerous fractures

Ravinement en montagne : processus, mesures, modélisation, régionalisation

Cemagref (Agricultural and Environmental Engineering Research Centre) in Grenoble and the Scientific Group "Draix, mountain erosion studies" held a conference in Digne, in the French Alps, from October 15 to 17, 2003, devoted to the theme of gully erosion in mountain areas. The aims of this international seminar were to evaluate the state of the art in research on erosion by water in uplands, to allow scientists to meet and exchange ideas on related topics, and to initiate further collaborations, with particular emphasis on field work in research basins. This meeting, which encompassed 32 oral presentations and 24 posters, was attended by 80 participants from 12 countries. A one-day scientific excursion to the Draix experimental site was dedicated to multidisciplinary researches on mountain erosion and upland floods.Gully erosion represents an important sediment source in river systems and accounts for as much as 70 to 90% of the overall sediment production of a catchment. In mountain areas, the steep slopes enhance gully processes, accelerate sediment transfer from uplands to valley bottoms and generate natural hazards : mudflows, overflowing of heavily loaded floods, silting up of reservoirs, for example. Therefore, there is a need for monitoring and for experimental and modelling studies in order to forecast the effects of climatic or land use changes on sediment dynamics in mountain catchments. In mountain environments, the development of appropriate measurement and observation techniques is quite difficult. Original devices were presented, operating from the small plot scale to the large gully and catchment scales.Several presentations focused on the processes and factors controlling erosion and gullying in mountain areas : these include mechanical breakdown and chemical weathering of rocks, bed load and suspension sediment transport, in which threshold effects have a determining influence. Mass movements are also a substantial source of sediments in steep upland basins. These may occur as superficial earth flows affecting the weathered layer, and as deep-seated landslides or rock-block slides. Several papers examined the susceptibility of the terrain to the mass movements, the conditions for landslide initiation, the relations between mass movements in black marls and rockfalls in the overlying limestone levels, and the formation and propagation of debris flows. The major role of a few paroxystic events in sediment production was pointed out by several authors. Particle detachment, mass movement initiation, and sediment transport are processes subjected to threshold effects. The connectivity in the basin network may vary from one event to the other. The studies on sediment load rates during floods showed that the availability and location of sediment stocks in the channel and gully bottoms play a greater role than the transport capacity of the flow. The role of the vegetation cover in diminishing sediment yields is widely accepted. Therefore, it is still necessary to understand how the aerial parts and roots of vegetation interact with runoff and erosion in order to model the evolution of gully systems subject to climatic or land use changes. These studies show that erosion modelling becomes more complex as one goes from the plot or slope scale to the catchment scale, which necessitates a transport-distance approach. At this scale, the question of connectivity between flow paths becomes crucial and depends on the time scale of the study.The present issue of the journal publishes five papers related to erosion measurement and remote sensing, to erosion dynamics at the catchment or at a regional scale, and interactions between erosion, and vegetation and restoration works.The paper by D. Raclot et al. presents a first attempt of at using low-altitude aerial photographs taken from a drone for erosion surveys in the Draix area. The relatively poor vertical accuracy (70 cm) does not allow yet for the measurement of short-term ablation, but the good quality and resolution of the pictures, and the possibility for of repeated acquisition offer good opportunities for the survey of landform evolution, especially in remote areas of the studied catchment. Two others additional papers (E. Gauché, J. Collinet & P. Zante) present fruitful methodologies for mapping and topographic survey of gully development. Recent progress in GIS and DEM techniques complete the quantitative observation of erosion processes. Addressing scale issues, the paper by J. Collinet and P. Zante presents different techniques in Tunisia covering rainfall simulation on plots in the flat upstream areas, monitoring of gully evolution on the intermediate slopes, sedimentation in reservoirs at the outlets. The authors show the importance of the processes linking these three slope interfaces. In Morocco, E. Gauché found contrasting rhythms of landscape evolution between mountain slopes and alluvial deposits downstream. Lithology and relief play a role, which is reinforced by land use change to a point that an erosion crisis may be identified, mainly due to the decrease of the extent of tilled areas. The paper by F. Rey presents an experimental bioengineering device for trapping sediments in gullies in marls of the French Alps and shows how brush layers 1.2 m thick across a gully may trap 0.1 to 0.3 m3 per year. The paper by M. Cohen and F. Rey discusses the paradox between positive vegetation dynamics at the regional scale and active erosion observed at the local scale and shows that processes vary depending on spatial scales : for instance, gully erosion may co-exist with vegetation spread on interfluves.Temporal and spatial scales as well as connectivity in soil erosion systems appear as key questions for further research on gully erosion, including appropriate measurement techniques, process analysis, modelling, and upscaling

Suivi hydrologique et sédimentaire des petits bassins versants miniers : guide méthodologique

International audience- Le CNRT Nickel et son environnement, conscient que les milieux naturels calédonienssubissent, du fait de l'activité minière, de fortes dégradations liées à une mauvaise gestiondu ruissellement sur les surfaces dénudées, a financé un projet intitulé "Fonctionnementdes petits bassins versants miniers" qui s'est déroulé sur la période 2010-2014.- Ce projet a permis une amélioration des connaissances sur le fonctionnement hydrologiqueet sédimentaire des bassins versants miniers, grâce aux travaux conduits sur le site pilote dePoro mais également sur différents bassins versants étudiés dans le cadre de collaborationsavec les compagnies minières partenaires du projet (KNS, SLN, VALE). Ce projet aégalement permis de tester in situ les dispositifs, matériels et protocoles de mesure lesmieux adaptés pour réaliser les suivis hydrologiques et sédimentaires sur les petits bassinsversants miniers.- Dans ce projet multipartenaires coordonné par l'Université de la Nouvelle Calédonie (UNC),IRSTEA a assuré la coordination de la mise en place des suivis et du recueil des donnéesainsi que l'exploitation scientifique des données recueillies. Ce travail a donné lieu à unrapport scientifique dont la diffusion est assurée par le CNRT. Le bureau d'études A2EPs'est, lui, vu confier l'élaboration d'une note méthodologique sur l'acquisition et labancarisation des mesures de terrain (hydrologie, qualité des eaux, transport solide). Cedocument a permis de préciser l'organisation des données souhaitable pour un réseau demesure ainsi que les méthodes et procédures de suivi adaptées au contexte de la Nouvelle-Calédonie.- Au terme de ce projet, le CNRT a demandé que les rapports rendant compte du projet globalsoient complétés par un guide méthodologique destiné aux professionnels qui sont tenus demettre en place des suivis hydrologiques et sédimentaires dans le cadre des autorisationsd'exploiter.- Le présent document a donc pour objectif de rassembler les éléments techniques permettantde mettre en place les dispositifs de mesure et d'assurer la production de données de qualitéqui permettront à la fois de mesurer les impacts des rejets actuels sur les milieux situés enaval et de poursuivre l'amélioration des connaissances sur le fonctionnement des bassinsversants miniers indispensable à l'amélioration de la conception et du dimensionnement desdispositifs de réduction des impacts.- Ce document s'appuie sur les rapports fournis par IRSTEA et A2EP, complétés par lesformations assurées par IRSTEA au cours du projet ainsi que par les éléments rassembléspar le groupe "instrumentation" du projet. Il a fait l'objet d'échanges avec les servicestechniques de la DAVAR et de la DIMENC et est diffusé sous l'égide du CNRT

L'érosion des Terres Noires dans les Alpes du sud : contribution à l'estimation des valeurs annuelles moyennes (bassins versants expérimentaux de Draix, Alpes de Haute Provence, France) / Erosion of the Terres Noires (Black Earth) in the southern French Alps : A contribution to an assessment of mean annual values (Draix experimental catchment areas)

Abstract: In the southern French Alps, an extensive area is covered by Jurassic marls known as Terres Noires (Black Earth). Erosion, resulting from the Mediterranean mountain climate acting upon this sensitive lithology, has led to the development of a «badlands» topography. Experimental catchment areas have been set up in Draix, 1 5 km north-east of Digne, to study the hydrological, ecological, and economic consequences of this erosion. The general aim of the experimental catchment areas is to obtain a better understanding of the phenomena associated with erosion by mountain torrents. Particular attention is paid to the influence of the vegetative cover. Since 1984, measurements have been conducted which make it possible to estimate sediment yields from the areas during floods. The paper presents a summary of the data collected, which it is hoped can be used to estimate mean annual values. As the values are for catchment areas of different size and with different proportions of vegetative cover, comparisons can be made to gain insights into the influence of these variables. In the two catchment areas devoid of vegetation, the specific mean annual rates range from 150 to 190 tonnes/hectare per year, and they appear to be independent of the size of the catchment area. The figures are close to the highest in the world. Expressed in terms of denudation of the soil layer (weathered marl), these figures are equivalent to a rate of 1 1 to 1 5 mm/year and confirm the results obtained by ORSTOM and BRGM (Bureau de Recherche Géologique et Minière) in the Buëch valley on the same type of lithology. In the catchment area which is almost entirely wooded, the specific sediment yield (again calculated per unit area) is 40 times less.L'érosion des Terres Noires dans les Alpes du Sud : contribution à l'estimation des valeurs annuelles moyennes (bassins versants expérimentaux de Draix, Alpes-de-haute-Provence) Erosion of Terres Noires (Black Earth) in the southern French Alps: A contribution to an assessment of mean annual rates (Draix experimental catchment areas) N. Mathys, S. Brochot, M. Meunier Résumé : Dans les Alpes du sud de la France, les Terres Noires, marnes jurassiques, couvrent une surface importante. Le climat, méditerranéen à tendance montagnarde, favorise sur cette formation de lithologie sensible une érosion avec un faciès de badlands. Les conséquences hydrologiques, écologiques, et économiques de cette érosion justifient l'existence de bassins versants expérimentaux. Situés à Draix, à 1 5 km au nord-est de Digne, ces bassins versants expérimentaux sont dédiés à la progression des connaissances dans les disciplines impliquées par le large thème de l'érosion torrentielle. Un accent particulier y est mis sur l'étude de l'influence de la couverture végétale. Les mesures, pratiquées à partir de 1984, permettent d'évaluer la production de sédiments à l'exutoire des bassins pendant les crues. Cet article présente une synthèse des données recueillies, conduite dans l'optique taille ou de couverture végétale variables, des comparaisons sont alors effectuées. Sur les deux bassins dépourvus de couverture végétale, les valeurs spécifiques (rapportées à la surface dénudée) moyennes annuelles obtenues sont comprises entre 150 et 190 t/ha.an ; elles sont indépendantes de la superficie du bassin. Ces valeurs approchent les records mondiaux. Exprimées en ablation équivalente de la surface du sol (marne altérée), on retiendra les chiffres de 1 1 à 1 5 mm/an. Ils rejoignent les résultats obtenus dans la vallée du Buëch sur la même lithologie par l'ORSTOM et le BRGM. Sur le bassin boisé en quasi-totalité, cette production spécifique - toujours rapportée à la surface dénudée - est 40 fois moins élevée.Mathys Nicolle, Brochot Sylvie, Meunier Maurice. L'érosion des Terres Noires dans les Alpes du sud : contribution à l'estimation des valeurs annuelles moyennes (bassins versants expérimentaux de Draix, Alpes de Haute Provence, France) / Erosion of the Terres Noires (Black Earth) in the southern French Alps : A contribution to an assessment of mean annual values (Draix experimental catchment areas). In: Revue de géographie alpine, tome 84, n°2, 1996. pp. 17-27

La méthode ANETO : un outil pour la prédétermination des débits de crue des bassins versants torrentiels des Pyrénées françaises Peak discharge prediction on torrential catchments of the French Pyrénées range : the ANETO approach

Pour améliorer la prévention et la protection contre les risques générés par les crues torrentielles en montagne, les acteurs locaux ont besoin de connaissances sur les différents processus mis en jeu, notamment sur l'évaluation des débits de crue. Les auteurs présentent ici une méthode combinant l'exploitation statistique des mesures hydrométriques et pluviométriques et la prise en compte de données climatologiques et géomorphologiques pour déterminer de façon plus réaliste les débits de crues des bassins versants torrentiels des Pyrénées françaises.<br>Flood runoff estimations on torrential basins suffer from numerous uncertainties. In order to guide practitioners to predict realistic values of peak discharge, a method based on statistical regressions and geographic analysis had to be designed by the Restoration of Mountainous Territories department of the National Forest Office (ONF-RTM). The region for application of the method corresponds to torrential basins of the French Pyrenees, ranging from 4 to 500 km² in area. It takes into account classical hydrological parameters, and integrates climatic and geomorphologic characteristics which significantly increase the performance of the model

Analyse et modélisation à différentes échelles des mécanismes d'érosion et de transport de matériaux solides (cas des petits bassins versants de montagne sur marne (Draix, Alpes-de-Haute-Provence))

Le travail a pour objectif de déterminer quels sont les flux solides et liquides produit par un petit bassin versant de montagne en réponse à un épisode pluvieux. Il s'appuie sur des observations réalisées sur les bassins de recherche de Draix, étudie les processus et facteurs intervenant dans la formation des écoulements et quantifie leurs rôles respectifs. Différentes échelles spatiales d'analyse sont utilisées : la parcelle, le versant ou la ravine éle mentaire et le bassin versant. La forte non-linéarité des réponses est mise en évidence au moyen d'une analyse statistique approfondie des données disponibles (1985-2003). Ce travail statistique est complété par l'interprétation d'observations et d'expérimentations de terrain spécifiques : expérimentations de simulation de pluie et observations sous pluies naturelles à l'échelle de la parcelle et de la ravine, suivi de l'état des stocks de sédiments dans les lits. Enfin, une première ébauche de modélisation spatialisée, à l'aide du modèle ETC (Erosion des torrents en crue) développé au Cemagref, a été réalisée, vérifiant la capacité du modèle à reproduire le fonctionnement temporel et spatial observé.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF