Reconstruction de l'activité des glissements de terrain au moyen d'une approche dendrogéomorphologique (Moyenne vallée de l'Ubaye, Alpes de Haute Provence)

Landslides constitute a common mass movement process and a widespread hazard in mountain where they cause damage and destruction to settlements, transportation corridors, or even lead to the loss of life. A major obstacle for the knowledge of past landslide activity is the lack of historical data with satisfying spatial resolution over medium to long timescales and on a continuous basis. In wooded shallow landslides, the use of tree rings may greatly help the documentation of past events and may allow reconstruction of precise chronologies of landslide reactivations over considerable periods of the past. 13 wooded shallow landslides located in the Ubaye valley were analysed using dendrogeomorphic approach. This method enhances the historical chronicles. It permits to quantify return periods of landslide reactivations and to improve our knowledge concerning the spatial behaviour of the process. Coupled with a statistical approach based on a Poisson probabilistic model, it permits to quantify and to map the probability of landslide reactivations. Finally, the coincidence between landslide reactivations and extreme precipitation was examined in order to improve existing threshold values for the triggering of major landslides in the French Alps.Les glissements de terrain sont à l'origine de la destruction de nombreuses infrastructures routières, de bâtiments et causent la perte de vies humaines. L'observation des glissements de terrain est limitée par l'absence d'archives historiques précises. Sur les glissements de terrain superficiels forestiers, la dendrogéomorphologie permet de reconstruire, avec une résolution temporelle saisonnière et une emprise spatiale décamétrique, l'activité du processus, à partir de l'analyse des perturbations anatomiques contenues dans les cernes de croissance. 13 glissements de terrain superficiels forestiers, localisés dans la vallée de l'Ubaye ont été étudiés. L'analyse dendrogéomorphologique permet de densifier les chroniques historiques, d'obtenir des périodes de retour et d'améliorer nos connaissances sur le comportement spatial du processus. L'approche statistique basée sur un modèle de Poisson, permet de quantifier et de cartographier des probabilités de réactivation. La thèse proposera d'évaluer la robustesse de la reconstruction dendrogéomorphologique ainsi que le rôle des précipitations dans le déclenchement des évènements passé

Reconstitution of landslide activity using dendrogeomorphology

Les glissements de terrain sont à l'origine de la destruction de nombreuses infrastructures routières, de bâtiments et causent la perte de vies humaines. L'observation des glissements de terrain est limitée par l'absence d'archives historiques précises. Sur les glissements de terrain superficiels forestiers, la dendrogéomorphologie permet de reconstruire, avec une résolution temporelle saisonnière et une emprise spatiale décamétrique, l'activité du processus, à partir de l'analyse des perturbations anatomiques contenues dans les cernes de croissance. 13 glissements de terrain superficiels forestiers, localisés dans la vallée de l'Ubaye ont été étudiés. L'analyse dendrogéomorphologique permet de densifier les chroniques historiques, d'obtenir des périodes de retour et d'améliorer nos connaissances sur le comportement spatial du processus. L'approche statistique basée sur un modèle de Poisson, permet de quantifier et de cartographier des probabilités de réactivation. La thèse proposera d'évaluer la robustesse de la reconstruction dendrogéomorphologique ainsi que le rôle des précipitations dans le déclenchement des évènements passé.Landslides constitute a common mass movement process and a widespread hazard in mountain where they cause damage and destruction to settlements, transportation corridors, or even lead to the loss of life. A major obstacle for the knowledge of past landslide activity is the lack of historical data with satisfying spatial resolution over medium to long timescales and on a continuous basis. In wooded shallow landslides, the use of tree rings may greatly help the documentation of past events and may allow reconstruction of precise chronologies of landslide reactivations over considerable periods of the past. 13 wooded shallow landslides located in the Ubaye valley were analysed using dendrogeomorphic approach. This method enhances the historical chronicles. It permits to quantify return periods of landslide reactivations and to improve our knowledge concerning the spatial behaviour of the process. Coupled with a statistical approach based on a Poisson probabilistic model, it permits to quantify and to map the probability of landslide reactivations. Finally, the coincidence between landslide reactivations and extreme precipitation was examined in order to improve existing threshold values for the triggering of major landslides in the French Alps

Reconstruction de l'activité des glissements de terrain au moyen d'une approche dendrogéomorphologique (Moyenne vallée de l'Ubaye, Alpes de Haute Provence)

Les glissements de terrain sont à l'origine de la destruction de nombreuses infrastructures routières, de bâtiments et causent la perte de vies humaines. L'observation des glissements de terrain est limitée par l'absence d'archives historiques précises. Sur les glissements de terrain superficiels forestiers, la dendrogéomorphologie permet de reconstruire, avec une résolution temporelle saisonnière et une emprise spatiale décamétrique, l'activité du processus, à partir de l'analyse des perturbations anatomiques contenues dans les cernes de croissance. 13 glissements de terrain superficiels forestiers, localisés dans la vallée de l'Ubaye ont été étudiés. L'analyse dendrogéomorphologique permet de densifier les chroniques historiques, d'obtenir des périodes de retour et d'améliorer nos connaissances sur le comportement spatial du processus. L'approche statistique basée sur un modèle de Poisson, permet de quantifier et de cartographier des probabilités de réactivation. La thèse proposera d'évaluer la robustesse de la reconstruction dendrogéomorphologique ainsi que le rôle des précipitations dans le déclenchement des évènements passé.Landslides constitute a common mass movement process and a widespread hazard in mountain where they cause damage and destruction to settlements, transportation corridors, or even lead to the loss of life. A major obstacle for the knowledge of past landslide activity is the lack of historical data with satisfying spatial resolution over medium to long timescales and on a continuous basis. In wooded shallow landslides, the use of tree rings may greatly help the documentation of past events and may allow reconstruction of precise chronologies of landslide reactivations over considerable periods of the past. 13 wooded shallow landslides located in the Ubaye valley were analysed using dendrogeomorphic approach. This method enhances the historical chronicles. It permits to quantify return periods of landslide reactivations and to improve our knowledge concerning the spatial behaviour of the process. Coupled with a statistical approach based on a Poisson probabilistic model, it permits to quantify and to map the probability of landslide reactivations. Finally, the coincidence between landslide reactivations and extreme precipitation was examined in order to improve existing threshold values for the triggering of major landslides in the French Alps.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Quantification des vitesses d'érosion par dendrogéomorphologie : utilisation des racines déchaussées dans les badlands marneux des bassins expérimentaux de Draix (Alpes de Haute-Provence)

International audienceLes marnes noires du Jurassique couvrent de vastes surfaces dans les Alpes françaises du Sud. Partout où elles affleurent, elles sont sujettes à une érosion intense à l'origine de problèmes de sédimentation dans les cours d'eau et d'envasement dans les réservoirs hydro-électriques. Dans les badlands de Draix (Alpes de Haute-Provence), les vitesses d'érosion obtenues par différentes méthodes de mesure atteignent 5 à 10 mm/a. Dans cette étude, les mesures d'érosion quantifiées au moyen d'une approche dendrogéomorphologique sont validées par comparaison avec celles issues du suivi micrométrique d'un réseau de 47 tiges métalliques. Trente-neuf racines de Pinus sylvestris L. déchaussées ont été échantillonnées dans le bassin-versant du Moulin (8 ha) et l'impact de la dénudation sur les variations anatomiques des cernes annuels de croissance a été mesuré. L'année de déchaussement a pu être déterminée via l'étude de la diminution très rapide, de l'ordre de 60%, de la surface du lumen des trachéides. Les vitesses d'érosion mesurées au moyen de la dendrogéomorphologie atteignent en moyenne 5 mm/an et montrent une corrélation significative [r²(d)=0,39] mais non exclusive avec la pente locale. Elles concordent avec celles obtenues à partir de mesures micrométriques (5,7 mm/a en moyenne) et par cubages réalisés à l'exutoire du bassin-versant (4,7 mm/a). Dés lors, cette approche offre des perspectives particulièrement intéressantes pour quantifier et spatialiser rapidement les vitesses d'érosion moyennes sur plusieurs décennies, dans des bassins versants non instrumentés

Defining optimal sample size, sampling design and thresholds for dendrogeomorphic landslide reconstructions

International audienceTrees affected by mass movements record the evidence of geomorphic disturbance in their growth-ring series, and thereby provide a precise geochronological tool for the reconstruction of past process activity. At the tree scale, identification of past mass movements was typically based on the presence of growth anomalies and focused on the presence of scars, tilted or buried trunks, as well as on apex decapitation. In terms of sampling strategy, however, clear guidelines have been largely missing. Most previous work was based either on the sampling of visibly disturbed trees irrespective of their position at the study site or on the systematic sampling of trees evenly distributed along transects. Based on a dense dataset of 323 trees growing on an active landslide body, this study aims at defining guidelines for future tree-ring sampling of landslides. Using random extractions of trees and iterative mapping, we investigate subsets of the full tree-ring sample to define optimal sampling strategy, sample depth and trees for the development of frequency maps of landslide reactivations. We demonstrate that (i) the sampling of 50-100 trees can be sufficient to obtain satisfactory results on landslide frequency without including noise to the dendrogeomorphic record; (ii) increasing growth disturbance thresholds should be adjusted to sample size and are preferable to fixed values; (iii) an even distribution of sampled trees is crucial to increase the reliability of frequency maps, even more so if the reconstruction is based on small sample sizes; and that (iv) the selection of the most frequently disturbed trees is key to reduce uncertainties in the frequency maps. The optimization of sample sizes and the adjustment of sampling strategy will not only facilitate fieldwork and render analyses and interpretation more reliable, but will also ultimately allow reconstruction of time series of past mass movements with reasonable temporal efforts and excellent cost-benefit ratios

Assessment of forested shallow landslide movements coupling tree ring records from stems and exposed roots

The purpose of this study is to explore and illustrate the potential of exposed roots to reconstruct larger-scale landslide activity and thus to complement tree-ring data gathered from stems to reconstruct spatio-temporal patterns of landslide reactivation. Work was undertaken in a forested area of the Davids-Bas landslide, Barcelonnette (Southeastern French Alps) and based on growth disturbances (GD) from 48 stems and on anatomical changes (decrease of cell lumina) in 20 exposed root sections of heavily affected Mountain pine (Pinus uncinata Mill. ex Mirb.) trees growing on the scarps and upon the landslide body. A total of 95 GD and 20 anatomical changes were identified in the samples pointing to 7 movements of the landslide body since AD 1977. The study demonstrates that reconstructions of landslide reactivations obtained from exposed roots samples are not significantly different from those gathered from stems, but that the inclusion of exposed roots permits realization of frequency maps in sectors which could not be documented with classic dendrogeomorphic approaches focusing on tree-ring records from stems (scars, cracks). In addition, and even more importantly, the inclusion of exposed roots allowed assessment of the geomorphic evolution of the landslide at the local scale and to detect precursor signals of major reactivations in the form of crack widening before the main movement was registered in the tree stems. In that sense, the combined approach presented in this paper can be considered as a valuable tool for land-use planners and emergency cells in charge of forecasting future events and in protecting people and their assets from the negative effects of landslides.L’objectif de cette étude est de démontrer le potentiel des racines déchaussées pour la reconstitution de l’activité de glissements de terrain passés. L’étude a été réalisée sur le glissement de terrain des Davids-Bas, situé sur le versant sud du bassin de Barcelonnette (Alpes de Haute-Provence, France). L’analyse dendrogéomorphologique repose sur l’étude de 48 pins à crochets présentant des signes traumatiques caractéristiques d’une instabilité dont 11 ont fait l’objet de prélèvements additionnels au niveau du système racinaire. L’analyse des sections de racines et de tige a permis de dater 95 perturbations anatomiques depuis 1968 et de reconstruire 7 réactivations du glissement des Davids-Bas (1977, 1978, 1979, 1984, 1996, 2001 et 2004). Les racines déchaussées enregistrent des réactivations non détectées au niveau de la tige. Elles permettent donc d’affiner la cartographie des périodes de retour. D’autre part, le caractère abrupt des variations anatomiques observées révèle une exposition résultant de processus d’érosion brutaux et non d’une érosion continue. Il nous renseigne, de ce fait, sur la magnitude des réactivations passées. Enfin, l’analyse des sections racinaires prélevées au niveau des escarpements et de fissures de tension situées en amont de ces escarpements permet de reconstruire précisément l’évolution micro-topographique du versant. L’étude confirme l’apport des analyses couplées tiges/racines pour la reconstitution de la dynamique spatio-temporelle des glissements de terrain superficiels et suggère une prise en compte systématique des racines exposées dans les reconstructions futures

Contrasted effects of climate change on low-altitude relict <i>Pinus uncinata</i> stands in the Northern French Alps

This paper reports on climate-induced growth changes in relict, low-altitude mountain pines ( Pinus uncinata Mill. ex. Mirb.) from two refugia with cold microclimates located in the Northern French Alps. The P. uncinata stands analyzed grow at the lower bound of their ecological limit and are thus thought to be sensitive indicators of ongoing climate change. Using dendroecological approaches, we compare tree-ring growth at two closely spaced low-altitude stands in the Chartreuse massif (French Alps): La Plagne and Cirque de Bresson. La Plagne is a N-NW-exposed, ventilated slope with cold air circulating in the scree during summer, and the presence of sporadic permafrost as well as ground overcooling, whereas Cirque de Bresson is located on a small, S-exposed fan with sporadic avalanche activity. At both sites, growth responses of P. uncinata to changes in twentieth and twenty-first centuries temperature and precipitation conditions were investigated by means of moving correlation analyses. At Cirque de Bresson, a significant and rapid decline in tree-ring widths has been observed since the early 1990s. We attribute this decline to (i) increasing air temperatures at the beginning of the growing season (May–June) as well as to (ii) a decrease in soil water potential. At La Plagne, we do not detect any significant trend between the higher summer temperatures and tree growth, presumably as a result of the circulation of cold air in the scree slope, which is thought to maintain fresh and humid soil conditions and therefore favor tree growth. These forest stands provide prime examples on how dendroecology can contribute to the study of the dynamics and local variability of tree growth and climate change in relict forest populations with high ecological and conservation values.</p

High resolution, quantitative reconstruction of erosion rates based on anatomical changes in exposed roots at Draix, Alpes de Haute-Provence - critical review of existing approaches and independent quality control of results

International audienceExtensive areas of French Alps are underlain by Jurassic black marls. Wherever these "terres noires" crop out they become subject to intense erosion, causing major sedimentation in regional reservoirs and river systems. In the badlands near Draix (Alpes de Haute-Provence, France), measured sediment rates were obtained at the plot scale by surface elevation change-based methods and at the catchment scale by monitoring sedimentation in dams. In this study, we use a dendrogeomorphic approach based on anatomical changes in exposed roots of Pinus sylvestris L. to accurately quantify continuous denudation rates. A total of 123 cross sections (75 from buried and 48 from exposed roots of 23 trees) were sampled in the Moulin basin. The size and position of roots at the time of exposure was determined via anatomical variations in the annual growth rings of roots. In cross sections of buried roots, a sharp reduction of earlywood tracheid lumen area - a growth signature which has traditionally been used to determine the moment of root denudation - was observed as soon as erosion reduced soil cover to < 3 cm. As a consequence, estimates of eroded soil thickness had to be adjusted to take account of this bias. Bias-adjusted, averaged, medium term erosion rates derived from exposed roots vary between 6 and 7 mm y-1 at Moulin basin depending on the importance accorded to the uplift of roots after exposure. values are significantly correlated to slope angle and match with erosion rates derived from monitored iron stakes (5.7 mm y-1) or measurements of sediment yield in retention dams. (4.7 mm y-1) at the outlet of the Moulin basin. Besides demonstrating that the interpretation of anatomical signatures in tree roots to erosion have to be revised, this paper also shows that dendrogeomorphic analyses of roots are indeed a powerful tool for the quantification of minimal rates of soil erosion in environments where measurements of past activity are not available

Probability Maps of Landslide Reactivation Derived from Tree-Ring Records

International audienceProbability maps of landslide reactivation are presented for the Aiguettes landslide located in the southern French Alps based on results obtained with dendrogeomorphic analysis. Spatio-temporal activity was derived from tree-ring series of 223 disturbed trees. 355 growth disturbances were identified in the samples indicating 14 reactivation phases of the landslide body between 1898 and 2011. Probabilities of landslide reactivation were computed and illustrated using a Poisson distribution model. For example, in the toe, the probability increases from 0.28 for a 5-year period to 0.99 for a 100-year period. This method differs from conventional approaches, which have demonstrated to have limitations in the prediction of spatiotemporal reactivation of landslides. Based on extensive data and therefore allowing determination of quantitative probability maps of reactivation, this approach is considered a valuable tool for land managers in charge of protecting and forecasting people as well as for those responsible for land-use planning and management

Assessment of forested shallow landslide movements coupling tree ring records from stems and exposed roots: Utilisation des racines déchaussées pour la reconstruction de l’activité passée des glissements de terrain superficiels

International audienceThe purpose of this study is to explore and illustrate the potential of exposed roots to reconstruct larger-scale landslide activity and thus to complement tree-ring data gathered from stems to reconstruct spatio-temporal patterns of landslide reactivation. Work was undertaken in a forested area of the Davids-Bas landslide, Barcelonnette (Southeastern French Alps) and based on growth disturbances (GD) from 48 stems and on anatomical changes (decrease of cell lumina) in 20 exposed root sections of heavily affected Mountain pine (Pinus uncinata Mill. ex Mirb.) trees growing on the scarps and upon the landslide body. A total of 95 GD and 20 anatomical changes were identified in the samples pointing to 7 movements of the landslide body since AD 1977. The study demonstrates that reconstructions of landslide reactivations obtained from exposed roots samples are not significantly different from those gathered from stems, but that the inclusion of exposed roots permits realization of frequency maps in sectors which could not be documented with classic dendrogeomorphic approaches focusing on tree-ring records from stems (scars, cracks). In addition, and even more importantly, the inclusion of exposed roots allowed assessment of the geomorphic evolution of the landslide at the local scale and to detect precursor signals of major reactivations in the form of crack widening before the main movement was registered in the tree stems. In that sense, the combined approach presented in this paper can be considered as a valuable tool for land-use planners and emergency cells in charge of forecasting future events and in protecting people and their assets from the negative effects of landslides