Trajectories of avalanche risk resulting from socio-environmental changes in the high valleys of the French Alps

Les avalanches de neiges sont répandues dans les zones montagneuses. Elles menacent les personnes, détruisent les bâtiments et bloquent les routes. Historiquement, les approches visant à réduire le risque d’avalanche étaient basées seulement sur l’aléa, qui est une composante du risque. Récemment, des analyses de risques plus complètes ont émergé associant aléas, exposition et vulnérabilité. Cependant, leurs applications restent le plus souvent statiques. En effet, elles négligent les évolutions à long terme du risque résultantes des évolutions simultanées de ses trois composantes. Elles ignorent également les variations spatio-temporelles, à l’échelle locale, des systèmes sociaux et naturels (par exemple, dynamique des populations, économie, évolution des écosystèmes, changement climatique) ainsi que leurs interactions (par exemple, l’exploitation forestière). Par conséquent, la variabilité locale des trajectoires de risque d’avalanche ne peut pas être prise en compte. Enfin, les estimations de risque générées dans un cadre d’analyse quantitatives négligent généralement les changements d’occupation des sols. En particulier l’évolution du couvert forestier qui peut potentiellement modifier l’activité des avalanches et par conséquent le risque d’avalanche. Ainsi, l’objectif de cette thèse est tout d’abord de développer une approche qualitative intégrative. Cette approche combine les informations issues des sciences naturelles et sociales pour évaluer l’évolution à long terme du risque d’avalanche et de toutes ses composantes (aléa, vulnérabilité et exposition), en fonction de l’évolution des facteurs socio-économiques, environnementaux et climatiques. Ensuite le second objectif est d’étudier dans quelle mesure les particularités socio-économiques et environnementales locales peuvent entraîner des disparités spatiales et temporelles dans les trajectoires de risque. Enfin, le dernier objectif est de proposer des estimations quantitatives du risque d’avalanche qui prennent en compte les changements du couvert forestier dans les trajectoires d’avalanche. La thèse se focalise sur les Alpes Françaises, pendant la période 1860-2017, qui est une zone d’avalanche très active et qui a connu d’importants changements socio-économiques et environnementaux aux fils des années.Snow avalanches are prevalent processes in mountain areas, threatening people, destroying buildings and blocking roads. Historically, approaches to reduce avalanche risk were based on the sole hazard component of risk. Recently, more comprehensive risk analyses emerged that couple hazard, exposure and vulnerability. However, existing implementations remain more often than not static, neglecting long term changes in the risk resulting from the simultaneous evolution of its three components. They also ignore the small-scale spatio-temporal patterns in the social (e.g. population dynamics, economy) and natural systems (e.g. evolution ofecosystems, climate change) as well as in their interactions (e.g. forest logging). Consequently, local variability in avalanche risk trajectories cannot be accounted for. Eventually, risk estimates generated within a quantitative risk framework generally neglect land cover changes, notably forest cover evolution, that can potentially alter avalanche activity and, hence, avalanche risk.On this basis, the aims of this PhD are to (i) develop an integrative qualitative approach combining knowledge from natural and social sciences to assess long term changes in avalanche risk and in all its components, hazard, vulnerability and exposure, as function of changes in their socio-economic and environmental drivers, (ii) investigate to which extent local socioeconomic, land cover and climatic peculiarities may lead to spatial and temporal disparities in risk trajectories and (iii) propose quantitative avalanche risk estimates that take into account changes in forest cover within avalanche paths. Herein, the focus is on the high mountains of the French Alps for the 1860-2017 period, a highly active avalanche area that witnessed important socio-economic and environmental changes over the years. All in all, this PhD illustrates how strongly snow avalanche risk evolves in space and time, as function of changes in its components and drivers. Ultimately, the work proposed may be of great interest for stakeholders looking to elaborate effective risk protection strategies that consider the complex dynamics of the human and natural systems

Trajectoires du risque avalancheux résultant de changements sociaux-environnementaux dans les hautes vallées des Alpes françaises

Snow avalanches are prevalent processes in mountain areas, threatening people, destroying buildings and blocking roads. Historically, approaches to reduce avalanche risk were based on the sole hazard component of risk. Recently, more comprehensive risk analyses emerged that couple hazard, exposure and vulnerability. However, existing implementations remain more often than not static, neglecting long term changes in the risk resulting from the simultaneous evolution of its three components. They also ignore the small-scale spatio-temporal patterns in the social (e.g. population dynamics, economy) and natural systems (e.g. evolution ofecosystems, climate change) as well as in their interactions (e.g. forest logging). Consequently, local variability in avalanche risk trajectories cannot be accounted for. Eventually, risk estimates generated within a quantitative risk framework generally neglect land cover changes, notably forest cover evolution, that can potentially alter avalanche activity and, hence, avalanche risk.On this basis, the aims of this PhD are to (i) develop an integrative qualitative approach combining knowledge from natural and social sciences to assess long term changes in avalanche risk and in all its components, hazard, vulnerability and exposure, as function of changes in their socio-economic and environmental drivers, (ii) investigate to which extent local socioeconomic, land cover and climatic peculiarities may lead to spatial and temporal disparities in risk trajectories and (iii) propose quantitative avalanche risk estimates that take into account changes in forest cover within avalanche paths. Herein, the focus is on the high mountains of the French Alps for the 1860-2017 period, a highly active avalanche area that witnessed important socio-economic and environmental changes over the years. All in all, this PhD illustrates how strongly snow avalanche risk evolves in space and time, as function of changes in its components and drivers. Ultimately, the work proposed may be of great interest for stakeholders looking to elaborate effective risk protection strategies that consider the complex dynamics of the human and natural systems.Les avalanches de neiges sont répandues dans les zones montagneuses. Elles menacent les personnes, détruisent les bâtiments et bloquent les routes. Historiquement, les approches visant à réduire le risque d’avalanche étaient basées seulement sur l’aléa, qui est une composante du risque. Récemment, des analyses de risques plus complètes ont émergé associant aléas, exposition et vulnérabilité. Cependant, leurs applications restent le plus souvent statiques. En effet, elles négligent les évolutions à long terme du risque résultantes des évolutions simultanées de ses trois composantes. Elles ignorent également les variations spatio-temporelles, à l’échelle locale, des systèmes sociaux et naturels (par exemple, dynamique des populations, économie, évolution des écosystèmes, changement climatique) ainsi que leurs interactions (par exemple, l’exploitation forestière). Par conséquent, la variabilité locale des trajectoires de risque d’avalanche ne peut pas être prise en compte. Enfin, les estimations de risque générées dans un cadre d’analyse quantitatives négligent généralement les changements d’occupation des sols. En particulier l’évolution du couvert forestier qui peut potentiellement modifier l’activité des avalanches et par conséquent le risque d’avalanche. Ainsi, l’objectif de cette thèse est tout d’abord de développer une approche qualitative intégrative. Cette approche combine les informations issues des sciences naturelles et sociales pour évaluer l’évolution à long terme du risque d’avalanche et de toutes ses composantes (aléa, vulnérabilité et exposition), en fonction de l’évolution des facteurs socio-économiques, environnementaux et climatiques. Ensuite le second objectif est d’étudier dans quelle mesure les particularités socio-économiques et environnementales locales peuvent entraîner des disparités spatiales et temporelles dans les trajectoires de risque. Enfin, le dernier objectif est de proposer des estimations quantitatives du risque d’avalanche qui prennent en compte les changements du couvert forestier dans les trajectoires d’avalanche. La thèse se focalise sur les Alpes Françaises, pendant la période 1860-2017, qui est une zone d’avalanche très active et qui a connu d’importants changements socio-économiques et environnementaux aux fils des années

Respective influence of geomorphologic and climate conditions on debris-flow occurrence in the Northern French Alps

International audienceWe investigate the influence of geomorphic and climate conditions on the spatiotemporal variability in debris flow occurrence using a dedicated hierarchical Bayesian model. The case study includes 104 catchments from the French Alps with debris flow occurrences since 1970. We identify from a list of 21 variables two geomorphic and two climate variables that explain 72% of the total variance. Lithology is identified as the primary driver (42%) of the spatiotemporal variability of debris flow activity. To analyze the effect of the duration of the observation on inferred relations, we run the model using debris flow occurrence data and climate variables from the last 12 and 23 years of the study period, respectively. The model is significant as soon as at least 23 years of observations are considered. To detect the influence of spatial variability in debris flow frequency, we partition our data set into two subsamples composed of catchments with at least or less than 4 debris flows over the period. This reveals that precipitation during the event and the 2 days before exceeding 20 mm in total are the most important variables for both subsamples. It explains at least 57% of the total variance and 73% of its climate component. We conclude that difference in debris flow occurrences from one catchment to another is mostly due to geomorphologic conditions, whereas homogeneous groups of catchments react strongly to climatic conditions in a rather similar way

Impact of land cover on avalanche hazard: how forest cover changes affect return periods and dynamical characteristics simulated by a statistical-numerical avalanche model

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Impact of land cover on avalanche hazard: how forest cover changes affect return periods and dynamical characteristics simulated by a statistical-numerical avalanche model

International audienc

Impact of land cover on avalanche hazard: how forest cover changes affect return periods and dynamical characteristics simulated by a statistical-numerical avalanche model

International audienc

Impact of land cover on avalanche hazard: how forest cover changes affect return periods and dynamical characteristics simulated by a statistical-numerical avalanche model

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One and a half century of avalanche risk to settlements in the upper Maurienne valley inferred from land cover and socio-environmental changes.

International audienceChanges in mountain landscape can affect avalanche activity, causing changes in risk, potentially enhanced by a transition of the socio-environmental system and its underlying dynamics. Thus, integrative approaches combining biophysical and social sciences are required to assess changes in risk in all its dimensions. This study proposes a holistic methodology combining land cover change detection using advanced image processing techniques, geohistorical investigations and qualitative modelling of risk changes in order to infer the evolution of avalanche risk and its drivers in the upper Maurienne (French Alps) from 1860 to 2017. Results show that a continuous increase of forested areas associated with the retraction of agro-pastoral zones followed a period of land abandonment and depopulation. However, reforestation within avalanche paths remains largely incomplete and mostly absent in the majority of release areas, making a decrease in avalanche occurrence and propagation unlikely. This, combined with marked urban sprawl partially concentrated in avalanche prone areas, locally increased the exposure of residential settlements to avalanches. Hence, even if new defense structures have been set up, our analysis indicates that avalanche risk in the upper Maurienne increased through the study period. Even if local specificity related to physical dissimilarities and/or distinguished socio-economic trends always exist, our results may be valid for many high alpine valleys. Our approach is also transferable to other natural hazards, notably in wider mountain environments, as a contribution to the elaboration of effective adaptation strategies in a context of increasing risks related to combined climate change and socio-economic transitions

Evolution of avalanche risk inferred from a diachronic analysis of landcover change in the French Alps

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Impact of land cover on avalanche hazard: how forest cover changes affect return periods and dynamical characteristics simulated by a statistical-numerical avalanche model

International audienc