Upslope migration of snow avalanches in a warming climate.

Snow is highly sensitive to atmospheric warming. However, because of the lack of sufficiently long snow avalanche time series and statistical techniques capable of accounting for the numerous biases inherent to sparse and incomplete avalanche records, the evolution of process activity in a warming climate remains little known. Filling this gap requires innovative approaches that put avalanche activity into a long-term context. Here, we combine extensive historical records and Bayesian techniques to construct a 240-y chronicle of snow avalanching in the Vosges Mountains (France). We show evidence that the transition from the late Little Ice Age to the early twentieth century (i.e., 1850 to 1920 CE) was not only characterized by local winter warming in the order of +1.35 °C but that this warming also resulted in a more than sevenfold reduction in yearly avalanche numbers, a severe shrinkage of avalanche size, and shorter avalanche seasons as well as in a reduction of the extent of avalanche-prone terrain. Using a substantial corpus of snow and climate proxy sources, we explain this abrupt shift with increasingly scarcer snow conditions with the low-to-medium elevations of the Vosges Mountains (600 to 1,200 m above sea level [a.s.l.]). As a result, avalanches migrated upslope, with only a relict activity persisting at the highest elevations (release areas >1,200 m a.s.l.). This abrupt, unambiguous response of snow avalanche activity to warming provides valuable information to anticipate likely changes in avalanche behavior in higher mountain environments under ongoing and future warming.

Holistic rockfall risk assessment in high mountain areas affected by seismic activity: Application to the Uspallata valley, Central Andes, Chile.

Over large regions exposed to natural disasters, cascading effects resulting from complex or concatenated natural processes may represent a large portion of total risk. Populated high-mountain environments are a major concern, and methods for large-scale quantitative risk analyses are urgently required to improve risk mitigation. This article presents a comprehensive quantitative rockfall risk assessment over a large archetypal valley of the Andean mountains, in Central Chile, which integrates a wide spectrum of elements at risk. Risk is expressed as an expected damage both in monetary terms and casualties, at different scales relevant for decision making. Notably, total rockfall risk is divided into its main drivers, which allows quantifying seismically induced rockfall risk. For this purpose, the local seismic hazard is quantified and the yield acceleration, that is, acceleration required to initiate rockfall, is determined at the regional scale. The probability of failure is thereafter derived in terms of annual frequency of rockfall initiation and integrated in the quantitative risk assessment (QRA) process. Our results show the significant role of seismic activity as the triggering mechanism of rockfalls, and highlight elements at risk that have a major contribution to the total risk. Eventually a sensitivity analysis is conducted to (i) assess the robustness of obtained risk estimates to the data and modeling choices and (ii) identify the most influential assumptions. Our approach evidences the feasibility of large-scale QRAs in sensitive environments and opens perspectives for refining QRAs in similar territories significantly affected by cascading effects and multihazards.

Towards a holistic paradigm for long-term snow avalanche risk assessment and mitigation.

In mountain territories, snow avalanches are a prevalent threat. Long-term risk management involves defining meaningful compromises between protection and overall sustainability of communities and their environment. Methods able to (i) consider all sources of losses, (ii) account for the high uncertainty levels that affect all components of the risk and (iii) cope for marked non-stationarities should be employed. Yet, on the basis of a literature review and an analysis of relations to Sustainable Development Goals (SDGs), it is established that snow avalanche risk assessment and mitigation remain dominated by approaches that can be summed up as deterministic, hazard oriented, stationary and not holistic enough. A more comprehensive paradigm relying on formal statistical modelling is then proposed and first ideas to put it to work are formulated. Application to different mountain environments and broader risk problems is discussed.

Assuming accuracy, pretending influence? Risks of measuring, monitoring and reporting sustainable development goals.

From the local to global level, indicators and reports are produced and published to support the transition towards sustainable development. Building from two European-level science-policy workshops, this perspective essay discusses the types of risks involved with such sustainability reporting. The analysis is rooted in the framework of the UN 2030 Agenda and sustainable development goals (SDG). As a globally adopted framework, it provides an example of how risks are either recognised and framed, or non-recognised. Well recognised risks include data availability for SDGs and siloed preparation of indicators, while risks receiving less attention are ritualistic reporting lacking a critical evaluation of the limitations of the SDG framework itself. These different risks are likely to reinforce each other. A specific risk is a too narrow focus on one-way communication aiming to inform individual policy decisions. Risks related to SDGs are best managed with iterative, integrative and interactive knowledge production fostering holistic understanding.

Sustainable Development Goals and risks: The Yin and the Yang of the paths towards sustainability.

The United Nations 2030 Agenda and Sustainable Development Goals (SDGs) define a path towards a sustainable future, but given that uncertainty characterises the outcomes of any SDG-related actions, risks in the implementation of the Agenda need to be addressed. At the same time, most risk assessments are narrowed to sectoral approaches and do not refer to SDGs. Here, on the basis of a literature review and workshops, it is analysed how SDGs and risks relate to each other's in different communities. Then, it is formally demonstrated that, as soon as the mathematical definition of risks is broadened to embrace a more systemic perspective, acting to maintain socio-environmental systems within their sustainability domain can be done by risk minimisation. This makes Sustainable Development Goals and risks "the Yin and the Yang of the paths towards sustainability". Eventually, the usefulness of the SDG-risk nexus for both sustainability and risk management is emphasized.

Force fluctuations on a wall in interaction with a granular lid-driven cavity flow.

The force fluctuations experienced by a boundary wall subjected to a lid-driven cavity flow are investigated by means of numerical simulations based on the discrete-element method. The time-averaged dynamics inside the cavity volume and the resulting steady force on the wall are governed by the boundary macroscopic inertial number, the latter being derived from the shearing velocity and the confinement pressure imposed at the top. The force fluctuations are quantified through measuring both the autocorrelation of force time series and the distributions of grain-wall forces, at distinct spatial scales from particle scale to wall scale. A key result is that the grain-wall force distributions are entirely driven by the boundary macroscopic inertial number, whatever the spatial scale considered. In particular, when the wall scale is considered, the distributions are found to evolve from nearly exponential to nearly Gaussian distributions by decreasing the macroscopic inertial number. The transition from quasistatic to dense inertial flow is well identified through remarkable changes in the shapes of the distributions of grain-wall forces, accompanied by a loss of system memory in terms of the mesoscale force transmitted toward the wall.

Impacts of land-use and land-cover changes on rockfall propagation: Insights from the Grenoble conurbation.

Several studies have debated the incidence of global warming on the probability of rock instability, whereas the impacts of land use and land cover (LULC) changes on rockfall propagation and associated hazards have received comparably little interest. In this study we evaluate the impacts of LULC changes on rockfall hazards on the slopes above the village of Crolles (Chartreuse massif, Grenoble conurbation, French Alps) through a three-level approach: (i) diachronic landscape analysis for four different periods of the past (i.e. 1850, 1956, 1975, and 2013), (ii) computation of 3D rockfall simulations taking explicitly account of reconstructed LULC changes, and (iii) resulting changes in rockfall hazards over time. We reveal that the disappearance of viticultural landscapes (relating to the decline of cropping areas during the interwar period) and intense afforestation of the steepest upper portion of the slope resulted in a significant increase of rockfall return period associated to a gradual decrease of mean kinetic energy at the level of the urban front of Crolles. According to the Eurobloc methodology, the degree of hazard decreased significantly despite the continuous and rapid urban sprawl on the slopes. These results underline that forests can indeed have significant protection function but also call for a more systematic inclusion of LULC changes in hazard assessments in the future.