Référence
bibliographique complète 
ECKERT, N. 2009. Assessing the impact of climate change on snow avalanche activity in France over the last 60 winters using hierarchical Bayesian spatiotemporal change point models. In Anderssen, R.S., R.D. Braddock and L.T.H. Newham (eds) 18th World IMACS Congress and MODSIM09 International Congress on Modelling and Simulation. Modelling and Simulation Society of Australia and New Zealand and International Association for Mathematics and Computers in Simulation, July 2009, pp. 26042610. ISBN: 9780975840078. http://www.mssanz.org.au/modsim09/G2/eckert.pdf 
Abstract: [...]
Motsclés 
Snow Avalanches, Climate Change, Hierarchical Bayesian Modelling, Change Point Models, Avalanche occurrences, Runout Altitudes. 
Organismes / Contact 
UR ETNA, Cemagref Grenoble, 38 402 Saint Martin d’Hères, France – email: : nicolas.eckert@cemagref.fr 
(1)
 Paramètre(s) atmosphérique(s) modifié(s) 
(2)
 Elément(s) du milieu impacté(s) 
(3)
 Type(s) d'aléa impacté(s) 
(3)
 Soustype(s) d'aléa 
Temperature, precipitation  Avalanche  Dry snow avalanche and wt snow avalanche 
Pays
/ Zone 
Massif
/ Secteur 
Site(s) d'étude 
Exposition 
Altitude 
Période(s)
d'observation 
France  Alpes & Pyrénées  19462005 
(1)
 Modifications des paramètres atmosphériques 

Reconstitutions 

Observations 

Modélisations 

Hypothèses 
Informations complémentaires (données utilisées, méthode, scénarios, etc.) 
(2)
 Effets du changement climatique sur le milieu naturel 

Reconstitutions 

Observations 

Modélisations 

Hypothèses 
Sensibilité du milieu à des paramètres climatiques 
Informations complémentaires (données utilisées, méthode, scénarios, etc.) 

(3)
 Effets du changement climatique sur l'aléa 

Reconstitutions 

Observations 
Abstract: Avalanche runout altitudes are studied in the whole French territory. A change in runout altitude regime has occurred in France around 1976. Between 1946 and 1976, a decrease of 55 m has affected the mean runout altitude, but the probability of a high magnitude event has remained constant. After the change point, the mean runout altitude has regained its initial state, whereas the probability of a high magnitude avalanche has been divided by two. A retreat of avalanche is therefore engaged in France since nearly 30 years. This especially concerns high magnitude events, since the return period associated with an avalanche reaching its minimal altitude on a mean path has increased from 20 to 40 years over the last 30 years. Avalanche occurrences and runout altitudes are therefore differently influenced by changes in constraining climatic factors, so that the interest of a joint temporal modelling of the two phenomena would be limited. One possible explanation is that dry snow avalanches are progressively replaced by wet snow avalanches because of climate worming, thus keeping constant the number of events, but reducing their magnitude by modifying snow rheology. To confirm this statement, further research is needed to compare and explicitly correlate the obtained annual effects with climatic data such as precipitation and temperature series. This will allow improving our knowledge on climate change in the alpine space and its consequence on avalanche hazard. Fluctuations of avalanche occurences in the Northern French Alps: No monotonic trend such as a constant increase or decrease over the period studied is visible. This indicates that there is for the moment no irreversible change in avalanche activity in the northern French Alps. On the other hand a combination of different time structures exists. First, memory effects can be suspected, with many sequences of consecutive years showing close values, for example between 1949 and 1954. Second, cyclic variations seem to occur, with a succession of four cycles of around 15 years, with maxima around 1951, 1965, 1980 and 1995. Finally, abrupt variations also exist, with very low values directly following very high values, for example, between 1962 and 1963 or between 1987 and 1988. The interannual trend [trendg_{t} which corresponds to the succession of the different regimes] provided by the shifting level model consists therefore in a complex combination of pseudoperiodic cycles and abrupt changes [...]. The structured variability explains 42% of the interannual variability, and therefore approximately 5% of the total variability of avalanche occurrences. [...] The relatively good model fit obtained is rather the result of model’s flexibility than of the presence in the data of successive significantly different regimes. Fluctuations of avalanche runout altitudes in France [Alps and Pyrenees]: Obtaining the mean runout altitude fluctuations implies inverting the RAI. This highlights that, in contrast to the mean annual probability of reaching the valley floor, the mean annual runout altitude computed with [the present] model depends on the path considered. With the mean altitude of the valley floor, 1223.3 m asl, the mean behavior at the state level is obtained. Under this assumption, the mean runout altitude has decreased, from a little more than 1400 m asl in 1946 to nearly 1350 m asl in 1977. Since the change point, it has increased again, up to nearly 1400 m asl in 2005. The mean avalanche runout altitude is therefore not different for now than it was 60 years ago in France. However, a clear increase seems to be engaged for nearly 30 years, which is consistent with the general context of climate warming. Moreover, the 1960s and 1970s in France were a short cold period differing from the general context of glacial retreat since the end of the Little Ice Age, which is also coherent with the decrease of mean runout altitudes at that time. These results give good confidence in the ability of the mean RIA to be a meaningful climatic indicator. For the probability of reaching the valley floor, the decrease is strong and continuous since the change point. If the return period corresponding to the valley floor is computes, it appears that it has increased from 20 years in 1980 to nearly 40 years in 2005. The same minimal altitude from a French path is now reached two times less often than 25 years ago. This is important in terms of hazard mitigation, since it suggests that French mountain valleys are now globally less exposed to avalanche hazard than they were earlier. For instance, this indicates that the usual assumption of an underlying stationary process while computing reference scenarios from the available data may lead to overpessimistic decisions. Discussion and conclusion: One possible explanation in accordance with Martin et al. (2001)’s work is that dry snow avalanches are progressively replaced by wet snow avalanches because of climate worming, thus keeping constant the number of events, but reducing their magnitude by modifying snow rheology. To confirm this statement, further research is needed to compare and explicitly correlate the obtained results with climatic data such temperature series. This will allow improving our knowledge on climate change in the alpine space and its consequence on avalanche hazard. Conversely, it will then also be possible to use snow avalanches as proxy indicators that point out further signals of climate change. [...] 
Modélisations 

Hypothèses 
Snow avalanches are mainly ruled by temperature fluctuations, heavy precipitations and wind regimes, so that climate change is likely to modify the frequency and magnitude of both ordinary and extreme events. [...] 
Paramètre de l'aléa 
Sensibilité des paramètres de l'aléa à des paramètres climatiques 
Informations complémentaires (données utilisées, méthode, scénarios, etc.) 
Frequency and magnitude  Snow avalanches are mainly ruled by temperature fluctuations, heavy precipitations and wind regimes, so that climate change is likely to modify the frequency and magnitude of both ordinary and extreme events. [...] [Avalanche occurrences] g_{t} models annual variations in the relative risks, which similarly affect all the townships of the region considered. The time series of g_{t} is therefore directly related to the climatic fluctuations and is presumably a good indicator of the impact of climate change on avalanche activity.

This study focuses on avalanche occurrences and runout altitudes in France over the last six decades. For both variables, a hierarchical spatiotemporal modelling framework is proposed to quantify the interannual fluctuations possibly resulting from climate change. First, the regional annual component is isolated from the total variability using a nonlinear analysis of variance. Second, the latent structured time trend is distinguished from the random noise with different time series shifting level submodels. The hierarchical structure obtained takes into account the uncertainty related to the estimation of the annual component for the quantification of the time trend. Bayesian inference is performed using Markov Chain Monte Carlo simulations [MCMC]. [...] In two recent papers, a hierarchical Bayesian framework has been used to study the fluctuations in France of avalanche occurrences (Eckert et al., 2009a) and runout altitudes (Eckert et al., 2009b). These two variables quantify frequency and magnitude of damageable events and are therefore crucial for hazard assessment. For both of them, the best time series model has been selected among a relatively large class using the DIC criterion. In this paper, the results provided in each case by the best model are summarized. Then, the annual and structured effects are analyzed, looking for possible correlations between avalanche occurrences and runout distances fluctuations over the last 60 years. French avalanche data: Fluctuations of avalanche occurences in the Northern French Alps: For avalanche occurrences, a spatiotemporal approach is conduced at the scale of the township. [...] A non homogenous Poisson observation model is postulated, which is classical for rare discrete events. [Parameters] quantify the spatiotemporal variability of avalanche activity. To detect spatiotemporal patterns, the annual observations are compared to a mean behavior expected in each township from the local topographical and nivological characteristics under the assumption of space and time stationarity. Moreover, a hierarchical model is used to share information between the different townships. The logrelative risks are therefore decomposed into different effects: a spatially structured term, a locally unstructured term, a temporal term (g_{t}) and an interaction term. [...] Fluctuations of avalanche runout altitudes in France [Change point model]: 
(4)  Remarques générales 
(5)
 Syntèses et préconisations

Finally, possible unstationarities are for the moment not taken into account in the avalanche engineering practices. Reference scenarios for hazard management are for instance always computed under the assumption of a stationary process. These results show that this should be reconsidered in the near future given the strong modification of runout altitude regime that has occurred during the last 25 years. 
Références citées :
Eckert, N., Parent, E., Kies, R. and Baya, H (2009a), A spatio temporal modelling framework for assessing the fluctuations of avalanche occurrences under a changing climate: application to 60 years of data in the northern French Alps. Submitted.
Eckert, N., Baya, H. and Deschâtres, M. (2009b), Assessing the impact of climate change on snow avalanche runout altitudes in France over the last 61 years using a hierarchical change point model. Submitted.
Martin, E., Giraud, G., Lejeune, Y. and Boudart, G. (2001), Impact of climate change on avalanche hazard. Annals of Glaciology, 32, 163167.