Accounting for Non-stationarity in Extreme Snow Loads: a Comparison with Building Standards in the French Alps

In a context of climate change, trends in extreme snow loads need to be determined to minimize the risk of structure collapse.We study trends in annual maxima of ground snow load (GSL) using non-stationary extreme value models. Trends in return levels of GSL are assessed at a mountain massif scale from GSL data, provided for the French Alps from 1959 to 2019 by a meteorological reanalysis and a snowpack model. Our results indicate a temporal decrease in 50-year return levels from 900 m to 4200 m, significant in the Northwest of the French Alps until 2100 m. Despite this decrease, in half of the massifs, the return level in 2019 at 1800 m exceeds the return level designed for French building standards under a stationary assumption. We believe that this high number of exceedances is due to questionable assumptions concerning the computation of current standards. For example, these were devised with GSL, estimated from snow depth and constant snow density set to 150 kg m−3, which underestimate typical GSL values for the full snowpack.

Non-stationarity of extreme ground snow load in the French Alps

<p>In a context of climate change, assessing trends in hazards related to extreme events is urgent. Specifically current methods to compute European standards for snow load actions on structures do not account for the non-stationarity due to climate change. We present the first analysis of extreme ground snow load trends for the whole French Alps. Our method is based on non-stationary generalized extreme value (GEV) distribution, time derivative of return level and likelihood ratio test. Thanks to Météo France reanalysis and snowpack models, we study moutain massif scale data available every 300m of altitude from 1958 to 2017. We detect an overall decreasing trend for annual maxima of ground snow load between 900m and 2700m, which is significant in the Northwest of the French Alps until 1800m. Despite decreasing return levels, in 2017 half of massifs at altitude 1800m still exceeds standard return levels. We underline the importance of snowpack modelling and limitations of approaches relying on ground snow load computed with snow depth annual maxima and an hypothesis on snow density.</p>