Non-stationary extreme value analysis of ground snow loads in the French Alps: a comparison with building standards

Erwan Le Roux,Nicolas Eckert,Samuel Morin,Guillaume Evin,Juliette Blanchet

doi:10.5194/nhess-20-2961-2020

Abstract

Abstract. 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 50-year return levels of ground snow load (GSL) using non-stationary extreme value models. These trends 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 to 4200 m, significant in the northwest of the French Alps up to 2100 m. We detect the most important decrease at 900 m with an average of −30 % for return levels between 1960 and 2010. Despite these decreases, in 2019 return levels still exceed return levels designed for French building standards under a stationary assumption. At worst (i.e. at 1800 m), return levels exceed standards by 15 % on average, and half of the massifs exceed standards. We believe that these exceedances are due to questionable assumptions concerning the computation of standards. For example, these were devised with GSL, estimated from snow depth maxima and constant snow density set to 150 kg m−3, which underestimate typical GSL values for the snowpack.

Highlights

Extreme snow loads can generate economic damages and casualties
We compute the relative change of 50-year return levels of ground snow load (GSL) between 1960 and 2010, quantify the uncertainty of return levels in 2019 to compare them with the stationary return levels designed for French standards (Sect. 4.3)
The largest decrease is found at 900 m with −30 % for return levels between 1960 and 2010

Summary

Introduction

Extreme snow loads can generate economic damages and casualties. For instance, more than USD 200 million in roof damages occurred during the Great Blizzard of 1993 (O’Rourke and Auren, 1997). The observed height of accumulated snow is called snow depth (in m). The density of this snow can vary widely between precipitation particles (ρSNOW ≈ 100 kg m−3) and a ripe snowpack (ρSNOW ≈ 500 kg m−3). Multiplying snow depth by snow density gives the surface mass of snow (in kg m−2). Surface mass of snow corresponds to the snow water equivalent (SWE), which is the height of water (in mm) we could obtain if we melt all the snow in a 1 m2 area. Snow load is the pressure exerted by this surface mass of snow (in N m−2 or Pa) and equals the SWE times the gravitational acceleration (g = 9.81 m s−2)

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