engleski

Mapping of maximum snow load values for the 50 years return period for Croatia.

Snow load is one of the important climatic elements that are part of the technical building regulations, together with minimum and maximum temperatures and wind. In particular, the maximum snow load at the ground for the 50 years return period has to be estimated and map has to be supplied as an appendix to the national technical building regulations. The method for the estimation of this parameter at the station locations is presented, following with the geostatistical mapping procedure for estimating this parameter for the whole Croatian territory. Snow load is defined as a product of the snow density, snow depth and the gravitational acceleration. It is often considered as the largest design load for the roof system of the buildings. Hence, careful estimation of the snow load values is important in order to avoid both unnecessary construction cost but also the risk of roof failure (Luna et al. 2003). Although the snow depths are measured at large number of meteorological station, the snow loads are not easy to estimate because of small number of snow density measurements. To overcome this, snow density was fitted to snow depth data using linear regression according to the daily pairs of values from the 13 meteorological stations. Since snow density changes with elevations and seasons, the stations have been divided to two subsets according to the station elevations (lower than 600 m, and 600-1000 m height) and the regression equations are built also on monthly basis (Jonas et al. 2009). Including the highest Zavižan station (altitude: 1594 m) to the group of higher stations, downgraded the estimations by shifting the densities of the rest of the station in the group to to high values. So the Zavižan station has been used to model the snow densities only for one station that is higher than 1000 m. These modelling results allowed for estimating the snow load on the 98 low elevation stations and seven higher elevation stations based on the monthly maximum snow depths and the estimated snow densities. Based on the monthly maximum snow loads the series of maximum annual snow loads can be calculated for 118 locations. Maximum estimated snow load is 2.6 kNm-2 for lower elevation stations and 4.0 kNm-2 for higher elevation stations. Maximum measured snow load of 11.6 kNm-2 is from Zavižan station. Based on the occasional snow episodes on the coastal stations, the estimated snow load is below 0.3 kNm-2 what is consistent with results from other Mediterranean countries. This series of annual maximum snow loads are the basis for the estimation of the annual maximum snow load values for the 50 years return period, by means of the generalized extreme value (GEV) theory. The parameters of the GEV distributions have been estimated with VGAM package that fits many models and distributions by maximum likelihood estimation and is available in R open-source environment for statistical computing and visualisation. For most of the stations, data are close to the Gumbel family of the GEV distributions. Finally, the estimated annual maximum snow load values for the 50 years return period on 118 stations are the input for the geostatistical procedure of mapping this parameter for the 56 000 km2 of Croatian territory. Further on, snow water equivalent, which is an important parameter for the snow hydrological evaluations, can be calculated from the estimated snow load map, by dividing the snow load with gravitational acceleration.

snow load ; snow density ; snow depth ; regression-kriging ; spatial prediction ; generalized extreme value theory

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