Impact of snow state variation for design flood simulations in glacierized catchments

A continuously running hydrological energy balance model was used to analyse the influence of different snow covers on the formation of large floods within glacierized catchments. Model outputs from three different catchments in the Austrian Alps with glacierizations ranging from 30 to 39% were statistically interpreted to cover a wide range of possible combinations of areal snow cover distribution and mean snow water equivalent (SWEm) and to define initialisation values for extreme runoff scenarios. These model states consider snow conditions between less than 10% snow coverage up to almost complete snow coverage, with different snow depths represented by minimum, mean or maximum levels of SWEm. For the utilization of extreme runoffs, these snow cover states were combined with design storm input data. An analysis of the resulting flood hydrographs showed that the maximum runoff values occur with minimal snow cover conditions. With increasing snow coverage and SWEm, flow magnitudes tend to decrease while the relevant precipitation time increases significantly. The influence of topographical catchment features, snow cover and SWEm on the simulated scenario discharge is discussed. The contribution of the different runoff components to the modelled peak flows confirms the assumption that floods in glacierized catchments are a combination of directly discharging rain and ice melt.