Numerical models are often used as supporting tools for natural hazard assessment and hazard mapping. We apply a physically based numerical model RAMMS (CHRISTEN et al. 2010), which solves the two-dimensional shallow water equations for granular flows using the Voellmy-Salm friction relation (VOELLMY 1955; SALM 1966) to analyse the influence of different commonly used starting conditions for debris flows. In a first step the Voellmy friction coefficients were calibrated by back-calculating a granular-type small event at Dorfbach, Randa (Canton Valais, Switzerland) while in a second step the two different starting conditions, namely block release (all debris material is accelerated at once) and input hydrograph (inflow of material is controlled according to a given discharge hydrograph) were compared by analyzing the resulting discharge curves at a specific location.
Simulations showed that block releases often result in too large flow heights compared to input hydrographs. For the calibrated parameters used here a transition occurs at a relatively small volume of approx. 5000 m³ and an initial release height of only 0.32 m. Since input hydrographs are often based on measured discharge hydrographs in the field or on empirical relationships we infer that input hydrograph simulations represent more realistic starting conditions than block release simulations for channelized debris flows and that there is an overestimation of flow heights for block release simulations over quite a large range of volumes.