Observations of debris-flow events all over the world cover a wide range of phenomenologically similar processes, consisting of different concentrations of water, fine and coarse sediment, and frequently wooden debris. For this reasons, empirically derived coefficients to be used in prediction models to estimate debris-flow dynamics often show a wide degree of scatter. Two of such empirically derived concepts, originally developed for pure water flows, are presented in this study, showing similar deviations from hydrostatic stress assumption in subcritical flow conditions. The first concept is used to estimate debris-flow velocities, based on superelevation data. Based on our experimental results as well as observations from real debris-flow events at the field monitoring station at Illgraben (canton Valais, Switzerland) we show that the empirical coefficient used in the superelevation equation to account for non-Newtonian flow effects correlates with the Froude number – the dimensionless ratio between gravitational and inertia forces in the flow. Interestingly, a similar relationship – the second concept presented – has been found in recent studies to estimate the maximum impact pressure of a debris-flow event. Our results suggest that for debris flows and decreasing Froude numbers inertia forces become more important and the hydrostatic pressure distribution may be an unrealistic assumption for empirically based prediction models in subcritical conditions.