Beyond rainfall thresholds: hydrologic controls on debris-flow initiation in alpine channels

Abstract. Debris flows are a severe natural hazard in mountain regions. As a spatially and temporally highly irregular phenomenon, meaningful quantitative description and generalization of the underlying processes in contrasting environments as well as associated robust local prediction of debris flows is hindered by a lack of sufficiently detailed observations. As a consequence, debris flows are frequently predicted based on simple regional precipitation (or rainfall) intensity-duration thresholds that lump many channels over larger spatial domains, irrespective of potentially different underlying debris flow trigger processes due to hydro-climatic or geomorphic differences between the channels. Based on long-term debris flow records of up to 18 years, hydro-climatic observations and modelled hydrological variables across three contrasting active debris flow channels (ID1-ID3) in the Austrian Alps, this study (1) analyses differences between the local debris flow regimes in these channels, based on their strength of temporal coupling with hydro-climatic and hydrological extremes, (2) quantifies differences between regional and local debris flow initiation thresholds that emerge from these differences in their temporal coupling, and (3) identifies the pairs of threshold model variables that allow the most reliable debris flow predictions in each of the three study channels. We have found that the three study channels are characterized by considerable difference in seasonal debris flow timing. In channel ID1, debris flows events are largely confined to early summer, occurring, on average, several weeks before the wettest conditions in each year, thus being only weakly coupled to hydro-meteorological extremes. In the close-by channel ID2, debris flows occur throughout summer, on average a few days after the wettest conditions of any specific year. The high debris flow frequency in this channel in addition suggests multiple cycles of sediment exhaustion and re-supply to the channel over one season. Together, this is evidence for a stronger coupling and a highly dynamic balance between the availabilities of sufficient sediment and in-stream transport capacity, respectively. Due to the geographical proximity of and thus similar hydro-climatic conditions in ID1 and ID2, the differences in debris flow regimes can largely be attributed to differences in landscape characteristics and thus geomorphic predisposition. In contrast, in channel ID3, debris flows are largely confined to late summer and temporally coincide with the wettest annual conditions, suggesting a strongly coupled debris flow regime. The differences between ID3 and the other channels arise from the combined influence of hydro-climatic and geomorphic predisposition. These differences in debris flow regimes are reflected in the differences between the individual local thresholds across the three channels and the systematically superior performance of local thresholds to detect debris flows than regional thresholds across all channels. It was further found that traditional precipitation and rainfall thresholds were consistently outperformed by thresholds that explicitly account for the sum of rainfall and snowmelt. The overall strongest thresholds were found to be the ones based on channel discharge, providing a direct descriptor of in-stream transport capacity. Together the results provide evidence for the benefit of local thresholds and explicitly accounting for hydrological variables in threshold-based debris flow prediction models.