Respective influence of geomorphologic and climate conditions on debris-flow occurrence in the Northern French Alps

We investigate the influence of geomorphic and climate conditions on the spatiotemporal variability in debris flow occurrence using a dedicated hierarchical Bayesian model. The case study includes 104 catchments from the French Alps with debris flow occurrences since 1970. We identify from a list of 21 variables two geomorphic and two climate variables that explain 72% of the total variance. Lithology is identified as the primary driver (42%) of the spatiotemporal variability of debris flow activity. To analyze the effect of the duration of the observation on inferred relations, we run the model using debris flow occurrence data and climate variables from the last 12 and 23 years of the study period, respectively. The model is significant as soon as at least 23 years of observations are considered. To detect the influence of spatial variability in debris flow frequency, we partition our data set into two subsamples composed of catchments with at least or less than 4 debris flows over the period. This reveals that precipitation during the event and the 2 days before exceeding 20 mm in total are the most important variables for both subsamples. It explains at least 57% of the total variance and 73% of its climate component. We conclude that difference in debris flow occurrences from one catchment to another is mostly due to geomorphologic conditions, whereas homogeneous groups of catchments react strongly to climatic conditions in a rather similar way.