Debris flows rainfall thresholds in the Apennines of Emilia-Romagna (Italy) derived by the analysis of recent severe rainstorms events and regional meteorological data

Recent severe rainstorms events in October 2014 and September 2015 triggered more than a hundred debris-flows in the western part of the Apennines of Emilia-Romagna (Italy). In this work, we tested a novel method to define debris flows rainfall thresholds for the 2014 and 2015 rainstorms (which have been used as reference events) and to extend these results across the Apennines of Emilia-Romagna, making use of long-term rainfall data of the regional raingauges network. Results are compared, for validation, to rainfall rates recoded during other past rainstorm-debris flows events (which have been used as validation events). At first, the method involves a spatial discriminant analysis between the spatial distribution of debris flows and the high frequency weather-radar rainfall data for the 2014 and 2015 reference events. The analysis defines rainfall cutoff values over rainfall durations from 30′ to 6 h, related to verification indices in the ROC curves, which are used as debris flows rainfall thresholds. Exceedance ratios are calculated between the computed rainfall thresholds and the rainfall rates at 10 years return periods at corresponding rainfall durations computed for raingauges located in the areas affected by the 2014 and 2015 events. The ratios are then used as multipliers of the rainfall rates at 10 years return periods over rainfall durations from 30′ to 6 h calculated for all other raingauges in the regional study area. To spatialize thresholds calculation to the regional scale, the computed thresholds are interpolated across the Apennines of Emilia-Romagna. The research resulted in the assessment of two-levels debris flows rainfall thresholds curves which seem to be adequate to discriminate rainfall rates recorded during past debris flows events used for validation. Discussion evidences advantages and limits of our approach, compares results to existing debris flows thresholds and highlights their possible use in a multi-stage warning procedure at regional scale.