A combined triggering-propagation modeling approach for the assessment of rainfall induced debris flow susceptibility

Rainfall-induced shallow slides can evolve into debris flows that move rapidly downstream with devastating
consequences. Mapping the susceptibility to debris flow is an important aid for risk mitigation. We
propose a novel practical approach to derive debris flow inundation maps useful for susceptibility assessment,
that is based on the integrated use of DEM-based spatially-distributed hydrological and slope stability
models with debris flow propagation models. More specifically, the TRIGRS infiltration and infinite
slope stability model and the FLO-2D model for the simulation of the related debris flow propagation and
deposition are combined. An empirical instability-to-debris flow triggering threshold calibrated on the
basis of observed events, is applied to link the two models and to accomplish the task of determining
the amount of unstable mass that develops as a debris flow. Calibration of the proposed methodology
is carried out based on real data of the debris flow event occurred on 1 October 2009, in the Peloritani
mountains area (Italy). Model performance, assessed by receiver-operating-characteristics (ROC) indexes,
evidences fairly good reproduction of the observed event. Comparison with the performance of the traditional
debris flow modeling procedure, in which sediment and water hydrographs are inputed as
lumped at selected points on top of the streams, is also performed, in order to assess quantitatively
the limitations of such commonly applied approach. Results show that the proposed method, besides
of being more process-consistent than the traditional hydrograph-based approach, can potentially provide
a more accurate simulation of debris-flow phenomena, in terms of spatial patterns of erosion and
deposition as well on the quantification of mobilized volumes and depths, avoiding overestimation of
debris flow triggering volume and, thus, of maximum inundation flow depths.