Comparison of different methods for debris-flow run-out analyses. Insights from the case of Nus (Valle D’Aosta, Italy)

Vittorio Chiessi, Renato Ventura, Carlo Esposito, Gabriele Scarascia Mugnozza


The presented study compares some methods for assessing potential debris-flow paths (and, thus, related spatial hazard) on Alpine alluvial fans in order to evaluate their potential applications. The proposed methods range from empirical to numerical approaches. In particular, the different techniques were used to back-analyse a debris-flow event; their reliability was then assessed by comparing the results with the actual mapped effects of the reference event.

The investigated area is located in the western Alpine arc, in the municipality of Nus (Aosta, Italy), which was hit by a major flood on October 2000. Slope phenomena were dominantly of the debris-flow type and affected a wide portion of Nus, which lies on an alluvial fan in the catchment area of the St. Barthélemy river.

The main effects of the debris-flow (in terms of invaded areas, erosional and depositional zones) were surveyed and  mapped. Such an activity, together with the availability of detailed maps of the event produced by Civil Protection authorities, allowed us to calibrate the input parameters of each adopted methodology. Not much confidence should be put in the results of empirical methodologies as they can be significantly dependent on hardly determinable input parameters, or underestimate the level of hazard and be excessively dependent on previous phenomena. The comprehensive hydraulic methodology produced more reliable results, even if the quality and quantity of data required make this methodology applicable to a limited number of cases.

Preference should thus be given to a step-by-step approach which discriminates the fans with a high level of hazard (requiring comprehensive modelling) from those where simplified modelling might be justified. The alternative approach proposed in this study is a volumetric method, which does not need very sophisticated data and takes into account the actual topographic surface and slope gradient. However, crucial to this approach is an adequate estimation of the magnitude of the expected event.

Finally, due to its reliability, the hydraulic method was used to perform a forward modeling of event scenario for similar events in the current topographic setting, as modified by the 2000 event.


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