Assessing debris flow hazards associated with slow moving landslides: methodology and numerical analyses

Clayey slow-moving landslides are characterized by their capability to suddenly change behaviour and release debris-flows. Due to their sediment volume and their high mobility, they are far more dangerous than those resulting from continuous erosive processes and associated potential high hazard magnitude on alluvial fans. A case of transformation from earthflow to debris-flow is presented. An approach combining geomorphology, geotechnics, rheology and numerical analysis is adopted. Results show a very good agreement between the yield stress values measured by laboratory tests, in the field according to the morphology of the levees, and by back-analyses using the debris-flow modelling code, Bing. The runout distances and the deposit thickness in the depositional area are also well reproduced. This allows proposing debris-flow risk scenarios. Results show that clayey earthflows can transform under 5-years return period rainfall conditions into 1 km runout debris-flows of volumes ranging between 2,000 to 5,000 m(3).