Abstract
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 m3.





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Acknowledgments
This work was supported by grants from the French Ministry of Research in the ACI-CatNat contract MOTE (MOdélisation, Transformation, Ecoulement des coulées boueuses dans les marnes) and from the Centre National de la Recherche Scientifique under the INSU-PNRN contract ECLAT (Ecoulement, Contribution de LAves Torrentielles dans les basins versants marneux). Contribution INSU N°354. Contribution EOST N° 2003-40-UMR7516.
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Malet, JP., Maquaire, O., Locat, J. et al. Assessing debris flow hazards associated with slow moving landslides: methodology and numerical analyses. Landslides 1, 83–90 (2004). https://doi.org/10.1007/s10346-003-0005-x
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DOI: https://doi.org/10.1007/s10346-003-0005-x