Modelling of erosion processes associated with rainfall-triggered lahars following the 2011 Cordon Caulle eruption (Chile)

In this work we investigate the sheet and rill erosion process as a possible lahar initiation mechanism in two catchments located in Villa la Angostura, Argentina (Southern Andes), in association with the 2011 eruption of Cordón Caulle (Chile). A few days after the climactic phase of the eruption, several lahar events occurred in Villa la Angostura where 15 cm of tephra mantled the slopes. The physically based model LISEM (openLISEM, 2013a, b) was applied to two small catchments where sheet and rill erosion of tephra deposits was observed. Model input parameters related to vegetation, soil surface and infiltration were obtained from field measurements, geotechnical tests and the literature. A total of 20 scenarios were simulated using a range of values of rainfall intensity and duration and hydraulic conductivity; only 16 of these scenarios resulted in erosion. Results show that hydraulic conductivity (Ks) and rainfall intensity significantly affect the runoff volume, with the smallest Ks (i.e. the tephra deposits associated with the largest fraction of fine ash) being associated with the largest eroded volumes and the largest discharges of water and sediment. The scenarios with the highest hydraulic conductivity did not show erosion for any modelled rainfall intensity. Nonetheless, we conclude that sheet and rill erosion were not the principal mechanism for rain-triggered lahars associated with the Cordón Caulle eruption, but mostly shallow landslide and similar erosion mechanisms contributed to lahars in Villa La Angostura. In fact, the eroded volumes calculated with LISEM are significantly smaller than the volume calculated with shallow landslide models for the same catchments (TRIGRS).