Abstract

--- - Secondary lahars represent one of the major threats at active or quiescent volcanoes. Therefore, it is important to stablish their frequency, distribution, and triggering mechanism to define hazard-scenarios. - In this work, an analysis of a lahar, that occurred on February 4th, 2010 in the Nexpayantla ravine, on northwestern sector of Popocatepetl volcano is presented. A geomorphological and textural analysis coupled with rain data and satellite imagery were used to determine its origin and to delineate inundation areas. This lahar was triggered by a 100 mm/day precipitation and initiated as a stream flow, that transformed into a debris flow, and ended as a stream flow. The transformation from streamflow to debris flow was due to sediment entrainment by laminar erosion and mass wasting processes. In contrast, its subsequent dilution was favored by changes in channel geometry, the high wood content, and the lack of fine sediment. Modelling of this lahar by numerical simulations allowed to calculate a flow depth of up to 1.7 m. Satellite imagery revealed that the deposit observed in the field represents only 68 % of the actual inundation area. Precipitation data indicates a 27-years return period for rains similar in magnitude to the one that triggered this lahar. Further detailed studies of secondary lahars will allow to better constrain hazard-scenarios at Popocatepetl volcano.