Lahars triggered by tropical storms at Volcán de Colima, Mexico: flow characteristics and morphological impact on the volcano slopes

Rain-triggered lahars represent a frequent phenomenon at Volcán de Colima, Mexico. Hurricane Jova, on October 12th, 2011, was an anomalously rainfall event resulting in more than 240 mm of rainfall in 24 hours. This event triggered several lahars in the main ravines and induced multiple landslides on the valley sides. Based on data recorded at a seismic monitoring station along the Montegrande ravine, we were able to evaluate the lahar and obtain physical parameters of the flow and associated deposits. The lahar started around 7:20 a.m. (GTM) and lasted for approximately three hours. Five main flow pulses were detected. The first three pulses were closely spaced, and the final two occurred after one hour and 30 minutes, respectively. The event is classified here as a multi-pulse lahar, falling within the hyperconcentrated to debris flow regime. Three main depositional units were recognized along the ravine. The two lower units are up to 50 cm thick, massive, and mostly composed of sand and gravel. The upper unit is up to 1.5 m thick, massive with clasts embedded in a sandy matrix. The deep erosion observed along Montegrande ravine is related to the large magnitude (flow depth and velocity) of these flows, and to their long duration, with three hours of continuous scouring along the ravine. Immediately after the Jova event, dozens of landslides occurred along the ravine, some of which dammed the river and affected the flows during the 2012 lahar season. These landslides both initially trapped lahars and later provided debris for subsequent flow events, even during periods of low rainfall accumulation. The event here described is a clear example of large magnitude lahars at Volcán de Colima during tropical rainfall associated to hurricanes hitting the Mexican Pacific coast, as more recently occurred in 2015 with the transit of Hurricane Patricia. The findings of this study contribute to a better assessment of hazard scenarios in the case of extreme hydrometeorological events at Volcán de Colima and to understanding how their impact can drastically alter the hydrological balance of the volcano.