The Parraguirre ice–rock avalanche 1987, semi-arid Andes, Chile – a holistic revision

Abstract. Chile is particularly exposed to mountain hazards along the Andean Cordillera. The impact and frequency of devastating debris flows are expected to increase in the future under climatic warming and urban expansion. To inform monitoring, mitigation and adaptation measures, it is crucial to understand the characteristics of past events in this region. The Parraguirre rock avalanche of 29 November 1987 is a prominent example as it developed into a devastating debris flow, reaching 50 km downvalley and causing severe damage and loss of human lives. Its destructive power is related to the large water volume involved. The origin of this water is largely unknown – so is the initial trigger volume and the total mass transfer downvalley. We therefore retrace the past event using data and techniques that are now at hand. These include historic topographic maps, aerial imagery, meteorological and hydrological records, and multi-phase mass-flow modelling. We find a trigger volume of 17.0±1.4×106 m3 and a total fluid flood volume of 16.0×106 m3 – both significantly larger than previous estimates. Moreover, a first estimate of the solid mass transfer exported from the Parraguirre catchment of 38.1±15.2×106 m3 is presented. The high water content cannot be explained by entrainment of soil water and snow cover alone but requires a substantial contribution from glacier ice. Furthermore, our simulations corroborate the damming hypothesis of Río Colorado and, thereby, reconcile the observed wave pulses, arrival times and run-out distance. Apart from the geo-tectonic preconditioning, we define the Parraguirre rock avalanche as a meteorological compound event. The reason for this is that the spring of 1987 was outstanding in terms of the snowpack height, which preconditioned high snowmelt rates during particularly anomalous warm days at the end of November. Such preconditioning can be readily accounted for in monitoring and early warning procedures for mountain hazards.