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. Impact and frequency of devastating debris-flows is 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 November 29, 1987 is a prominent example as it developed into a devastating debris flow reaching 50-km down-valley 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 down valley. We therefore retrace the past event using new findings from remote sensing, climate and hydrological records as well as process-based modelling. Important corrections are at order. We find a trigger volume of 17.0 ± 1.4·10⁶ m³ and a total fluid flood volume of 16.0·10⁶ m³. The solid mass transfer out of the Parraguirre catchment amounts to 38.1 ± 15.2·10⁶ m³. The high water content cannot be explained by entrainment of soil water and snow cover alone but requires 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 pre-conditioning, we forward the Parraguirre rock avalanche as a meteorological compound event. The reason is that the spring of 1987 was outstanding in terms of the snowpack height, which preconditioned high snow-melt rates during particularly anomalous warm days at the end of November. Such pre-conditioning is readily accountable in monitoring and early warning procedures for mountain hazards.