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Executive Summary A relatively small eruption of Nevado del Ruiz volcano, located in the Central Cordillera of Colombia, South America, took place on November 13, 1985. The materials ejected during the eruption melted part of the glacial ice cap at the summit of the volcano, releasing a series of lahars (volcanic mudflows and debris flows). The lahars descended through steep, well-defined, and relatively narrow river canyons, reaching speeds of up to 45 km/hr. Major flows descended the eastern side of the volcano through the valleys of the Azufrado, Lagunillas, and Gualí rivers. The flow through two of these channels, the Azufrado and Lagunillas rivers, merged approximately halfway down the mountainside to form a large flow that continued along the Lagunillas River valley. This flow disgorged through a narrow canyon onto a gently sloping alluvial fan and adjacent floodplain that extend along the eastern front of the mountains. The lahar flow devastated the city of Armero, built on the alluvial fan approximately 2.0-2.5 km downstream from the mouth of the Lagunillas River canyon. Estimates varied and claimed that between 20,000 and 24,000 people perished at Armero, most of them crushed or buried in their homes. Another lahar flow descended the western slope of the volcano through the narrow canyon of the Chinchina River, destroying 400 houses and causing as many as 1,800 deaths near the town of Chinchina. The risk of an eruption like that which occurred on November 13, 1985 was well recognized prior to the event. Extensive effort went into defining the risks and the areas subject to disaster and to promoting emergency preparedness for evacuation and disaster relief. Although many did much to prepare, it seems clear that the risk under which Armero lived its last days was never recognized by local city officials. Additionally, the official
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warning system in place the night of the disaster failed to get word to the citizens of Armero, most of whom were in bed for the evening after ashfall and other volcanic events of the day were thought to have ceased. An estimated 5,000 people did survive and were displaced by the disaster. The continuation of sporadic volcanic activity recorded since the November 13, 1985 eruption, combined with the extensive ice cap still remaining around the summit, could result in future lahars with equally devastating consequences for the large population still living in proximity to the volcano. The study team characterized the magnitude and extent of the flows, catalogued much of the damage, and analyzed the nature of the geologic processes that led to both the initiation of the lahars and the extensive damage resulting from them. In addition, the team interviewed many survivors and participants in the emergency preparedness and recovery efforts. The lessons learned can be summarized as follows: The Nevado del Ruiz volcano continues to present a threat to the region from the formation of additional and potentially larger lahars to other volcanic-related phenomena such as ash fallout. Longer-term research to continuously assess the Ruiz hazard is needed. Special care must be given to properly educate as well as warn those members of the community who will be required to take action during an emergency situation to effectively communicate the potential risk. Risk communication and warnings are taken more seriously when accompanied by materials that provide adequate information, specifically about both the nature of the hazard and the steps that must be taken to mitigate the effect of the hazard. These factors are particularly important for hazards that have not manifested themselves in the lifetimes of the people who face them. The recovery efforts in many ways had characteristics similar to other disaster recovery efforts. The study and analysis of these efforts would have been enhanced had collection of socioeconomic data been undertaken prior to the event. A need to warn quickly and effectively must be emphasized.
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