Wardman, J; Sword-Daniels, V; Stewart, C; Wilson, TM; (2012) Impact assessment of the May 2010 eruption of Pacaya volcano, Guatemala. (GSN Science Reports 2012/9 ). GNS Science: New Zealand.

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Abstract

This report summarises the field observations and interpretations of a reconnaissance trip to Guatemala in September 2010. The purpose of this trip was to investigate the impacts of the 27 May 2010 eruption of Pacaya volcano, located approximately 30 km SSW of Guatemala City. This eruption was of particular interest as it presented an opportunity to study an event with parallels to an eruption of the Auckland Volcanic Field and its consequences for the city of Auckland. A further interesting feature of this event was that a major tropical storm arrived immediately after the eruption, providing an opportunity to study the interaction between two co-occurring natural disasters. The 27 May 2010 eruption of Pacaya volcano began shortly after 14h00. The paroxysmal phase started shortly after 19h00 and lasted approximately 45 minutes. This phase generated a plume that was directed towards the north. At Cerro Chino, 1 km from crater, large ballistic fragments (up to half a metre in length) fell, killing one news reporter, injuring many others and destroying buildings, vehicles and equipment. This took local communities and civil defence by surprise as previous tephra falls had been to the west and southwest of the crater and preliminary civil defence efforts had been focussed on those areas. Three communities located 2.5-3.5 km to north of crater were particularly badly affected by the fall of ballistic clasts. Roofs in these towns were extensively damaged by ballistic blocks and to a lesser extent by tephra accumulation. The tephra plume travelled to the north, and Guatemala City was covered in an estimated 2-3 cm of coarse basaltic tephra which local residents described as being like ‘black sand’. The majority of the report is concerned with describing impacts of the tephra fall on Guatemala City. A prompt and efficient citywide cleanup was initiated by the city’s municipality to remove tephra from the 2100 km of roads in the capital. An estimated 11,350,000 m3 of tephra was removed from the city’s roads and rooftops. The possibility of using the tephra for aggregate in cement production was investigated, but it was found to be too friable (low mechanical strength). It was disposed of in landfills around the city. Despite the cleanup operations, considerable quantities of tephra were washed into the city’s underground drainage network from where it was very difficult to remove. Blockages of stormwater drains led to surface flooding of the city’s road network which persisted for months afterwards. Tephra also entered the city’s many wastewater treatment plants, both by direct deposition and through sewer lines. There was no option but to clean out all these systems, an expensive and time-consuming job. A number of accidents happened during the cleanup operations. Limited data available from hospital emergency department admission records indicates that most of these were caused by people falling from their roofs, and other heights, while cleaning up the tephra. The eruption did not cause any discernible increase in respiratory illnesses above normal wintertime levels. This is probably due to several factors: the grain size of the tephra was coarse, with no material present in very fine fractions that can penetrate into the lungs, and the eruption happened in the evening and in rainy conditions and thus most people were indoors. The eruption appeared to have minimal effect on the functioning of two of Guatemala City’s large public hospitals, other than exacerbating pre-existing drainage and flooding problems for one of them as tephra blocked downpipes, gutters, drains and sumps. For electricity and water supplies, effects of the eruption on continuity of supply were minor, although problems were experienced. A geothermal plant close to the volcano was badly damaged by falling ballistic clasts, and had to be closed for repairs and cleaning for three weeks. Flashover was also a problem for distribution lines. Cleaning of tephra from substations was mostly unnecessary because of the arrival of the tropical rainstorm shortly afterwards. For the city’s water supplies, a large storage tank was contaminated by tephra and had to be cleaned out, and there was also abrasion damage to air-cooled motors and groundwater pumps, but generally there was little overall disruption to the continuity of supply beyond normal variations. Probably the most significant disruption caused by the tephra fall was the closure of the international airport for five days, to allow cleanup of the runway and apron. A complication of the cleanup operation was that the tephra was extremely abrasive, and in the process of cleaning a new bituminous runway surface was destroyed and all markings on the runway and apron were removed also. A similar, though more minor problem, was reported while cleanup of the large flat roofs of one of the public hospitals was underway, when a waterproof coating was damaged by abrasion. Development of cleaning methods to minimise abrasion damage may be worth considering for future eruptions of this type. The arrival of a major tropical storm immediately after the eruption generally added to the difficulties experienced by organisations and individuals involved in the response. The storm had a much larger and more widespread impact on the country, resulting in 160 deaths and over 168,000 people requiring evacuation, compared to two deaths (plus two more indirect deaths due to accidents while clearing tephra) and just over 3,000 people evacuated as a result of the eruption. While the heavy rains made some of the impacts of the eruption worse (in particular, it washed the tephra into underground drainage networks before the cleanup was complete, which has in turn worsened drainage problems in the city), it also dampened down the tephra, minimised the corrosive potential of the tephra by washing away its chemically active surface coating), and suppressed fires.

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