REPORT:
Lahar Hazards at Agua Volcano, Guatemala

Introduction

On October 30, 1998 between 10:30 and 11:00 a.m., residents south of Casita heard a roaring noise like helicopters or thunder. Some thought an earthquake was occurring. Three to five minutes thereafter, a wave of muddy debris 1.0-1.3 km wide and an average of 3.5 m deep destroyed all traces of two towns (figure 2). Observations by survivors record an enormous flood on the slopes of the volcano and a wall of mud on the volcano apron. A person on the volcano slopes saw a "black curtain of water with trees." On the apron of the volcano where the flow spread out, survivors describe the flow as -- "an infernal wave of mud,rocks,and trees," or "enormous mass of mud." [2]. The debris flow moved about 10 km from its source. It also generated floods that moved an additional 10 km downstream,destroying roads and bridges and inundating homes.

No large debris flows occurred at Agua volcano during Hurricane Mitch, but historical accounts indicate that such events have occurred there in the past. On September 11, 1541, heavy rains caused a debris flow that inundated Cuidad Vieja (plate 1), killed more than 600 people, and destroyed the town [1]. Cuidad Vieja was the capitol city of Guatemala in 1541. After the destruction of Cuidad Vieja, the capital moved to Antigua Guatemala. The following eyewitness accounts record this event.

Este año a sido de muchas aguas y habiento llovido jueves y viernes y sábado con mucho viento y no mucha agua el dicho lavada serían como es dicho dos horas de noche. Vino muy gran tormenta de agua de lo alto del volcán (Agua) que esta encima de la ciudad fue tan súpito que no . para poder remediar las muertes y danos que sucedieron fue tanta la tormenta de la piedra que trago por delante el agua y la mucha madera y árboles que los que lo uimos estábamose admirados entro por la casa de la del Adelantado don Pedro Alvarado que aya gloria que llevo todas las paredes y tezado como esetaba mas de en tire de ballesta . (Marroquín)[1].

El memorabilisimo [suceso] de 11 de septiembre de 1541,,que totalmente arruinó la ciudad vieja de Guatemala . continuándose por muchos días,,que a no prevenir al aviso de un ruidoso estruendo como por debajo de tierra,hubieran causado los terremotos,que hubo después de la ruina de la ciudad,aun más muertas,que en las que en ella hubo, siendo en número de más de seiscientas personas.Bastantemente expresan las juntas,y funciones de Cabildo esta tribulación;pues en todas las que se tuvieron para conseguir mudar la ciudad,dicen,estar casi continuamente temblando la tierra (Vázquez 1937:1:154)[1].

[English translation:] This has been a year of much rain,and having been raining Thursday,Friday and Saturday,with strong winds and not much water,the mentioned "lavada " [wash,means heavy rain or the debris flow?] happened two hours into the night..A great storm of water came from atop the volcano (Agua)which is above the city,it was so sudden that [we ] did not ...prevent the deaths and damages that happened;the stone storm [debris flow ] was so huge that it swallowed the water in front of it,and much wood and trees,those who saw it were impressed,it entered through the house of the "Adelantado " don Pedro Alvarado,, and it took away all the walls and the roof .(Marroquín)[1].

The well remembered [event] of September 11th, 1541, that totally destroyed old Guate- mala City [this first capital is now named Cuidad Vieja] . lasting many days..Were it not for the noisy uproar underneath the earth that the earthquakes caused,many more deaths would have taken place than those that happened there,a number greater than six hundred people.The meetings and functions of the town Council state this tribulation,since during each meeting they had to achieve the move of the City,they say,the earth was constantly trembling.(Vázquez 1937:1:154)[1].

Although many have suggested that the 1541 debris flow resulted from a crater-lake breakout the accounts above and others in reference note [1] suggest that the 1541 debris flow at Agua was remarkably similar to the 1998 debris flow at Casita. In each case, the event occurred in the latter half of the rainy season, when the ground was water saturated. In each case, several days of heavy rain triggered a landslide, which in turn generated a debris flow. In each case, survivors reported that the ground shook immediately prior to the arrival of the debris flow, and many people mistook the event for an earthquake. In each case, the debris flow destroyed population centers on the apron of the volcano.

In prehistoric time, Agua has erupted explosively to form widespread ash-fall deposits (called tephra), hot ash flows (called pyroclastic flows) and lava flows. Although the volcano has erupted numerous times during the past 80,000 years, the most recent eruptions presently dated are more than 10,000 years ago [1]. If such eruptions were to occur now, many people and costly infrastructure would be at risk. In the aftermath of a voluminous eruption of ash or pyroclastic flows, volcanic debris flows would certainly occur during the rainy seasons. This report describes the hazards of landslides and lahars in general, and discusses potential hazards from future landslides and lahars at Agua volcano in particular. The report also shows, in the accompanying lahar-hazard-zonation map, which areas are likely to be at risk from future landslides and lahars at Agua. A broader volcano-hazard assessment of the volcano is in progress [1].

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