REPORT:
Lahar Hazards at Mombacho Volcano, Nicaragua

Introduction

In late October and early November 1998, torrential rains of Hurricane Mitch caused numerous slope failures in Central America, the most catastrophic of which occurred at Casita volcano, Nicaragua on October 30, 1998. At Casita, five days of heavy rain triggered a 1.6-million-cubic-meter rock and debris avalanche that generated a 2-to 4-million-cubic-meter debris flow that swept down the steep slopes of the volcano, spread out across the volcano's apron, destroyed two towns, and killed more than 2500 people [2]. The avalanche did not dam the upper drainages or impound water. Rather, it appears that the Casita debris flow evolved directly as the avalanche moved down slope [2]. On October 30, 1998 between 10:30 and 11:00 AM, 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 Mombacho volcano during Hurricane Mitch, but historical accounts indicate that such events have occurred there in the past. In 1570, an intense rainstorm caused an avalanche and debris flow on the south side of the volcano that inundated a town named Mombacho, killed more than 400 people, and obliterated the town [1]. The town of Mombacho no longer exists and its exact location is not known, but it was apparently high on the volcano or very near the open crater to the south (plate 1). The following account records this event.

Cuatro leguas de esta ciudad [Granada ] estaba un pueblo de indios que se llamaba Mombacho,junto á un volcán pequeño que el año de 70,con un tormenta muy grande de viento y agua que hizo un noche reventó,y un lado del cayó todo la gente que había en él no escapó sino un solo vecino de la Ciudad de Granada que se llamaba Caravallo,y dos indias viejas; quedando seis ó siete españoloes con todos los demás indios enterrados.Por el otro lado salió tan gran tempestad de agua y piedra que en más de seis leguas por aquella parte hizo grande daño en los cacaotales y estancias de ganados (Lopez de Velasco 1970:87) [1].

[English translation]

Four leagues (22,288 meters) away from this city [Granada] there used to be a village of natives called Mombacho, near a small volcano that blew out in the year 70, during a very strong storm at night with rain and wind. One side of it fell(*)over and all the people who had been on it there did not escape except for a neighbor of the city of Granada called Caravallo, and two old native women that were spared; six or seven Spaniards and all others natives were buried by it. Such a great quantity of water and stone came out from the other side that it caused great damage over more than six leagues (33,412 meters) in that part of the cacaotales (cacao plantation grounds) and over the livestock areas (Lopez de Velasco 1770:87)[1].

*Translator's Note: No indication of what fell over. (Something or maybe "one side fell down " in the first instance. "Blew out or burst "=("reventó"). Comment: In general text difficult to follow. Missing and incorrect words.

Many have suggested that an earthquake caused 1570 debris flow as in the following 19th century account:

A causa de un terremoto,collapse la pared sur del volcán [Mombacho], provocando un alud de barro y piedras. El pueblo indígena de Mombacho queda sepultado muriendo sus 400 habitantes (Incer n.d.).

[English translation]

Due to an earthquake, the South face of the volcano [Mombacho] collapsed, causing an avalanche of mud and stone. The native village of Mombacho was buried, 400 of its inhabitants dying (Incer n.d.).

The account below,however suggests that the earthquake happened the night before the debris flow rather than on the same stormy night:

Before the landslide took place:

... solía temblar mucho y muy a llenado la tierra en aquella comarca [Grandada], y las noche antes que se reventa, dicen que temblaban y se meneaban las sabanas y prados circunvecinos, como se menea el agua e el mar poco ante que vea la calma, y que en las casas de Granada no quedaron aquella noche tejas en los tejados, y que muchas paredes y casas se cayeron; ya no tiembla tanto por allí la tierra ni tan á menudo (Dos Religiosos 1965)[1].

[English Translation]

... There used to be many earthquakes and a lot of dirt in that area [Granada], and during the night before it blew out, it is said that, there were earthquakes and that the savannas and meadows trembled liking the movements to those of the sea water before it reaches its calm. It is also said that, on the houses of Granada there were no tiles left on the roofs, and that many walls and houses fell down; there are no more earthquakes in that area or not so frequently (Two religious persons 1965)[1].

Taken together the accounts above suggest that the 1570 debris flow at Mombacho was remarkably similar to the 1998 debris flow at Casita. In each case, an intense rainstorm triggered a land-slide, which in turn generated a debris flow, and the debris flow destroyed population centers near the volcano. Also like Casita, Mombacho is a deeply dissected volcano, with numerous areas of rock that has been weakened by hydrothermal alteration.

In prehistoric time, Mombacho has erupted explosively to form widespread ash-fall deposits (called tephra), and huge debris avalanches. The volcano has also erupted numerous lava flows. Although the volcano has erupted numerous times during the past, the chronology of its activity is poorly known [1]. On at least three occasions huge debris avalanches have occurred at the volcano. These debris-avalanche deposits extend about 10 km from the summit to the southeast, northeast and south (plate 1). Each covers 20 to 30 km2. The oldest of these edifice failures was to the southeast and produced an unnamed group of isletas, the partly inundated hummocks that are characteristic of debris avalanche deposits. This avalanche also dammed Laguna de Pichicha. Its failure scarp has been filled in by subsequent volcanism and is now no longer recognizable as such. A prominent scarp open to the north-northeast (figure 2) is the source of the debris avalanche that formed Las Isletas de Granada,Isla Guanabana and Peninsula de Aseses. The youngest debris avalanche came from the south crater of Mombacho volcano (figure 2). It now occupies a lobe up to 6 km wide that is marked at its distal southern margin by Río El Pital and is characterized by numerous lakes that fill closed depressions on its surface (plate 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 debris, 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 Mombacho 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 Mombacho.

Return to:
[Report Menu] ...