Lahars

Lahars have been responsible for a large number of deaths in volcanic eruptions, and although a few eruptions have caused very large death tolls due to lahars (e.g. Awu, 1711, Galunggung, 1822, Kelut, 1919, Nevado del Ruiz, 1985), smaller death tolls (<50) are common (Table 8).

Galunggung, 1822

At Mt. Galunggung in West Java in 1822, an eruption occurred through the Crater Lake, creating a large avalanche and voluminous lahars to the southeast of the volcano. Hot water and bluish mud were spread up to 38 km from the volcano, burying people in their houses. The following extract describes the event at a distance of c. 50 km form the volcano (from Anon, 1832):

"The rivers, obstructed with these enormous masses of burning lava, were transformed into boiling streams, the overflowing of which arrested at every step the wretched fugitives, and doomed them to a painful and cruel death, in the midst of rivers of liquid fire, or of boiling water. The rivers above. and the Tjilosse and the Tji-Konnir floated down in great numbers the bodies of men and animals, and the inhabitants of the lower districts saw whole houses, with their tenants still living, floating down these swollen and heated streams, without the possibility of giving them relief.. Further from the seat of the volcano the scene was still more dreadful. There were to be seen the lifeless and half consumed bodies of men, women and children, who had escaped wounded from the burning flood, or had been intercepted in their flight, and the still living who were seeking in vain asylum amidst the general destruction. On the 9th, 10th and 11th it rained without interruption, and the wretched inhabitants who had fled from their houses, were left without shelter, and were intercepted in their flight by the overflowing of rivers, and the destruction of the bridges."

Agung, 1963

Lahars were generated following the 1963 eruption of Gurung Agung in Bali, by heavy rainfall remobilising freshly erupted loose material. A lahar inundated the village of Subugan where many had sought refuge in the mosque. About 200 people were killed here while 20 survived in the ceiling of the building. Other deaths occurred in rivers during the rainy season when people were not careful enough when crossing the lahar areas.

Table 8. Lahar events causing deaths, adapted from Blong (1984) and Neall (1996).

Kelut

Mt. Kelut is another volcano with a crater lake, situated on the Island of Java in Indonesia. Numerous eruptions through the lake have occurred, inundating the surrounding lowlands with lahars. In 1586, such an event caused more than 10 000 fatalities. In 1905 a diversion dam was constructed to try diverting lahars from the most populated areas. However, in 1919 all the lake water was expelled in an eruption and the dam was swept away in lahars up to 58 m deep. The lahars inundated 131 km2 destroying more than 100 villages and causing 5160 fatalities. Following 1919, several engineering projects were carried out to lower the Crater Lake level from 78 to around 20 million m3. A violent eruption in 1966 expelled the remaining water to form voluminous lahars which again destroyed the reconstructed diversion dam and radiated outward in several river valleys. Around 45 km2 was inundated and 208 people killed. Further engineering works reduced the lake volume to 4.3 million m3 and an eruption in 1990 generated less lahars, reducing the death toll to 32 people.

Öraefajökull

Öraefajökull is one of the subglacial Icelandic volcanoes that generate huge volumes of meltwater when erupting, to cause enormous floods - termed jökulhlaups. The largest of these occurred in 1362, completely destroying two parishes and sweeping away between 50 and 100 people. An eye-witness reports the following (from Bárdarson, 1971):

"Then came the third crash, and at the same moment the glacier exploded with enormous noise. Water and ice filled every ravine in the mountain and washed away all the people and livestock in the settlement below, or buried them all in deep mud, sand and glacial debris inside the farmhouses."

The district later became known as Öraefi - meaning wasteland. Another eruption in 1727 was smaller, but caused widespread damage to the Öraefi district and the loss of three lives, described in the following extract (from Thorarinsson, 1958):

"Two women and a boy took refuge on a roof of the highest; but they had hardly reached it, when, being unable to resist the force of mud that was forced against it, it was carried away by the deluge of water, and as far as the eye could reach, the three unfortunate persons were see clinging to the roof. One of the women was found afterwards among the mud of the jökulhlaup, but burnt, and as it were parboiled; her body was so damaged and tender but it could scarcely be touched."

Nevado del Ruiz, 1985

The 1985 eruption of Nevado del Ruiz caused the fourth worst volcanic disaster of recorded history, but was only a very small eruption. Pyroclastic flows and surges scoured ice and snow on the upper flanks of the volcano to generate lahars. As the lahars travelled they entrained more rock debris and water to increase their bulk by a factor of up to 4. The now huge lahars swept across valley floors and destroyed much of the town of Armero, 72 km from the volcano. An estimated 21 559 people were entombed by lahars at Armero, with >1000 victims in other areas.

A student who survived the lahars at Armero reported the following (from, Pierson et al., 1990 and Voight, 1990):

"The ash began falling at 10.45 pm; we had 5 minutes waking up our colleagues and the professor. We didn't hear any kind of an alarm; we turned on the radio to see what they said, and the Mayor was talking and he said not to worry, that it was a rain of ash, that they had not reported anything from the Nevado, and to stay calm in our houses. We were listening to it when suddenly it went off the air; then we were trying to tune it in, and at that moment the electric power went out, and that's when we started hearing the noise in the air, like something toppling, falling. We became scared and went out to the street. and when we went out the cars were swaying. motorcycles were toppling over.. and then we were running; we were about to reach the corner but a river of water was coming down the streets. The water that was coming was cold, surely it was the dam. Most of us students ran back to the hotel, which was a three-storey building with a terrace, built of concrete and very sturdy; we though it would withstand, that it was nothing more than a flood, so we went up to the terrace. From the terrace we saw the interior of the hotel, the inner courtyard. We called to our colleagues to come up. They started coming up, and suddenly I felt some blows. I looked towards the back of the hotel and saw like foam, something coming out of the darkness, and some explosions (from ruptured gas pipes). It was the mud getting to the hotel;. it crashed against the rear of the hotel and started crushing in the walls.. Then the ceiling slab fractures and we felt the floor falling out. I didn't know where I was. I was covered with mud. there was a little girl who I thought was decapitated, but what happened was that her head was buried in the mud. A lady told me, "look, that girl moved a leg". Then I moved toward her and my legs sank into the mud which was hot but not burning, and I started out to get the little girl out, but when I saw her hair was caught, that seemed to me the most unfair thing in the world. suddenly we felt like (the mud) was pushing us at an incredible speed, and we felt like we were floating - we were on a water tank, pretty much empty. Luckily, five of us ended up on it, travelling on it; (we) were at the front of the mud, and we saw a wave that was going to fall on us, around 3 or 4 metres high, we continued advancing at an impressive speed, all in darkness (except for the headlights of floating cars); gas pipes were exploding and (debris) was all around us. We would crash against something, and then go on toward something else. then suddenly it began to slow down. Then the tank didn't move any further. it was already midnight, and we sat down to wait. All night long ash was falling. At 4.30 or 5.00, more or less, the noise increased again, then we thought that another mudflow was coming, and sure enough, towards the side you could see something shining that was moving. . And it started to be light, and that's where we lost control because we saw that horrible sea of mud, which was so gigantic. there were people buried, calling out, crying for help, and if one tried to go to them, one would sink into the mud. . So now you must start counting time as before Armero and after Armero. it's like living and being born again. when you analyse that, you realise that a world definitely exists that is stranger than the normal world in which we live."

Thousands of the injured managed to reach higher ground, but 1000-2000 residents still remained trapped alive in the Armero mud. Of these only 65 had been rescued by noon the next day - the viscous mud was so difficult to cross. The terrifying magnitude of the tragedy was cruelly revealed as the mud de-watered in the weeks following the lahars; as its level dropped more bodies were exposed every day. This disaster was all the more tragic because it could have been avoided. The hazards of the eruption were foreseen by scientists but their warnings were not acted upon - authorities were unwilling to bear the economic or political cost of an early evacuation or a false alarm. A volcanic hazard map was prepared but not circulated widely, and last minute evacuation orders were not transmitted in time.

A scientific appraisal of the way in which the events of Nevado del Ruiz were handled by authorities concluded with the following statement (from Voight, 1990):

"Thus the catastrophe was not caused by technological ineffectiveness or defectiveness, nor by an overwhelming eruption, or by an improbable run of bad luck, but rather by cumulative human error - by misjudgement, indecision and bureaucratic short-sightedness. Armero could have produced no victims, and therein dwells its immense tragedy."