Triggering mechanisms

Lahars can be triggered by a variety of different mechanisms, both eruptive and non- eruptive. The former type include eruptions through crater lakes, melting of snow and ice, admixing of water with pyroclastic flows, and phreatic explosions. Non-eruptive mechanisms include the sudden release of large amounts of water from crater lakes or subglacial lakes due to edifice collapse and heavy rainfall on volcanic slopes. Examples of each of these mechanisms is given below as evidence of the widespread variety and effects of such flows.

1.4.1.1 Eruptions through crater lakes

A prime example of lahars generated by eruptions through a crater lake have occurred at the Indonesian volcano of Gunung Kelut in Java. Over the last 1000 years, 29 lahars of this type have been responsible for the loss of more than 15,400 lives. The dangers posed by these flows are exacerbated by the natural levees in the river that force inundation of the lowlands adjacent to the channels (e.g., Alzwar (1985)).

Mt. Kelut has the capacity for a very large crater lake. Prior to the eruption of 1901, the most reliable estimate of lake volume was 78×106 _m3_{; post-eruption, the}

volume dropped to around 40×106 _m3 _{(Alzwar (1985); Neall (1996)). The following}

eruption occurred in 1919, and emptied the lake of all water, forming lahars with depths of up to 58 m, travelling up to 38 km from source, inundating a total of 131 km2_{, and}

killing 5,160 people (Alzwar (1985); Neumann van Padang (1951)). After this, the government instigated plans to protect against future disasters of this magnitude by draining the lake of the bulk of its content. The eruption of 1923 illustrated that a step-wise drainage of the lake would be better, after 5 workers were killed; finally, the lake was drained to a volume of less than 2×106 _m3 _{(Neall (1996)). The next}

eruption in 1951 evaporated the waters, and no sizable lahars occurred. The tunnels, however, were blocked, the crater deepened, and the risk of future flows increased. The tunnel drainage system was re-established in 1954 and further tunnels built to drain the deeper crater (Zen and Hadikusumo (1965)). These systems were again damaged after the 1966 eruption that triggered the expulsion of c. 20×106 _m3 _{of water from the}

lake and killed more than 200 people (Neall (1996)). Another eruption in 1990 ejected only small volumes of water that produced limited lahars, killing 32 people (Verstoppen

(1992)). Subsequent extrusion of a lava dome in 2007-2008 displaced nearly the entire lake volume without eruption or major lahar flows (Smithsonian Institute (2008)). Despite this, history has shown that once the dome is removed, the lake will return and highly destructive lahars may still eventuate.

1.4.1.2 Melting of snow and ice, and the admixing of pyroclastic flows

As mentioned previously in this Chapter, the eruption of Nevado del Ruiz, Columbia, on 13th _{November 1985 produced a series of lahars that caused the deaths of more than}

23,000 people. The eruption involved pyroclastic surges and flows and a small Plinian tephra; the total volume erupted was estimated at 1.9×107 _m3 _{(Calvache (1990)).}

Pierson et al.(1990) found evidence that the majority of snow and ice melt was caused by two pyroclastic flows, which scoured channels in the ice c. 100 m wide and 2-4 m deep. The material removed was entrained by the flows and melted, generating dilute slurries of water and debris. These lahars were also enlarged by the entrainment of avalanches predominantly comprised of snow, ice, and rock debris. Peak discharge of these cumulative flows was estimated at c. 48,000 m3_{/s at 9.6 km from source,}

decreasing to c. 27,000 m3_{/s above Armero, at c. 74 km from source, where over}

21,500 people were buried in the deposits (Pierson et al. (1990)). The worst part of this disaster was that the great loss of life could have been prevented. The hazards posed by an eruption were forecasted by scientists, but their recommendations for evacuation were not initiated by the government.

1.4.1.3 Phreatic explosions

Phreatic explosions that involve no magma include the 1888 eruption of Bandai-san Volcano in Honshu, Japan. This event comprised 15-20 explosions directed to the north that followed 15-30 minutes of violent seismicity. The first explosion occurred on the flanks of the volcano, with later explosions concentrated around the base and accompanied by a lateral blast (Williams and McBirney (1979)). The northern flank of the cone collapsed and produced a debris avalanche with a volume of 1.5 km3 that, in turn, initiated lahars along major stream channels, covered 34 km3_{, and buried 7}

villages (Glicken and Nakamura (1988)). Blong (1984) stated that the events of, and subsequent to, the collapse killed a total of 461 people and injured a further 70.

1.4.1.4 Collapse of crater lakes

Collapses of crater lakes have been documented at several volcanoes, including Mt. Kelut in Indonesia in 1875, Agua in Guatemala in 1541, and Mt. Ruapehu in New Zealand in 1953. It is the event at Mt. Ruapehu on the evening of 24th _{December 1953}

that is the most well documented, and occurred when part of the barrier of ice and ash at the lowest rim of the lake, emplaced during eruptions in 1945, collapsed suddenly. Around 340,000 m3 _{of water was released, and formed a lahar in the Whangaehu River}

with a depth of about 7 m. The flow was predominantly constrained to the river channel, but was responsible for the destruction of the Tangiwai railway bridge (Healy

(1954)). Unfortunately, the collapse of the railway bridge coincided with the passage of the night express train from Wellington to Auckland. The train was unable to stop, and plunged into the river, killing 151 people. The flow itself carried a train carriage 2.4 km downstream (Neall (1996)), two 5-ton blocks from the bridge 60 yards downstream, and the 126-ton base of a pier 70 yards downstream (Stilwell et al. (1954)). As a response to the Tangiwai Disaster, a lahar warning system was installed upstream of the reconstructed bridge and is still in operation today. This disaster highlights the unpredictable nature of this type of event, and the necessity for adequate warning systems.

1.4.1.5 Heavy rainfall

After most eruptions, the slopes of volcanoes are covered with easily erodible, loose pyroclasts and ash. These deposits are vulnerable to remobilisation by rainfall, par- ticularly in tropical climates where heavy rainfall occurs seasonally. The heavy rain, combined with easily available and entrainable material, can lead to significant and sizable lahar hazards. Mt. Pinatubo in the Philippines, for example, produced many large lahars in the aftermath of its 1991 eruption (e.g., Marcial et al. (1996)). Ty- phoons released large volumes of water over the Philippines, and were the initial cause of the more than 200 lahars that occurred between 12th June and 10th September and killed a total of 83 people (US Department of Commerce (1992)).

There are many other examples of heavy rainfall causing lahars found around the world. In 1963, renewed activity at Iraz`u Volcano in Costa Rica produced large volumes of ash, which covered a wide area around the volcano. Several lahars triggered by heavy rainfall occurred after this, including the December 1963 flow in the Reventado River that killed 20 persons and destroyed over 300 houses (e.g.,Waldron (1967)). In 1902, a few hours prior to the eruption of Mt Pel´ee that killed 28,000 people in Saint-Pierre, a lahar swept down the slopes and killed 400 people in the town of Le Prˆecheur (Tanguy

(1994)). Yet other examples occur yearly at such places as Mt. Merapi and Mt. Semeru in Indonesia, where seasonal rainfall occurs and causes almost daily lahar flows following numerous ash eruptions (e.g., Lavigne et al. (2000a); Lavigne et al. (2000b);

Thouret et al. (2007)).