It only takes 40 millimeters of sustained, heavy rain for an hour and a half for lahar to occur and if Albay experiences scattered rainshowers due to the tail-end of a cold front just like last weekend, there is a high possibility of lahar.
Renato Solidum, Philippine Institute of Volcanology and Seismology (Phivolcs) director, said that even if a river channel has old volcanic deposits, rainwater may still plow those deposits, causing lahar.
“Ideally, where there are new deposits, you expect lahar, but people should understand that the slopes of Mayon volcano is covered with loose sand, ash, or rocks that can be re-mobilized even though they are old,” he said.
This could be catastrophic. Unlike in Pinatubo, the soil in Mayon Volcano could be mixed with rock, making it more dangerous for people who would be caught in the lahar.
In their January 27 advisory, Phivolcs listed the major river channels of Buyuan, Miisi, Basud, San Vincente, Buang, Quirangay, and Masarawag-Maninila, but Solidum said that the aside from those listed, other river channels should be watchful for lahars or sediment-laden stream flows.
Lahar vs stream flow
Solidum said that the pitfall of many is how to describe a flow of water with varying degrees of solid.
“If the solid content is 20 percent or greater, we can call that lahar,” Solidum said.
He said that flows of water consisting less than 20 percent are just flood or a stream flow.
He further explained that lahars with 20 percent to 60 percent solid content are hyperconcentrated lahars or ‘lahar na malabnaw.’ If it consists more than 60 percent solid, it is then ‘lahar na malapot’ or debris flow, which he describes as cement-like.
“If there is so much sediment, what you will see as the lahar is moving is it will push the boulders in front,” he said.
In Mayon Volcano, because of the boulder front, the boulders collide and produce a sound, signaling volcanologists or residents nearby that a lahar is approaching.
Solidum said that when they monitor the volcano, as they listen, they look for a spot where they can easily get a sample of the lahar. One of the villages where lahars usually occur is Matanag in Legazpi City.
Hot and cold lahar
Hot lahar consists new deposits from the volcano and are usually visible due to the presence of steam. In Budiao Church, immediately after the February 1, 1814 Plinian eruption that killed 1,200 Albayanos, the lahar was hot.
Solidum said that hot lahar has hot surface like cement and are usually white due to precipitate.
He however said that whether it is hot or cold, it is still dangerous and warned residents to stay away from lahar.
Part 1: Mariton Bornas
Part 2: Ed Laguerta
Part 4: Don’t judge Mayon’s alert level by the pics you see online
Part 5: Non-local Heroes
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