Punongbayan, director of the Philippine Institute of Volcanology and Seismology (Phivolcs), said the collapse occurred at the crater wall’s lowest point leading to the Maraunot River in Botolan, Zambales.
A year ago, the Department of Public Works and Highways constructed a canal at the crater’s Maraunot notch to slowly drain water from the two-kilometer wide crater lake to prevent the wall from collapsing and endangering communities.
Last July 10, Punongbayan said the water flowing through the canal finally eroded the materials and gradually released some 160 million cubic meters of lahar through Pinatubo’s western slopes.
As a result, the narrow canal has been replaced by a 3.7-meter wide rivulet, releasing as much as four cubic meters of water per second, he said.
Had the canal not been built, he said the crater’s western wall could have given way and sent at least 320 million cubic meters of water mixed with lahar sediments toward populated areas in Botolan.
The July 10 incident, he said, deposited four meters’ thick of lahar materials at Sitios Poon Bato and Malomboy, but there was no damage to properties nor injuries to Aetas who live there.
"Nature merely did what we all along wanted to happen at the Maraunot notch, where the canal that was dug up was not adequate to drain the rising waters of the crater lake," Punongbayan said.
DPWH regional director Alfredo Tolentino said the breakout saved the government some P1.2 million initially earmarked for the canal’s widening. — Ding Cervantes