Lahar threat seen lurking in La Niña

SOLIDUM. INQUIRER PHOTO

CLARK FREEPORT—The government’s chief volcanologist on Friday advised settlements around Mt. Pinatubo to be vigilant for lahar flow as the La Niña phenomenon expected to strike next month might trigger intense typhoons in the second half of 2016.

“Lahar flow (volcanic sediments washed down by rains) is a lingering threat and it is not yet over,” said Dr. Renato Solidum, director of the Philippine Institute of Volcanology and Seismology (Phivolcs), in a forum organized here by the Capampangan in Media Inc.

“They should not let their guard down,” he said, referring to residents living near the Crow Valley-O’Donnell and Sacobia-Bamban rivers in Tarlac; Abacan, Pasig-Potrero and Porac-Gumain rivers in Pampanga; and Marella-Santo Tomas, Maloma and Balin Baquero-Bucao rivers in Zambales.

Volcanic debris

Until 1997, rain in past lahar seasons washed down volcanic debris, which were estimated by the United States Geological Survey at 3 cubic kilometers (equivalent to 300 million dump-truck loads), by the Phivolcs at 5.5 cubic kilometers and by the Department of Public Works and Highways (DPWH) at 6.7 billion cubic meters.

These were ash, sand, boulders, rocks, pumice stones and sulfur dioxide, which make rivers shallow.

Solidum said around 50 percent of these materials must have receded in middle reaches of rivers and could be remobilized by downpours that occur for nine straight days and nights.

The Zambales rivers were almost the same level as dikes, he said.

The FVR Megadike, which was built in 1997 to trap lahar in Pampanga province, could take in 10 cubic kilometers of lahar based on a scientific model done in 2008.

‘Be vigilant’

Solidum expressed concern for families in 18 Bacolor villages enclosed in the megadike. “They shouldn’t be there,” he said, confirming that the megadike had never been challenged by a typhoon as strong as Ondoy that hit the country in 2009.

The danger was less on the side of Tarlac province because a secondary explosion in 1993 diverted the flow to the Abacan River.

“Dikes should be maintained by dredging and [should be] built strong enough. The DPWH should monitor antilahar structures year-in, year out,” Solidum said.

“People should not panic but be vigilant instead. Really, that kind of scenario [of lahar being remobilized downstream] is not over,” he added.