No lahar threat, says Phivolcs

SAN FERNANDO, Pampanga, Philippines – The Philippine Institute of Volcanology and Seismology (Phivolcs) clarified yesterday there were no indications that tons of lahar debris on the slopes of Mt. Pinatubo are poised to flow and bury parts of this province.

In an interview with The STAR, Phivolcs chief volcanologist Perla de los Reyes said a team of scientists who inspected the Pasig-Potrero River last Friday found the lahar embedded in the channel remained intact.

The river, which runs down from the slopes of Mt. Pinatubo, used to be an active lahar channel.

De los Reyes said the scientists inspected the river from the area of the Gapan-San Fernando-Olongapo Road downstream up to the so-called Delta 5 lahar monitoring station on the slopes of Mt. Pinatubo some 20 kilometers away.

The supposed lahar threat from the Pasig-Potrero caused some panic among local folk after some people were reported to have claimed that lahar was on its way to a packed local mall. The rumors were immediately contained by local officials.

Huge volumes of volcanic debris were deposited on the Pasig-Potrero channel during the series of lahar flows that followed the Pinatubo eruption in 1991.

The heavy rains brought by the recent southwest monsoon triggered fears that these lahar deposits might be re-mobilized.

Last Friday, San Fernando Mayor Oscar Rodriguez expressed apprehension that strong rains would push the volcanic debris downstream and bury the southeastern portions of this city, as well as the towns of Minalin and Sto. Tomas.

He recalled that in the 1990s, the entire town of Bacolor was literally buried by volcanic materials during the lahar avalanche from the Pasig-Potrero.

“There is no re-mobilization (of lahar debris). It was only water with siltation that flowed,” said De los Reyes.

De los Reyes said the so-called transverse dike across the downstream portion of the Pasig-Potrero remained intact, although some 70 meters of the “tail dike,” also in the lower parts of the river, was damaged.

“The tail dike was after all built to capture the volcanic materials flowing down. With no lahar materials flowing, the tail dike has served to dam up water instead,” she said.

The dikes are within the U-shaped megadike built starting 1997, to shield heavily populated communities from lahar flow.

De los Reyes, however, said that downward movement of lahar along the channel is possible during “torrential rains.”  

She said the recent rains brought by the southwest monsoon were not torrential and did not significantly disturb the deposits in the channel.

While she saw no immediate danger from lahar flow at the Pasig-Potero, De los Reyes nevertheless noted that some people continue to live within the megadike system.

“It’s not for us to tell them to move out, but we must remember that the megadike was built precisely to catch lahar flows within its system to protect outside communities,” she said.