In an interview with Punto, Philippine Institute of Volcanology and Seismology (Phivolcs) director Rene Solidum urged yesterday government engineers to monitor structures, especially old ones, which were constructed along former lahar channels emanating from Mt. Pinatubo.
Solidum cited studies showing that the upper reaches of the lahar-laden Pasig-Potrero River now tends to flow towards this city and that severe rains could lead to an overflow of the existing anti-lahar megadike a few kilometer’s from this city’s downtown and posh subdivisions.
In 1996, the government constructed a U-shaped “megadike” at the Pasig-Potrero River to contain the flow of lahar which has already buried most of Bacolor town by then. The dike prevented lahar from spreading to other threatened parts of Pampanga, including the cities of Angeles and San Fernando.
While volcanic debris on the upper slopes of Mt. Pinatubo have almost been depleted, volcanic deposits on the river channels could still be remobilized, particularly during the rainy season.
Solidum said that in case of “extreme rains”, millions of cubic meters of combined water and volcanic materials could still flow through the Pasig-Potrero. Because of this, there is a need for continuous dredging of the river’s channel to prevent its flows from overflowing or damaging the megadike.
He said that quarrying of lahar materials could be done within the Pasig-Potrero to preserve the channel’s depth, but stressed the need for government engineers to manage such quarrying.
Solidum , however, said that a possible overflow at the section of the megadike towards Angeles City is expected to be “minor” magnitude.
This, even as Barangay Cuayan chairman Arturo Candelaria has sounded the alarm over breached portions of the Taug dike along the Abacan River in this city. The Abacan River also used to be a lahar channel until a “secondary explosion” on Mt. Pinatubo’s slopes in 1993 triggered changes that blocked lahar deposits from flowing through it and the Sacobia River in Tarlac, diverting lahar flows to the Pasig-Potrero River.
Solidum said, however that floodwaters, not lahar flows, could threaten this city in case the dike is breached.
He urged, however, the Department of Public Works and Highways (DPWH) to monitor old anti-lahar structures, as he recalled a series of small anti-lahar dams, or sabo dams, which were constructed at the upper reaches of the Abacan River after Mt. Pinatubo’s eruption.
“These were constructed as check dams to slow down lahar flows, “ he said, although he noted that the volume of volcanic debris trapped in them was not significant. The dams were made of rocks held together by sheets of cyclone wires.
At the same time, Solidum reiterated there is no danger from Mt. Pinatubo’s 2.5-kilometer wide and over 100-meter deep crater lake which developed after the volcano’s eruption.
“The lake’s waters are continuously drained through the Maraunot notch towards Botolan, Zambales to the South China Sea. The lake is on a bedrock,” he said.