Pampanga’s defense vs lahar holds despite pond formation

A POND has formed at the FVR Megadike’s base because a spillway has been blocked by sand and other debris. TONETTE T. OREJAS / Inquirer Central Luzon

CITY OF SAN FERNANDO—Water draining from Mt. Pinatubo through the Pasig-Potrero River has formed a pond at the base of the eastern FVR Megadike across Barangay Alasas in this Pampanga capital.

But experts assure the public there is nothing to fear.

The facility’s Spillway 3, designed to slow the flow of lahar and water, has not taken in all of the river water. It is 95 percent complete and is expected to be turned over to the government before the month ends.

Levy Laus, chair emeritus of the Pampanga Chamber of Commerce and Industry, sent Marni Castro, a flood mitigation advocate, to check if the presence of water at the dike posed a threat to lives and property.

“Definitely, water [has been detected at] the San Fernando side,” said Castro who inspected the site on Tuesday.

The FVR Megadike, named after former President Fidel V. Ramos, was built in 1996 to trap volcanic materials washed down by rains from the slopes of Mt. Pinatubo after its eruptions in June 1991.

Meant primarily to save the Pampanga capital and nearby towns, the U-shaped, 56-kilometer-long megadike enclosed around 30 villages of Bacolor, Santa Rita and Porac.

But old lahar deposits, including sand, have accumulated in an area leading to the first of three dike collection pools that take in river water. Each pool is 106 meters long and 15 meters wide.

The deposits have been blamed for the excess water that formed a pond, but experts said the materials actually serve as natural barriers and should  not be dredged out or hauled off.

The megadike itself is sound, according to Gerry Rita, consultant of the Japanese engineering firm Nippon Koei. “Water is not leaking (on the land side of the megadike),” where crops and livestock continue to thrive, Rita said.

The eastern side of the dike is not likely to breach or erode because of the pond, said Leo Mendoza, assistant director of the Department of Public Works and Highways’ Mount Pinatubo Emergency Project Management Office.

Mendoza said that side of the facility was reinforced with sheet piles made of 20-meter-tall steel to protect the dike’s base.

Spillway 3 was built in the vicinity of the megadike’s failed transverse dike, which would serve as a dam in the middle of two dikes.

The P290-million Spillway 3 was built after several portions of the San Fernando-Santo Tomas-Minalin Tail Dike breached in August 2012.

Mendoza said the Japanese-designed spillway, the first of its kind in the Philippines, has three drops that slow down the flow through three pools.

The spillway is also fitted with an irrigation canal that can provide 260 cubic meters of water per second to Bacolor through Barangay San Isidro.

But Castro urges the megadike’s operators to keep a close watch over the spillway during the rainy months to determine if the structure is efficient or if water will flow back or develop ponds.

Spillway 1, located in Santa Rita and worth P270 million, is not operational. Water from the Pasig-Potrero River has yet to be diverted there, Mendoza said.

The Pasig-Potrero River took in 500,000 cubic meters out of 6.6 billion cubic meters of pyroclastic materials that Mt. Pinatubo spewed, according to Nippon Koei.

“Over time, we will see 70 percent of the water flowing to Spillway 1 and 30 percent to Spillway 3,” Mendoza said. The construction of Spillway 2 has been canceled.