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Monday, August 17, 2009

News: Botolan folk told: Watch out for typhoons

Botolan folk told: Watch out for typhoons
By Tonette Orejas

CITY OF SAN FERNANDO, Pampanga—Filipino-American geologist Kelvin Rodolfo has asked his province mates in Botolan, Zambales to be constantly aware of typhoons and monsoon warnings from the weather bureau, urging them to map out evacuation plans to avoid floods and lahar.

In an e-mail from Chicago, Rodolfo said the August 6 flashfloods, which displaced some 7,000 families in 10 villages in Botolan, happened because the Bucao River received runoff from a huge watershed that is still filled with lahar from Mt. Pinatubo’s 1991 eruptions.

“All the runoff from rain [fall] on 270 square kilometers (sq km) on Pinatubo itself, plus 390 sq km from areas outside Pinatubo to the north, mainly the Balintawak-Cabatuan river sytem,” Rodolfo said of what drains to Bucao River before spilling to the South China Sea.

Dr. Renato Solidum, chief of the Philippine Institute of Volcanology and Seismology (Phivolcs), said the flood was caused by heavy rains in the watershed of Bucao River.

That watershed, he said, includes not only the slopes of Mt. Pinatubo but also the mountains west and north of the volcano.

Strong current heavy with lahar and mud breached almost a kilometer of the Bucao dike, swamping 10 villages up to more than 1.52 meters of floodwater.

The volcanic sediments that settled on the village of Carael, which were more than two meters high as of Sunday, caused the river to flow back to the other villages before heading out to the sea, said Alfredo Tolentino, director of the Department of Public Works and Highways in Central Luzon.

“People need to be constantly aware of typhoon and monsoon weather warnings from [the Philippine Atmospheric, Geophysical, and Astronomical Services Administration] and should have a standing plan as to high ground to go to, and a good route to it,” Rodolfo said.

The study, “Two years of lahar on the western flank of Mt. Pinatubo: Initiation, flow processes, deposits, and attendant geomorphic and hydraulic changes,” provides scientific data and findings on what lies ahead of Botolan and other Zambales towns located on the western side of the volcano.

The study, the most extensive of its kind for a Philippine volcano, was done by Rodolfo, Jess Umbal and Rosalito Alonso (University of Illinois at Chicago), Cristina Remotigue (National Institute of Geological Sciences, University of the Philippines Diliman), Ma. Lynn Paladio-Melosantos (Phivolcs), Jerry Salvador and Digna Evangelista (Mines and Geosciences Bureau), and Yvonne Miller (University of Geneva).

The study said about two-thirds of the five to six cubic kilometers of rocks, ash, sand, mud, and gases that Pinatubo spewed out in 1991 settled on the western slopes.

“[Lahar deposits] have filled channels almost to capacity, so future flows are expected to avulse out of the present lahar field along new routes,” it said.

The sediments were “funneled principally” into the Bucao and Santo Tomas rivers. These had filled the channels, spilled out and buried villages and farms.

“Considering the vast quantity of fresh pyroclastic debris, lahar, and excess sedimentation may continue for five to 10 years or longer,” it said.

The Bucao River, with a total watershed of 659 sq km, drains the west and northwest of the volcano and its terrain. The river’s Pinatubo portion, spanning 270 sq km, is “by far the largest catchment basin on the volcano.”

The southern part of the Bucao watershed, on the other hand, is drained by nine tributaries of the upper Balin Baquero River, which is joined by the Maraunot River downstream.

Rodolfo said monsoon rains must be watched because 70 to 80 percent of at least 3,800 millimeters of annual rainfall in the province is delivered by the southwest monsoon (habagat) from June to September.