MANILA— The number of spillway gates opened at Magat Dam in Isabela Province has been reduced to 2 from 7 on Friday, the weather bureau’s Hydrometeorology Division said Saturday, as historic flooding hit Cagayan Valley.

PAGASA hydrologist Richard Orendain said the volume of water released by the dam was one of the contributing factors to the widespread flooding currently experienced in Cagayan and Isabela.

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“Hindi lang naman Typhoon Ulysses ang nakaapekto doon pati 'yung frontal system. Mas malakas 'yung ulan nung frontal system doon sa Isabela and Cagayan area kaya hindi pa nagre-release si Magat Dam mayroon nang area na baha,” Orendain said.

(It was not only typhoon Ulysses but also the frontal system that affected that area. The frontal system was much stronger in the Isabela and Cagayan area that’s why there was flooding already in some areas even if Magat Dam had yet to release water.)

Video courtesy of the Philippine Coast Guard via PTV

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Orendain said areas upstream of the Cagayan River experienced strong rains and flooding was also reported in some parts.

“Nadagdagan lalo nung dumating itong si bagyong Ulysses. Nadagdagan ulit 'yung ulan at lumakas lalo 'yung frontal system,” he said.

(This increased when Ulysses arrived. Rain increased and the frontal system got stronger.)

Magat Dam started releasing water on Nov. 9 in preparation for Typhoon Ulysses.

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As of 11 a.m. Saturday, engineer Carlo Ablan of the Flood Forecasting and Weather System of the National Irrigation Administration said the two gates has a combined opening of 4 meters, discharging 1,365 cubic meters per second of water.

The water inflow is, meanwhile, recorded at 1,271 cms.

The dam’s water level is at 192.20 meters, close to its highest elevation level at 193 meters. It had opened all 7 gates on Friday.

Orendain explained that releasing water was necessary to ensure that the dam won't break.

“'Yung dam operation kasi hindi puwedeng paabutan ng spilling level kung alam mo na malaki 'yung pag-ulan doon sa watershed. Kailangan agapan mo 'yan otherwise baka pumutok 'yung dam. Nakakatakot 'yun kaya dinahan-dahan nila ng release,” explained Orendain.

(In dam operation, you cannot let it reach up to the spilling level when you know that there is a big amount of rain in the watershed. You need to prevent that otherwise the dam could break. That is scary that’s why they release water slowly.)

But there is process that must be followed first before water is released, he said.

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PAGASA, he said, will issue a preparatory phase where dam operators would reply if all their warning equipment and spillways are working.

“Ibig sabihin na pag nag-report sila sa amin na ok lahat ng equipment nila, maghihintay na lang kami ng instruction kapag magbubukas sila saka sila magpapadala ulit ng notice of dam discharge,” he said.

(Once they report to us that their equipment is working then we will wait for instructions on when they are going to open and they will send us notice of dam discharge.)

The notice specifies the time of release, the number of gates to be opened, and the total amount of water discharged.

Notice of additional dam discharge is sent once they plan to open more gates, and it should contain the same information, he said.

“Habang nagdadagdag sila may notice munang ipapadala sa amin para ma-notify din 'yung Office of Civil Defense at ma-notify din ng Office of Civil Defense 'yung mga regional offices nila sa affected area,” he said.

(While they open more gates they have to send us notice for us to notify the Office of Civil Defense and in turn, the Office of Civil Defense could notify its regional offices in affected areas.)

The weather bureau has also been issuing flood bulletins, providing its forecast and warnings in areas which may be affected by flooding.

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In its Cagayan River Basin Flood Bulletin No. 12 issued at 5 a.m. Saturday, slow recession on water level was monitored in Tumauini Station (Isabela); Buntun Station (Tuguegarao, Cagayan); and Ninoy Aquino Bridge (Tuao, Cagayan). The water level is above critical and widespread flooding still persisted.

Low-lying areas in Tumauini, Delfin Albano, Sto. Tomas, Cabagan, Sta. Maria and San Pablo in Tumauini Station in Middle Cagayan River may be affected, it said.

In Lower Cagayan River, low-lying areas likely to be affected are Peñablanca, Tuguegarao City, Enrile, Solana, Iguig, Amulung, Alcala, Baggao, Lasam, Gattaran, Lal-lo, Camalaniugan, Appari, Tuao, Piat and Sto. Niño.

Meanwhile, the water level at Pangal Station (Echague, Isabela), Maris Station (Ramon, Isabela), both in Upper Cagayan River, and Gamu Station (Gamu, Isabela) in Middle Cagayan River, monitored “continuous recession on water level.”

While the water is above alarm, flooding still persists and may affect the areas of San Agustin, Jones, Echague, Alicia, Angadanan, Cauyan City, Naguilian, Reina Mercedes, San Mateo, Cabatuan, Aurora, Luna, Gamu, Ilagan City and San Mariano.

Residents near mountain slopes and low-lying areas particularly along rivers are advised to take appropriate action.