MANILA, Philippines — The National Irrigation Administration (NIA) on Sunday said it followed proper protocols in releasing water from the Magat Dam’s reservoir after a think tank blamed the agency for the severe flooding in Isabela and Cagayan when Typhoon “Ulysses” (international name: Vamco) swamped the provinces last week.
Infrawatch PH said operators of the dam in Ramon town, Isabela province, “rushed to open [the dam] gates only at the height of Ulysses” when it should have made sufficient water drawdown two to three days before the typhoon struck as part of its mandate.
Environment Secretary Roy Cimatu on Sunday said it had yet to be established if the opening of the floodgates of Magat Dam should be blamed for the flooding. He noted, though, that before the typhoon arrived, the Cagayan River was already swollen by rain dumped on its 18 tributaries, many originating in nearby provinces affected by earlier storms since October.
In a statement, the NIA said its team opened the gates as early as Nov. 9 and continued to release water from the reservoir until now to prevent overspilling that may result in dam breakage.
“[The] prereleases or the calibrated opening of dam gates must be made before the occurrence of a typhoon or unusual weather disturbances to minimize, if not to avoid, large Magat Dam discharges before spilling level will be reached,” it said.
The decision to open spillways rests on the NIA, which oversees the operation of the Magat River Integrated Irrigation System.
Interior Secretary Eduardo Año had proposed that during typhoons and other calamities, the National Disaster Risk Reduction and Management Council should be the one to approve the release of excess waters by dams.
He said that depending on the rainfall forecast, the dam floodgates could be opened even before storms hit.
Heavy rainfall
The NIA noted that before Ulysses, five typhoons had swept across Cagayan, as well as the province of Isabela and almost the entire Luzon, which caused rain for almost two months.
“During this period, two gates were opened or an equivalent of 4 meters were opened to reduce water containment at the Magat reservoir. However, as water inflow from our Magat watersheds continually increases, additional gates must be gradually opened to maintain the safe level of the reservoir,” it said.
As Ulysses crossed the Luzon landmass last week, it brought heavy rainfall over the watersheds. The monitoring station of the Philippine Atmospheric, Geophysical and Astronomical Services Administration (Pagasa) in Bayombong town, Nueva Vizcaya province, which is upstream of Magat Dam, recorded 111.4 millimeters of rainfall for a 24-hour period on Nov. 11.
As of 4 p.m. on Sunday, the dam reservoir’s water level was pegged at 192.26 meters above sea level, slightly increasing from an elevation of 192.15 masl recorded earlier in the morning and closer to its spilling level of 193 masl. As of 4 p.m., water was being discharged at 431 cubic meters per second.
Pagasa weather specialist Oyie Pagulayan said this was the lowest recorded inflow so far, compared to the earlier records during the onslaught of Ulysses.
Despite the sparse rainfall over Magat’s reservoir, the increase in dam water elevation may be due to water still flowing from its 4,143-square-meter watershed in Nueva Vizcaya and Ifugao provinces, Pagulayan said in an interview.
Shutting the only open gate will depend on the amount of water flowing into the dam in the next few days, she said. “If we close it now, the elevation may rise again, and we may resort back to increasing the [opened gates],” she said. “Our goal here is to maintain and stabilize the water level.”
Attempting to store the water while very near its spill mark of 193 masl could prove more dangerous for residents downstream, Pagulayan said.
River overflows
According to the NIA, “the continuous rains upstream of Magat Dam contributed to high inflows, while rivers along the Cagayan River … also reached [their] maximum capacity, causing river overflows not only to the Cagayan River but also in adjacent areas.”
The dam is a catch basin of 4,143 square kilometers of watershed connected to eight upstream rivers—the Alimit, Lamut and Ibulao in Ifugao, and the Abian, Matuno, Santa Cruz, Marang and Santa Fe in Nueva Vizcaya, according to the agency.
It noted that Magat River is only one of 18 river tributaries of the whole Cagayan basin and the only river system that could delay the surge of floodwater because of the dam.
The NIA said Magat Dam operators were able to regulate the gush of water from these sources. Without these infrastructure, it stressed that flooding might have worsened in low-lying areas that could have claimed more lives and more damage to property.
The Magat River Integrated Irrigation System has protocols on dam discharge and flood warning, which the NIA said were “strictly observed” during the flood operation.
“The system, as its practice, is making sure that all concerns regarding the dam status are addressed immediately and properly … [the team] is in close coordination with the Office of Civil Defense, Provincial Disaster Risk Reduction and Management Council, Pagasa, Regional Agriculture and Fisheries Extension Network group and other concerned local government offices prior, during and after water releases,” the agency said.
It added that different mediums of communication were used to disseminate information and updates on the dam’s water releases, including television, radio, print, social media, emails, SMS and even sirens.
“All stakeholders are kept posted and updated as to the actual scenario of the water volume that may be flowing into their place on an hourly basis through SMS,” the NIA said.
As parts of Cagayan Valley reeled from the flooding, Pagulayan said it was also necessary to look at the other rivers that drain into the Cagayan River basin, which could also contribute to the swelling of the country’s longest river.
“If we look at the whole river basin, Magat only contributes 15 percent,” she said. “Even if Magat releases water, the other 85 percent also [come] from other rivers with varying drainage areas that flow toward the Cagayan River basin.”
—With reports from Villamor Visaya Jr. and Jerome Aning