Metro Manila (CNN Philippines, July 26) — Three of Luzon’s major dams opened spillway gates after the southwest monsoon dumped heavy rains over the weekend.
The Ambuklao Dam in Benguet province released water as the reservoir hit close to its spilling level of 752 meters early Monday morning, according to the Philippine Atmospheric, Geophysical and Astronomical Services Administration. Water discharged from the dam flows to its downstream Binga dam.
Luzon’s biggest dam, the San Roque Dam in Pangasinan, serves as the catch basin for the Ambuklao-Binga dam systems. San Roque Dam’s water level on Sunday was way below its spilling level of 280 meters, enough to contain spills from the other two dams.
NAPOCOR’s warning said the Ambuklao Reservoir’s “water elevation was above 750.10 meters and rising amid heavy rainfall” brought about by the monsoon.
Meanwhile, the Ipo Dam rose to as high as 101.40 meters — past its 101-meter spilling level — forcing the dam operator to release water on Sunday noon.
Ipo is not built for flood control and is part of the Angat-Ipo-La Mesa water reservoir that supplies 90% of Metro Manila’s water requirements. It is located in Norzagaray, Bulacan, downstream of Angat River.
The monsoon rains, combined with water released from Ipo Dam, submerged homes and farm lands in villages along the riverbanks in the towns of Calumpit, Pulilan, and Hagonoy on Sunday, according to ground surveillance and data collected by CNN Philippines.
Spillovers from the tributaries of Angat River make other surrounding towns Norzagaray, Angat, San Rafael, Bustos, Baluiag, and Plaridel vulnerable to flooding too when Ipo Dam spills water.
“IPO Dam Management conducted spilling operation at 12:29 p.m. with approximate discharge of 40.9 cms (centimeters),” read an advisory from PAGASA.
The PAGASA is just the messenger, as the decision-making powers on when to open a dam’s floodgates or spillway gates rest on dam operators like the Metropolitan Sewerage and Waterworks System (MWSS) or the private water concessionaires if the dam is used as a water reservoir or irrigation, and the NAPOCOR or hydropower plant operator if the dam is built for generating energy. Confusion over who should be calling the shots on when to release dam water in recent years had led to finger-pointing in the wake of flood-related tragedies.
The catch basin for Ipo Dam is the La Mesa Dam, water from which spills into Tullahan River that snakes through the cities of Quezon City, Caloocan, Malabon, Navotas and Valenzuela. Residents in these areas reported flooding but the PAGASA advisory was clear that the La Mesa Dam did not open its spillway gates.
Several barangays in Rodriguez in Rizal province were also inundated by knee-deep floods over the weekend, forcing thousands to seek shelter in schools and other evacuation sites. But the American colonial-era Wawa Dam was neither to blame. The dam has been decommissioned in the late 60s and has no spillway gates to control rainwater. When Wawa Dam overflows, water levels at the upper Marikina River rise, affecting the towns in Rizal (Rodriguez, Antipolo, Cainta, San Mateo) and Marikina.
