If dam authorities did not spill water on September 26, the dams could have broken and flooding could have been worse.

(Editor’s Note: This piece is part of our series on “Disasters: the Search for Solutions”.  To contribute to this series, email editorial@abs-cbnNEWS.com)

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MANILA - The afternoon of September 26 was very memorable for me. My heart was beating in excitement as I was going to cook for my mom’s birthday celebration that evening—something I love doing.

But my excited heart beats turned to nervous palpitations as our home felt the rage of the floodwaters brought about by the tropical storm Ondoy.
 
Like an unwelcome visitor, floodwaters entered our home at around one in the afternoon. It reached knee-deep after 20 minutes and it was hip-deep after an hour.

It was all unexpected. We lived in the same place for 14 years and the only “shocking” things that ever happened were a leaking roof and termite attacks. Even neighbors who lived in our part of Quezon City for more than 20 years were surprised; it never even flooded in the front of their houses. They were all devastated.

But other people had it worse. Homes in different parts of Metro Manila were flooded; some were even lost, especially in cities of Pasig and Marikina. Rizal and Laguna provinces were also “deeply” affected. (See the Ondoy situation map for Metro Manila)

Blame game

Like many in the city, I began to ask who should be blamed for the disaster.

That night, a friend called telling me that the flooding was caused, really, by waters released by dams during the storm. I believed her, then.

My friend insisted that it was impossible to flood during the afternoon because high tide was over, so it could only be from the dams.

Dam authorities interviewed on television tried to explain why the dams could not have been the cause of the deluge. But many remained unconvinced.

What further added to the suspicion were eyewitness accounts attesting that the floodwaters that entered their homes had underlying strong currents.

But were the dam authorities really to blame? Or, given circumstances, was the flooding that left Metro Manila and its surrounding provinces in a state of calamity really unavoidable?

Far from Manila

Philippine Atmospheric Geophysical and Astronomical Services Association (Pagasa) Hydro-meteorological division Chief Dr. Susan Espinueva said that 2 dams released water during the storm: Magat dam in Ramon, Isabela and Angat dam in Norzagaray, Bulacan.

But although both dams released water during the afternoon of the storm, Metro Manila was not part of both dams’ spillways.

“Both rivers did not contribute to the Metro Manila flooding. They should look at the maps first,” she told abs-cbnNEWS.com/Newsbreak, alluding to politicians who claimed that flooding in Metro Manila was caused by the Angat dam.

Although Angat dam is closer to Metro Manila, she insisted that it only affects flooding in certain municipalities in Bulacan. (See: Angat Spillway Map)

The map shows that whenever Angat spills water, it affects the municipalities of Angat, Baliuag, Bustos, Norzagaray, Plaridel, Pulilan and San Rafael in Bulacan.

On the other hand, Espinueva said it is impossible that water released from Magat dam caused the Metro Manila flooding. “It is farther (away) and affects municipalities of Isabela,” she said.

The National Power Corporation (NPC) and the National Irrigation Authority are the agencies responsible for the spilling operations of both dams.

“Pagasa’s role is to recommend to them to have pre-spilling operations of the dams and to assist them in mapping out and warning flood-prone areas near the dam,” she said.

Pre-storm spilling operations

In a note posted on botomoipatrolmo.multiply.com, Multiply user RD suggested that enough space should be left in each dam to accommodate at least 3 to 5 days of continuous rain. This way, he said, water release from the dam will not exacerbate flooding caused by heavy rains.

This is actually being done already, according to dam authorities.

As a matter of procedure, spilling operations are done before any storm hits the country, Espinueva said. These operations affect flood prone areas near the dams. These areas are called target areas.

Spillway map of Magat dam

Source: Pagasa Flood Forecast Branch

For instance, 4 hours before each pre-spilling operation is done at the Magat dam, pre-recorded tapes advising nearby residents to evacuate to a higher place are played in strategic places (called warning stations) near the target areas.

The locations of warning stations are indicated in the maps (see: Spillway Map of Magat dam). Warning Stations Type A are marked with yellow circles while Warning Station Type B are the red, ribbon-like markings on the map.

Further, before the pre-spilling operations, municipal and provincial governments are already warned, she said.

There is also a method to the pre-spilling operations.

Initially, water is released at a rate of 100-150 cubic meters per second (cms). This is gradually increased every hour until the reservoir water level (RWL) in the dam is lowered. The RWL pertains to the water level in the dam.

Before increasing the rate at which water is released, dam authorities are also supposed to consult and coordinate with people from the target areas or other lower areas if the rivers’ water levels are already high. If it is already high, they minimize water outflow from the dam, she said. If the water levels are low, they continue to release more water.

Anticipating Ondoy, Espinueva said that pre-spilling operations at the Magat dam started around 1 in the afternoon or mid-afternoon of September 25. “There was no storm or rain that day in Isabela (when the pre-spilling operation was done).”

The following day, Ondoy hit some parts of Luzon. “I should say that the pre-spilling operations were done right on time,” Espinueva added.

Espinueva said the same procedure for pre-spilling operations is done in all dams that are monitored by Pagasa, including the Angat dam.

“Flooding is to be expected in the target or affected areas that have been warned that is why they are warned and told to move to higher places,” Espinueva said.

2 days before Ondoy

The spilling water level in Angat dam is 212 meters (m) in height. The height of the dam is 220 m. “Before 212 m of water is reached, we recommend NPC to have pre-spilling operations,” Espinueva said.

Spilling operations were done in Angat dam 2 consecutive days before the storm hit the country. Each time, the dam released 500 cms of water, Espinueva said.

As of September 25, 2009, Pagasa Flood Forecast hydrograph showed that water level in the Angat dam was maintained at 210 meters high.

Source: Pagasa Flood Forecast Branch, as of October 4, 2009

“The extra meter-space allowed for storage of rain. We anticipated the heavy rains,” NPC Corporate Communications Manager Dennis Gana said.

During the storm, Angat dam obtained 3,000 cms of water inflow and then released a total of 500 cms from 1pm to 11pm on September 26.

Many regarded the release as one of the causes of the flooding. But instead of regarding it as added volume to the floodwaters already plaguing Metro Manila and its environs, one should regard it as a way to prevent further flooding, according to Gana.

That release, he said, was necessary to avoid breaking of the dam—a possibility that could have worsened the flooding.

In effect, Gana said, “the dam prevented the entry 2,500 cms into the river systems. This lessened the flooding. This did not contribute to the flooding.”

Preserving Metro Manila’s drinking water

This leaves one more question—why didn’t dam authorities release more water prior to the storm?

There is a good reason behind this. “We cannot release more than 500 cms of water during pre-spilling operations because the dam might not perform its expected functions,” Gana said.

Angat dam is the primary source of Metro Manila’s drinking water, Gana said. Water from the dam is also used for irrigation and power generation, he added.

“Sometimes, the storms veer away from the target areas. If we release a lot of water it might go below the rule curve of the dam and there will be water shortage if the rains don’t come to fill the dam,” Gana said.

The rule curve is the minimum water level of each dam that will allow the dam to perform its expected functions, Espinueva said.

La Mesa dam helped prevent flood, too

Espinueva said that it is possible that the La Mesa dam, which is in Quezon City, overflowed during the storm because the spilling level of the dam is only 80.15 m.

She explained that the La Mesa dam receives water from the Angat dam, passing through Ipo dam’s underground tunnel to fill its reservoir with Metro Manila’s water supply.

“It has no gate, the dam has no spilling or pre-spilling operations,” she said. Instead of spilling, the dam’s water level is lowered because the water goes to the faucets of residents of the Metro, she said.

Espinueva said heavy rains could have caused water from La Mesa dam to overflow. She maintained, however, that even this couldn’t have been the reason for flooding.

“Whether La Mesa dam is located in Metro Manila or not, because of the month’s worth of rain that poured during that Saturday, there will be flooding,” Espinueva said.

She added that even the La Mesa dam also helped in preventing more floods. “The dam would be able to catch and store rainwater, instead of it going straight to streets,” she said.

Strong currents

Espinueva said that the water from the Sierra Madre mountain range, part of which is located in Rizal province, could have been the cause of flooding in Metro Manila.

“The Tullahan River in Valenzuela also overflowed because it rained hard, this added to the flooding,” she said.

What then accounted for the strong flood currents that witnesses felt as the flood waters entered their homes? And why did flood waters rise so fast at one point?

It was not the water from dams, Espinueva said. “Usually, when waters from mountains go down to low areas, there is no current because the water goes only in one direction,” she said.

The reason for the strong current is that floodwaters from different places overflowed to other places.

“Water came from the mountains, and then the low areas got flooded. When the low areas are completely flooded, water overflows to another low area. The current from the different sources of floodwaters are fighting—this caused the strong current. Not the release of waters from the dam,” Espinueva said. 

Espinueva also explained that floodwaters rose fast when Ondoy hit because there was so much rain water that fell on both “upstream and downstream areas.”

Upstream areas are Antipolo and Montalban or the mountainous areas, she explained, while downstream area refer to the rest of Metro Manila.

She said that, “usually it rains only on the upstream area or only on the downstream area—floodwaters rise but on a more steady or slower pace.”  This was not the case on September 26, when Ondoy hit. – abs-cbnNEWS.com/ Newsbreak