DAM SPILL: What are the factors to open the gates to release water?

Published November 27, 2020, 7:25 AM

by Ellalyn De Vera-Ruiz

  • The Philippines has a Flood Forecasting and Warning System for Dam Operation (FFWSDO) which was formulated in 1978.
  • This consists of a network of telemetered gauging stations, warning stations, supervisory control offices of dam sites linked with supervisory centers in the central offices of the National Power Corp. (NPC), National Irrigation Administration (NIA), and PAGASA.
  • Under the FFWSDO, the dam operators, NPC and NIA, are in charge of providing the “dam discharge warning,” while PAGASA provides the “flood warning” to the communities that are likely to be affected by the release of water from the dams.
  • PAGASA provides the meteorological condition – like forecast typhoon track, wind speed and direction, time of landfall, and the forecast rainfall, among others.

What are the factors that necessitate the opening of a dam’s gates to release water? What agency is tasked to assess the conditions and make that decision? How long before the spilling of water is the alarm sounded to warn communities to evacuate?

These questions have become part of many conversations in the past weeks after the massive flooding in Cagayan and Isabela provinces caused many deaths and damage to property. The release of excess water from Magat Dam’s reservoir after typhoon Ulyssess dumped heavy rainfall was cited as one of the factors to what local officials called the “worst flooding” in their area.

Magat Dam (NIA-MARIIS-DRD-Flood Forecasting and Warning System Dam Operation / FILE PHOTO / MANILA BULLETIN)

On the other hand, dam administrators said it was necessary to release water to prevent Magat Dam from breaking. The inflow of water had elevated the dam’s water level.

On other times of the year, dams have to be filled up to their respective “normal high water levels” to ensure sufficient water supply during the months of January to May, which are normally dry months. But during the rainy months, when there is excessive water inflow during heavy rainfall, water releases from dam reservoirs are conducted.

There is a flood control and warning system in place to warn communities to evacuate especially if the expected water inflow is heavy. There are protocols to follow after the evaluation of factors leads to the decision to release water.

A dam discharge and warning to the public will be issued and disseminated through warning stations (sirens or deployment of personnel or patrol cars), commercial radio stations and local television network, and other means such as SMS, fax, or radio to all agencies concerned.

Here are the facts on how the system works:

The flood control and warning system consists of a network of telemetered gauging stations, warning stations, supervisory control offices of damsites linked with supervisory centers in the central offices of the National Power Corp. (NPC), National Irrigation Administration (NIA), and PAGASA.

The telemetering system is composed of rain and water level gauge stations installed in selected places in dam watersheds, and radio telecommunication system for transmission of data.

This system was formulated 42 years ago, in 1978, after a similar catastrophic event prompted a flood forecasting and warning system to provide flood warnings during an impending release of impounded water through spillways.

That is known as the Flood Forecasting and Warning System for Dam Operation (FFWSDO), hydrologist Rosalie Pagulayan of Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA), said.

According to Pagulayan, the passage of typhoon “Kading” (Rita) in 1978 brought unprecedented rainfall in the watershed of Angat in Bulacan.

A part of Angat Dam is seen at Norzagaray, Bulacan.
(JANSEN ROMERO / MANILA BULLETIN)

As a result, Angat Dam then released floodwater from its reservoir contributing to the massive inundation of Central Luzon that left at least 100 people drowned.

Lead agencies

During the early implementation of the warning system, PAGASA was the lead agency as it has the mandate to provide flood warnings for major and principal river basins across the country, and the dams are within the drainage area of the major rivers, Pagulayan pointed out.

Over the years, other agencies such as NPC and NIA were also tapped to help PAGASA implement the FFWSDO.

At present, the agencies directly involved in its implementation are the NPC and NIA who operate and manage the major dams in Luzon, and the PAGASA through its Flood Forecasting Branch which operates the flood forecasting and warning systems.

The Department of Public Works and Highways, Office of Civil Defense (OCD), and the National Water Resource Board (NWRB), on the other hand, act as the monitoring agencies of the FFWSDO.

What are dams for?

Dams were constructed for many reasons –power generation, irrigation, and domestic water supply. Dams also have a flood mitigating function to hold and control huge amount of rainfall from going straight to the rivers and downstream communities.

Role of PAGASA

Under the FFWSDO, the dam operators, NPC and NIA, are in charge of providing the “dam discharge warning,” while PAGASA provides the “flood warning” to the communities that are likely to be affected by the release of water from the dams, Pagulayan explained.

PAGASA provides the meteorological condition – like forecast typhoon track, wind speed and direction, time of landfall, and the forecast rainfall, among others, she said.

Aside from flood forecasting data, PAGASA also provides dam operators with climatological condition during a La Niña or El Niño, which could affect the rainfall pattern within the watershed.

Knowing the rainfall pattern will allow the dam administrators to effectively manage the stored water and volume of anticipated water inflow.

Protocols for spilling operations

The spilling operation of these dams are guided by protocols and manual operations.

For instance, for Magat Dam, NIA coordinates with the PAGASA for information regarding the “expected date and time of landfall of the typhoon, speed and direction,” as well as the “intensity of rainfall” and “any disturbance aside from a tropical cyclone.”

Before the arrival of a typhoon, NIA also inspects its manual and checks its warning stations and equipment.

It also activates its warning stations, “six hours before pre-release” of water from the reservoir.

Two to three days before the expected landfall of a typhoon, a drawdown of Magat reservoir’s water level is also conducted. At this point, NIA informs concerned agencies, such as the Office of Civil Defense, Philippine National Police, local government units, media, and non-government organizations.

Rainfall intensity

Under Magat Dam’s protocol during a typhoon, water inflow is computed based on the amount of telemetered rainfall observed and recorded.

“The rainfall intensity is of particular importance for the dams as it is utilized to compute for the expected inflow and the resultant water level based on the estimated inflow,” Pagulayan pointed out.

A warning on intensive rainfall is issued by PAGASA. If Magat Dam’s inflow increases and exceeds 1,600 cubic meters per second, floodwater may begin to rise.

Six to eight hours standard warning time

Pagulayan noted that there are actions in the flood operations manuals that are common to all the dams. These are the sounding of sirens, sending written messages, and the deployment of personnel and patrol cars to the target areas of the dams.

In the case of Ambuklao and Binga dams, in the event that a tropical cyclone enters the “typhoon marking area 2,” the operation of the spillway is reverted back to NPC.

Binga Dam in Itogon, Benguet (NAPOCOR / MANILA BULLETIN)

But for all the major dams, the spillway operations, provision of appropriate warnings, and deployment of personnel will be done by the designated flood operation manager.

“While the lag time for the arrival of discharged water to the downstream community varies, the standard warning time is between six and eight hours prior to the gate opening,” Pagulayan said.

She also made clear that “no spilling is also conducted during nighttime.”

Updating of protocols

PAGASA established a Technical Cooperation Project for the Strengthening of Flood Forecasting and Warning System for Dam Operation in 2009-2012.

The project, which was established with financial assistance from the Japan International Cooperation Agency, revisited important aspects of dam operation, particularly the manuals that were being used.

It also conducted river survey to check the carrying capacity of the recipient channel in the event of spillway operation and the conduct of communication drill.

Though the manuals have been updated, Pagulayan cited that “there is a further need to incorporate the changing weather patterns that brings excessive or above normal rainfall to the watershed, which often necessitates the conduct of spillway operation.”

This handout aerial photo taken and received on November 14, 2020, from the Philippine Coast Guard shows residents sheltering on the roof of their flooded house in Cagayan province, north of Manila, on November 14, 2020, days after Typhoon Vamco hit parts of the country bringing heavy rain and flooding.
(Photo by Handout / Philippine Coast Guard / AFP / MANILA BULLETIN)

At present, NIA, along with other concerned government agencies, has committed to review existing protocols on Magat Dam discharges and is looking into the possibility of transferring the authority of water releases to the National Disaster Risk and Reduction Management Council (NDRRMC).

NIA, which is being blamed for the recent flooding in Cagayan and Isabela, said residents living near Cagayan River and concerned government officials, as well as agencies, were informed about the discharge of water on November 9, two days before the landfall of typhoon Ulysses.

However, due to excessive rainfall from previous weather systems and typhoon Ulysses, Magat Dam had to open seven gates on November 12 to discharge excess water from its reservoir.

On November 18, after the massive flooding in Cagayan and Isabela had captivated the attention of the public, there were calls to review operation manuals. In response, NIA, NDRRMC, PAGASA, OCD, NWRB, and the Department of the Interior and Local Government have started to review existing protocols on water discharges from Magat Dam.

 
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