When and how much water should San Roque Dam spill?
By Gabriel CardinozaPhilippine Daily Inquirer
That question was the focus of discussion during an out-of-town session of the provincial board at the site of San Roque Dam in San Manuel, Pangasinan, two weeks ago.
Vice Gov. Jose Ferdinand Calimlim, the presiding officer, decided to hold the session there, hoping to arrive at a definitive solution to prevent, if not minimize, flooding in communities downstream of the Agno River every time the dam opens its spillway gates.
The Agno River traverses 19 towns and San Carlos City, from San Manuel to Lingayen town, where it finally merges with the Lingayen Gulf.
The opening of the dam’s spillway gates and the subsequent water release are dictated by the flood operation rule (FOR), according to a 55-page manual that National Power Corp. (Napocor) has been using since the facility started operating in 2003.
Virgilio Garcia, field in-charge of Napocor’s flood forecasting and warning system for dam operations (FFWSDO), said the gates would only be opened when all of the following conditions exist: the reservoir elevation is 280 meters above sea level (masl), a typhoon hits Luzon or the Visayas, when the water entering the reservoir is 500 cubic meters per second (cms) or more, and rainfall volume is 60 millimeters (mm) or more in a day.
“The basic principle in spilling is that the outflow (water spilled by the dam) should always be lesser than the inflow (water coming into the dam),” Garcia told the board members.
This way, he said, water would be stored in the dam’s flood control component. The dam has a maximum water elevation of 290 masl. Its flood control component is from 280 masl to 290 masl, which can hold about 140 million cubic meters of water.
Aside from catching run-off water from the Cordillera and the Caraballo mountains, the dam captures water released by Binga Dam in Benguet, which is upstream of the Agno River. Binga Dam, in turn, catches water released by Ambuklao Dam farther up the river.
Ironically, this was the same flood control component that mainly contributed to the massive inundation in the province in October 2009 when the dam’s water level reached more than 289 masl due to the huge amount of rain dumped by Typhoon “Pepeng.”
Napocor had no choice then but to open the six spillway gates, releasing water at the rate of 5,072 cms, more than enough to breach dikes along the Agno River.
The massive flooding killed 60 people and destroyed about P4 billion worth of property, crops and fish.
A Department of Energy investigating committee headed by Dr. Guillermo Tabios, director of the University of the Philippines’ Hydraulics Research Center and member of the National Water Resources Board, later found that the dam operators violated the FOR when they allowed the water elevation to reach more than 284 masl before deciding to spill.
“The dam operators, under the protocol, should have spilled reservoir water to provide for a safety margin for another flood. Not spilling more water at that time ran the risk that there is very little flood control storage available—thus rendering inutile the flood control function of the reservoir, which is exactly what happened,” the committee said in its report.
Trauma
But no one has been punished for the devastation and the loss of lives so far.
Provincial administrator Rafael Baraan said Gov. Amado Espino Jr. has always been worried every time it rains, especially with the unpredictable weather because of climate change.
“We are worried about what would happen if the same scenario as Pepeng takes place. We are worried about the immediate effects of a deluge of the same magnitude,” Baraan said.
This is why, he said, even before Napocor decided to release water from the dam in the last two years, Espino had always been in touch with Napocor officials to ask them to start spilling.
When the skies cleared two weeks ago, among the towns along the Agno River where flooding was reported were Bayambang, Lingayen, Bugallon, Aguilar and Mangatarem, and San Carlos City mainly because of the swollen Tarlac and Camiling rivers that flow into the Agno River in Bayambang and Mangatarem.
“We understand the flood mitigating function of the San Roque Dam. We understand that if all things fail and it rains so hard downstream and upstream, it still has a 10-meter high holding capacity in order to slow down flooding at the lower [river basin],” Baraan said.
Former Pangasinan Rep. Mark Cojuangco, who chaired the House ad hoc committee on dams management and safety which was convened after the “Ondoy” and Pepeng floodings in 2009, said experts who attended the hearings agreed, though informally, on the need for a preemptive water release strategy at San Roque Dam.
“If you know that a typhoon is coming, you should start releasing water that the downstream river system can withstand so that when the typhoon comes, you will have a space for the water that the typhoon will dump,” Cojuangco told the board members.
Cojuangco’s committee had drafted the “Proposed Protocol for Operations of the San Roque Dam System,” but Congress failed to pass it because it was overtaken by the 2010 elections.
The proposal called for the declaration of a typhoon mode of operations for San Roque Dam once the weather bureau declares the start of the rainy season or when the dam’s water level passes the 270 masl level.
Once in the typhoon mode, a preemptive water release would be accomplished by generating power to maximize water outflow from the plant turbines, by spilling water through the spillway or the low level discharge, in case the facility cannot dispatch because of downed or toppled power lines, and by spilling according to a schedule that will reflect the proximity or lead time of an incoming weather event.
Volume of water
How much water the dam should spill was still a big question.
Cojuangco said he was concerned that the dikes downstream could only withstand a certain volume of water released. “The biggest stumbling block in coming up with a meaningful protocol is defining the outflow that will not damage downstream communities,” he said.
Unfortunately, Tabios said, flood control projects in the lower Agno River basin have a low level of protection. “[They’re] based on a 10-year flood return period. If you have [excess water] from the San Roque Dam, you can only release up to 2,600 cms, assuming that the river is full,” he said.
This is because of the rivers downstream flowing into the Agno River between the dam and Bayambang.
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Tags: Flood , Pangasinan , San Roque Dam , water
