Delayed Dam Project could have saved city from Sendong

Feb 13, 2015

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A hydroelectric power dam project on the Cagayan River that has been delayed for 15 years running could have possibly mitigated the deadly flash floods which ravaged Cagayan de Oro City some four years ago and killed thousands of residents.

 

Engr Cereal Donggay presents the Bulanog-Batang hydroelectric power project to the CDO City Tourism Council 10 Feb at the City Mayor's Office Conference Room (CIO photo by Rhoel Condeza)

“The Bulanog-Batang Hydroelectric Power Project has a huge 82-million cubic meter reservoir impounding area,” said Engr. Cerael C. Donggay, president and CEO of Greenergy Inc. during a project presentation to the City Tourism Council held Tuesday, 10 February at the City Mayor’s Office conference room.

 

“With adequate warning, we can release water from our reservoir to accommodate a huge volume of flood waters coming from the Bulanog and Batang Rivers which at the very least, could have mitigated the deadly flash floods which killed thousands of people during the Sendong disaster,” he said.

 

A weather specialist of the Philippine Atmospheric, Geophysical & Astronomical Services (PAGASA), who was previously assigned with the agency’s post-event assessment team in Misamis Oriental following a storm surge event, affirmed this but requested anonymity.

 

Impounded Water Area

“The 82 million cu.m. storage, I think is just large and somehow, Sendong is not the only baseline, if ever consider a higher threshold of capacity,” he said.

 

In comparison, he cited how the Pantabangan dam’s reservoir has a gross capacity of 2,996,000,000 cu. m., the biggest in the country (Bulanog-Batang has 2.7% of this capacity). The Angat dam’s reservoir has a gross capacity of 850,000,000 cu.m. (10 times bigger than Bulanog-Batang) with the La Mesa Dam reservoir at 50.5 million cu. m. the closest to it in size.

 

“Angat dam always releases its water during flood season,” he added. “Pantabangan and Angat are both ‘in-basin.’ Meaning, both are inside the Pampanga River Basin. They are both channeled by Pampanga River and Angat River and connected downstream before reaching Manila Bay. Somehow, the construction of the dam will mitigate/minimize the effect of flash floods/flooding in the downstream area of Cagayan river than without the dam as compared.”

 

Tropical Storm (TS) Sendong (International name: Washi), which has been reclassified as a “severe tropical storm,” was the second tropical storm to hit Mindanao in 2011 and the 19th tropical cyclone that entered the Philippine Area of Responsibility (PAR) in the same year.

 

The Lumbia PAGASA Station recorded a one-day rainfall of 180.9 mm which is higher than the monthly average of 117 mm during December in the period 1977-2005.

 

The huge rainfall and steep slopes of surrounding peaks compounded the destructive power of the flash floods.

The following figures show the rainfall measurements in Talakag station during 1-hour, 3-hour, 6-hour and 24-hour periods:

 

Maximum Rainfall Intensity Talakag station

 

Maximum 1-hour        60.6 mm

Maximum 3-hour     112 .0 mm

Maximum 6-hour     199.0  mm

Maximum 24-hour   230.5 mm (Dec 16, 2011)

 

In Capehan station, upstream of river Bubunawan (a tributary of Cagayan river), the 24-hour precipitation recorded on 16 December 2011 was 475 mm.

 

The huge quantity of rainfall that fell on the Cagayan river basin was most likely due to the orographic effect (relating to mountains, especially with regard to their position and form) caused by the mountains which hemmed in its tributaries upstream.

 

Rainfall estimates from NASA’s Tropical Rainfall Measuring Mission (TRMM) satellite, near-real time Multi-satellite Precipitation Analysis (TMPA) at the NASA Goddard Space Flight Center, Greenbelt, Md. were compiled for the period of Dec. 13 to 20, 2011 for the southern Philippines. TRMM is a joint mission between NASA and the Japanese space agency JAXA.

Rainfall totals are on the order of 200 to over 250 mm (~8 to 10 inches) along Mindanao’s east coast where Washi made landfall, but the highest amounts are along the northwest coast, where totals are on the order of 300 to over 400 mm (~12 to over 16 inches). (Steve Lang/SSAI/NASA’s Goddard Space Flight Center, Greenbelt, Md.)

 

Total estimated rainfall from 2011 Tropical Storm Washi (Sendong), as estimated by NASA TRMM satellite. Image credit NASA-TRMM project.

According to the Department of Public Works and Highways, the maximum flood height in 16 December 2011 was 7 to 9 meters over its average height during the last 75 years. (During the floods in January 2009 the level of the rivers reached 2 m). http://typhooncommittee.org/docs/publications/ESCAPWMOTD_0007.pdf

 

The capacity, volume or storage of a reservoir is usually divided into distinguishable areas. Dead or inactive storage refers to water in a reservoir that cannot be drained by gravity through a dam’s outlet works, spillway or power plant intake and can only be pumped out. Dead storage allows sediments to settle which improves water quality and also creates hydraulic head along with an area for fish during low levels.

 

On the other hand, active or live storage is the portion of the reservoir that can be utilized for flood control, power production, navigation and downstream releases.

 

A reservoir’s flood control capacity is the amount of water it can regulate during flooding. The surcharge capacity is the capacity of the reservoir above the spillway crest that cannot be regulated (Votruba, Ladislav; Broža, Vojtěch (1989). Water Management in Reservoirs. Developments in Water Science 33. Elsevier Publishing Company. p. 187. ISBN 0-444-98933-1.)

 

The planned 120-meter high dam is planned to be completed in five years at the junction of the Bulanog and Batang Rivers which feed into the Cagayan River. A cross section of the project presented by Mr. Donggay shows that 26-million cu.m. (32 percent) of the dam’s total reservoir capacity of 82-million cu.m. is ‘active storage’.

 

Bulanog-Batang Dam Cross-Section

“The idea is we are in a trend of Climate Change and there were events that happened unexpectedly like (TS) Sendong and (Typhoon) Pablo that hit our area, plus the expected combination of Habagat with Tropical Cyclone is inevitable,” the PAGASA expert said.

Mr. Donggay did not explain if the active storage reservoir of 26-million cu.m. included provisions for flood control reservoirs.

 

Also known as an “attenuation” or “balancing” reservoirs, flood control reservoirs collect water at times of very high rainfall, then release it slowly over the course of the following weeks or months. In some cases such reservoirs only function a few times in a decade and the land behind the reservoir may be developed as community or recreational land.

 

A new generation of balancing dams is being developed to combat the climatic consequences of climate change. They are called “Flood Detention Reservoirs”. Because these reservoirs will remain dry for long periods, there may be a risk of the clay core drying out reducing its structural stability. Recent developments include the use of composite core fill made from recycled materials as an alternative to clay.

 

However, the PAGASA expert said the flood control measure local governments can implement remains the transfer of residents to higher and safer grounds.

CDO Before and After Sendong

“The best solution, if possible by political will, is to move the people away from the downstream section of the Cagayan River to avoid hazards that lead to risks of lives and properties.”

 

“I’ve been there two times in the downstream area before. Informal settlers aggravate the normal flow of the river and they were the headache of the local government,” he added.

 

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