Solutions to flooding

DEMAND AND SUPPLY - Boo Chanco (The Philippine Star) - November 23, 2020 - 12:00am

The infuriating thing about our annual flooding problem is that there has been no shortage of solutions offered through the years. Our officials simply failed to act on the proposals.

An example is the Manggahan Floodway, where the government failed to implement the other part of the project which is the Paranaque spillway. As a result, we didn’t see any improvement.

One solution offered by Carlos Arcilla of UP is the use of capture tunnels in the Marikina-Montalban watershed to reduce the discharge of floodwater flowing into Marikina and returning the captured water volume to the Sierra Madre divide for discharge eventually to the Pacific Ocean.

Arcilla wrote in the Inquirer that “this solution should be cheaper and could be integrated in the overall scheme of flood water management, resulting in a much lesser discharge of floodwater into the metropolis, which would reduce the kind and amount of intervention.

“These capture tunnels might be more feasible than a proposed solution of making a channel from the Marikina Valley directly to the Manila Bay, potentially running over infrastructure and residents.”

I guess Arcilla is not too hot about the Paranaque spillway proposed by a study group led by environmental architect Jun Palafox back in 1977.  Jun was commissioned to help develop an urban plan for Metro Manila under the Metro Manila Transport, Land Use, and Development Planning Project.

Two projects were proposed as part of the plan’s flood control management: the ManggahanFloodway and Parañaque Spillway deals. But only the Manggahan Floodway was built.

The Manggahan Floodway-Paranaque Spillway Complex was supposed to effectively contain and channel floodwaters. Under the plan, the Laguna Lake will act as the initial floodwater container and the Manila Bay a secondary container.

When the Pasig and Marikina rivers reach critical levels, excess water is diverted to Laguna Lake through the Manggahan Floodway, while the Paranaque Spillway will flush out excess water from Laguna Lake towards Manila Bay to protect the 29 Laguna lakeshore towns.

The Manggahan Floodway and the Paranaque Spillway were designed to be simultaneously constructed, but only the floodway was built. As a result, every time flood waters are diverted into the Laguna Lake via the Manggahan Floodway, Laguna Lake becomes a toilet without a flush.

Palafox said “that was what exactly happened during the onslaught of Typhoon Ondoy, where 4,600 cubic meters per second of flood waters came down from the mountains and flooded more than 80,000 hectares of low-lying urban land in Metro Manila and around Laguna Lake. Singapore, in comparison, has only 71,000 hectares of land.”

Now they are saying the spillway cannot be built because the right-of-way is all built-up.

But for San Miguel’s Ramon S. Ang, that is not a problem. He still wants to build a $2-B Metro Manila Spillway, but through an underground tunnel. He plans to use similar boring machines to be used for the Manila Subway project.

Mr. Ang will build it at no cost to the government. He just wants the government to give San Miguel the right to generate power from the shifting tides.

Responding to my text inquiry, he described his proposal as a tunnel of 15 meters in diameter and about 20 kilometers from Laguna de Bay to Manila Bay. He said with two spillways, it will solve Metro Manila’s flooding problem.

How? He explained that Laguna de Bay will be able to work as a water storage again for the Sierra Madre mountains and Marikina River, and then flow out to Manila Bay. Because Pasig River is silted, RSA said “that’s why we need these two new spillways.”

RSA got his inspiration for this proposal from Sihwa Lake, an artificial lake in South Korea. It was created to provide reclaimed land for the nearby metropolitan area, flood mitigation, and secure irrigation water by converting the coastal reservoir to fresh water.

But water quality deteriorated due to a number of factors. The Koreans decided to construct the Sihwa Lake Tidal Power Station, the world’s largest tidal power installation, with a total power output capacity of 254 MW.

The tidal units essentially produce power by exploiting the gap in the water level between the sea and Lake Sihwa. As the tide rises, saltwater flows through the turbines from the Yellow Sea and into Lake Sihwa, creating electricity.

But at low tide, the gates are raised and the turbines revert to sluicing mode, allowing the lake to be emptied and no electricity to be produced.

The most remarkable impact has been the recovery of water quality and ecosystems. The continuous circulation of water between the lake and the outer sea during the power generation process has improved the water quality.

RSA said he submitted a proposal to DPWH during PNoy’s time and up to now has not been given a response. At no cost to the taxpayers, the only concern of government regulators should be the environmental impact of the project.

But if we are looking at dredging as one solution, Mr Ang said he also plans to dredge major rivers in Bulacan to mitigate flooding in the province, including the P1 billion Tullahan River dredging project.

“Dredging the Tullahan is a major component of our flood mitigation strategies. Essentially, it will clean up and deepen the river system, increasing its capacity to handle heavy rains and allowing for floodwaters to flow more freely and drain into the Manila Bay,” Ang said.

He explained that the Tullahan River is also a spillway for water coming from Angat and Ipo Dams going to the Manila Bay, hence the need to maintain its depth.

San Miguel has also started planting the first 10,000 mangroves it intends to plant in Hagonoy, Bulacan, as part of a larger plan to plant and grow almost 200,000 mangroves over 76 hectares of coastal area throughout Bulacan and Central Luzon.

So many things to do and many at no cost to the taxpayers. The Duterte administration should break from the past and act on these flood control projects right away.

Boo Chanco’s e-mail address is bchanco@gmail.com. Follow him on Twitter @boochanco

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