Metro Manila cannot be guaranteed to become completely flood-free even after billions of pesos are poured into new catchments, pumping stations and other mitigation projects, the Metropolitan Manila Development Authority (MMDA) said Wednesday.

MMDA Chair Don Artes tempered expectations as the agency unveiled plans for 39 “sponge city” and nature-based flood mitigation projects across the capital, stressing that increasingly severe rainfall can overwhelm even extensive flood-control systems.

“We cannot guarantee flood-free Metro Manila because you saw our rainfall events that keeps getting stronger,” Artes said during a Palace press briefing on 2 September.

He pointed out that flooding still occurs in other countries despite their investments in catchments and other mitigation infrastructure.

“What we can assure you is that these projects will be efficient in mitigating the flood,” Artes said, adding that the objective is to address flooding locally, area by area, rather than promise that water will disappear entirely across Metro Manila.

The MMDA earlier identified 39 sites from an initial pool of 107 locations after nearly three years of study and consultations with local governments, experts and landowners.

Rather than relying solely on traditional concrete flood-control structures, the projects will use a combination of underground cisterns, permeable surfaces, sunken gardens, lagoons and other systems designed to temporarily hold rainwater.

Artes explained that the basic principle is to keep rainfall close to where it falls for a period of time, preventing huge volumes of water from simultaneously entering drainage systems, creeks and rivers.

“The principle is to hold the water temporarily to their source, so that our drainage system, river system, and estuaries will not be overwhelmed,” he said.

Taft, Roxas Boulevard test case

The MMDA expects the approach to have a noticeable impact in some of Metro Manila's perennial flooding hotspots.

Artes cited the Taft Avenue-Roxas Boulevard corridor, where catchments are planned around Faura, Remedios, Abad and Vito Cruz, including larger facilities at Rizal Memorial, Remedios Circle and Rajah Sulayman Park.

These will work alongside the rehabilitation of three existing pumping stations along Roxas Boulevard and a planned new pumping station in Vito Cruz.

“I think substantially, substantially, the flooding situation in Taft and Roxas Boulevard will improve,” Artes said.

Another project being eyed beneath Camp Aguinaldo illustrates the scale of the proposed catchments.

The planned structure would cover 5,000 square meters, reach about 10 meters underground and temporarily hold as much as 50,000 cubic meters of rainwater.

According to Artes, the facility would prevent that volume of water from immediately entering the creek system shared by surrounding areas, including runoff from Camp Crame, giving existing waterways more time to discharge water before additional runoff arrives.

Designed for stronger rains

The push comes as the government confronts repeated episodes of severe flooding following intense rainfall.

During the same briefing, Malacañang said President Ferdinand Marcos Jr. had instructed agencies and local governments to prepare infrastructure and communities for increasingly extreme weather associated with climate change.

Artes said the 39 sites were selected through hydrological studies rather than simply placing catchments wherever land was available.

For example, MMDA rejected locations that may offer large spaces but would require water to be diverted into them, making the intervention less efficient and more expensive.