THE name might evoke visions of a beloved children’s cartoon character, but Metro Manila’s Sponge City Project may well help improve flood management in the capital region through nature-based solutions.
Earlier this month, the Metropolitan Manila Development Authority (MMDA) said it was in discussions with the Department of Finance about possible next steps after the agency, with support from the Asian Infrastructure Investment Bank, conducted site visits and paid courtesy calls on local governments to gather insights into flooding and possible solutions under the sponge city approach.
The sponge city concept is a nature-based urban design strategy aimed at managing flood risk and water scarcity. Instead of relying solely on “gray infrastructure” (pipes and concrete channels) to shunt water away as fast as possible, a sponge city is designed to absorb, store, permeate and purify rainwater. Done right, this approach can be integrated with and ease the pressure on conventional drainage systems and help absorb, store and control rainwater from heavy downpours.
The sponge city approach shifts from “fighting” water to “living” with it. This is achieved through three primary physical interventions: absorption, storage and slow release.
Absorption can be achieved by replacing concrete with permeable pavements and rain gardens. Storage can come by way of bioswales or linear, vegetated ditches, wetlands, and underground cisterns to hold water during peak intensity. Slow-release mechanisms can gradually release filtered water back into the water table or drainage systems to prevent flash flooding.
This isn’t all just theory. This approach has been successfully implemented in Wuhan, Singapore, and Berlin.
Wuhan was one of the first pilot cities in China’s national Sponge City program, launched in 2015. Facing chronic and severe waterlogging, the city transformed the Yangtze River Beach Park, replacing hard embankments with tiered landscapes of permeable grass, trees and rain gardens. They also installed massive underground storage tanks and permeable “eco-ditches” in residential neighborhoods.
During the record-breaking rains of 2020, areas that previously suffered from waist-deep flooding remained largely functional. The city has successfully absorbed about 70 percent of its annual rainfall in the pilot zones.
Meanwhile, Singapore’s Active, Beautiful, Clean Waters program is a global gold standard for the sponge city model. At Bishan-Ang Mo Kio Park, a straight, concrete drainage canal was “de-concretized” and turned into a meandering 3.2-kilometer natural river with floodplains. When it rains heavily, the park doubles as a conveyance channel, letting water spread out safely.
Berlin has pioneered the sponge city approach in high-density European urban settings, particularly in the Rummelsburg district. Berlin mandates green roofs on most new buildings. These roofs can retain up to 80 percent of annual rainfall. Furthermore, the city disconnected many buildings from the traditional sewer system, forcing them to manage runoff on-site through courtyard infiltration basins.
This reduced the load on Berlin’s combined sewer system, significantly lowering the risk of sewage overflows into the Spree River during storms.
These experiences can be both inspiring and instructive to the local effort to move away from purely gray infrastructure, such as pumps and dikes, toward “blue-green” systems.
The MMDA said the Metro Manila Sponge City Project was expected to run for about five years and targeted to begin next year. The program will include several components, such as water-retention interventions and the development of green public spaces, designed to help manage stormwater.
San Juan River Linear Park is a major pilot site where the river easement is being transformed into a park designed to absorb overflow. Instead of concrete walls, nature-based embankments and bioswales are being integrated to manage runoff before it hits the main channel.
The MMDA is also coordinating with local government units to identify strategic locations for massive underground and surface-level reservoirs to capture excess rainfall during peak intensity.
Marikina is integrating sponge city principles into a “Local Integrated Flood Mitigation Roadmap” that includes dedicated retention basins to temporarily store floodwaters; upstream coordination to manage gradual water releases from the Wawa Dam to prevent sudden surges in the Marikina River; and outfall efficiency by redesigning drainage so community runoff flows directly into the river through filtered channels, reducing the duration of localized flash floods.
Other cities outside Metro Manila are also moving toward a sponge city approach. For instance, Cagayan de Oro, which was hard hit by floods as a result of Tropical Storm Basyang earlier this year, is moving its flood management strategy away from traditional gray infrastructure by making the urban landscape more porous, allowing the city to absorb, store and repurpose rainwater.
It is too early to say how well these efforts will pay off, but given the experiences elsewhere in the world, there is reason for optimism.