Analysis of ‘Sponge City’ Technology and Success Cases for Preventing Flash Downpours and Urban Floods

Atmospheric River phenomena and localized sudden downpours caused by climate change have exposed the clear limitations of traditional drainage systems reliant on concrete and sewage networks. A ‘Sponge City’—a concept where an entire city absorbs, stores, and purifies rainwater like a sponge before recycling or slowly releasing it when needed—is a key climate adaptation technology that simultaneously achieves urban flood prevention and water resource management.

This article analyzes the structural core principles, representative global best practices, and implementation impacts of Sponge Cities.

1. Structural Operating Principles of a Sponge City

Traditional cities have high ratios of impervious surfaces, such as asphalt and sidewalk blocks, causing most rainwater to runoff along the surface and concentrate in sewer pipes (the cause of flooding). A Sponge City applies four major physical and ecological mechanisms with the goal of securing permeability and restoring natural water circulation.

[Traditional Urban Drainage System]

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Rainfall ➔ Impervious Surface (Asphalt) ➔ Surface Runoff Surge ➔ Sewer Overload ➔ Urban Flash Floods

[Sponge City Drainage System]

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Rainfall ➔ Permeable Pavement & Green Roofs ➔ Rain Gardens / Bioswales (Absorption & Purification) ➔ Subsurface Retention Basins (Storage) ➔ Slow Release or Water Recycling

① Permeable Pavement and Infiltration

Function: Installing porous concrete or permeable blocks on sidewalks, parking lots, and roadways.
Principle: By allowing rainwater to infiltrate into the ground immediately rather than flowing along the surface, urban peak discharge is reduced by more than 40% to 60%.

② Rain Gardens and Bioswales

Function: Constructing stormwater retention zones composed of soil, native plants, and gravel layers in roadside zones or park lowlands.
Principle: While temporarily storing rainwater, plant roots and soil microorganisms naturally purify initial stormwater pollutants (oil, fine dust, etc.) before infiltrating it deeper into the soil.

③ Green Roofs and Artificial Wetlands

Function: Creating vegetation layers on building rooftops and constructing large-scale artificial wetlands and waterfront parks within urban areas.
Principle: Green roofs retain 50% to 70% of the initial rainfall hitting a building to delay runoff timing. Artificial wetlands serve as massive urban retention basins, simultaneously mitigating urban heat island effects.

④ Subsurface Retention Infrastructure

Function: Building large-scale underground rainwater storage basins (tunnels & cisterns) beneath parks, sports fields, and roads.
Principle: When extreme downpours exceeding soil infiltration limits occur, rainwater is temporarily impounded. During droughts or normal periods, this water is recycled for garden irrigation and street cleaning, or discharged in a controlled manner based on river water levels.

2. Analysis of Representative Global Best Practices

① Pingxiang & Wuhan, China: Sponge City Pilot Projects

Background: Regions that previously suffered from habitual flooding during concentrated heavy rain, as well as severe water pollution.
Applied Technologies:
- Created large-scale vegetated waterfront zones along urban rivers and replaced over 80% of total roads with permeable asphalt and vegetated swales.
- Converted urban parks into “waterfront disaster prevention parks” designed to impound water during floods.
Performance & Outcomes: Inundation damage areas decreased by more than 90% even during sudden downpours of 50–70 mm per hour, achieving an annual rainwater recycling rate of over 70%.

② Singapore: ABC Waters (Active, Beautiful, Clean Waters) Program

Background: An environment facing simultaneous flood risks and water shortages due to limited land area and high proportions of impervious surfaces.
Applied Technologies (Bishan-Ang Mo Kio Park):
- Fully renaturalized a river previously designed as a concrete drainage channel into a winding natural stream and artificial wetland.
- During normal periods, it serves as a waterfront park for citizens; during heavy rainfall, the riverbed widens to transform into a massive rainwater retention basin.
Performance & Outcomes: Flood risks in surrounding river areas were eliminated, establishing a virtuous cycle where purified rainwater is recycled for urban water supplies.

③ Berlin, Germany: Potsdamer Platz & Integrated Stormwater Management

Background: Berlin faced high risks of urban flooding due to elevated terrain constraints and river drainage limits.
Applied Technologies:
- Implemented zero-surface-runoff conditions for rainwater during building permitting, mandating green roofs and self-contained rainwater storage facilities for all commercial and residential buildings.
- Created artificial lakes and waterfront vegetation zones across the Potsdamer Platz district, enabling 100% of rainwater falling within the complex to be treated and consumed on-site.
Performance & Outcomes: Reduced stormwater runoff into public sewers to virtually zero and established a model that self-supplies eco-friendly water even during droughts.

3. Implementation and Development Direction for Korean Sponge Cities

South Korea is also expanding the deployment of large-scale underground retention infrastructure and eco-friendly permeable pavements in regions vulnerable to sudden downpours, such as Gangneung and Pohang, starting with the Sinwol-dong Deep Rainwater Storage and Drainage Tunnel in Seoul.

Combining Underground High-Capacity Drainage Tunnels with Upper Sponge Structures: A “Three-Dimensional Upper and Lower Sponge City” system is needed—initially delaying runoff with green roofs and rain gardens, while collecting water in underground tunnels during extreme ultra-heavy rain events.
Strengthening Urban Redevelopment Requirements: Systematic guidelines must be codified into law to establish caps on impervious surface ratios and mandate minimum installation ratios for permeable pavements during new urban development and redevelopment projects.

Sponge City