Rainwater infiltration facilities

Needs Address

The need to stabilize river flow volumes in the event of extreme increase in rainfall. 

Adaptation effects

• Reduction and prevention of water-related damage such as inundation damage due to increased river flow volumes resulting from climate change.
• Elimination or mitigation of water shortages due to climate change.

Overview and Features

A rainwater infiltration facility is designed to reduce the amount of stormwater runoff into rivers by allowing rainwater to penetrate underground by facilities constructed underground. 

Major sites for consideration:

• Public facilities such as parks, schools, city hall, municipal offices, etc.
• Detached housing, multi-unit housing (construct on a smaller scale)
• Large commercial facilities (e.g., supermarket parking lots in suburban areas)

Types:

• Infiltration inlet (seepage pit, etc.): A structure filled with crushed rock on the sides and bottom, allowing rainwater that has collected inside to penetrate into the ground. Types include perforated concrete, porous concrete, and light/easy-to-handle plastic. 
• Infiltration trench: A culvert/conduit made with penetration holes on its sides or made of porous materials, and with its surrounding area covered with crushed stone, allowing collected rainwater to penetrate into the ground. Types include concrete (perforated concrete, porous concrete), as well as light/easy-to-handle plastic. 
• Permeable pavement: Structures that allow rainwater to permeate into the ground through the main surface or seams/edges of parking lots or other surfaces, and are made of porous materials such permeable asphalt, concrete, and blocks. 

Cost

Initial costs: 

• Infiltration inlet: About 70,000 yen per unit
• Infiltration trench: About 10,000 yen per meter
• Permeable pavement: About 7,000 yen per square meter

Subsidy programs: 

• Some local governments in Japan offer related subsidy programs. Some of them subsidize about half of the cost of installing the facilities, up to a certain maximum amount. 
• Besides what is mentioned above, legislation in Japan provides tax incentives, and low-interest financing is available from the Development Bank of Japan.

Energy source

None required. 

Ease of maintenance

• Infiltration inlet: Regular cleaning is required to remove soil, leaves, etc.
• Infiltration trench: An infiltration inlet is required at each end, and sediment must be prevented from flowing into the trench. 
• Permeable pavement: Regular cleaning is required to remove accumulated sludge, etc., from the pavement surface. 

Considerations

Technicians from consulting and construction firms are required at the design and construction phases. 

Co-benefit, suitability for developing countries

Infiltration facilities are one technique to limit the rainwater runoff into rivers, but they also function to recharge groundwater. 

Information Resources

• Association for Rainwater Storage and Infiltration Technology (see section on catchment storage and infiltration facilities)
  http://www.arsit.or.jp/ (in Japanese)
• Association for Rainwater Storage and Infiltration Technology (see manual on installation of rainwater storage and infiltration facilities for detached homes) 
  http://www.arsit.or.jp/ (in Japanese)
• Zama City: Subsidy Program for Installation of Rainwater Infiltration Facilities
  http://www.city.zama.kanagawa.jp/www/contents/1190789604474/index.html