LABS FOR RAINWATER SOCIETY
Implementation
In order to realize this city vision, we developed parts technologies and land design technologies for building sites. With respect to parts technologies, we developed simple methods for measuring soil infiltration amounts, rainwater tanks with holes, and soil improvement techniques. We used these parts technologies as we created green infrastructure site plans in order to convey the rainfall society concept to private homes, housing complexes, condominiums, schools, and stores. With respect to flood control, the project's objectives were to use storage and infiltration to suppress outflow whenever possible for precipitation levels of 100 mm, to promote the use of rainwater, to have owners and other related parties propose plans through intergenerational co-creation, and to keep overall costs to 100,000 yen or less per m3 of outflow suppression in times of flooding.
Rainwater green infrastructure was implemented for two private homes, one store, and at one university site.
These tests found that runoff rates were greatly reduced for all sites after implementing the measures. Sufficient consideration was also given to the use of rainwater in the tests. The rainwater green infrastructure installed in Fukuoka City fell within the target cost bounds, but costs were somewhat high for housing in Tokyo, with its small lots.
Although the cost per cubic meter and outflow suppression ratios varied by target precipitation level, the study produced interesting results: the amount of suppressed outflow was higher the higher the precipitation level, while the cost per cubic meter of suppressed outflow was lower the higher the precipitation level. Determining how to set target precipitation levels is an issue that requires future consideration. The study found that decentralized rainwater facilities could be installed for all of the building sites, that installation through intergenerational co-creation, based on the characteristics of individual properties, was effective, and that therefore this was an attractive method of facility installation.
In order to assess impressions of rain gardens, from June to November 2017 a questionnaire was administered to visitors to the Convivial Center for Rainwater Harvesting (private house1). Roughly 90% of the respondents indicated that they found rain gardens to be attractive, and roughly 70% indicated that they would like to have a rain garden in their own home. Respondents indicated that they found planted vegetation and potted vegetation, the collection of water in jugs, and soil infiltration to be especially attractive aspects of rain gardens. This showed that this is a technology that is effective not only for collecting rainwater, but also for enjoying gardening and traditional water use, and that it has cultural qualities such as traditional aesthetics and potential for unseen future value. Rainwater infiltration technologies and water retention technologies used in past comprehensive flood control projects are single-purpose technologies focused exclusively on flood control and water utilization. They do not provide cultural value, nor do they have potential for unseen future value. This appears to be one reason that they have failed to achieve a high level of penetration in existing residences. This traditional and attractive implementation method was made possible by co-creation by diverse participants in workshops and other events. The research team refers to these technologies as intergenerational co-creation technologies, and considers them important results of this research.