LABS FOR RAINWATER SOCIETY

The existing residence that is part of the project is a roughly 50 year old private house, on a 249 m2 lot, with 161.37 m2 of roof space. It is being used as the Convivial Center for Rainwater Harvesting -- a watershed and community base. The basic concept of flood control at this site was to disconnect the street inlet from the drain spout, allowing the water to instead infiltrate fertile garden soil.

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<Planning and design>

We divided the space into different areas based on the roof and water-collecting drain spout, and calculated the water balance resulting from storage and infiltration for the watersheds of the rain falling in each area based on precipitation data from the Hibaru Sports Park weather observation station. The building is on a residential home soil foundation created roughly 50 years ago, but as the result of ongoing garden care, such as tree planting and flower planting, by the site's owner, the foundation created when the building was erected has since been covered by fertile soil. Soil augers were used on three locations on the property, and it was confirmed that the bedrock is located roughly 70 cm below the surface. Working from the assumption that the bedrock consists of granite soil, we hypothesized that the soil infiltration rate was 20 mm/h.

Assuming 50% of the 70 cm of depth were crevices, we calculated the flood control effectiveness in terms of the amount of water retained by the soil. In order to promote infiltration into the soil, we planned basic infiltration trenches using mesh pipes and improvements to street inlets, as well as developing elemental technologies such as adjustment tanks for controlling discharge volume. In order to maintain soil quality, we designed various methods to collect water, such as using traditional pots and casks, connecting them to outdoor storage boxes that can be purchased easily in hardware stores, and using them as storage tanks. We also proposed a hot foot bath of rainwater heated in solar water heaters as a fun way to use rainwater.

We calculated that while 49.3 m3 of precipitation across the entire property resulted in roughly 34 m3 of runoff before implementing these measures, it would be possible to lower it to 13 m3 after the measures, a runoff decrease of roughly 75%. This reaffirmed that the greatest runoff prevention results can be achieved through infiltration.

 

<Implementation>

From December 2016 to February 2017 we carried out construction work to implement this plan. Major work such as pipework (cutting the drain spout and connecting it to the collection tank, etc.), carpentry work (wood deck work, etc.), and landscaping work (stonework, etc.) was contracted out to specialists. Other work, such as vegetation planting and small-scale gardening work, was primarily handled through a DIY approach of intergenerational collaboration between members of the community and the watershed area, environmental organizations, children, and university students. However, we were unable to install the 18 rainwater retention tanks (for use by toilets) behind the building as planned. The primary reason for this is that we had planned to reuse containers initially used to transport foods, connecting them together, but were unable to acquire the containers. Furthermore, there were additional concerns related to the Waterworks Act, which prohibits cross-connection with waterworks when reusing rainwater as water for toilets and requires the installation of meters for calculating sewage processing fees. 5.5 m3 of other water containment was installed in order to secure sufficient capacity for planned water utilization. This water was used for day-to-day vegetation watering, cleaning, and emergency purposes.

In order to verify river improvement effectiveness, from May 2017, the amount of data that fell on the property was monitored, as was the amount of water runoff in the area around where construction was performed. During the measurement period, the rainfall record was 37.6 mm in one hour and 297.6 mm in one day on July 6, 2018. Although this was less rainfall per hour than the target rainfall (measured in July 2009), the total amount of rainfall was extremely high. In the area around where the construction was performed, following the rain event, the total amount of runoff was confirmed to be less than 0.05 m3 12 hours after the hour in which the rainfall volume peaked. It is highly likely that this was temporary runoff from ponds or other storage facilities, but given that over the course of a day 51 m3 fell on the target area, this result confirmed that the measures taken were extremely effective in curbing and delaying rainfall runoff. Furthermore, the total cost of construction, including the cost of materials, was roughly 1,630,000 yen, which is equivalent to 77,000 yen per 1 m3, lower than the target cost of 100,000 yen per 1 m3.

In order to assess attitudes towards rain gardens, from June to November 2017 a survey was administered to visitors. Forms containing questionnaire questions were placed in the Center's consultation, and visitors were free to fill out the questionnaires if they desired. A total of 54 questionnaire answer forms were submitted, with the majority of respondents being in their 40s or 50s. Roughly one in four of the respondents were comparatively young, in their 20s, 30s, or in university. Roughly 90% of respondents indicated that they found rain gardens to be attractive, and roughly 70% responded that they wished to try building 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.