LABS FOR RAINWATER SOCIETY

The project implemented a plan to convert the home of a member active in Tokyo's Zenpukuji River area into a rainwater residence. Kaeru Kai members, children, architects, and others with close personal connections to Zenpukuji River pooled ideas used in the creation of plans for rainfall storage, parking space improvement, etc. Through close coordination with local landscapers, part of the construction in this project was performed this year.

The basic rainfall discharge approach was to connect a drain spout that collected the rainwater that fell on the roof to a rainwater drainage pipe, through a rainwater tank, etc., and for the overflow to infiltrate the soil on the property. The area in which infiltration was not possible would also be modified, as much as possible, to enable infiltration. In order to make improvements to areas in which infiltration is not possible, we needed to clarify the locations of buried pipes and equipment, so, based on the recommendations of a landscaper, we dug up soil adjacent to the non-infiltration area to visually confirm their locations and perform leveling. We confirmed the locations of gas pipes, water pipes, and sewer pipes. We also evaluated the quality of the soil and confirmed the presence of topsoil (andosol), backfilled soil, and, underneath that, Kanto loam layer bedrock (at a depth of 30cm or more). We performed simple double ring infiltration testing to measure the soil's infiltration capacity, and found that it was an extremely high 420 mm/hr.

Information about the property is shown in the table. During 100 mm rains, the amount of rainfall on the property is 10 m3. The infiltration capacity of standard Kanto loam is said to be 140 mm//hr (according to Tokyo Rainfall Collection and Infiltration Technical Guidelines). Using this value, we calculated the runoff rate before measures were implemented to be 64%.

 The plan is shown in the figure. The drain spout leading from the roof was connected to a rainwater tank and collected. Overflow was directed to a bioswale (a vegetation planting space that allows infiltration) and a front garden pond made using planter boxes. We removed concrete in the parking lot in locations where doing so would not affect pipework, and bore several roughly 10 cm diameter holes to increase the area's water infiltration capabilities. Furthermore, we raised the level of the unpaved space in front of the entryway, provided it with a slope to carry away rainwater, and planted vegetation (we assumed it would improve water infiltration capabilities to 200 mm/hr). As the result of these plans, assuming that in the event of 100 mm of rain, 30% of the rain that fell on the concrete area would flow into the holes, the overall rainwater runoff rate would fall to 19%, a suppression of 5 m3 of runoff. We implemented necessary construction this year. In the future we plan to perform rainwater tank and bioswale work (digging, planting, etc.), primarily using a DIY approach. This section of the project involved 900,000 yen in construction expenses -- 820,000 in flood control construction expenses excluding rainwater tanks for water utilization. At 198,000 yen per 1 m3, this exceeded the target of 100,000 yen.