LABS FOR RAINWATER SOCIETY

Rain gardens are one kind of green infrastructure technology. They are gardens (planted spaces) in urban environments that temporarily collect rainwater that fell on asphalt or roofs and allow it to infiltrate the soil. They offer many benefits and functions, such as alleviating urban weather conditions, assisting with flood control, securing spring water, contributing to biodiversity, improving water quality, fostering community interaction, and providing opportunities to experience nature in one's daily life. Therefore, a rain garden was built in the Rainwater Science Center as an implementation of elemental technology. The design conditions were as indicated below.

 

<Soil depth conditions>

 There are rainwater storage tanks installed in the Rainwater Science Center capable of accommodating the discharge of the planned rainfall management capacity of 60 mm/hr (simplified for convenience sake; actual capacity: 59 mm/h) (effective use of existing facilities). We decided to provide the rain garden itself a capacity of 198 mm of rainwater (design rainfall value). We investigated what depth of soil would be necessary based on these conditions. We performed mathematical analysis measured the change over time of the volumetric water content of the soil given its infiltration index for vertical rainfall. In our calculations, for the soil surface border conditions, we used the amount of rain measured at the Hibaru observation station as the design rainfall, for the bottom edge we assumed no water inflow or outflow (concrete), and we assumed a soil depth of 100 cm. In our initial conditions, we applied -100 cm of pressure from the soil surface to the base (representing the soil in its dry state). For the calculation time, we assumed roughly 6 hours of design rainfall. During initial, light rainfall, water was retained by the soil, with a seepage line approximately 20 cm below the surface. As the amount of rainfall increased, the volumetric water content of the soil increased, and at the peak time, the seepage line was roughly 40 to 50 cm, indicating that the soil was almost saturated. This showed that when soil was placed, open spaces in the soil could temporarily retain water and delay runoff. A soil depth of approximately 40 to 50 cm would likely be capable of retaining the design rainfall. The maximum design rainfall intensity is 24 mm/10 min=0.004 cm/s, which is lower than the saturated hydraulic conductivity of sand, so the soil surface is unlikely to become waterlogged, but in the case of high intensity rainfall, there is a distinct possibility of waterlogging as the result of the soil itself becoming saturated, so the sidewalls were provided with some leeway to catch rainfall that could not infiltrate the soil. Typical values for sand were used in the parameters used in the numerical calculation.

 

Actual implementation was performed in the rain garden in 2018. A layer of soil with a depth of roughly 40 cm was installed, a brook channel was created, holes were created, and runoff water was drawn from the rainwater tank. The following plants, potential natural vegetation that grows in waterfronts and along rivers in Fukuoka, were planted.