Rainwater storage/infiltration function of rain gardens for management of urban storm runoff in Japan

Rain gardens have recently gained popularity in Japan for the management of urban storm runoff. However, due to limited expertise regarding the technical capacity of such facilities, the level of their optimality in urban development remains unclear. To help elucidate rain garden capacity, research on two such facilities (referred to here as RG1 and RG2) was conducted in Kyoto, Japan, to clarify the characteristics of their storage/infiltration function. Rainfall and groundwater levels were monitored for 134 days between summer and autumn (a period incorporating the typhoon season) with the consideration of groundwater levels and soil water characteristics in six soil layers. Soil constitution and hydraulic conductivity in a saturated state were also evaluated to help clarify the infiltration function of the rain gardens. The results showed that RG2 had a higher water storage volume (109.54 m³) than RG1 (92.14 m³) due to the difference in area and soil porosities. Across both the rain gardens, the saturated hydraulic conductivity of each layer averaged to 0.227 cm/s, indicating a low risk of ponding. The soil available water capacity of dressing layers and existing soil layers at the greening site were excellent (> 12%) and good (between 8 and 12%). The results of the study indicated favorable storage/infiltration functions in both gardens and the potential to control more than 60% of storm runoff. The application of rain gardens may, therefore, be recommended in other urban areas of Japan.