Considering a practical scenario for mainstreaming rain gardens in Japan

: As a consequence of increased urbanisation, floods threaten inhabitants of various cities in Japan. Although a conventional way of treating urban flooding is to construct sewage systems, alternative approaches have to be developed because of their expense in the context of the current financial crisis. Rain gardens have recently been recommended as a best management practice for the treatment of stormwater runoff in Northern European countries, the USA, etc. While demand for rain gardens has been increasing in Japan, there is a lack of knowledge about policy planning for the development of rain gardens. This present study aims to 1) visualise a possibility for the development of rain gardens as a map and evaluate its effect in terms of storm water management in most urbanised area “ Delta Zone ” in Hiroshima-City, Japan, 2) discuss a method for policy planning for mainstreaming rain gardens as a kind of green infrastructure. By use of GIS technology, the areas under the conditions (elevation < 5.0 m, slope angle < 1.1 degrees and GWL < -2.0 m) were selected as potential areas for rain garden installation. Next, 1) green spaces, 2) sandy spaces, 3) car parks on the potential areas were identified because those spaces are easier to install rain gardens than building and road areas. As s result, it was indicated that “ Delta Zone ” could obtain a rainwater infiltration function to treat 20 mm/h stormwater event if only 10 % of those spaces turn into rain gardens. In addition, a practical scenario to mainstream rain gardens in Japan was discussed.