Use of rain gardens for stormwater management in urban design and planning

Ecosystem services in urban areas have deteriorated year on year due to increased urbanization. As a consequence of increased urbanization, 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, especially in local governments. Rain gardens, a type of low-impact development, 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 their functions and characteristics. The present study aims to explain the function of rain gardens through a verification experiment and field survey. This study shows that rain gardens are suitable for dealing with low flow volumes of rainwater, although their suitability could be slightly changed by soil properties and catchment area. Our field survey indicated that rain gardens must be connected to a sewage system. In addition, like other green infrastructure, rain gardens require maintenance. The most frequent type of maintenance is the disposal of garbage, as garbage can block channels running between a rain garden and a conventional sewage system. Moreover, garbage build up can lead to the retention of rain water for more than 36 h, and this water can become a habitat for mosquito larvae.