The rain garden (or bioretention basin) is an effective, space-efficient, and versatile feature that adapts well in the urban environment. It is the quintessential ABC Waters design feature for treatment of surface runoff.
“Rain garden” and “bioretention basin” are often used interchangeably to refer to the same thing. But they may also be used to differentiate one that is in the ground (“rain garden”) from another that is contained within a structure (“bioretention basin”).
A rain garden receives runoff from its catchment areas and allows the water to pond on its surface. The pool of water slowly percolates through the media. At the bottom, the cleansed water is collected and channeled for release. Excess water from the rain garden overflows through a sump and gets discharged downstream.
There are many considerations when designing for a rain garden, including some of the basic spatial parameters such as treatment area, overflow provision, and invert levels. These parameters are essential in determining form, water effect, and location during design development.
The most efficient shape and size sometimes may not be the best looking. As long as the minimum treatment area requirement is met, one may oversize the rain garden using free forms with the intent of achieving balance between functional effectiveness and design aesthetics. However, excessive over-sizing may affect plant health due to the lack of nutrients and water.
Excess water from the rain garden presents a design opportunity to use the overflowing water as a feature, for instance, by cascading it over a row of spouts. Compared to the overflowing water from the top of the rain garden, treated discharge from the bottom provides a more frequent and prolonged flow of clean water which can also be used to good effect through clever design details.
Invert levels are critical when siting a rain garden. For it to work, the rain garden must receive runoff onto its surface and discharge from its base. Therefore, how well a site allows this will affect the layout of rain gardens and consequently, influence the spatial character of the site. Take a site that is relatively flat for instance – because of the lack of gradient, rain gardens here need to be close to both where it receives water from and discharges water to. Consequently, multiple “localised” rain gardens will have to be distributed across the site. A sloping site or a site with an e-deck, on the other hand, is much less constrained by drainage issues and therefore, can also have rain gardens that are aggregated.
The brief discussion above covers some basic but important aspects of rain garden design. Additional considerations include treatment performance modelling, hydraulic calculation, media composition, soil properties, plant selection, integration with other ABC Waters design features to form a treatment train, integration with architecture and landscape, interface or integration with detention and harvesting systems, and modification of features for discharge control.