A natural filter for
urban stormwater.
Rain gardens combine temporary water storage, specialized soil media, and plants.
Water enters
Rainwater runoff from roofs, driveways, paved areas and other hard surfaces is directed into the rain garden through a carefully positioned inlet. Rather than allowing stormwater to flow immediately into conventional drainage infrastructure, the rain garden captures water close to where it falls.
The inlet and surrounding landscape are designed to guide water into the system while helping control flow and minimise erosion during heavier rainfall.
Water is detained
Once inside the rain garden, stormwater temporarily collects within a shallow surface storage area. This detention slows the movement of water and reduces the rate at which it enters the downstream stormwater network.
By temporarily holding and gradually releasing rainfall, the system can help reduce peak stormwater flows during rainfall events. The rain garden is designed to drain between events rather than remain permanently filled with water.
Media and roots filter
As the stored water moves downward, it passes through specially selected filter media and the active root zone of the plants.
The filter media physically captures sediment and suspended material, while the soil, microorganisms and vegetation contribute to natural treatment processes. Depending on the system design, rain gardens can help reduce pollutants and nutrients carried in urban stormwater.
The plants also play an important role. Their roots help maintain the structure and biological activity of the filter media while creating an attractive landscaped feature above the stormwater treatment system.
Water drains or overflows
After passing through the filter media, treated water is managed according to the site’s drainage conditions and system design. Where appropriate, some water may infiltrate into the surrounding soil. Other systems use a perforated underdrain to collect filtered water and discharge it to an approved drainage point.
For rainfall events that exceed the rain garden’s temporary storage capacity, a dedicated overflow provides a controlled pathway for excess water. This allows the system to manage everyday rainfall while safely accommodating larger storm events.
Together, the inlet, detention area, vegetation, filter media, drainage and overflow create an integrated system that captures, slows, filters and manages stormwater within the landscape.