Rain-gardens

A rain-garden is an infiltration system which filters stormwater, removing pollutants and allowing water to infiltrate into the soil. In doing so, it helps to protect the stream, but also provides a valuable “self-watering garden” for the local landscape.

A rain-garden infiltration system is simply like a garden, but with the underlying soils carefully specified to maximise the infiltration of water into the subsoil and ground water.

A rain-garden infiltration system is essentially made up of two components, a planted ponding zone and a sub-surface infiltration component.

Ponding - The ponding zone acts as a buffer, capturing and storing the inflows from a storm event, to allow them to be infiltrated into the soil below.

Infiltration - The sub-surface infiltration zone is made up of several layers:

-     a) an filtration layer at the top: made up of a loamy sand, this both filters the stormwater, and also provides a good soil media for plant growth;

-     b) a lower storage layer (typically made up of scoria): its porosity is high so that stormwater is stored at each rain event and can infiltrate slowly into the surrounding soils;

-     c) in addition, there is normally one or two transition layer(s) between the filtration and storage layer, which are used to prevent the migration of fine particles of sand into the scoria zone.

Plants play a vital role in the rain-garden, helping to promote evapotranspiration, as well as to maintain soil porosity. As the aim is to allow stormwater to infiltrate into the surrounding soil, the rain garden must not be lined, unless it close to infrastructure.

 In this photo, stormwater is shown collecting in the ponding zone after rain, before slowly seeping into the infiltration layer below.

Rain-garden infiltration systems can be designed to suit any number of gardens styles, from the more formal, modern look (below) to the rustic or natural look (top and middle right).

It is also possible to build self, contained rain-gardens, in planter boxes (bottom). Although these typically do not encourage infiltration.

Alternatively, infiltration systems can be built as hidden systems, under lawns and gardens (bottom right, an infiltration trench under construction).