With increasing urbanisation, landscaped areas are dwindling with more and more hardstand being added to the spreading concrete jungles. Walk down most inner Sydney suburbs and grassy patches are a rare sight.
All of this hardstand area only adds to the stormwater runoff generated during a storm, with efficient guttering and downpipes taking stormwater downstream where it enters receiving waterways, taking with it fine sediments and various types of nutrients and pollutants.
Prior to urbanisation, when water was treasured, stormwater runoff had the opportunity to infiltrate into the land, increasing soil fertility, accommodating plant grown and contributing to the wider ecosystem.
There is a way to reconnect this disconnected system between water and the land.
Raingardens are essentially not much more than your standard planter box or landscaped area, the main differences being that they get water from a nearby downpipe or from surface runoff directed to the landscaped area. Specific plants that can survive through periods of no watering and periods of excessive watering are planted in engineered filter media, with slotted sub-soil drainage lines picking up any excess water.
Raingardens are made up of a series of layers. Water moves vertically, particles are generally retained on top and nutrients are absorbed by plants.
The table below gives a rough guide of area draining to the raingarden to raingarden size:
Key considerations for designing a raingarden
- Location: Choose an area that can receive runoff from a nearby downpipe or a depressed area where an area of hardstand can be graded towards the raingarden. Consider the slope and grade of surrounding catchment, size and characteristics of the catchment, existing drainage point, site function, site users, and maintenance and safety issues.
- Type: Are you next to a building? A waterproof container may be your best choice. Do you have clay soils? An impervious geotextile should be installed around the base of the raingarden to stop runoff from reacting with the clay soils.
- Design: Vegetation requires a strong root system that can sustain both dry periods and periods of inundation with aesthetically pleasing foliage. Include the following:
- Mulch layer – a 50-millimetre multi-functional layer that helps to support plant growth, improve soil health and reduce erosion
- Filter layer – minimum of 300 millimetres thick with a medium that supports plant growth and is rich in organic matter
- Transition layer – a 100-millimetre thick layer made up of sand that drains at a faster rate than the filter layer
- Drainage layer – a 150-millimetre thick layer that has the highest hydraulic conductivity and consists of a slotted pipe with five millimetres of washed gravel
It is key to consider the following possible problems when creating a raingarden:
- Avoid using wood chips as mulch layer. This will float and block the downstream drainage network
- Avoid using scoria for the drainage layer as it releases fines and can block downstream drainage pipes
- The use of a filter sock/geotextile on the slotted pipe can increase the potential for blockages
- Make sure that the site can accommodate for the designed depth of the raingarden
- Using sand that is too fine or too coarse in the transition layer can have significant implications
- Protect the filter area from sediment runoff during construction as this has dire consequences on the performance of the system
- Beware scouring at the inlet
- There can be a disconnect between designer, contractor and person/organisation undertaking the long-term maintenance of the system.