Raingarden | microburstsoftware

Raingardens

A system with vegetation and layered filter media that captures, retains and treats stormwater before slowly releasing it to receiving waterways. Treated water may discharge via a drainage layer or infiltrate to the underlying soil.

Raingarden design

Raingardens are effective treatment systems for removing stormwater pollutants like nutrients, reducing the volume and frequency of stormwater flows and passively irrigating garden plants. Raingardens are a good option for providing WSUD treatment in built-up areas where space is limited.

Raingardens are a secondary treatment and an upstream swale or buffer can help to reduce litter or sediment entering the raingarden if flows from driveways, roads or garden areas enter the raingarden.

Always check that the raingarden is clear of underground services such as gas or pipelines.
In most cases raingardens can be left unlined to help infiltration into the surrounding soils. Infiltration can be increased using a gravel filled trench called an infiltration storage below a raingarden. Line raingardens so they don’t leak or infiltrate water if interaction with soils and groundwater needs to be prevented to:
• protect nearby buildings, foundations or infrastructure (within 2-3 m)
• avoid creating problems with saline groundwater or reactive clays

A saturated zone or submerged zone below the raingarden can provide a water storage to help plants survive dry periods through passive watering and improve treatment effectiveness.

In STORM the user should select 'lined' if the raingarden is lined and no or minimal infiltration from the raingarden to surrounding soils will occur. If it is unlined, the relevant soil type for the site should be selected. This refers to the soils at a depth of at least 500 mm (that is the soils that will exist below the raingarden) and the infiltration rate from the raingarden will depend on this soil type.

The user can vary the following parameters in STORM:

The area of the raingarden. Increasing the areas increases treatment effectiveness.
The depth of the filter media. This can be set to 0.3 or 0.5. Usually a depth of 0.3 would only be used for an irrigated roof garden or raingarden with a submerged zone to ensure adequate soil moisture.
The depth of the extended detention depth. This can be 0.1, 0.3 or 0.5 m.
The inclusion of a submerged zone. This provides a store of water to help plants survive dry periods and improves performance. The submerged zone is assumed to be at least 0.3 m deep.

Suitable locations for raingardens

Raingardens are a good option for providing WSUD treatment in built-up areas where space is limited.

Raingardens are an effective treatment for:

Individual allotments
Subdivisions
Streetscapes

There are situations when you should not use a raingarden, such as:

on flat sites where there is limited difference between the ground surface and outlet drain water level - raingardens ideally need a height difference of at least 800 mm between the pond water surface level and drainage pipe invert)
large catchments with high flows because these may result in:
- scouring of the raingarden surface
- the deposit of large sediment loads
- difficulty in spreading water evenly over a large surface area
catchments with high sediment loads such as industrial areas with disturbed surfaces or an area where sediment clogging is considered a high risk
when there are construction sites upstream
in an area where the catchment flows are constant which may stop the raingarden from drying out - raingardens should become wet and then dry out regularly to prevent algal growth.

Raingarden maintenance

Maintaining a raingarden is similar to maintaining any garden with a few extra things to look out for.

Inlet

Check that water can freely enter the raingarden through the inlet and that there is no litter or sediment blocking it. If not, clear the blockage.

Check that water is not scouring a pathway where it enters. If it is, re-profile and replant. Place gravel or stones if needed to protect soil from erosion.

Raingarden surface

Check the surface level is even and at least 5 cm below the raingarden boundary. If not, smooth out surface so water spreads evenly across it.

If sediment has accumulated and is impeding flow so it ponds for more than 2-3 hours after rainfall, remove fine sediment.

Plants

Plants are essential for a raingarden to work. If the plants are unhealthy, dying or missing they should be replaced. Try to identify the cause of the problem. If it is too much or too little water or 'wet feet', choosing different native species may help. Recommended plants for raingardens.

Overflow

Check the overflow is clear and water can enter it freely. This is very important as a blockage could lead to localised flooding or ponding of water.UnderdrainageIf you can access it, check water reaches the underdrainage and flows freely. You may be able to test this by pouring a few buckets of water into the raingarden and looking for water flowing in a pit immediately downstream..

For more information see Raingarden maintenance.