What are rain gardens?
Rain gardens (also referred to as bioretention schemes) can take the form of shallow landscaped depressions or raised planters and are used to reduce rainfall runoff whilst also mitigating the impact of pollution.
Rain gardens are flexible, ‘green’ surface water drainage features which can be incorporated into most development schemes and are an example of how SuDS components can be easily integrated into a development proposal without compromising the primary function of neighbouring land.
Rain gardens can replace or help to reduce the requirement for traditional soakaway crates or attenuation storage tanks within a development. They are suitable for both new developments and retrofit schemes on any soil type, and can even offer a garden drainage solution for developments on clay soils (providing that an outfall location to watercourse or sewer can be identified and connected to).
Rain gardens provide a wide range of benefits including:
Reducing rainfall runoff from a development,
Improving water quality by removing sediment and other pollutants through the filtration process,
Creating biodiverse habitats, and
Enhancing amenity value for local residents and site users.
Given the above, it is considered that rain gardens can be used on developments to meet the four pillars of SuDS Design: (1) Water Quality, (2) Water Quantity, (3) Amenity and (4) Biodiversity.
Are certain areas better suited for rain gardens?
Some areas within a development will be better suited for locating rain gardens than others. When choosing where to situate a rain garden in your design proposals the following best practise guidance should be considered:
An area in full sun or partial shade;
Within a natural topographic low point in relation to the surrounding area;
If infiltration is proposed locate at least 3m (10ft) away from the house to avoid any damage to foundations by percolating water;
In a well-drained area, on a very gentle incline (10% or less);
Where it is practical to install a pipe leading to the main drainage system below the rain garden in case of excessively heavy storms.
As a general rule a rain garden should be 5-10% of the catchment area for which it drains. In light of this, they are often a suitable solution for patio drainage and managing runoff from extension roof areas and carriageways.
On soils where infiltration is impeded, such as London Clay, rain gardens tend to be used in combination with other forms of sustainable drainage systems to create a treatment train as part of a Surface Water Drainage Strategy.
Rain Garden Design
Rain gardens are made up of three elements:
Freeboard
Topsoil
Sub-base
Freeboard
The freeboard is the space above the topsoil within a rain garden which can be used to store water. The freeboard depth is measured from the pathway/road level or top lip of the rain garden planter to the top of the topsoil.
To encourage water to flow into the rain garden and accommodate any resultant sediment accumulation a rain garden should be designed to have a freeboard depth of between 200 and 300mm.
Topsoil
The topsoil layer provides the first phase of infiltration. This layer is usually made up of a mixture of sand, soil and compost creating a substrate for vegetation to be planted.
The depth of the topsoil layer varies based on the type of vegetation proposed to be planted within the rain garden. For shrubs and herbaceous plants a minimum topsoil layer of 300mm is recommended. However, if it is proposed to turf or plant wildflowers in the rain garden a reduced depth of 200mm can be considered.
Given the composition of topsoil, this layer is not taken into account when calculating the attenuation storage volume provided by a rain garden.
Sub-base
The sub-base is the final layer within a rain garden and generally comprises of 100-500mm of gravel dependent on the water storage requirements.
Where the sub-soil is free draining it may be considered appropriate to not include the sub-base. However, in the majority of situations the sub-base will be required.
In instances where the sub-soil has a lower soakage rate, an under-drain can be installed within the sub-base to enable discharge into either another, downstream SuDS feature or to an outfall location. The use of an under-drain ensures that any stored water drains effectively and that the rain garden does not become waterlogged.
How much stormwater attenuation will a rain garden provide?
The following calculations provide an indication of the volume of water able to be stored within a rain garden with dimensions 1.0m x 1.0m x 1.0m.
Area of rain garden = 1.0m2
Depth of freeboard = 0.2m
Depth of topsoil = 0.3m
Depth of sub-base = 0.5m
The sub-base of a rain garden is generally made up of gravel which typically has a 30% void ratio (i.e. 30% of gravel volume is space available for storing water). Therefore, in this example, the total storage depth is 0.35m.
Given that the area of the rain garden is 1.0m2 and the storage depth is 0.35m the total storage volume provided by this example is 0.35m3 (calculations below).
Depth of Storage (m) = Depth of Freeboard (m) + 30% Depth of Sub-Base (m)
= 0.2 + (0.3 x 0.5)
= 0.35m
Volume of Storage (m3) = Depth of Storage (m) x Area of Rain Garden (m2)
= 0.35 x 1.0m2
= 0.35m3
Note: these calculations so not take into account any water lost through infiltration, evapotranspiration or connection to another surface water drainage component / outfall.