Rain Gardens

Code: 
U09
Sector: 
Urban
The complete description of the NWRM: 
Summary: 

Rain gardens are small-scale vegetated gardens used for storage and infiltration. The term ‘rain garden’ is often used interchangeably with ‘bioretention area’ (although the latter could also be applied more loosely to other measures such as filter strips or swales).

Rain gardens are typically applied at a property level and close to buildings, for example to capture and infiltrate roof drainage. They use a range of components, typically incorporated into the garden landscape design as appropriate. These components may include:

  • Grass filter strips to reduce incoming runoff flow velocities and to filter particulates. For example, these may be used at the base of roof drainage downspouts to slow and filter roof runoff as it enters the rain garden.
  • Ponding areas for temporary storage of surface water prior to evaporation, infiltration or plant uptake. These areas will also promote additional settling of particulates.
  • Organic/mulch areas for filtration and to create an environment conducive to the growth of micro-organisms that degrade hydrocarbons and organic matter. These may be particularly effective where rain gardens are used to treat excess highway runoff.
  • Planting soil, for filtration and as a planting medium. The clay component of the soil can provide good adsorption for hydrocarbons, heavy metals and nutrients.
  • Woody and herbaceous plants to intercept rainfall and encourage evaporation. Planting will also protect the mulch layer from erosion and provide vegetative uptake of pollutants.
  • Sand beds to provide good drainage and aerobic conditions for the planting soil. Infiltration through the sand bed also provides a final treatment to runoff.

The filtered runoff is then either collected and returned to the conveyance system (using an underdrain) or, if site conditions allow, infiltrated into the surrounding ground. They aim to capture and treat stormwater runoff from frequent rainfall events, while excess runoff from extreme events is passed on to other drainage features as part of a SuDS ‘train’.  Rain gardens should be planted up with native vegetation that is happy with occasional inundations. Rain gardens are applicable to most types of development, and can be used in both residential and non-residential areas. They can have a flexible layout and should be planned as landscaping features, enhancing the amenity value.

Illustration(s): 

Rain garden

Source: Andras Kis’ presentation, NWRM Workshop 1

Possible benefits with level: 
Benefits Levelsort descending
ES7 - Flood risk reduction High
PO9 - Take adequate and co-ordinated measures to reduce flood risks High
PO11 - Better protection for ecosystems and more use of Green Infrastructure High
BP5 - Increase evapotranspiration High
BP6 - Increase infiltration and/or groundwater recharge High
BP14 - Create terrestrial habitats High
ES1 - Water storage Low
PO2 - Improving status of physico-chemical quality elements Low
PO4 - Improving chemical status and priority substances Low
PO8 - Prevent groundwater status deterioration Low
BP7 - Increase soil water retention Low
BP8 - Reduce pollutant sources Low
BP17 - Absorb and/or retain CO2 Low
ES3 - Natural biomass production Low
ES8 - Erosion/sediment control Low
ES4 - Biodiversity preservation Medium
ES5 - Climate change adaptation and mitigation Medium
ES6 - Groundwater/aquifer recharge Medium
ES9 - Filtration of pollutants Medium
ES10 - Recreational opportunities Medium
ES11 - Aesthetic/cultural value Medium
PO5 - Improving quantitative status Medium
PO7 - Prevent surface water status deterioration Medium
PO14 - Prevention of biodiversity loss Medium
BP1 - Store runoff Medium
BP2 - Slow runoff Medium
BP9 - Intercept pollution pathways Medium
BP10 - Reduce erosion and/or sediment delivery Medium
BP16 - Reduce peak temperature Medium
Case study(ies): 
Last updated: 09 Jun 2015 | Top