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

Case studies per NWRM