Rain Garden Study


Sixteen, full-scale rain gardens are located in an area that was previously a gravel parking lot. Each cell is approximately 6m x 6m (20ft x 20ft) with a bottom area of 3m x 3m (10ft x 10ft). All the rain gardens have the same bioretention soil mix which is 60% sand and 40% compost. This study focuses to a large extent on plants and the influence of plants on the soil mix. Eventually, as funds become available, the rain gardens will be instrumented to continuously monitor flow and water quality treatment.

There are four plant treatment replicated four times. The treatments include:

  • Shrubs
  • Shrubs and small trees
  • Grasses (managed meadow)
  • No plants

The facility collects stormwater from roads and rooftops in a large cistern (approximately 11,356 liters or 3,000 gallons) allowing for the application of stormwater at various hydraulic and pollutant loads. During storms stormwater is distributed by gravity from the cistern through weirs at known volumes to the rain gardens. Each rain garden has an under-drain and a two stage control structure to control hydraulic residence time, saturated zone and ponding depth. 

Between storms dead storage in the tank can be used to mix synthetic stormwater mixtures and dose the rain gardens with desired volumes and pollutant concentrations. The flexibility built into the collection and delivery system will also allow for the application of various pollutants of concern as scientists and agencies in the region determine research needs.

The measured infiltration rate for the rain garden subgrade is 0.0076 cm/hr (0.003 in/hr).

Some specific questions assessed in the rain garden study include:

  • What are the different evapotranspiration rates of typical rain garden plants?
  • What is the ability of rain gardens to maintain adequate infiltration rates over-time and resist clogging from applied sediment loads?
  • Do different plant palettes influence soil structure and, therefore, influence infiltration rates differently?
  • What is the pollutant uptake of selected shrubs and grasses?
  • How does the biogeochemical structure of the bioretention soil mix evolve over time and does this influence nutrient cycling and management?
  • What is the stormwater volume reduction on soils with poor infiltration rates?