What is a Rain Garden?
Rain Gardens are landscape features designed to treat stormwater runoff from hard surface areas such as roofs, roads and parking lots. They consist of sunken garden spaces where runoff can pond and infiltrate into deep constructed soils and then into the native soils below.
Where native soils have low infiltration rates, rain gardens often have a drain rock reservoir and perforated drain system to take excess water to the storm drain system. The constructed soils of the rain garden, and an overlying mulch layer, are designed to replicate many of the pollutant removal mechanisms that operate in forested ecosystems.
Stormwater enters the rain garden via an inlet pipe or sheet flow. Small storm events can usually be temporarily stored until they infiltrate into the ground. Most rain gardens are designed to pond no more than 2-3 inches above the soil bed.
While usually designed as stand-alone facilities without conveyance, new designs are evolving that group a series of rain gardens along linear features such as roads, and which include weirs and surface conveyance similar to bioswales.
The soil mix in rain gardens is usually about 1200mm deep and has a high proportion of sand and organics. Most sources now recommend adding a 50-75mm layer of organic mulch above the soil for erosion control, pollutant removal and to maintain infiltration capacity.
Types of Rain Gardens
These rain gardens are used where all inflow is intended to infiltrate into the underlying subsoil. Sources suggest candidate sites have soil permeability greater than 30 mm/hr. An overflow for large events is provided by pipe or swale to the storm drain system.
Full Infiltration with Reservoir
Infiltration gardens with a reservoir have a drain rock reservoir so that surface water can move quickly through the installed growing medium and infiltrate slowly into subsoils from the reservoir below. This type of garden is a candidate for sites with subsoil permeability greater than 15 mm/hr.
These rain gardens are designed so that most water can infiltrate into the underlying soil, while any surplus overflow is drained by perforated pipes that are placed near the top of the drain rock reservoir. This type of garden is suitable for sites with subsoil permeability of greater than 1.0 mm/hr and less than 15 mm/hr.
Partial Infiltration with Flow Restriction
Flow restrictor rain garden variations are used where subsoil permeability is less than 1 mm/hr. The added feature is a flow restrictor assembly with a small orifice which slowly decants the top portion of the reservoir and rain garden. This type of rain garden provides water quality treatment and some infiltration, while acting like a small detention facility.
Advantages of Rain Gardens
Rain gardens can be sized to temporarily store runoff from smaller to medium sized storm events in the depression area itself, the constructed soils, and any constructed reservoir.
Studies have shown that vegetated soils remove more stormwater pollutants than non-vegetated soils through processes of absorption, filtration, sedimentation, infiltration, phytoremediation, volatilization, surface resistance and thermal attenuation. Bioretention systems have demonstrated excellent removal for heavy metals, with some research showing the most uptake occurs in the mulch layer. Estimates from research suggest that metal accumulation would not create any environmental concerns for at least 20 years in these systems.
Hydrocarbons are removed via sorption during the storm event, then biodegradation in the mulch layer by microbial populations. Some US research on older bioretention facilities is showing that they maintain soil functions that actually enhance pollutant processing capability over time. Phosphorous removal rates are also high.
Bioretention areas can be used to treat highly contaminated runoff from some land uses where pollutant concentrations exceed those typically found in stormwater. Rain gardens can be used for these types of land uses (stormwater “hot spots”) as long as an impermeable liner is used at the bottom of the filter bed.
Because stormwater in rain gardens is detained for period of time, it has a chance to infiltrate, replenishing soils and replicating the natural hydrology. Bioretention areas can be applied in almost any soils or topography, since runoff percolates through a man-made soil bed and the design can return excess flows to the stormwater system.
Bioretention facilities are ideally suited to many ultra-urban areas, such as parking lots. While they consume a fairly large amount of space (approximately 5 - 20 percent of the area that drains to them), they can be fit into existing parking lot islands or other landscaped areas.
Some aquatic wildlife is sensitive to changes in water temperature. Bioretention is a good option for cold water streams because water ponds in them for only a short time, decreasing the potential for warming. Rain gardens have been shown to decrease the temperature of runoff from certain land uses, such as parking lots.
If runoff from a portion of the site can be diverted to a rain garden, the size of other on-site stormwater techniques can either be reduced accordingly, or its effectiveness will be enhanced.
Bioretention can be used as a stormwater retrofit, by modifying existing landscaped areas, or if a parking lot is being resurfaced. In highly urbanized areas, this is one of the few retrofit options that can be employed.
Maintenance is similar to other landscaped garden beds but care must be taken to remove any sediments that accumulate, to check for erosion at the inlet and for any blocking of outlets.
Vegetated infiltration basins can have an informal or formal design and are easily integrated into the overall landscape or site design. To avoid becoming a breeding ground for mosquitoes, sources recommend that rain gardens be designed to drain no more than a set number of hours (approximately 30-72) after a rainfall event has ended.
Some sources note that costs of building rain gardens compare favourably with conventional stormwater management facilities. When comparing the cost of rain gardens with other stormwater management techniques, the cost of the landscaping components can be left aside as many municipalities would require still landscaping for commercial and multi-family development.
The use of bioretention areas may reduce the need for other BMPs that require large tracts of contiguous land. As a result, the true cost of the practice is less than the construction cost reported. Similarly, maintenance activities conducted on bioretention areas are not very different from maintenance of a landscaped area; however, bioretention areas may actually lower utility costs by requiring less watering than similarly landscaped areas.
Limitations of Rain Gardens
Bioretention systems are generally applied to small sites (less than 2 ha), and in a highly urbanized setting. Literature suggests rain garden areas of about 10-20% of the upstream impervious area. An optimum rain garden size is about 50 square metres, draining 250 square metres of impervious area. When used to treat larger areas, they can clog. In addition, it is difficult to convey flow from a large area to a bioretention area.
Smaller, distributed rain gardens are better than single large scale facilities. Many jurisdictions specify separation distances for locating rain gardens near wells, property lines, building foundations, footing drains and steep slopes. Rain gardens cannot be placed over utility crossings unless trench dams are installed. Rain gardens should be separate from the ground water table to ensure the table never intersects with the bed of the bioretention facility. This prevents possible ground water contamination.
Rain gardens are best used on relatively shallow slopes (approximately 5%). However, sufficient slope is needed to ensure that the bioretention area can be connected with the storm drain system. This stormwater management practice is most often applied to parking lots or residential landscaped areas, which generally have shallow slopes.
Because rain gardens can look just like standard landscaped garden beds, there is some concern by local governments and designers that, over time, owners and maintenance companies will not understand their dual function, possibly making alterations that prevent gardens from functioning properly.