Stormwater planters

This is an image map of a stormwater planter, clicking on components will load the appropriate article.

An above ground planter with downspout and overflow illustrated.

Over underground infrastructure, soils prone to subsidence, or on sites considered to be pollution hot spots, it may be necessary to prevent all infiltration. Stormwater planters are "filtration-only" BMPs, similar to bioretention cells, that can be squeezed into tight urban spaces, adjacent to buildings and within the usual setbacks required for infiltrating facilities. Stormwater planters can also be used as a means of providing building-integrated LID by capturing a portion of the rainwater from the rooftop. This type of cell can be constructed above grade in any waterproof and structurally sound container, e.g. in cast concrete or a metal tank.

Overview

Stormwater planters are an ideal technology for:

• Sites which cannot infiltrate water owing to contaminated soils or shallow bedrock,
• Zero-lot-line developments such as condos or dense urban infill.

The fundamental components of a stormwater planter are:

• a planting bed of filter media,
• suitable vegetation,
• decorative aggregate or stone,
• An underdrain,
• An impermeable liner,

The design may benefit from:

• A level spreader

Planning Considerations

Stormwater planters may be integrated into the landscape similarly to bioretention practices. See bioretention planning.

Additional site opportunities

As they do not require connection to the earth for infiltration purposes, stormwater planters can also be used in elevated locations. They are sometimes used in retrofit applications on otherwise impermeable surface, as raised beds or planters surrounding buildings. They can be employed to capture downspout disconnection or even upon terraces or vertical surfaces of buildings.

Design

A flow-through planter comprises a ponding zone, mulch layer, filter media for planting, and a supporting gravel drainage layer

This article is specific to flow-through stormwater planters, vegetated systems that do not infiltrate water to the native soil.
If you are designing a planted system which does infiltrate water, see advice on Bioretention: Sizing.

The dimensions of a stormwater planter are largely predetermined according to the function of the component. As they do not contain a storage reservoir the planters rely more upon careful selection of materials. Both the filter media and the perforations of the pipe play critical roles for flow control.

Component	Recommended depth (with underdrain pipe)	Typical porosity (n)
Ponding (dp)	150 to 450 mm	1
Mulch	75 ± 25 mm	0.7 for wood based 0.4 for stone
Filter media (dm)	300 mm to support turf grass (and accept only rainwater/roof runoff) 600 mm to support shrubs, flowering perennials and decorative grasses 1000 mm to support trees	0.35 for Blend A - Drainage rate priority; 0.4 for Blend B - Water quality treatment priority
Pipe diameter reservoir	Is equal to underdrain pipe diameter	0.4
Pipe bedding (db)	50 mm (although commonly omitted altogether).	0.4

Storage media

filter media

Underdrain

Stormwater planters differ from full and/or partial infiltration bioretention practices in that the storage function is provided only by the water retention capacity of the filter media. As such, there is no storage reservoir and the only purpose to the aggregate layer is to drain water to the perforated pipe. For this, a medium aggregate as described in choker layer is recommended as it negates the need for a separating layer to the filter media. Design details can be found here Underdrains for non-exfiltrationg practices.

Planting

• Planters must be designed in a way that insulates the soil through freezing temperatures, or plant species that can survive the winter season in raised planters must be used.
• Stormwater planters routinely capture only rainwater flowing from adjacent rooftops. This means that salt may be less of a concern than in Bioretention: Parking lots or Bioretention: Streetscapes.
• The plant lists are still a good place to start when selecting species for LID in Ontario.
• A more formal aesthetic for the planting design is appropriate for the urban hardscape setting.

Liners

An impermeable liner is incorporated into non-infiltrating practices such as stormwater planters, and may be applied in permeable pavements installations where separation from the native soils and groundwater is required.

• Waterproof containment can be created using concrete or a plastic membrane/liner (HDPE or EPDM are common materials).
  • When the membrane is being used directly in the ground, punctures from stones can be prevented by compacting a layer sand (30 - 50 mm) over the soil prior to installing the membrane.
  • Alternatively, a manufactured cushion fabric (geotextile) can be employed for this purpose.
  • The top surface of the membrane must also be protected from stone and gravel being used for inside the BMP. Again, sand or a cushion fabric may be used.
• When a pipe is used to provide drainage from the practice to an outlet structure or storm sewer, a 'pipe boot' or flange should be sealed to both the pipe and the liner to prevent leaks.

Surface

As stormwater planters are often quite small and receive very rapid flow, both a level spreader and the use of stone mulch are strongly recommended.

Gallery

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  Stormwater planters are ideal for situating alongside buildings to capture rainwater from roof runoff. LSRCA headquarters, 2017.

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  Stormwater planter, treating downspout runoff, at Waterview Rec Center, Philidelphia
  Photo credit: PWD

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  Rain garden, Illick Hall, SUNY College of Environmental Science and Forestry, Syracuse, NY
  Photo credit: DASonnenfeld

Performance

Starting after TRIECA (end March) members of STEP will be undertaking a literature review on the performance of our most popular BMPs. The results will be combined with the information we have to date from the development of the Treatment Train Tool and agreed performance metrics established. Until then, please feel free to continue to ask questions via email or the feedback box below.

See Also

• Bioretention: Parking lots
• Bioretention: Streetscapes

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• Shrubs
• Perennials
• Grasses
• Forebays
• Overflow
• Mulch
• Filter media
• Choking layer
• Reservoir

Proprietary links

A number of precast modules exist to contain treatment media. As many of these systems are enclosed water balance calculations may be erroneous where evapotranspiration is constrained. In our effort to make this guide as functional as possible, we have decided to include proprietary systems and links to manufacturers websites.
Inclusion of such links does not constitute endorsement by the Sustainable Technologies Evaluation Program.
Lists are ordered alphabetically; link updates are welcomed using the form below.

• Filterra

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A vegetated practice that collects and treats stormwater through sedimentation and filtration. Contributions to water cycle/water balance are through evapotranspiration only; no infiltration.

The technique of removing pollutants from runoff as it infiltrates through the soil.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.

a top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales.

The engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

The natural ground material characteristic of or existing by virtue of geographic origin.

An underlying bed filled with aggregate or other void-forming fill material that temporarily stores stormwater before infiltrating into the native soil or being conveyed by an underdrain pipe.

A perforated pipe used to assist the draining of soils.

The porosity (n) of a mixture is the ratio of the volume of void-space to the total or bulk volume of the mixture. It is closely related to the concept of void ratio (e) where void ratio is the ratio of the volume of void-space to the volume of solids. n = Volume of voids/Total volume of mixture = e/(1+e)

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

A perforated pipe used to assist the draining of soils.

Mineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm.

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

Lake Simcoe Region Conservation Authority

A vegetated practice that collects and treats stormwater through sedimentation and filtration. Contributions to water cycle/water balance are through evapotranspiration only; no infiltration.

A lot level bioretention cell designed to receive and detain, infiltrate and filter runoff, typically used for discharge from downspouts.

Sustainable Technologies Evaluation Program

The accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

The temporary storage of stormwater to control discharge rates, and allow for sedimentation.

A bioretention BMP featuring an impermeable liner and underdrain that prevents infiltration of runoff into the underlying native soil; provides sedimentation and filtration of urban runoff as it passes through the mulch layer, engineered filter media and vegetation root zone.

A bioretention cell that features an impermeable liner that collects and treats stormwater through sedimentation and filtration only (i.e., no infiltration).

Ground depression acting as a flow control and water treatment structure, that is normally dry.

Refers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer.