Planters: Sizing

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 saturated hydraulic conductivity of the filter media and the number and size of perforations of the underdrain pipe play critical roles for flow control. Options for maintaining separation between filter media and clear stone aggregate surrounding the perforated pipe include a 100 mm deep choker layer or sheet of geotextile filter fabric.

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	0.4 for sandier Blend A - Drainage rate priority; 0.35 for more loamy Blend B - Water quality treatment priority
Choker layer	100 mm	0.4
Perforated pipe diameter	150 to 200 mm	0.4
Clear stone aggregate layer below perforated pipe	50 mm (although commonly omitted altogether).	0.4

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.

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

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 parameter that describes the capability of a medium to transmit water.

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.

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.

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.