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 satruated hydraulic conductivity of the filter media and the number and size of perforations of the underdrain 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	0.4 for sandier Blend A - Drainage rate priority; 0.35 for more loamy 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

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 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.