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 filter media and the perforations of the pipe play critical roles for flow control.

Component	Recommended depth (with underdrain pipe)	Typical void ratio (VR)
Ponding (dp)	≥ 300 mm	1
Mulch	75 ± 25 mm	0.7 for wood based 0.4 for aggregate
filter media (dm)	300 mm to support turf grass (and accept only rainwater/roof runoff) 600 mm to support flowering perennials and decorative grasses 1000 mm to support trees	0.3
Pipe diameter reservoir	Is equal to underdrain pipe diameter	0.4
Pipe bedding (db)	50 mm (although commonly omitted altogether).	0.4

Calculate the maximum overall depth

• Step 1: Determine what the planting needs are and assign appropriate depth of media, using the table above.
• Step 2: Select an underdrain pipe diameter (typically 100 - 200 mm), assign this as an 'embedding' depth.
• Step 3: Calculate the maximum possible storage reservoir depth beneath the pipe (d_s):

$d_{s}=f'\times 38.4$

Where:

• f' = Design infiltration rate in mm/hr, and
• 38.4 comes from multiplying desired drainage time of 96 hours by void ratio of 0.4

Additional step for system without underdrain

• Step 4: Determine maximum permissible ponding depth (d_p):

$d_{p}=f'\times 19.2$

Where:

• f' = Design infiltration rate in mm/hr, and
• 19.2 comes from multiplying desired drainage time of 48 hours by void ratio of 0.4. Note that conceptually the drainage of the ponded area is limited by ex-filtration at the base of the practice.

• Step 5: Sum total depth of bioretention, and compare to available space above water table and bedrock. Adjust if necessary.

Calculate the remaining dimensions

• Step 6: Multiply the depth of each separate component by the void ratio and then sum the total to find the 1 dimensional storage (in mm).
• Step 7: Calculate the required total storage (m³):

$Storage=RVC_{T}\times A_{c}\times C\times 0.1$

Where:

• RVC_T is the Runoff volume control target (mm),
• A_c is the catchment area (Ha),
• C is the runoff coefficient of the catchment area, and
• 0.1 is the units correction between m³ and mm.Ha.

• Step 8. Divide required storage (m³) by the 1 dimensional storage (in m) to find the required footprint area (A_p) for the bioretention in m².
• Step 9. Calculate the peak flow rate (Q_p, in L/s) through the filter media:

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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 perforated pipe used to assist the draining of soils.

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

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.

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.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

The period between the maximum water level and the minimum level (dry weather or antecedent level).

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.

The upper surface of the zone of saturation, except where the surface is formed by an impermeable body.

Subsurface water level which is defined by the level below which all the spaces in the soil are filled with water; The entire region below the water table is called the saturated zone.

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.

The land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.