Bioretention: Sizing

To size a bioretention facility using this page, the design infiltration rate (q' in mm/hr, after correction) of the native underlying soils must estimated, and the maximum available excavation depth (d in m) is the primary constraint to sizing.
If the space on the ground is the tighter constraint to design, try this article on sizing bioretention for space.

Step 1

The bioretention facility will comprise three vertical zones:

1. a 'bowl' permitting water to pond occasionally,
2. a layer of filter media to support plant growth, and
3. a reservoir of stone to store and facilitate infiltration of additional water.

Firstly, all three zones will be sized together within the limited depth to find the footprint area of the whole bioretention facility. To do this the mean void ratio is estimated to be 0.35. To calculate the require facility area or footprint where the depth is constrained: $A_{p}={\frac {IiD}{V_{R}d+q'D}}$

Where:

• D = Duration of design storm in hrs
• i = Intensity of design storm in mm/hr
• q' = Design infiltration rate in mm/hr
• V_R = Void ratio (porosity), as measured (or default to 0.35 for all aggregates)
• R = Ratio of catchment area (A_c) to BMP footprint area (A_p) syn. I/P ratio.
• A_p = Area of the infiltration practice in m²
• d = Depth of infiltration practice in m.

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

A body of water smaller than a lake, often artificially formed.

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)

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

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)

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)

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

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.

The ratio of the impervious catchment (drainage) area to the pervious (footprint) area of the receiving BMP.