Permeable pavements: Sizing

The following calculation is used to size the stone storage bed (reservoir) used as a base course. It is assumed that the footprint of the stone bed will be equal to the footprint of the pavement. The following equations are derived from the ICPI Manual

For full infiltration design, to calculate the total depth of clear stone aggregate layers needed for the water storage reservoir

The equation for the maximum depth of the stone reservoir (d_{r, max}, m) is as follows:

$d_{r,max}={\frac {\left[\left(RVC_{T}\times R\right)+RVC_{T}-\left(f'\times D\right)\right]}{n}}$

Where:

• RVC_T = Runoff volume control target (m)

$RVC_{T}=D\times i$

• D = Duration of the design storm event event (hr)
• i = Intensity of the design storm event (m/hr)
• R = A_i/A_p; the ratio of impervious contributing drainage area (A_i) to permeable pavement area (A_p). Note that the contributing drainage area should not contain pervious areas. R should not exceed 2.
• f' = Design infiltration rate of underlying native soil (m/hr)
• n = Porosity of the stone bed aggregate material (typically 0.4 for 50 mm dia. clear stone)

On highly permeable soils (e.g., infiltration rate of 45 mm/hr or greater), a maximum stone reservoir depth of 2 metres is recommended to prevent soil compaction and loss of permeability from the mass of overlying stone and stored water.

For partial infiltration design, to calculate the depth of the storage reservoir needed below the invert of the underdrain pipe

For designs that include an underdrain, the depth of the storage reservoir below the invert of the underdrain pipe (d_r) can be calculated as follows: $d_{r}={\frac {f'\times t}{n}}$

Where:

• f' = Design infiltration rate (mm/hr), and
• t = Drainage time (hrs), e.g. 72 hours, check local regulations for drainage time requirements.
• n = Porosity of the stone bed aggregate material (typically 0.4 for 50 mm dia. clear stone)

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Where the total contributing drainage area of the pavement (A_c) and total depth of clear stone aggregate needed for load bearing capacity are known (i.e., storage reservoir depth is fixed) or if available space is constrained in the vertical dimension due to water table or bedrock elevation, the minimum footprint area of the water storage reservoir, A_r can be calculated as follows:
$A_{r}={\frac {D(i-f')\times A_{c}}{d_{r}\times n}}$

Where:

A_c = A_i + A_p

Then increase A_r accordingly to keep R, the ratio of impervious contributing drainage area to water storage reservoir (i.e., permeable pavement) area, between 0 and 2 to reduce hydraulic loading and avoid premature clogging (assumes A_r = A_p).

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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.

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.

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 hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

The total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds.

An alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.

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

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

A perforated pipe used to assist the draining of soils.

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

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 total mass of a pollutant entering a waterbody over a defined time period.

The net amount of something (e.g. chemical, such as phosphorus), calculated as the product of concentration and volume in a given time. Some BMPs significantly reduce loading of pollutants to the environment by reducing volume more so than concentration.