# 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 ^{[1]}

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

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

Where:

*RVC*= Runoff volume control target (m)_{T}

*D*= Duration of the design storm event event (hr)*i*= Intensity of the design storm event (m/hr)*A*_{i}= Impervious contributing drainage area (m^{2})*A*_{p}= Permeable pavement area (m^{2})*f'*= Design infiltration rate of underlying native soil (m/hr)*C*= Runoff coefficient of impervious contributing drainage area (e.g., 0.9 for asphalt)*n*= Porosity of the stone bed aggregate material (typically 0.4 for 50 mm dia. clear stone)*R*=*A*/_{i}*A*_{p}

It is important to note that R, the ratio of impervious contributing drainage area (*A _{i}*) to permeable pavement area (

*A*), should not exceed 2 to limit hydraulic loading and help avoid premature clogging. Also important to note is that the contributing drainage area should not contain pervious areas that are sources of sediment that can lead to premature clogging.

_{p}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[edit]

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:

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)

Where the total 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:

Where:

A_{c} = A_{i} + A_{p}

Then increase A_{r} accordingly to keep R between 0 and 2, which reduces hydraulic loading and helps avoid premature clogging. This assumes that the water storage reservoir area and permeable pavement area are the same (A_{r} = A_{p}).

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- ↑ Smith, D. 2006. Permeable Interlocking Concrete Pavements; Selection, Design, Construction, Maintenance. 3rd Edition. Interlocking Concrete Pavement Institute. Burlington, ON.