Permeable pavements: TTT

Permeable pavements is found within the LID toolbox

Surface
Berm height (mm)	This is the height of the curb which constrains the overland sheet flow of water. Where curb cuts or other outlet exists at the lowest point of the pavement, the suggested value is 0.
Surface roughness (Manning’s n)	Lower numbers indicate less surface obstruction and result in faster flow. Suggested range for pavement 0.01 – 0.02
Surface slope (%)	Typically between 1 – 4% (>2% recommended Effective grading)
Pavement
Thickness (mm)	This is the thickness of just the pre-cast blocks (or depth of asphalt/concrete poured in place).
Void ratio	This most commonly refers to the jointing material used between precast blocks. Suggest 0.4 unless otherwise tested. Where a pervious product is poured in place, an appropriate figure should be obtained from the manufacturer.
Impervious surface fraction	This is the proportion of the total pavement taken up with the pre-cast blocks and will vary between products; an example value may be 0.85. Where a pervious product is poured in place, this value will be 0.
Permeability (mm/hr)	This is the permeability of the joint material in block systems. Where the permeability of the bulk surface is measured and known instead, the impervious fraction can be adjusted to model a block surface as continuously permeable instead.
Clogging factor	0.5 to model a matured system?
Soil (Bedding layer?)
Thickness (mm)	Depth of bedding layer
Porosity (fraction)	Suggested value 0.4 unless otherwise tested (see OPSS aggregates)
Field capacity (fraction)	Suggested range 0.10 - 0.12 for sand
Wilting point (fraction)	Suggested value 0.03 for sand
Conductivity (mm/hr)	Suggested range 100 – 250 mm/hr for sand
Conductivity slope	Suggested value 45 for sand
Suction head (mm)	Suggested value 50 for sand
Storage
Thickness (mm)	Depth of all aggregate bases
Void ratio	Suggested value 0.4 unless otherwise tested
Seepage rate (mm/hr)	Infiltration rate of native soil
Clogging factor	0.5 to model a matured system?
Design drawdown time (hrs)	Maybe 72 or 96 hours? See Drainage time
Drain (underdrain)
Flow coefficient	Suggested value 1
Flow exponent	Suggested value 1
Offset height	This is the height from the base of the cell to the height at which the drain discharges. In some designs this may be the height of the perforated pipe within the storage layer. In other designs this height is adjusted by creating an upturn in the discharge pipe. Permeable pavements

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

A mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

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

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

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)

Mineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm.

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

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

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

A perforated pipe used to assist the draining of soils.