Permeable pavements: TTT

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Permeable pavements is found within the LID toolbox
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 [1]

Surface slope (%) Typically between 1 – 4% (>2% recommended Effective grading)
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[1]
Wilting point (fraction) Suggested value 0.03 for sand[1]
Conductivity (mm/hr) Suggested range 100 – 250 mm/hr for sand
Conductivity slope Suggested value 45 for sand [1]
Suction head (mm) Suggested value 50 for sand [1]
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
  1. 1.0 1.1 1.2 1.3 1.4 Oregon State Univ., Corvallis. Dept. of Civil, Construction and Environmental Engineering.; Environmental Protection Agency, Cincinnati ONRMRL. Storm Water Management Model Reference Manual Volume I Hydrology (Revised). 2016:233. Accessed August 23, 2017.