# Permeable pavements: TTT

From LID SWM Planning and Design Guide

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 ^{[1]} |

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 asphaltA mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces./concrete poured in place). |

Void ratioThe 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) | 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. |

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

PorosityThe 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) (fraction) | Suggested value 0.4 unless otherwise tested (see OPSS aggregates) |

Field capacity (fraction) | Suggested range 0.10 - 0.12 for sandMineral 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.^{[1]} |

Wilting point (fraction) | Suggested value 0.03 for sandMineral 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.^{[1]} |

Conductivity (mm/hr) | Suggested range 100 – 250 mm/hr for sandMineral 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. |

Conductivity slope | Suggested value 45 for sandMineral 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. ^{[1]} |

Suction head (mm) | Suggested value 50 for sandMineral 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. ^{[1]} |

Storage | |

Thickness (mm) | Depth of all aggregateA 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. bases |

Void ratioThe 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) | Suggested value 0.4 unless otherwise tested |

Seepage rate (mm/hr) | Infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour. of native soilThe natural ground material characteristic of or existing by virtue of geographic origin. |

Clogging factor | 0.5 to model a matured system? |

Design drawdown timeThe period between the maximum water level and the minimum level (dry weather or antecedent level). (hrs) | Maybe 72 or 96 hours? See Drainage time |

Drain (underdrainA perforated pipe used to assist the draining of soils.) | |

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 |

- ↑
^{1.0}^{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. https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100NYRA.txt Accessed August 23, 2017.