Bioretention: TTT

Bioretention cells are found within the LID toolbox

Surface
Berm height (mm)	Sometimes referred to as the bowl depth
Surface roughness (Manning’s n)	Lower numbers indicate less surface obstruction and result in faster flow Suggested ranges: Mown grass (dependent on density) 0.03 – 0.06 Stone 0.03 – 0.05 Planted (highly dependent on density) 0.05 – 0.15
Surface slope (%)	If the slope > 3% a series of Check dams or weirs should be included in the design.
Soil (bioretention filter media)
Thickness (mm)	Depth of filter media
Porosity (fraction)	Suggest range 0.3 – 0.35 unless otherwise tested
Field capacity (fraction)	Suggested range 0.10 - 0.12
Wilting point (fraction)	Suggested value 0.03
Conductivity (mm/hr)	Suggested range 25 – 250 mm/hr
Conductivity slope	Suggested value 45
Suction head (mm)	Suggested range 50 - 60
Storage
Thickness (mm)	Depth of storage aggregates layer
Void ratio	Suggest value 0.4 unless otherwise tested
Seepage rate (mm/hr)	Infiltration rate of native soil
Clogging factor	Maybe up to 0.5 to account for some anticipated maturation.
Design drawdown time (hrs)	Also known as '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 others this height is adjusted by creating an upturn in the discharge pipe.Bioretention: Partial infiltration

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 compacted earthen wall that diverts runoff or creates shallow ponding of runoff. In some cases, runoff ponds behind the berm and gradually flows through it or is infiltrated.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

The engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

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)

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 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)

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.