Bioretention: TTT

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Bioretention cells are found within the LIDLow 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. toolbox
Surface
BermA 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. height (mm) Sometimes referred to as the bowl depth or ponding depth (e.g. 300 mm)
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[1]
  • 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 (bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. filter mediaThe 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.)
Thickness (mm) Depth of filter mediaThe 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.
Porosity (fraction) Suggest 0.4 unless otherwise tested
Field capacity (fraction) Suggested range 0.10 - 0.12 [1]
Wilting point (fraction) Suggested value 0.03 [1]
Conductivity (mm/hr) Suggested range 25 – 250 mm/hr
Conductivity slope Suggested value 45 [1]
Suction head (mm) Suggested range 50 - 60 [1]
Storage
Thickness (mm) Depth of storage aggregatesA 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. layer
Void ratio Suggest 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 Maybe up to 0.5 to account for some anticipated maturation.
Design drawdown timeThe period between the maximum water level and the minimum level (dry weather or antecedent level). (hrs) Also known as '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 others this height is adjusted by creating an upturn in the discharge pipe.Bioretention: Partial infiltration
  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. https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100NYRA.txt Accessed August 23, 2017.