Bioretention: TTT

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Bioretention cells are found within the LID toolbox
Berm height (mm) Ponding depth, maximum (e.g. 300 mm), sometimes referred to as '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[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 (bioretention filter media)
Thickness (mm) Depth of filter media
Porosity of filter media Suggest 0.35 unless otherwise tested
Field capacity (fraction) Suggested range 0.10 - 0.12 [1]
Wilting point (fraction) Suggested value 0.03 [1]
Hydraulic conductivity (mm/hr) Suggested range 25 – 250 mm/hr
Conductivity slope Suggested value 45 [1]
Suction head (mm) Suggested range 50 - 60 [1]
Thickness (mm) Depth of storage aggregates layer
Porosity of storage reservoir aggregate 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 drainage time (hrs) Maximum permissible time to fully drain the storage reservoir.
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
  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.