Bioretention: Full infiltration

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Fully infiltrating bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. with an inspection well, but no underdrainA perforated pipe used to assist the draining of soils.

Overview

This type of bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. facility provides the highest level of stormwater volume control. Although this design does not include an underdrainA perforated pipe used to assist the draining of soils., an overflow pipe, a weir or other outlet structure must be included.

Planning considerations

Inlets

All forms of bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. benefit hugely from the inclusion of pretreatmentInitial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms. in the overall treatment trainStormwater management following the hierarchical approach: Source Control measures, Conveyance Control measure and End of Pipe treatment to achieve the water quality and water balance target for lot level development of the preferred strategy.A combination of lot level, conveyance, and end-of-pipe stormwater management practices.. The best type of pretreatmentInitial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms. will depend on how the water enters into the facility. Mostly commonly this would be:

  • sheet flow
  • concentrated overland flowOpen space floodway channels, road reserves, pavement expanses and other flow paths that convey flows typically in excess of the capacity of the Minor Drainage System., or
  • concentrated piped flow.

Distributed sheet flow is preferable to reduce the erosive energy of the water, and to promote infiltration across the surface of the practice.

Overflow and active storage

A plan should be in place for the conveyance of excess flow in the event that the facility is inundated during an extreme event. This may be to permit controlled overland flowOpen space floodway channels, road reserves, pavement expanses and other flow paths that convey flows typically in excess of the capacity of the Minor Drainage System. to another facility, or to direct the flow to a pipe. In this case the invert of this pipe must be below the surrounding landscape to prevent accidental spilling. The depth between the top of the filter media/mulch and the invert of the overflow pipe is active storage of limited duration ponding and is sometimes referred to as the bowl depth. To prevent mosquito activity (and fear of mosquito activity), surface ponding time should be limited to 24 hours and should be calculated as 1D infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface..

Design

Calculating Size

BioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. facilities can be sized as other forms of subterranean infiltration facility. Infiltration: Sizing and modeling

Modeling

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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

Materials

Back to Bioretention
  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.