Bioretention: Full infiltration

Fully infiltrating bioretention with an inspection well, but no underdrain

Overview

This type of bioretention facility provides the highest level of stormwater volume control. Although this design does not include an underdrain, an overflow pipe, a weir or other outlet structure must be included.

Planning considerations

Inlets

All forms of bioretention benefit hugely from the inclusion of pretreatment in the overall treatment train. The best type of pretreatment will depend on how the water enters into the facility. Mostly commonly this would be:

• sheet flow
• concentrated overland flow, 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 flow 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 infiltration.

Design

Calculating Size

Bioretention facilities can be sized as other forms of subterranean infiltration facility. Infiltration: Sizing and modeling

Modeling

Bioretention cells are found within the LID toolbox

Surface
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 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
Wilting point (fraction)	Suggested value 0.03
Hydraulic 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
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

Materials

• Reservoir gravel
• Choking layer
• Geotextiles
• Pipes
• Biomedia
• Erosion control
• Mulch
• Plants and Planting design

Back to Bioretention

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

A perforated pipe used to assist the draining of soils.

Initial 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.

Stormwater 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.

Open 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.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

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

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.

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 parameter that describes the capability of a medium to transmit water.

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.

An underlying bed filled with aggregate or other void-forming fill material that temporarily stores stormwater before infiltrating into the native soil or being conveyed by an underdrain pipe.

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 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.