Bioretention: Construction

This section is also applicable to: Bioswale Rain garden

Construction Tasks Table

Construction Stage	Construction Task	Bioretention	Bioswale	Rain garden
Pre-Construction	Verification of Siting and LID Design	x	x	x
	Communication and Utilities Coordination	x	x	x
	Installation of ESC and Construction Fencing	x	x	x
	ESC Inspection and Maintenance (ongoing)	x	x	x
Excavation and Mass Grading	Clearing and Grubbing	x	x	x
	Excavation	x	x	x
	Rough Grade	x	x	x
	Verification of Grading/Survey*	x	x	x
	Scarification (if applicable)	x	x	x
Backfill Granular, Utilities and Pipes	Geotextile (if applicable)	x	x	x
	Impermeable Liner (if applicable)	x	x
	Underdrain (if applicable)	x	x
	Overflow or Overflow Drain (if applicable)	x	x	x
	Monitoring Well (if applicable)	x	x
	Clean Out Port	x	x	x
	Storage Reservoir	x	x
	Stone Choker Layer (if applicable)	x	x
	Curbing (if applicable)	x	x
	Pre-treatment and Inlet	x	x	x
	Soil/Filter Media	x	x	x
Finishing Grades: Inlet, Outlet, Biomedia and Plants	Finishing Grading	x	x	x
	Riprap/Large Stone (if applicable)	x	x	x
	Plant Verification and Installation	x	x	x
	Mulch Placement	x	x	x
	Stabilizing Contributing Drainage Area - Planting Adjacent Vegetation	x	x	x
	As-built Survey	x	x
	Permanent Fencing (if applicable)	x	x
Post-Construction	Identify and Address Deficiencies	x	x	x
	Assumption/Certification Protocols	x	x	x

Construction Tasks Described

Pre-Construction (KM)

Verification of Siting and LID Design
Installation of ESC and Construction Fencing
ESC Inspection and Maintenance (ongoing)

Excavation and Mass Grading (KM)

Clearing and Grubbing
Excavation
Rough Grade
Verification of Grading/Survey*
Scarification

Backfill Granular and Pipes (JC)

Geotextile
Geotextiles are filter fabrics that can be installed to separate dissimilar soils and prevent the migration of materials. As such, proper installation of geotextiles is important for ensuring the proper division of materials.

Construction Steps:

• Roll out fabric on the flattened sub-grade surface
• Provide a minimum overlap of 300 mm between adjacent lengths of geotextile. If the manufacturer specifies a greater overlap, follow the manufacturer’s specification
• Fabric should be secured in place with stakes along the edge of the sub-grade surface and where lengths of fabric overlap
• Provide 500 mm of excess at the ends of each length of fabric
• Excess material should be folded on top of the infiltration medium/choker course once installed
• Provide cuts for trees where appropriate

Key Inspection Points:

• Inspect the geotextile to make sure it is clean and free of damage
• Ensure that the geotextile delivered to the site matches the design specifications and is approved by the engineer prior to installation
• Check that sufficient overlap is provided between lengths of fabric
• Confirm that geotextiles have been applied as per the manufacturer’s guidelines
• Confirm that structural considerations for geotextiles that are applied adjacent to infrastructure

Mistakes to Avoid:

• Wrinkles in the fabric – follow the manufacturer’s procedure for installation to ensure geotextiles lie smooth on sub-grade
• Unnecessary application of geotextile – mixing of media is negligible where structural loads are not expected, so geotextile does not need to be applied. Also geotextiles should not be used where root growth is encouraged to penetrate different layers of media

For more information on geotextiles, see the page on geotextiles.

Impermeable liner

Underdrain
Overflow or Overflow Drain
Monitoring well

Backfill Granular and Pipes (SPC)

Storage Reservoir
Stone Choker Layer
Curbing
It is very important to make sure that the contractor responsible for curb construction understands curb cut designs and elevations. This is often a new technique for contractors and they may not understand the overall concept of water in the gutter line being directed behind the gutter.
Common mistakes to avoid:

• Elevated curb cuts and reverse slopes (sloping from back of curb towards instead of depressing from gutter line towards the back)
• Curb cut size (width)
• Placement of curb cuts on steep slopes or the down slope side of a catchbasin

Pre-treatment and Inlet
Pre-treatment structures are most cost effective when they slow down incoming flows, collect sediment for easy clean out, and release water to the bioretention facility in a non-erosive way. Some pre-treatment structures/strategies include:

• Vegetation: If sized correctly, it works more effectively in areas with more dispersed flow. However, in concentrated flow and sediment areas it might not be the best option. Sediment removal can be more difficult in areas of dense vegetation. An advantage of using vegetation as pre-treatment is that it can be easily integrated into the landscape design.

• Concrete curbing

Items to ensure during construction:

• Verify that’s correct pretreatment device (jelly fish, vegetation, curbing, etc.)
• Correct size and location
• Correct elevation and footing
• Tied into the curb? or could happen before the curbing?
• Does it work?

For more information about pre-treatment strategies and their design, visit these page: Pretreatment and Pretreatment features

Soil Media / Filter Media

For small LID facilities, apply soil media in 150-300 mm lifts until desired top elevation of bioretention, bioswale or rain garden area is achieved. Thoroughly wet each lift before adding the next and wail until water has drainer through the soil before adding the next lift. Avoid manual or machine compaction. If possible, equipment should not be operated within the infiltration practice. If required, allow soil to dry before equipment is allowed back into the practice.

For large LID facilities, slinger trucks are recommended for spreading soil in even layers and reducing the need to move soil media manually or by backhoe. If possible, equipment should not be operated within the infiltration practice. If necessary, appropriate low ground pressure rated equipment can enter after at least 45 cm of bioretention, bioswale or rain garden soil is installed and has properly dried.

Soil/filter media’s design parameters and assumptions should be confirmed through in-situ permeability testing (e.g., permeameter measurements to determine hydraulic conductivity). Results of permeability testing should be reviewed by the designer and, if required, changes to the LID design may be needed.

All exposed soil areas that are not being actively worked must have temporary erosion protection or permanent cover within 7 days for slopes 3:1 or greater and 14 days for slopes 3:1 or greater and 14 days for slopes 3:1 or flatter. This should apply to all exposed soil areas year-round and until the site is stabilized.

Upon material arrival to the site, make sure to conduct:

• Chain of custody
• Visual inspection
• Grab a sample, ribbon test

Finishing Grades: Inlet, Outlet, Biomedia, Plants (SPC)

Finish Grading
The finish grading process is another critical handoff moment as a number of elements such as curbs, sidewalks, soils and vegetation start to come together to create the functional and aesthetic value of a site. The same supervision and communication that was put into controlling ESC in earlier stages is also required during finish grading. In many cases, more attention to detail is needed for elements in this phase that will create or not the success of the final product.

Riprap (Large Stone)
Plant Material Verification and Installation

Mulch Placement
Stabilizing Contributing Drainage Area - Planting Adjacent Vegetation

Finishing Grades: Inlet, Outlet, Biomedia, Plants (JC)

As-built Survey

Address Deficiencies and Assumption Protocols (JC)

Permanent Fencing
Identify and Address Deficiencies
Assumption/Certification

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

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.

A lot level bioretention cell designed to receive and detain, infiltrate and filter runoff, typically used for discharge from downspouts.

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.

Gravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices.

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.

A perforated pipe used to assist the draining of soils.

a top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales.

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.

Box like underground concrete structure with openings in curb and gutter designed to collect runoff from streets and pavement.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

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

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

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.

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.

A lot level bioretention cell designed to receive and detain, infiltrate and filter runoff, typically used for discharge from downspouts.

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.

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