Bioretention: Construction

This section is also applicable to: Bioswale Rain garden

Overview

Insert blurb here

Construction Tasks Table

Construction Stage	Construction Task	Bioretention	Bioswale	Rain garden
Pre-construction	Design verification and site walk-through	x	x	x
	LID construction notes	x	x	x
	Construction: tendering and contract	x	x	x
	Construction: communication, inspection plan, and utilities coordination	x	x	x
	Construction: erosion and sediment control measures	x	x	x
	Construction: mobilization, access, staging, and perimeter controls	x	x	x
Excavation and Mass Grading	Clearing and Grubbing	x	x	x
	Excavation and Rough Grade	x	x	x
	Final Excavated Grade and Verification	x	x	x
Backfill Granular, Utilities and Pipes	Geotextiles (if applicable)	x	x	x
	Underdrains (if applicable)	x	x
	Impermeable Liner (if applicable)	x	x	x
	Overflows (if applicable)	x	x	x
	Monitoring Wells (if applicable)	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
Finishing Grades: Inlet, Outlet, Biomedia and Plants	Soil Media and Amendment	x	x	x
	Finishing Grading	x	x	x
	Karge Stone and Riprap (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 Surveys	x	x
	Permanent Fencing (if applicable)	x	x
Post-Construction	Identify and Address Deficiencies	x	x	x
	Final Certification	x	x	x

Pre-Construction

When the design is 90% complete, the project manager (PM) should conduct a walk-through of the site to verify that the designs match site conditions. This site walk-through is a critical step in the transition from design to construction and helps the project manager plan for future pre-bid or pre-construction meetings. Think carefully about how a contractor will be able to build the LID practice. At this stage, determine if the design drawings and construction notes address specific LID construction requirements, and determine whether they contain inaccuracies or are missing detail. The table below identifies details key points and considerations for the walk-through.

Design verification points and key considerations

Verification point	Key considerations
Adjacent land uses	Have land uses changed while the design was in progress? Are there any planned land-use changes on adjacent properties and is their drainage connected to the site? Will adjacent land uses create dirty runoff that may flow onto the construction site or into the LID feature? Is additional erosion and sediment control (ESC) measures required beyond what is marked in the ESC plan?
Contributing and receiving drainage areas	Where is water coming onto the site and where is water leaving the site? Are any environmentally sensitive areas downstream? Are there any apparent changes while the design was in progress? If so, should these changes motivate modifications to the design, particular ESC measures, etc.?
Seasonal maintenance operations	Will winter snow storage and salt or sand application impact the LID feature?
Location of LID features	Do the proposed LID locations match the site? Does the footprint of the LID fit within the site's constraints?
Existing and planned infrastructure/utilities	Confirm accuracy of design drawings. Are there any unmarked utilities not detailed within the drawing set?
Property boundaries and building foundations	Confirm accuracy of design drawings. Do fence lines align with property boundaries?
Natural heritage features and existing vegetation	Do the designs accurately capture existing natural heritage features? Which areas on the site should be protected during construction, and will these exclusion areas make the LID facility more difficult to construct? Where are the drip lines of the site's trees? Are there any invasive species on the site which could colonize the LID practice?
Access, sequencing, staging, and constructability	Where will the contractor access the site? What is the ideal construction sequence, and where could materials and supplies be stored during construction? Do site characteristics constrain standard construction practices? Will it be a difficult job? Do constructability issues require a field change or amendment to the design or project tender?

LID construction notes

LID design drawings need detailed construction notes within the design drawings. These notes should include:

• sequencing and staging areas
• material specifications
• testing requirements
• installation procedures
• elevations and invert details for inlet, outlet, and overflow features
• grading details for conveyance features
• erosion and sediment control requirements
• inspection and maintenance responsibilities during construction and prior to assumption

The last two bullets require further comment. LID facilities are most vulnerable to sedimentation and clogging during their own construction and construction of adjacent and connected lands. LID construction contracts should specify a sufficient combination of ESC measures and inspection and maintenance of the LID facility during the construction and assumption phases to ensure that it will function without remediation. Contracts for larger-scale projects, e.g., where the LID facility in question is part of a road reconstruction project or subdivision development, should contain clear terms covering responsibilities for keeping sediment from entering newly constructed LID features and removing it if necessary.

The project contract and tendered drawings, including details, notes, ESC requirements, material specifications, and testing requirements should align. Given that most standard contract templates do not include wording specific to LID construction, considerations for LID can be placed under special provisions within the contract.

Details within the drawings and contract should also be accurately reflected in the tender bid form. A detailed bid form that reflects the entirety of the work allows contractors to deliver accurate bids, lessens the likelihood of project shortcuts, and helps to ensure transparency amongst bids. Ultimately, clear bid pricing reduces the potential for misunderstandings and conflicts and attracts experienced contractors.

Minimum contractor requirements

Many contractors are unfamiliar with building LID practices, so project managers should consider minimum contractor requirements. Consider mandating contractor training, attendance at pre-bid meetings, LID project references, and bid bond insurance as prerequisites.

Be sure to verify all design changes

A simple departure from the design—installing a T-junction (top) in place of a Y-junction (below) would have made it extremely difficult or impossible to inspect this underdrain and flush it out. (Image sources: CVC)

After securing a contractor, a pre-construction meeting between the project manager, project engineer, and contractor will facilitate an efficient job site and reduce the potential for miscommunication. These meetings should cover:

• LID design details and construction notes
• material specifications, inspections, and chain-of-custody
• access routes and storage areas
• protection of LID practices: phasing, ESC, and perimeter fencing
• equipment requirements and recommendations
• verification of field changes
• plant establishment and warranty-period maintenance
• ownership and assumption protocols
• project inspection plan
• verification processes for field changes to the design and material substitutions

The table below expands upon the bullets above and provides helpful guidance on communication topics at the pre-construction meeting. Given the lead-up time needed for testing requirements of specified materials, these discussions should occur soon after the contract is awarded, ideally 2 months prior to project construction.

Pre-construction communication topics

Communication topic	Guidance
LID design details and construction notes	Provide overview of LID design, ideally through a design brief. Discuss LID design details and construction notes, and ensure details and notes are understood by all parties involved. Discuss project phasing and general timeliness.
Material specifications, inspections, and chain-of-custody	Material substitutions must be confirmed by the project engineer. Material testing requirements and their timelines must be understood by all parties, including who is responsible for testing. Material acceptance protocols, with chain of custody, should be understood by all parties, including who is responsible if the material fails. Biomedia manufacturing and testing should start prior to construction, ideally 2 months in advance.
Access routes and storage areas	Discuss access to the site along with material and equipment storage with the contractor. Material storage should be downstream of LID features. Heavy equipment routes should be agreed on to minimize impact to the LID feature and protected areas.
Protection of LID practices during construction: phasing, ESC, and perimeter controls	Review and discuss the ESC plan. Discuss frequency of ESC inspection, protocols for repair and replacement, and parties responsible. Discuss communication protocols for sediment contamination. Discuss rehabilitation protocols for sediment contamination and parties responsible.
Equipment requirements and recommendations	Wide track, low-ground pressure vehicles are recommended to minimize compaction. Excavation should occur from outside of LID feature whenever possible. See Excavation. Gravel and biomedia should be installed by a slinger truck outside of practice whenever possible. Avoid heavy machinery within LID footprint whenever possible.
Verification of field changes	The project engineer must verify all field changes to materials and design. In the design engineer’s absence, the project manager should verify field changes.
Plant establishment and warranty-period maintenance	Discuss planting timelines at the pre-construction meeting. Avoid planting in the summer months (June, July and August). Fall planting is the ideal scenario. Plants will require regular maintenance (irrigation and weeding) to ensure establishment, especially over the first year. Ensure that plant maintenance and its frequency is understood by responsible parties. Plant deficiencies should be addressed prior to warranty period initiating.
Ownership and assumption protocols	Discuss how testing will confirm LID practice functionality according to assumption protocols.
Project inspection plan	Project inspection plan should be detailed and understood by all parties. Sign off by all parties involved on photos, video, and social media post may be required.

Inspection plan

The inspection and maintenance page gives detailed guidance on how to conduct construction inspections for LID practices. Generally, construction inspections should be continuous as the work progresses. If this isn’t possible, critical inspection points are:

• site preparation
• excavation
• installation of pipes, granular, and biomedia
• finishing grades
• surface treatment installation: pavers, plants, porous concrete, etc.
• after rain events (see ESC inspection and maintenance)
• whenever sub-contractors or utilities begin work (hand-off points)

The last element—inspection of sub-contractors—requires further comment. For example, if a sub-contractor is installing the curbing, they need to understand the purpose of the curbing and curb cuts, i.e., that they are meant to direct water into the LID facility, not to the closest catch basin. Without this knowledge, the sub-contractor may mistakenly build the curbs as they usually do: to direct water into a catchbasin.

Utilities coordination

Similarly, if the LID practice is in close proximity to sub-surface utilities, the utility owner may use their own contractors to perform related work. If this is the case, these sub-contractors must also be made aware of the LID practice’s purpose, how compaction affects performance, keeping the LID practice clear of excavated dirt and generally how their work could impede the construction and performance of the LID practice. In general, communication between the contractors, project managers, and project engineers should be frequent and open.

Grooves perpendicular to the grade help prevent the development of rills. If left unchecked, rills can develop into gullies. (Photo source: CVC, 2022)

STEP’s Erosion and Sediment Control Guide contains detailed guidance on the design, installation, inspection, maintenance, and decommissioning of ESC measures. This section gives practical advice on structural and non-structural ESC for contractors, inspectors, and those who are new to construction and ESC practices.

Establish required perimeter controls, including erosion and sediment control (ESC) measures, prior to construction and adjust accordingly during construction. ESC measures come in two basic types: structural ESC, which slows and holds runoff from the disturbed area, dissipating erosive forces, promoting ponding, and allowing suspended particles to settle out; and non-structural ESC, which primarily minimizes the exposure time of stripped soils.

For non-structural ESC measures, be sure to:

• Stage clearing and grubbing to minimize exposure of stripped soils.
• Watch the weather and adjust plans accordingly.
• Manage and demarcate heavy equipment routes away from LID practices.
• Use simple, common-sense methods, such as scarifying exposed soils perpendicular to the grade to prevent the development of rills.
• Create and use a pollution prevention plan for fuels or solvents.

Generally, all exposed soils that are not being actively worked on 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 lesser. This should apply to all exposed soil areas year-round and until the site is stabilized.

The table below gives guidance on common mistakes made when installing structural ESC measures.

Best practices and common errors when installing structural erosion and sediment control measures

Structural ESC measure	Do	Don't	(Pass) Photo Example	(Fail) Photo Example
Sediment fencing	Stake it in, support it, and double it up adjacent to natural heritage features.	Leave vulnerable points with insufficient supports.	This sediment fencing has been doubled up and staked in. As a result, it kept a large amount of sediment from entering the adjacent natural heritage feature. It is in need of immediate maintenance, however. (Photo source: CVC)	This sediment fencing was not staked in at the likely point of failure, resulting in the release of sediment into an adjacent natural heritage feature. (Photo Source: CISEC)
Inlet controls	Support and stake in inlet controls such as sediment socks. Use catch basin inserts as a final line of protection.	Leave inlets to the stormwater management system unprotected or unsupported	This filter stock has been staked in and supported. (Source: CVC)	This catch basin was left unprotected, resulting in the release of sediment into the stormwater management system. (Photo source: CISEC)
Stockpiles	Sling soil and aggregates into the facility when possible. Install soil and aggregates upon arrival to the site when possible.	Store materials within the stormwater management system, e.g., in the roadway. Leave stockpiles unprotected when wet weather is expected.	(Photo source: CVC)	If it were to rain, fines from this stockpile would go straight into the downstream catchbasin. (Photo source: CVC)
Protecting features during construction, including using sacrificial filter fabric	Use sacrificial filter fabric when necessary to protect features, and leave ESC measures in place until all construction in the LID's drainage area is complete.	Remove ESC measures early or allow sediment to enter the LID footprint.		ESC measures at this bioretention feature were removed before construction in its drainage was complete, allowing sediment into it.

ESC measures require ongoing attention to ensure integrity and performance. ESC inspections should take place, at minimum, during installation, after rainfall events, and at the critical inspection points discussed in the Pre-construction: communication, inspection plan, and utilities coordination (section).

At these inspections, keep an eye out for accumulated sediment, worn materials, compliance with the project's staging plan, and potential failure points.

Ponding in front of this catchbasin indicates that this ESC inlet control is working. Ponding water allows for suspended sediments to settle. (Photo source: CISEC)

This inlet control has worked well, but the sediment should be removed as soon as possible. (Photo source: CISEC)

In tandem with installation of ESC measures, setting up access and staging areas is one of the first steps in any construction project. Perimeter controls such as construction fencing are often combined with silt fencing. Safety fencing may be needed to restrict public access to the site, especially if the site sees lots of pedestrian traffic. Together, construction fencing and structural ESC measures should demarcate:

• the limits of construction
• heavy equipment routes
• equipment and material storage areas
• LID practice footprints
• natural heritage features
• access areas

Construction steps

1. Install perimeter fencing/silt fence in alignment with design drawings and ESC plan.
2. Demarcate staging and access points with fencing or surface markings in alignment with design drawings.

Key inspection points

• Ensure perimeter fencing and access/staging areas are in alignment with design drawings.

Mistakes to avoid

1. Departing from the plan to stage and limit access points, material storage areas, and equipment storage areas
2. Installing inadequate perimeter controls, e.g. inadequate sediment fencing or construction fencing

Excavation and mass grading

Clearing and grubbing is the stripping of vegetation and topsoil prior to excavation and rough grading within designated areas.

Scarifying native soils helps to increase infiltration rates. (Photo source: CVC, 2022)

Construction steps

1. Clear trees, large shrubs, stumps, etc. (grubbing).
2. Use appropriately sized machinery (e.g. a mini-excavator) to strip sod and ground-level vegetation (clearing).
3. Stockpile materials within designated areas if necessary. If the materials are being hauled offsite, stripped materials ideally would go straight into the back of a dump truck.
4. Secure stockpiles with appropriate erosion and sediment control (ESC) measures if the material must be stored before hauling offsite or will be reused.

Key inspection points

• Structural ESC measures are in place before clearing and grubbing begin.
• Natural heritage features, including trees, and construction limits are clearly marked and respected by the contractor.
• Contractor cleared and grubbed only within designated areas and according to the phasing plan.

Mistakes to avoid

• clearing more area than is needed
• insufficient marking of areas to be protected
• stockpiling materials without ESC controls or for extended periods of time
• leaving heavy equipment in the facility’s footprint
• making excessive passes with heavy equipment in the LID practice’s footprint without scouring soil afterward

Excavation to the specified depths in the drawings is critical to the LID feature’s future performance. It also requires heavy machinery which, if used incorrectly, can compact soils and lower their native infiltration rates.

Excavating from impervious surfaces adjacent to the LID practice minimizes compaction.

Construction steps

1. Mark out the limit of excavation.
2. Use predefined access routes to move heavy equipment.
3. Cut and stabilize access and staging areas.
4. Cut and stabilize channels and slopes.
5. Excavate the LID feature from the outside-in whenever possible. A standard excavator (10-45 tons) has a longer reach than mini (<6 tons) or midi excavators (6-10 tons) and can be the best choice despite its weight, since it may not need to enter into the facility’s footprint to complete the job. However, a mini excavator would be the best choice if entry into the LID feature’s footprint is necessary.
6. Excavate to within 100 mm of the final grade.
7. Check grades with appropriate equipment (transit levels, etc.).
8. Stockpile materials in a designated area if it must be stored before hauling.

Key inspection points:

• Confirm the limit of excavation.
• Use of excavation equipment specified in the contract, in the required manner (from the outside in, with a mini excavator, etc.).
• Confirm excavated rough grade and final grade prior to backfilling using survey equipment
• Use of toothed bucket and scouring the bottom of practice if applicable
• If appropriate, measure compaction using a cone penetration test or a cone penetrometer
• If appropriate, field verify infiltration rates at excavated bottom of practice using Guelph permeameter or double ring infiltrometer

Mistakes to avoid:

• making too many passes with heavy machinery within the LID footprint.
• leaving heavy machinery within the LID’s footprint (for breaks, lunch, overnight, etc.)
• using the wrong equipment
• over excavating
• smearing the bottom of the practice with the bucket, which can compact native soils

LID designs typically have specific grading requirements. Verifying depths and slopes is critical for the optimal performance of the facility. For example, if the design calls for a channel with a 1% slope, the channel may not function as designed if it has a 2% slope instead.

Always confirm elevations throughout construction. (Photo source: CVC)

Excavation to the sub-grade will differ depending on whether the design is for a full infiltration, partial infiltration, or no infiltration LID system. If it’s a full infiltration or partial infiltration practice, the sub-grade should be as level as possible to promote even infiltration across the facility’s footprint. For no infiltration designs, excavate sub-grade slopes to the design grade in the direction of the underdrain, typically 1 to 2%.

As with all infiltrating LID practices, avoid compaction of sub-grade soils whenever possible. However, depending on the design and the local municipality, compaction of the sub-grade may be required for permeable pavement LIDs.

Construction steps

1. Excavate the final 100 mm, to the final design grade, using appropriate equipment. This is often done with hand tools.
2. Confirm excavation depths and slopes using appropriate equipment (transit levels, global navigation satellite systems, etc.)

Key inspection points

• Confirm final excavated depths and slopes.

Mistakes to avoid

• “eye-balling” depths and slopes

Backfill Granular, Utilities and Pipes

Geotextiles

Geotextiles are filter fabrics that can be installed to separate dissimilar soils and prevent the migration of materials. Geotextiles are recommended for installation at the bottom and side limits of the LID feature and should be implemented at the discretion of the engineer.

Construction Steps:

1. Roll out fabric on the flattened sub-grade surface.
2. Provide a minimum overlap of 300 mm between adjacent lengths of geotextile. If the manufacturer specifies a greater overlap, follow the manufacturer’s specification.
3. Secure fabric in place with stakes along the edge of the sub-grade surface and where lengths of fabric overlap.
4. Provide 500 mm of excess at the ends of each length of fabric. Fold excess materiel on top of the infiltration medium/choker course once installed.
5. Provide cuts for trees and shrubs where appropriate.
6. Assess on-site conditions to ensure that the tensile, tear, and puncture strength ratings of the geotextile are suitable for expected structural loads (e.g. traffic). If not properly considered, geotextiles can be adversely impacted by loading from adjacent infrastructure uses.

Key Inspection Points:

• Geotextile is clean and free of damage.
• Geotextile delivered to the site matches the design specifications and is approved by the engineer prior to installation.
• Sufficient overlap is provided between lengths of fabric.
• Geotextiles have been applied as per the manufacturer’s guidelines.
• Structural considerations for geotextiles have been made when applied adjacent to infrastructure and the geotextile can handle expected structural loads.
• Class II geotextiles conform to OPSS 1860.

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.

• 

  The shape of the LID feature required the geotextile to be rolled out prior to being laid on the subgrade surface. Photo taken during construction of IMAX Corporation Headquarters. (Photo Source: CVC, 2012)

• 

  Excess fabric is draped over the edges of the LID practice, and backfilling with granular is taking place. Photo taken during construction of IMAX Corporation Headquarters. (Photo Source: CVC, 2012)

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  Footprint with geotextile, granular, and underdrain. Photo taken during construction of IMAX Corporation Headquarters. (Photo Source: CVC, 2012)

Underdrains

Underdrain installed to subgrade. (Photo Source: CVC, 2010)

Underdrain installed with maintenance risers. Note the 'T' joints, which are no longer recommended. (Photo Source: CVC, 2010)

Underdrains are perforated pipes that collect subsurface water and convey water out of an LID facility once it reaches capacity. Their design is determined by the drainage requirements of the feature.

Construction Steps:

1. Install the underdrain to grade with a consistent slope of 0.5% to 1.0%.
2. Connect overflow risers with single-elbowed ‘y’ connections or 45° elbows.
3. Situate maintenance risers where siltation is expected in drain lines (such as at junctions or where grade and direction changes occur), in accordance with the design drawings.
4. Mark the locations of risers.
5. Use video inspections to confirm that the interior of the underdrain remains free of debris after construction.

Key Inspection Points:

• Size, type, and material of the underdrain conforms to the design specifications.
• Underdrain is perforated prior to arrival on-site.
• Drain material is resistant to UV radiation and any chemicals in the soil and groundwater.
• Clean-outs are positioned correctly and spaced appropriately for the size of the underdrain.
• Any manual perforations are approved by the design or field engineer.
• Pipe grade matches design specifications.
• Tie-in locations to municipal sewers meet municipal specifications.

Mistakes to Avoid:

• Incorrect overflow risers – ensure single-elbowed ‘y’ connections or 45° elbows are used for overflow risers instead of ‘T’ connections.
• Incorrect drain size – confirm underdrain has an internal diameter greater than or equal to 200 mm to reduce freezing and allow cleaning and camera inspections.
• Potential for clogging – Remove sock from underdrain prior to installation or geofabric wrap. A choker course is recommended, instead, as field change.

For more information on the design of underdrains, such as materials and connections, see the page on underdrains linked above.

Impermeable liners

Impermeable liners can be applied when LID features are not intended to infiltrate runoff into the underlying soil. They protect building foundations or other subsurface infrastructure. They also prevent infiltration through contaminated soils. Where building foundations next to the LID feature are not waterproofed, liners can be used to protect adjacent infrastructure.

Construction Steps:

1. Compact 30 – 50 mm of sand over the soil onto which the membrane will be installed to protect against punctures. Alternatively, a geotextile can be used in the place of sand.
2. Place lengths of liner on the bed of the facility as per the manufacturer’s specifications.
3. Provide 150 mm of overlap between adjacent lengths of liner.
4. Secure liner in-place and bond together the overlapping portions of liner as specified by the manufacturer.
5. Seal a flange to the pipe and liner when pipes provide drainage from the practice.
6. Compact sand or apply cushion fabric on top of the liner to protect against punctures.

Key Inspection Points:

• Liner matches design specifications.
• Liner installation procedures conform to manufacturer’s specifications.
• Minimum overlap between lengths of liner is achieved.
• Need for protection of building footprints and adjacent infrastructure confirmed through field verification of setbacks.
• Sand and/or geotextile are applied on both sides of the liner to protect against punctures.
• Lengths of liner are bonded together to ensure protection against groundwater penetration and contamination.

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  An impermeable liner was placed along one side of the LID facility to prevent infiltrated water from contacting the nearby watermain in the road right-of-way, which runs parallel to the feature. (Photo Source: CVC, 2020)

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  Impermeable Bentofix® liner being rolled out in LID practice area. (Photo Source: CVC, 2012)

Overflows or overflow drains

Overflows are features of inline facilities and convey larger storm events out of the LID feature.

Construction Steps:

1. Connect the overflow drain to the underdrain.
2. Fit a metal, domed grate to the overflow.
3. Install overflow pit to grade at the location specified in the contract documents.

Key Inspection Points:

• Overflow drain matches design specifications.
• Overflow pit is positioned at the maximum water surface elevation of the practice, as per the contract documents.
• Sufficient freeboard is provided between the overflow and inlet such that the inlet is not inundated by design storm flows.
• Overflow grate matches design specifications.
• Overflow grates located in high-traffic areas are screwed on or equipped with locks.

Mistakes to Avoid:

• Backflows: Ensure that backflows are avoided by confirming that inlet and overflow elevations match design specifications with surveys.
• Incorrect sequencing: Install overflow drain prior to backfilling with granular material and engineered soil.
• Incorrect grate material: Avoid using plastic overflow grates due to breakages and UV degradation.

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  Overflow pipe installed to an underdrain T-connection. (Photo Source: CVC, 2010)

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  Overflow installed with a plastic cap. Note that the use of plastic grates is discouraged. (Photo Source: CVC, 2012)

Monitoring wells

Monitoring well stack installed next to underdrain within the LID practice. (Photo Source: CVC, 2010)

Monitoring wells allow for measurement of water levels in the sub-surface components of LID practices. They are important for sampling procedures and ensuring that the LID feature are performing as intended.

Construction Steps:

1. Install a cap stand at the required elevation to anchor the monitoring well.
2. Wrap the perforated pipe with a geotextile sock (to prevent migration of soil into the well).
3. Anchor the perforated pipe to the bottom of the LID feature.
4. Install a lockable cap to the top of the standpipe to protect against vandalism.
5. Install monitoring equipment (e.g., a level logger) if required.

Key Inspection Points:

• Pipe is perforated, rigid, and matches design specifications.
• Well is installed according to contract documents.
• Geotextile sock is wrapped around the perforated portion of the well.

Mistakes to Avoid:

• Incorrect sequencing: Ensure that standpipe is installed prior to backfilling with granular material.

Storage reservoir

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The storage reservoir layer holds and directs the stormwater into the underlying/native soils. The LID facilities, if applicable, should be filled with uniformly graded, washed stone (20 mm – 50 mm) that provides 30 to 40% void space.

Construction steps:

1. Backfill material from outside of the LID facility to avoid compaction and sediment entering the facilities. Use a slinger truck if possible.
2. Place the material to the elevation and thickness as per the design specifications.

Key Inspection Points:

• Material arrival to the site:
  • Check chain of custody
  • Verify it meets specifications as per design
  • Verify no debris or fines within the aggregate (it’s a washed stone)
• Granular material should be 19 - 50 mm clear stone or as per design. See Aggregates for further details.
• Installed aggregate is at the correct elevation as per design.

Mistakes to Avoid:

• Accepting material that does not meet design requirements and specifications.
• Installing material with heavy equipment from the inside of the LID facility
• Installing frozen aggregate. Do not install frozen aggregate
• Leaving heavy equipment (excavator) for long periods of time within the LID facility.

For more guidance on materials specifications, refer to Aggregates, OPSS aggregates and (Ontario Provincial Standards, 2013)

Stone choker layer

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In LID facilities, a choker layer of ≥ 100 mm depth is recommended to prevent migration of finer filter media into the underlying storage reservoir aggregate. Similar to the storage reservoir material, this aggregate layer should be a washed 5 – 10 mm stone not containing any debris. Installation of aggregate choker layer should not be done when frozen.

Follow the construction guidance shown above in the section “Storage reservoir”

For more information on the choker layer, see the page: Choker layer and OPSS aggregates.

Curbing

Improper elevation at the curb cut, impeding the inflow to enter the LID facility (bioretention) (Photo source: CVC, 2015)

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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.

Construction Steps:

1. Place the right forms (rolled curve vs standard) in the inlet location.
2. Pour concrete.
3. Shape the inlet
4. Add the river stone on top of the fresh concrete (if applicable)
5. Provide sufficient curing time, according to (CSA, 2009)

Ensure curb granular base (granular A) does not spill over into LID infiltration area. If material spills over, remove as best as possible while still maintaining the 2:1 slope for curbing. (Photo source: CVC, 2012)

Key Inspection Points:

• Use of proper curb form by sub-contractor.
• Curb type aligns with design.
• Curb cut location, type and dimension aligns with design.
• Designated concrete wash out is in place and away from LID facility.

Mistakes to Avoid:

• Elevated curb cuts and reverse slopes (sloping from back of curb towards instead of depressing from gutter line towards the back).
• Wrong curb cut width size.
• Use of wrong curb form.
• Concrete wash out within or upstream of LID facility.
• Ensure curb granular base (granular A) does not spill over into LID infiltration area. If material spills over, remove as best as possible while still maintaining the 2:1 slope for curbing
• Lack of communication to concrete contractor or ready-mix driver explaining the function and importance of protecting the LID feature.

For more information on curb cuts, see these pages: Curb cuts, Curb cuts: Gallery and Bioretention: Streetscapes

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Pre-treatment and inlet

Pre-treatment structures are most cost effective when they slow down incoming flows, collect sediment for easy clean out, and slowly release water to the bioretention facility mitigating erosion. Pretreatment structures/strategies can include curb cuts, Aggregates, proprietary devices like filters or hydrodynamic separators, vegetation, concrete sumps, membrane filters, overland flow sumps, etc.

Construction Steps:

• Installation of pretreatment features will vary based upon type. Similarly, installation timeline will range with type and could occur at excavation and mass grading, curb work or at finishing grade. Given pre-treatment features are typically integrated with the LID inlet coordination amongst multiple sub-contractors is sometime needed. The following details steps for various pre-treatment types:
• Vegetation: Follow the guidance shown below in the section “Plant Material Verification and Installation”
• Curbing: Follow the guidance shown above in the section “Curbing”.
• Aggregate: Follow the guidance shown above in the section “Stone reservoir”.

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  Aggregate material (rock) installed as the inlet and pre-treatment device in the rain garden at Glendale P.S. in Brampton, ON. The runoff comes from a vegetated swale into the inlet, conveying it into the rain garden. (Photo Source: CVC, 2021)

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  Aggregate material (rock) installed as the inlet and pre-treatment device in the rain garden at Glendale P.S. in Brampton, ON. The runoff comes from a vegetated swale into the inlet, conveying it into the rain garden. (Photo Source: CVC, 2021)

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  Aggregate material (rock) installed as the inlet and pre-treatment device in the rain garden at Glendale P.S. in Brampton, ON. The runoff comes from a vegetated swale into the inlet, conveying it into the rain garden. (Photo Source: CVC, 2021)

Proprietary pre-treatment device

1. Excavate and prepare base for proprietary pre-treatment device according to design.
2. Install proprietary pre-treatment device according manufacturer directions.
3. Manufacturer representative may need to confirm proper installation and functioning through approved testing and inspection.
   

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  Installation of Jellyfish filter and sorbtive media vault at the IMAX bioswale project in Mississauga, ON. (Photo Source: CVC, 2012)

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  Installation of Jellyfish filter and sorbtive media vault at the IMAX bioswale project in Mississauga, ON. (Photo Source: CVC, 2012)

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  Installation of Jellyfish filter and sorbtive media vault at the IMAX bioswale project in Mississauga, ON. (Photo Source: CVC, 2012)

Key Inspection Points:

• Verify that the correct pre-treatment device (jellyfish filter, vegetation, curbing, etc.) is being installed.
• Verify that all components of the pre-treatment device are installed
• Verify correct size and location of pre-treatment device.
• Verify correct elevation, slope, and footing according to design
• Is it tied into the curb, downspout, or other inlet? Or could happen before the curbing?
• Wet weather performance check:
  • Does it work?
  • Is water entering the LID facility properly?
  • Is sediment and debris accumulating?
  • Is it dissipating erosive forces?

Mistakes to Avoid:

• Pre-treatment component parts are missing
• Grading/elevation errors that deviates from design
  • Incorrect pipe inverts causing short circuiting
  • Insufficient grade drop or slope into pre-treatment to ensure positive flow of water
  • Improper grading from pre-treatment to LID feature inhibiting positive flow
  • Insufficient sump depth to account for sediment and debris accumulation
• Using wrong concrete forms if concrete curbs are part of pre-treatment.

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

Finishing Grades: Inlet, Outlet, Biomedia, Plants

Soil media and amendment

Bioretention can be constructed over any soil type, but hydrologic soil group A and B are best for achieving water balance objectives. Facilities designed to infiltrate water should be located on portions of the site with the highest infiltration rates. STEP has detailed construction guidance on soil amendments available on its [Construction Specifications for Implementing Compost Amended Planting Soil in Ontario] webpage. These resources include specification details and a spreadsheet calculator for determining soil and amendment volumes.

Construction Steps:

1. Apply soil media in 150-300 mm lifts until desired top elevation of bioretention, bioswale or rain garden area is achieved.
2. Apply soil media with slinger trucks when possible. This type of equipment will reduce the need to move soil media manually or by backhoe.
3. Thoroughly wet each lift before adding the next and wait until water has drained through the soil before adding the next lift.
4. If amending soils:
   
   • Stockpile native soils in designated areas.
   • Mix selected additives with native soils using appropriate equipment.
   • Sling soils into the LID feature whenever possible.

Key Inspection Points:

• When material arrives to the site: perform chain of custody, visual inspection and ribbon test to ensure that material meets design requirements.
• 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. See Testing for more information regarding testing procedures.
• Ensure that soil media is being applied to the correct depth.
• Ensure that the native soil and amended material are well blended.
• Measure potentially compacted soils using a cone penetration test or a cone penetrometer.

Mistakes to Avoid:

• Machine compaction: Equipment should not be operated within the infiltration practice.
• Soil material: Accepting material that does not meet design specifications and requirements.
• Application: Applying the next lift without letting dry the previous lift of filter material.

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. Careful attention to detail is still needed in these final phases to ensure the success of the final product

Construction steps:

• Sol Media:

1. Backfill or sling material from outside of the LID facility at the lowest possible speed to avoid loss of component material.
2. Apply material in 150-300 mm lifts until the desired elevation and thickness, allowing for positive flow.
3. Wet material between lifts, allowing for drying before backfilling/slinging another lift.

Large stone and riprap

Riprap serves as a protective layer to: prevent sediment entering the underlying layers and clogging the LID facility, dissipate stormwater energy, and stabilize the terrain and slope.

Construction Steps:

1. Backfill the material from the outside of the LID facility to avoid compaction.
2. Ensure the material is being placed to the elevation and thickness to the design requirements.

Key Inspection Points:

• Arrival of material to the site: material meets specifications as per design, no debris in the aggregate and, if possible, washed or clean stone with little to no fine materials in it.
• Placement: material is backfilled to the right depth and elevation.

Common Mistakes to Avoid:

• Use of wrong material that does not meet design specifications. If there are changes in the material, it must be approved by the supervisor or project manager.
  

Riprap

Plant material verification and installation

CVC staff planting plant material at Our Lady of Fatima Catholic School in Brampton, Ontario. (Photo Source: CVC, 2022)

A healthy and vibrant vegetation structure provides itself with nutrients to sustain growth. Additionally, it holds, and retains water and oxygen, and binds and degrades pollutants. When selecting plants, consider the following:

• Native species need fewer inputs and are better adapted to local climate and soils.
• Larger plant stock should be considered and prioritized in areas of frequent ponding and concentrated flows.
• Trees, shrubs, grasses, sedges and/or rushes present unique benefits and limitations in LID facilities. For example, debris from trees/shrubs may block outlets, but their size and form of planting can provide superior establishment in challenging locations.
• Planted plants in a moderately diverse cluster of plants are more easily maintained and more visually accepted and pleasing to the general public.
• A planting plan should include species that tolerate harsh conditions (e.g., drought, water inundation and/or salt). Most riparian plant species will do well in rain gardens, bioswales and bioretention facilities.
  

Tree guards used at Haggert Ave bioswale in Brampton, Ontario. (Photo Source: CVC, 2019)

During installation:

• Time of planting: It is crucial for plant survival and thriving, avoid at all times planting during summer months with higher temperatures (July and August) and winter months (November - March). Consider planting early in the spring (April - June) or late in the fall (September - October).
• Planting depth: Plant each plug/tree or shrub to the desired depth as per design considerations. For small plants, it is recommended to use small tools such as a hand trowel.
• Plant substitution: Any changes in the species delivered during construction must be accepted by the designer and/or supervision team before installing.
• Trees/Shrubs protection: If any trees/shrubs need protection, ensure that they receive it by installing proper tree guards.
• Irrigation: It is recommended for the plant material to receive additional watering right after planting if no rainfall event occurs. Also, plant material should be watered during the summer months or during drought periods for two years after planting.
• Fertilizers: The use of these products is acceptable, as long as they are applied properly.
• Pesticides: It is recommended to only use if needed.

For more information regarding plant selection, refer to Plant lists and Plant selection.

Mulch placement

Wood mulch provides numerous functions to a bioretention practice including reducing soil erosion, filtration, protecting underlying soils from compaction, retaining moisture, and minimizing volunteer weed establishment. Communicate to contractor how to install and the mulch type in the plans and or specifications with notes, details, spot elevations, and other special features needed as per designed.

Equipment recommended for this task can be:

• Automated: Mulch blowers.
• Manual: Shovels, buckets, level rakes.
  

During installation:

1. Ensure that mulch depth should be no greater than 75 mm to maintain oxygen supply to underlying soils.
2. Ensure that the top of the mulch meets the finish elevation as per design.
   

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  Machinery placing mulch in a rain garden at Glendale Public School in Brampton, ON. Read about the Design and Build Overview in this case study.

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  Mulch layer as the final grade in the Haggert Ave bioswale project in Brampton, ON.

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  Elevation problem with the mulch close to the end of this rain garden at Mineola P.S. This elevation is not allowing runoff to fully enter the rain garden and causing ponding in this low spot.

Stabilizing contributing drainage area - planting adjacent vegetation

Similar to the plant material verification and installation task shown in Construction: plant material verification and installation page, any planting required to stabilize the contributing drainage area will need to meet the specifications and considerations shown in such page.

Additionally, if turf/grasses is required to stabilize the contributing drainage area, installation should be done as per the grower/nursery’s specifications and standards.

Natural lawn grass installed to stabilize the contributing drainage area at the Glendale P.S. rain garden in Brampton, Ontario. (Photo source: CVC)

As-built surveys

Throughout the construction process, it is sometimes necessary to deviate from the intended design of LID features and adapt the design to on-site conditions. The completion of a post-construction as-built survey is a standard operating procedure for engineering projects that captures any changes made to the feature’s design during construction. For an example of municipal standards for as-built surveys, refer to the City of Toronto's Engineering Survey Standards for Consultants

Survey Steps:

1. Use the same datum as the pre-engineering survey.
2. If a pre-engineering survey is not available, use a reference feature as the datum.
3. Identify the type, diameter, and material of exposed utilities.
4. Include data on critical feature elevations and existing utilities, such as:
   • Inverts of newly installed pipe
   • Vertical and horizontal bends in pipes
   • Existing public utilities
   • Existing private utilities
   • New and existing structures (e.g., catchbasins, manholes, chambers, etc.)

Inspection Points:

• Datum matches the pre-engineering survey or selected reference feature.
• Data provided in as-built survey matches the as-built standards of the local municipality.

Permanent fencing

Permanent fencing at Elm Drive in Mississauga.

Delineating features with permanent fencing can eliminate tripping and falling hazards and protect soil media and plantings.

Construction Steps:

1. Securely install fencing around the perimeter of the feature.

Inspection Points:

• Fencing size and material matches design.
• Fencing is installed in correct location as per design.

Post-Construction

Addressing deficiencies

Prior to passing responsibility to the landowner, contractors must ensure that any deficiencies in the LID feature’s construction are addressed. Keep in-mind what mistakes must be avoided during each construction task.

Final certification

Final inspection requirements are in-place to ensure that a final site-wide inspection is performed and that mitigation can occur before the contractor is relieved of responsibility. Final certifications are performed by knowledgeable personnel, such as the design engineer, who are qualified to ensure that LID features are operating as intended. This stage is critically important, as it is the last opportunity for issues to be resolved before the property owner assumes responsibility of the practice. Four (4) levels of inspection exist for final inspections of LID infiltration and filtration practices. The level of inspection, or combination of tests, that is selected is dependent on the goals of the assessment and feasibility of each inspection. A substantial amount of guidance is available in Low Impact Development Certification Protocols: Bioretention Practices.

Visual Inspection (Level 1)
This qualitative method is low cost and requires minimal effort to conduct. Visual inspection should be used as an initial step to determine if the LID practice is operating properly. Visual inspections can be performed during dry weather and wet weather to examine various aspects of LID performance, such as the following:

• draw-down time
• ponded water
• soil properties
• plant cover and plant health
• sedimentation

Surface Infiltration Capacity Testing (Level 2)
Level 2 includes the employment of quantitive methods of testing and data collection. These tasks are also cost-effective and can establish if the LID feature was built in conformance with the design specifications. Some level 2 tests include the following:

• soil sampling and testing
• elevation surveys
• infiltration testing
• synthetic runoff testing

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  Flood test for a bioretention LID facility. (Photo Source: CVC, n.d.)

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  Flood test for permeable pavers. (Photo Source: CVC, 2013)

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  Double-ring infiltrometer. (CVC, 2012)

Continuous Water Level Monitoring (Level 3)
Site infiltration rates can be measured with greater accuracy through continuous water level monitoring. This monitoring approach evaluates seasonal infiltration performance through the monitoring of temperature and groundwater levels. The following conditions and equipment are necessary for continuous water level monitoring:

1. A perforated observation well
2. A water level logger
3. A rain gauge within 5 km of the site

Using rainfall and water level data can provide an understanding of drawdown times after variously sized storms.

rainfall data and water level measurements can be used to assess infiltration performance. (Photo Source: CVC, 2015)

Long-term Monitoring (Level 4)
High-intensity monitoring is a comprehensive but expensive approach to assessing peak flow reduction and water volume reduction during natural runoff events. These reductions can be quantified by completing a water budget during natural storm events, specifically through the measurement of inflow and outflow. Pollutant load reductions can also be quantified with the appropriate technology. Long-term monitoring is especially recommended if the LID feature that is the subject of the test is the first of its kind in its given jurisdiction, if geologic conditions pose a concern, or if the feature is being implemented to protect sensitive and significant natural features.

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.

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.

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

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.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

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.

Mineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm.

Deposition of material of varying size, both mineral and organic away from its site of origin by the action of water, wind, gravity or ice.

Settling-out or deposition of particulate matter suspended in runoff.

A perforated pipe used to assist the draining of soils.

Credit Valley Conservation

Human application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47

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.

Ground depression acting as a flow control and water treatment structure, that is normally dry.

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

Sustainable Technologies Evaluation Program

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 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.

Soil particles with a diameter less than 0.050 mm.

The total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds.

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.

Soil or media particles smaller than sand and larger than clay (3 to 60 m)

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

An alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.

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.

The total mass of a pollutant entering a waterbody over a defined time period.

The net amount of something (e.g. chemical, such as phosphorus), calculated as the product of concentration and volume in a given time. Some BMPs significantly reduce loading of pollutants to the environment by reducing volume more so than concentration.

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.

The water below the surface, and typically below the groundwater table.

Mineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm.

Refers to a system that accepts all of the flow from a drainage area and conveys larger event flows through an overflow outlet.

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.

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 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.

(1) The wearing away of the land surface by moving water, wind, ice or other geological agents, including such processes as gravitation creep; (2) Detachment and movement of soil or rock fragments by water, wind, ice or gravity (i.e. Accelerated, geological, gully, natural, rill, sheet, splash, or impact, etc).

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.

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.

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

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

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.

The accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle.

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

The natural ground material characteristic of or existing by virtue of geographic origin.

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.

Human application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47

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

The technique of removing pollutants from runoff as it infiltrates through the soil.

The mathematical expression of the water balance.

The total mass of a pollutant entering a waterbody over a defined time period.

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.

Low impact development is a stormwater management and land development strategy applied at the parcel and subdivision scale that emphasizes conservation and use of on-site natural features integrated with engineered, small scale hydrologic controls to more closely mimic pre-development hydrologic functions.

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.