Inspection and Maintenance: Bioretention & Bioswales

Inspection & Maintenance Guidance of vegetated stormwater best management practices that tmeproarily store rainwater and snowmelt from roofs, pavement, parking lots, etc. in depressed planting beds, concrete planters or swales. TRCA, 2016

Overview

Bioretention treats stormwater by slowing it down, filtering it through soil and plant roots, soaking it into the ground and evaporating it back to the atmosphere. Runoff water is delivered to the practice through inlets such as curb cuts, spillways or other concrete structures, sheet flow from pavement edges, or pipes connected to catchbasins or roof downspouts. The planting bed and side slopes are typically covered with a mixture of plants, mulch and stone. Water in excess of its storage capacity overflows to another BMP or the municipal storm sewer. Filtered water is either infiltrated into the underlying soil to replenish groundwater, or collected by a sub-drain (i.e., underground perforated pipe) and discharged to the storm sewer system or another BMP. Depending on the permeability of the underlying soil or other constraints, it may be designed with no sub-drain for full infiltration, with a sub-drain for partial infiltration, or with an impermeable liner and sub-drain for a no infiltration practice. The sub-drain pipe may feature a flow restrictor (e.g., orifice cap or valve) for gradually releasing detained water and optimizing the amount drained by infiltration. Key components of bioretention practices for inspection and maintenance described further below.

Associated Terminology

• Bioretention cell: A flat-bottomed, depressed planting bed containing filter media soil, a gravel water storage layer and optional sub-drain pipe(s); Also known as a rain garden.
• Stormwater planters: A bioretention cell contained within an engineered (e.g., concrete) structure.
• Biofilter: Bioretention cell or swale with an impermeable liner or containment structure and sub-drain.
• Dry swale or Bioretention swale: A gently sloping, linear oriented bioretention practice designed to be capable of conveying water across an elevation gradient. Also known as a bioswale or dry swale.

How to Preserve Basic BMP Function

• Maintain grading of the filter bed (or grass filter strip if present) at curb cut inlets so at least 5 cm of the back of the curb is visible through regular sediment removal and regrading.
• To avoid over-compaction of the filter media soil, any maintenance tasks involving vehicle or foot traffic on the filter bed should not be performed during wet weather.
• For bioretention with sod (i.e., turf grass) as vegetation cover, maintain with a push mower or the lightest mulching ride mower available and core aerate and dethatch annually in the spring to help maintain permeability.
• Pruning of mature trees should be performed under the guidance of a Certified Arborist.
• Woody vegetation should not be planted or allowed to become established where snow will be piled/stored during Winter.
• Removal of sediment from the filter bed surface should be done with rake and shovel, or vacuum equipment to minimize plant disturbance. If a small excavator is to be used, keep it off the BMP footprint to avoid damage to side slopes/embankments and over-compaction of the filter media.

Construction Inspection Tasks

Construction inspections take place during several points in the construction sequence, specific to the type of LID BMP, but at a minimum should be done weekly and include the following:

1. During site preparation, prior to BMP excavation and grading to ensure the CDA is stabilized or that adequate ESCs or flow diversion devices are in place and confirm that construction materials meet design specifications
2. At completion of excavation and grading, prior to installation of pipes/sewers and backfilling to ensure depths, slopes and elevations are acceptable
3. At completion of installation of pipes/sewers, prior to backfilling to ensure slopes and elevations are acceptable
4. After final grading, prior to planting to ensure depths, slopes and elevations are acceptable;
5. Prior to hand-off points in the construction sequence when the contractor responsible for the work changes (i.e., hand-offs between the storm sewer servicing, paving, building and landscaping contractors)
6. After every large storm event (e.g., 15 mm rainfall depth or greater) to ensure Erosion Sediment Controls (ESCs) and pretreatment or flow diversion devices are functioning and adequately maintained. View the table below, which describes critical points during the construction sequence when inspections should be performed prior to proceeding further. You can also download and print the table here
   

Performance Inspection of a Rain garden. Photo source: TRCA.

Bioretention/Swales: Construction Inspections

Construction Sequence Step & Timing	Inspection Item	Observations*
Site Preparation - after site clearing and grading, prior to BMP excavation and grading	Natural heritage system and tree protection areas remain fenced off
	ESCs protecting BMP layout area are installed properly
	CDA is stabilized or runoff is diverted around BMP layout area
	BMP layout area has been cleared and is staked/delineated
	Benchmark elevation(s) are established nearby
	Construction materials have been confirmed to meet design specifications
BMP Excavation and Grading - prior to installation of pipes/sewers and backfilling	Excavation location, footprint, depth and slopes are acceptable
	Excavated soil is stockpiled outside the CDA
	Embankments/berms (elevations, slopes, compaction) are acceptable
	Excavation bottom and sides roughened to reduce smearing and compaction
BMP Installation – after installation of pipes/sewers, prior to backfilling	Structural components (e.g., foundation, walls) installed according to plans, if applicable
	Impermeable liner installed correctly, if applicable
	Installations of sub-drain pipes (e.g., locations, elevations, slopes), standpipes/monitoring wells are acceptable
	Sub-drain trench dams installed correctly (location, elevation)
Landscaping – after final grading, prior to planting	Filter bed depth and surface elevations at inlets are acceptable
	Maximum surface ponding depth is acceptable
	Filter bed is free of ruts, local depressions and not overly compacted
	Planting material meets approved planting plan specifications (plant types and quantities)
Note: for Observation Column: S = Satisfactory; U = Unsatisfactory; NA = Not Applicable*

Routine Maintenance - Key Components and I&M Tasks

Regular inspections (twice annually, at a minimum) done as part of routine maintenance tasks over the operating phase of the BMP life cycle to determine if maintenance task frequencies are adequate and determine when rehabilitation or further investigations into BMP function are warranted.

Table below describes routine maintenance tasks for bioretention practices, organized by BMP component, along with recommended minimum frequencies. It also suggests higher frequencies for certain tasks that may be warranted for BMPs located in highly visible locations or those receiving flow from high traffic areas (vehicle or pedestrian). Tasks involving removal of trash, debris and sediment and weeding/trimming of vegetation for BMPs in such contexts may need to be done more frequently (i.e., higher standards may be warranted).

Individuals conducting vegetation maintenance and in particular, weeding (i.e., removal of undesirable vegetation), should be familiar with the species of Plants: List specified in the planting plan and experienced in plant identification and methods of removing/controlling noxious weeds. Key resources on these topics are provided below at the links provided:

• Agriculture and Agri-food Canada’s Weed Info database
• Ontario Ministry of Agriculture, Food and Rural Affairs’ Ontario Weed Gallery
• Ontario Ministry of Agriculture, Food and Rural Affairs’ Noxious Weeds In Ontario list
• Ontario Invasive Plant Council’s Quick Reference Guide to Invasive Plant Species
• Oregon State University Stormwater Solutions, 2013, Field Guide: Maintaining Rain Gardens, Swales and Stormwater Planters, Corvallis, OR.
• Plants of Southern Ontario (book), 2014, by Richard Dickinson and France Royer, Lone Pine Publishing, 528 pgs.
• Weeds of North America (book), 2014, by Richard Dickinson and France Royer, University of Chicago Press, 656 pgs.

Bioretention/Swales: Key Components, Descriptions and Routine I&M Requriements

Component	Description	Inspection & Maintenance Tasks
Contributing Drainage Area (CDA)	Area(s) from which runoff directed to the BMP originates; includes both impervious and pervious areas.	Remove trash, debris and sediment from pavements (biannually to quarterly) and eavestroughs (annually); Replant or seed bare soil areas as needed.
Pretreatment	Devices or features that retain trash, debris and sediment; help to extend the operating life cycle; examples are eavestrough screens, catch basin inserts and sumps, oil and grit separators, geotextile-lined inlets, gravel trenches, grass filter strips and forebays.	Remove trash, debris and sediment annually to biannually or when the device sump is half full; Measure sediment depth or volume during each cleaning, or annually to estimate accumulation rate and optimize frequency of maintenance
Inlets	Structures that deliver water to the BMP (e.g., Curb cuts, spillways, pavement edges, catch basins, pipes).	Keep free of obstructions; Remove trash, debris and sediment biannually to quarterly; Measure sediment depth or volume during each cleaning or annually to estimate accumulation rate and optimize frequency of maintenance; Remove woody vegetation from filter bed at inlets annually.
Perimeter	Side slopes or structures that define the BMP footprint; may be covered by a mixture of vegetation, mulch and stone with slopes up to 2:1 (H:V), or concrete or masonry structures with vertical walls.	Confirm the surface ponding footprint area dimensions are within ±10% of the design and that maximum surface ponding depth meets design specifications; Check for side slope erosion or damage that compromises water storage capacity
Filter Bed	Flat or gently sloping area composed of a 0.5 to 1 m deep layer of filter media soil covered by a mixture of vegetation, mulch and stone where surface ponding and filtration of runoff occurs.	Check for standing water, barren/eroded areas, sinkholes or animal burrows; Remove trash biannually to quarterly; Rake regularly to redistribute mulch and prevent sediment crusts; Maintain 5 to 10 cm of mulch or stone cover to prevent weed growth and soil erosion; Repair sunken areas when ≥ 10 cm deep and barren/eroded areas when ≥ 30 cm long; Remove sediment when > 5 cm deep or time to drain water ponded on the surface exceeds 48 hours.
Vegetation	A mixture of deep rooting perennial plants, tolerant to both wet and dry conditions and salt (if receiving pavement runoff); can include grasses, flowers, shrubs and trees; roots uptake water and return it to the atmosphere; provide habitat for organisms that break down trapped pollutants and help maintain soil structure and permeability.	Routine maintenance is the same as a conventional perennial garden bed; In the first 2 months water plantings frequently (biweekly in the absence or rain) and as needed (e.g., bimonthly) over the remainder of the first growing season; Remove weeds and undesirable plants biannually to quarterly; Replace dead plantings annually to achieve 80% cover by the third growing season; Do not apply chemical fertilizers.
Overflow Outlet	Structures that convey overflows to another BMP or municipal storm sewer.	Keep free of obstructions; Remove trash, debris and sediment biannually to quarterly.
Sub-drain	Optional component; perforated pipe(s) surrounded by gravel and may be wrapped in geotextile filter fabric; installed below the filter media soil layer to collect and convey treated water to an adjacent drainage system; may also include a flow restrictor.	Include standpipes or access points to provide means of flushing the perforated pipe; Keep pipe and flow restrictor free of obstructions by flushing annually; Inspect flow restrictor frequently (e.g., biannually to quarterly).
Monitoring well	Perforated standpipe that extends from the bottom of the BMP to above the invert of the overflow outlet. Allows measurement of subsurface water level to track drainage performance over time.	Standpipes should be securely capped on both ends and remain undamaged.

Rehabilitation & Repair

Table below provides guidance on rehabilitation and repair work specific to bioretention and dry swales organized according to BMP component.

Technician conducting sediment removal to ensure infiltration rates for the practice are able to maintain > 25 mm/h. Photo Source: TRCA, 2018

Bioretention/Swales: Key Components, Typical Issues and Rehabilitation Requirements

Component	Problem	Rehabilitation Tasks
Inlets	Inlet is producing concentrated flow and causing filter bed erosion.	Add flow spreading device or re-grade existing device back to level. Rake to regrade damaged portion of the filter bed and replant or restore mulch/stone cover. If problem persists, replace some mulch cover with stone.
Filter Bed	Local or average sediment accumulation ≥ 5 cm in depth.	At inlets remove stone and use vacuum equipment or rake and shovel to remove sediment. For large areas or BMPs, use of a small excavator may be preferable. Restore grades with filter media that meets design specifications. Test surface infiltration rate (one test for every 25 m2 of filter bed area) to confirm it is > 25 mm/h. Replace stone, mulch and vegetation coverage (re-use/transplant where possible). If problem persists, add pretreatment device(s) or investigate the source(s).
	Surface ponding remains for >48 hours or surface infiltration rate is <25 mm/h.	Remove sediment as described above. Core aerate (for sodded bioretention); or remove stone, sediment, mulch, and plant cover and till the exposed filter media to a depth of 20 cm; or remove and replace the uppermost 15 cm of material with filter media that meets specifications. Test surface infiltration rate (one test for every 25 m2 of filter bed area) to confirm it is > 25 mm/h. Replace stone, mulch and plants (re-use/ transplant where possible).
	Damage to filter bed or slide slope is present (e.g., erosion rills, animal burrows, sink holes, ruts)	Regrade damaged portion by raking and replant or restore mulch/stone cover. Animal burrows, sink holes and compacted areas should be tilled to 20 cm depth prior to re-grading. If problems persist, consider adding flow spreading device to prevent erosion or barriers to discourage foot or vehicular traffic.
Vegetation	Vegetation is not thriving and filter media is low in organic matter (<3%) or extractable phosphorus (<10 mg/kg)	Remove stone, mulch and plant cover and uppermost 5 cm of filter media, spread 5 cm of yard waste compost, incorporate into filter media to 20 cm depth by tilling. Replace stone, mulch and plants (re-use/transplant where possible).
Sub-drain	Sub-drain perforated pipe is obstructed by sediment or roots.	Schedule hydro-vac truck or drain-snaking service to clear the obstruction.

Inspection Time Commitments and Costs

Estimates are based on a typical partial infiltration bioretention design (i.e., includes a sub-drain); estimates for other designs (i.e., full infiltration and no-infiltration) can be found in the Low Impact Development (LID) Stormwater Management Practice Inspection and Maintenance Guide

General time commitments and costs for inspection of bioretention and dry swale features with partial infiltration (in 2016 $ figures).

Per-task cost estimates for maintenance and rehabilitation of a partial infiltration bioretention feature (in 2016 $ figures).

Construction and life cycle cost estimates for bioretention and dry swale features with partial infiltration (in 2016 $ figures).

Estimates of the life cycle costs of inspection and maintenance have been produced using the latest version of the LID Life Cycle Costing Tool for three design variations (full infiltration, partial infiltration and no infiltration) to assist stormwater infrastructure planners, designers and asset managers with planning and preparing budgets for potential LID features.

Assumptions for the above costs and the following table below are based on the following:

• Capital costs included within the category of construction include those related to site assessment, and conceptual and detailed design related tasks such as borehole analysis and soil testing. All material, delivery, labour, equipment (rental, operation, operator), hauling and disposal costs are accounted for within the construction costs of the facility. Standard union costs were derived from the RSMeans database in 2010 and have been adjusted for 5 year inflation of 8.79% (2010 to June, 2015).
  • Costs include overhead and inflation to represent contractor pricing. It was assumed the practice is part of a new development (i.e., not a retrofit), thereby excluding (de)mobilization costs unless a particular piece of equipment would not normally have been present at the site. Additionally, it was assumed that excavated soil associated with construction of the BMP would be reused elsewhere on site. Overhead costs were presumed to consist of construction management (4.5%), design (2.5%), small tools (0.5%), clean up (0.3%) and other (2.2%).
• For maintenance frequencies and requirements and the life span of each practice are based on both literature and practical experience. Life cycle and associated maintenance costs are evaluated over a 50 year timeframe, which is the typical period over which infrastructure decisions are made.
• For bioretention, it is assumed that some rehabilitation (e.g., rehabilitative maintenance) work will be needed on the filter bed surface once the BMP reaches 25 and 50 years of age in order to maintain functional drainage performance at an acceptable level. Included in the rehabilitation costs are (de)mobilization costs, as equipment would not have been present on site. Design costs were not included in the rehabilitation as it was assumed that the original LID practice design would be used to inform this work. The annual average maintenance cost does not include rehabilitation costs and therefore represents an average of routine maintenance tasks, as outlined in the Table under section, Routine Maintenance - Key Components and I&M Tasks above. All cost value estimates represent the net present value (NPV) as the calculation takes into account average annual interest (2%) and discount (3%) rates over the evaluation time periods.

Inspection Field Data Sheet

Feel free to download (downward facing arrow on the top righthand side) and print (Pinter emoticon on top right hand side) the following Bioretention and Dry swale Inspection Field Data Form developed by TRCA, STEP and its partners for the Low Impact Development Stormwater Management Practice Inspection and Maintenance Guide.

The 8 page document prompts users to fill out details previously mentioned above on this page in other sections about various zones associated with Bioretention and Dry swale features (i.e. inlets, perimeter of the feature, filter bed, outlets, etc.) and describe why each area is a pass or fail, and if remediate action is required and under what timeframe it would be completed by. Furthermore, the forms prompt the reviewer to determine what type of inspection is being conducted for the feature in question: Construction (C), Routine Operation (RO), Maintenance Verification (MV), or Performance Verification (PV).

Staff conducting monitoring of bioretention's water level with the use of a continuous water level data logger located in the feature'smonitoring well. Photo Source: TRCA, 2018

State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include Source, Conveyance and End-Of-Pipe Controls.

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.

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.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

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

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.

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

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

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.

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.

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.

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.

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.

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.

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.

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

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

Water ponded on the ground surface.

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

A system flow of gully inlets, pipes, overland flow paths, open channels, culverts and detention basins used to convey runoff to its receiving waters. City of Toronto 45 Wet Weather Flow Management November 2006

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.

(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).

The rate at which stormwater percolates into the subsoil measured in inches per hour.

Decayed organic material used as a plant fertilizer. Compost helps to support healthy plant growth through the slow release of nutrients and the retention of moisture in the soil.

Toronto and Region Conservation Authority

Sustainable Technologies Evaluation Program

State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include Source, Conveyance and End-Of-Pipe Controls.

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.