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.

Key Components and I&M Tasks

Key Components, Descriptions and Routine Inspection and Maintenance Tasks

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, catchbasins, 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. References

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.

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.

Toronto and Region Conservation Authority

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.