Inspection and Maintenance: Vegetated Filter Strips

Inspection & Maintenance Guidance of Vegetated filter strips, which are a vegetated stormwater best management practice that are gently sloping, densely vegetated areas that treat runoff as sheet flow from adjacent impervious areas. Similarly, soil amendment areas are any landscaped area where the topsoil has been amended to enhance its water holding capacity (TRCA, 2013)

Overview

Vegetated filter strips (a.k.a. buffer strips and grassed filter strips) are gently sloping, densely vegetated areas that treat runoff as sheet flow from adjacent impervious areas. They slow runoff velocity and filter out suspended sediment and associated pollutants, and provide some infiltration into underlying soils. Originally used as an agricultural treatment practice, filter strips have evolved into an urban SWM practice. Vegetation may be comprised of a variety of trees, shrubs and native plants to add aesthetic value as well as water quality benefits. With proper design and maintenance, filter strips can provide relatively high pollutant removal benefits. Maintaining sheet flow into the filter strip through the use of a level spreading device (e.g., pea gravel diaphragm) is essential. Using vegetated filter strips as pretreatment practices to other best management practices is highly recommended. They also provide a convenient area for snow storage and treatment, and are particularly valuable due to their capacity for snowmelt infiltration.

Properly functioning vegetated filter strips and soil amendment areas reduce the quantity of pollutants and runoff being discharged to municipal storm sewers and receiving waters (i.e., rivers, lakes and wetlands). In addition to their SWM benefits, they provide aesthetic value as attractive landscaped features.

Key components of Vegetated filter strips to pay close attention to are the:

• Inlets
• Vegetated Slopes
• Soil amendments
• Slope & Channel Design
• Overflow outlets

Trash, debris and sediment builds up at these locations and can prevent water from flowing into or out of the practice.

Associated Practices

• Grass Swales: A parabolic or trapezoidal-sized bottom, swale that contains grassed sloping sides and a filter media bottom to both convey overland flow and provide water treatment, and are often subject to more frequent maintenance. They generally contain an outlet structure at the lowest point for water to be sent to another LID BMP or the storm system; sometimes referred to as a roadside ditch. Does not contain check dams.
• Swales: Swales are linear landscape features consisting of a drainage channel with gently sloping sides. Underground they may be filled with engineered soil and/or contain a water storage layer of coarse gravel material. Two variations on a basic swale are recommended as low impact development strategies, although using a combination of both designs may increase the benefit.
• Bioswales are sometimes referred to as 'dry swales', 'vegetated swales', or 'water quality swales'. This type of BMP is form of bioretention with a long, linear shape (surface area typically >2:1 length:width) and a slope which conveys water and generally contains various water tolerant vegetation.

Inspection and Testing Framework

Example of a properly constructed Vegetated Filter Strip, which includes proper vegetation condition, composition coverage that are important visual indicators that will have to be maintained and checked regularly by trained staff. Source: (GVRD, 2005)..

Visual Indicators Framework - Vegetated Filter Strips

Component	Indicators	Construction Inspection	Assumption Inspection	Routine Operation Inspection	Verification Inspection
Contributing Drainage Area
	CDA condition	x	x	x	x
Inlet
	Inlet/Flow Spreader Structural Integrity		x	x	x
	Inlet/Flow Spreader Structural Integrity	x	x	x	x
Perimeter
	BMP dimensions	x	x		x
Filter Bed
	Standing water		x	x	x
	Trash		x	x
	Filter bed surface sinking		x	x	x
Planting Area
	Vegetation cover	x	x	x	x
	Vegetation condition		x	x
	Vegetation composition	x	x	x

The image above shows a manual soil corer, a split soil core sampler kit which preserves the soil sample for further testing (i.e., bulk density) and acrylic core sample tubes. All examples of tools to be use for soil characterization testing. Source: (TRCA, 2016)..

Testing Indicators Framework - Bioretention/Swales

Component	Indicators	Construction Inspection	Assumption Inspection	Routine Operation Inspection	Verification Inspection
Testing Indicators
	Soil characterization testing	x	x		(x)
	Surface infiltration rate testing		x		(x)
Note: (x) denotes indicators to be used for Performance Verification inspections only (i.e., not for Maintenance Verification inspections)

Construction Inspection Tasks

Example of a properly designed and implemented vegetated grass swale in a parking lot. It's important to check on the BMP after large storm events (>15 mm) to determine it's still in good condition and functioning as designed. (Photo Source: MAPC, 2010)

During construction inspections the planting material (types and quantities) selected along with where they are oriented with the grass filter strips should follow the original planting plan. (Photo Source: MAPC and 495/MetroWest Partnership, 2017)

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 grading, prior to planting to ensure slopes and elevations are acceptable
3. 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
4. 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
   

Vegetated Filter Strips

Construction Sequence Step & Timing	Inspection Item	Observations*
Site Preparation - after site clearing and grading, prior to BMP 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 Grading - prior to landscaping	Excavation location, footprint, depth and slopes are acceptable
	Excavated soil is stockpiled outside the CDA
Landscaping – after final grading, prior to planting	Topsoil depth, degree of compaction and surface elevations at inlets and outlets are acceptable
	Area is free of ruts, local depressions
	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

Generally, routine maintenance will be the same as for any other landscaped area; weeding, pruning, and litter removal. Regular watering may be required during the first two years until vegetation is established. Routine inspection is very important to ensure that dense vegetation cover is maintained and inflowing runoff does not become concentrated within the practice. Vehicles should not be parked or driven on filter strips. For routine mowing of grassed filter strips, the lightest possible mowing equipment should be used to prevent soil compaction. For the first two years following construction the filter strip should be inspected at least quarterly and after every major storm event (> 25 mm). Subsequently, inspections should be conducted in the spring and fall of each year and after major storm events. Inspect for vegetation density (at least 80% coverage), damage by foot or vehicular traffic, channelization, accumulation of debris, trash and sediment, and structural damage to pretreatment devices. Trash and debris should be removed from pretreatment devices and the filter strip surface at least twice annually. Other maintenance activities include weeding, replacing dead vegetation, repairing eroded areas, dethatching and aerating as needed. Remove accumulated sediment on the filter strip surface when dry and exceeding 25 mm depth.

Table below describes routine maintenance tasks for Vegetated filter strips, 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 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
• 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.

Vegetated/Grass Filter Strips: Key Components, Descriptions and Routine I&M Requirements

Component	Description	Inspection & Maintenance Tasks	(Pass) Photo Example	(Fail) Photo Example
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.	CDA has not changed in size or land cover. Sediment, trash or debris is not accumulating and point sources of contaminants are not visible. (Photo Source: CSN, 2013)	Size of the CDA or land cover within it has changed from design assumptions. Accumulation of sediment on the CDA is visible.
Inlets & Overflow Outlets	Structures that deliver water to the BMP (e.g., Curb cuts, spillways, pavement edges, catch basins, pipes) or convey flow that exceeds the storage capacity of the BMP to another drainage system (i.e. other LID BMP, or storm sewer).	Keep free of obstructions; Remove trash, debris and sediment biannually to quarterly; Reconfigure splash block if displaced; Remove accumulated sediment	There is no evidence of damage to the gravel diaphragm inlet that would prevent runoff from entering the BMP nor excessive erosion of the filter bed. (Source: Aquafor Beech). There are no obstructions at the inlet and stormwater can freely flow into the BMP as sheet flow from the pavement and gravel diaphragm. (Photo Source: CSN, 2013	Splash pad has been displaced and could lead to excessive erosion of the filter bed. Concrete barriers (parking stall curbs) are preventing stormwater from entering the BMP as sheet flow from the pavement and isnteads heavily concentrating the flow (erosion already occurring. Sediment has accumulated at the inlet edge of the BMP.
Perimeter	Side slopes or structures that define the BMP footprint; may be covered by a mixture of vegetation, with slopes no higher than 3% - inspection is done to confirm the dimensions and footprint area of the BMP are acceptable.	For soil amendment areas, inspection involves confirming that topsoil depth and degree of compaction are acceptable in the specified areas; Check for side slope erosion/damage from vehicular/foot traffic.	The footprint area where soil amendments have been implemented does not significantly deviate from the final design and should not negatively affect its stormwater management treatment performance.	The footprint area where soil amendments have been implemented is significantly smaller than what was specified in the final design.
Filter Bed	Gently sloping (between 0.5 and 3%) vegetated area that receives runoff from adjacent impervious surfaces and is composed of a 0.2 to 0.3 metre deep uncompacted topsoil layer containing 5 to 15% organic matter by dry weight where filtration and evaporation of runoff occurs. Vegetated filter strips and soil amendment areas should not pond water on the surface during storm events.	Check for standing water, barren/eroded areas, sinkholes or animal burrows; Remove trash biannually to quarterly; Remove accumulated sediment when ≥ 5 cm depth; Re-grade and restore cover over any animal burrows, sunken areas when ≥ 10 cm in depth and erosion rills when ≥ 30 cm in length; Add stone cover to maintain 5 to 10 cm depth where specified in the planting plan	There are no erosion rills, gullies or bare soil areas on the filter strip surface. (Photo Source: Washington State Department of Transportation, 2018)	Scour erosion along the inlet edge is visible. Bare soil areas and bright green biofilm on the filter strip surface indicate that concentrated flow and surface ponding occurs regularly. Clear evidence of bed sinking on the lawn, creating preferential ponding areas which could cause vegetation to die off (Source: The Anxious Gardener) .
Vegetation	Healthy vegetation cover (i.e., grasses, herbs, shrubs, and trees) is relied upon to intercept, uptake and evapotranspire stormwater and to provide habitat for soil organisms that break down pollutants. Plant roots also help to maintain soil structure and permeability. Routine maintenance of vegetation is the same as a conventional lawn or planting bed.	Watering during first two months after planting; Watering for the remainder of the first two (2) growing seasons (i.e., May to September) after planting or until vegetation is established; Watering for the remainder of the BMP lifespan Mow grass to maintain height between 5 to 10 cm; Routine maintenance is the same as a conventional lawn; 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 annually to biannually if dieback is particularly high or droughts disease are prevalent Prune shrubs and trees Cut back spent plants Divide or thin out overcrowded plants	The vegetated filter strip is evenly covered with dense turf grass which helps to maintain its stormwater treatment function and aesthetic value. (Photo Source: Trinkaus Engineering))	Major portions of the filter strip contain bare soil or dead vegetation which reduces its aesthetic value and could be negatively affecting its stormwater treatment function

Tips to Preserve Basic BMP Function

• Because the risk of compaction is higher when topsoil is saturated, any maintenance tasks involving vehicle (e.g., ride mower) or foot traffic on the filter bed should not be performed during wet weather.
• Use push mower to maintain enhanced swales with grasses as vegetation cover or the lightest ride mower equipment available to minimize compaction of the filter bed.
• Use a mulching mower to maintain enhanced swales with grass as vegetation cover or leave clippings on the surface to help replenish organic matter and nutrients in the topsoil.
• 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 accumulated on the filter bed surface should be performed by hand with rake and shovel, or vacuum equipment where feasible. If a small excavator is the chosen method, keep the excavator off the BMP footprint to avoid damage to side slopes/embankments and compaction of the topsoil.

Rehabilitation & Repair

Table below provides guidance on rehabilitation and repair work specific to vegetated/grass filter strips and soil amendments organized according to BMP component.

This clever design incorporates a level spreading device after a curb cut has narrowed the flow path. This kind of treatment train approach would provide an opportunity to provide pretreatment at the point of concentration. This repair helps to limit the potential of erosion occurring within the feature. Photo credit: (MPCA, 204)

Vegetated Filter Strips and Soil Amendments Key Components, Typical Issues and Rehabilitation Requirements

Component	Problem	Rehabilitation Tasks
Inlets	Inlet or flow spreading device is producing concentrated flow and causing filter bed erosion.	Add flow spreading device or re-grade existing device back to level to promote sheet flow to the filter bed. Regrade damaged portion of the filter bed and replant. If problem persists, consider adding turf reinforcement devices or replace filter bed vegetation cover with stone at inlets.
Filter Bed	Topsoil is overly compacted.	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 planting soil that meets design specifications. Test surface infiltration rate to confirm it is > 15 mm/h. Replace stone 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 < 15 mm/h.	Remove sediment as described above. Core aerate (for grass swales); or remove stone, sediment and plant cover and till 5 cm of yard waste compost into the exposed planting soil to a depth of 20 cm; or remove and replace the uppermost 15 cm of material with planting soil that meets design specifications. Test surface infiltration rate to confirm it has been restored to > 15 mm/h. Replace stone 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 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	Plants not thriving and planting soil is low in organic matter (<5%) or available phosphorus (<12 mg/kg)	Remove stone and plant cover and uppermost 5 cm of planting soil, spread 5 cm of yard waste compost, incorporate into soil to 20 cm depth by tilling. Replace stone and plants (re-use/transplant where possible).

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

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.

Stormwater Management

a gently sloping, densely vegetated areas that treat runoff as sheet flow from adjacent impervious areas. They function by slowing runoff velocity and filtering out suspended sediment and associated pollutants, and by providing some infiltration into underlying soils.

A level spreading device placed at a runoff discharge location, perpendicular to flow, to maintain sheet flow and distribute runoff as evenly as possible across a pervious area or stormwater infiltration practice. A gravel diaphragm acts as a pretreatment device, settling out suspended sediments before they reach the 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 slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

Watercourses and Lake Ontario, to which Stormwater and Combined Sewer Overflows discharge.

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 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.

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.

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.

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.

A long narrow trench or furrow dug in the ground, as for irrigation, drainage, or a boundary line.

Structures constructed of a non-erosive material, such as suitably sized aggregate, wood, gabions, riprap, or concrete; used to slow runoff water. Can be employed in practices such as bioswales and enhanced grass swales.

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.

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.

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 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.

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.

Water ponded on the ground surface.

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

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

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.

Vegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales.

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.

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 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

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

Abiotic transfer of water vapour to the atmosphere.

The practice of adding organic material, such as mulch or compost to topsoil to improve fertility, and tilling of the native soils to reverse compaction and restore its water retaining capacity.

A body of water smaller than a lake, often artificially formed.

Water ponded on the ground surface.

a gently sloping, densely vegetated areas that treat runoff as sheet flow from adjacent impervious areas. They function by slowing runoff velocity and filtering out suspended sediment and associated pollutants, and by providing some infiltration into underlying soils.

Stormwater management following the hierarchical approach: Source Control measures, Conveyance Control measure and End of Pipe treatment to achieve the water quality and water balance target for lot level development of the preferred strategy.

A combination of lot level, conveyance, and end-of-pipe stormwater management practices.

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.

Vegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales.

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.

Sustainable Technologies Evaluation Program

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.