Inspection and Maintenance: Vegetated Filter Strips

From LID SWM Planning and Design Guide
Jump to navigation Jump to search
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[1])


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.

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

Associated Practices[edit]

  • 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[edit]

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).[2].
Visual Indicators Framework - Vegetated Filter Strips



Construction Inspection

Assumption Inspection

Routine Operation Inspection

Verification Inspection
Contributing Drainage Area
CDA condition x x x x
Inlet/Flow Spreader Structural Integrity x x x
Inlet/Flow Spreader Structural Integrity x x x x
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).[3].
Testing Indicators Framework - Bioretention/Swales



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[edit]

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)[4]

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)[5]

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

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[edit]

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:

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)[6]
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[6]
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.

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)[7]
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[8])

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[edit]

  • 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[edit]

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)[9]

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

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.
  • Core aerate; or remove stone, mulch and vegetation cover and till topsoil to a depth of 20 cm; or remove and replace with uncompacted topsoil that meets design specifications. Replace stone, mulch and vegetation cover (reuse/transplant where possible).
Topsoil organic matter or phosphorus content too low and vegetation not thriving.
Topsoil pH is out of specification range (6.0 to 7.8) and vegetation not thriving.
  • If soil pH is lower than 6.0, amend with ground limestone to raise the pH back to neutrality. If soil pH is higher than 7.8, amend with compost or sulphur to lower the pH back to neutrality.
Topsoil soluble salts content exceeds 2.0 mS/cm .
  • Flush affected area with fresh water.
Surface ponding remains for > 24 hours or surface infiltration rate is out of acceptable range.
  • Remove any accumulated sediment and core aerate. If problem persists, remove vegetation, till the topsoil to a depth of 20 cm to reduce compaction, or remove and replace the uppermost 15 cm of material with topsoil that meets design specifications. Replace vegetation cover (transplant where possible).
Damage to filter bed is present (e.g., erosion rills, animal burrows, local sinking, ruts)
  • Re-grade damaged portion and restore vegetation cover. Animal burrows, local sinking and compacted areas should be tilled to 20 cm depth prior to re-grading and planting.

Inspection Time Commitments and Costs[edit]

Estimates of the life cycle costs of inspection and maintenance have been produced using the latest version of the LID Life Cycle Costing Tool (LCCT) to assist stormwater infrastructure planners, designers and asset managers with planning and preparing budgets.

For vegetated filter strips and soil amendment areas, life cycle cost estimates have been calculated for two level-of-service scenarios: the minimum recommended frequency of inspection and maintenance tasks , and a high frequency scenario to provide an indication of the potential range. The LCCT has assumptions for vegetated filter strips and soil amendment areas based on an impervious to pervious area (I:P) ratio of 2.5:1 and inspection and maintenance recommendations for enhanced swales, for relevant components.

The life cycle cost estimates for vegetated filter strips and soil amendments are presented in the above table. No design variations scenarios were examined, therefore the Minimum Recommended and High Frequency maintenance scenarios are the only two scenarios examined for this BMP. It is assumed that no rehabilitation work will be needed to maintain acceptable drainage performance over a 50 year time period (TRCA, 2016)[10]

Assumptions for the above costs are based on the following:

  • The annual average maintenance cost represents an average of routine maintenance tasks, as outlined above in Routine Maintenance - Key Components and I&M Tasks All cost value estimates represent the NPV as the calculation takes into account average annual interest (2%) and discount (3%) rates over the evaluation time periods.
  • The CDA has been defined as 2,000 m2 of impermeable pavement (e.g., road or parking area) which drains by sheet flow to a vegetated filter strip that is approximately 800 m2 in area (I:P ratio of 2.5:1), 70 m long, 11.4 m wide and situated along the long edge of the paved area. The side slope is defined as 12.5:1 (8%). Water enters the BMP as sheet flow from a gravel diaphragm flow spreading device along the edge adjacent to the pavement. The BMP surface is planted with grass and does not include check dams, nor any pipes or culverts.
  • Estimates of the life cycle costs of vegetated filter strips and soil amendment areas in Canadian dollars per unit CDA ($/m2) are presented above. The [[Cost analysis resources|LCCT) allows users to select what BMP type and design variation applies, and to use the default assumptions to generate planning level cost estimates. Users can also input their own values relating to a site or area, design, unit costs, and inspection and maintenance task frequencies to generate customized cost estimates, specific to a certain project, context or stormwater infrastructure program.


  1. Estimated life cycle costs represent NPV of associated costs in Canadian dollars per square metre of CDA ($/m2).
  2. Average annual maintenance cost estimates represent NPV of all costs incurred over the time period and do not include rehabilitation costs.
  3. Rehabilitation cost estimates represent NPV of all costs related to repair work assumed to occur every 25 years including those associated with inspection and maintenance over a two (2) year establishment period for the plantings.
  4. Life cycle costs are very similar but slightly lower for BMPs constructed with filter sock or rock check dams, than concrete ones due to differences in material and labor unit costs.
  5. Rehabilitation costs are estimated to be between 24.4 to 28.1% of the original construction costs for High Frequency and Minimum Recommended maintenance program scenarios, respectively.
  6. It is assumed that no rehabilitation is needed to maintain acceptable drainage performance over a 50 year evaluation period.
  7. Maintenance and rehabilitation costs over a 25 year time period are estimated to be 3.52 to 5.42 times the original construction cost, for the Minimum Recommended and High Frequency maintenance scenarios respectively.
  8. Maintenance and rehabilitation costs over a 50 year time period are estimated to be 6.16 and 9.57 times the original construction cost for the Minimum Recommended and High Frequency maintenance scenarios respectively.

Inspection Field Data Sheet[edit]

Feel free to download (downward facing arrow on the top righthand side) and print (Pinter emoticon on top right hand side) the following Vegetated filter strips and Soil amendments Inspection Field Data Form developed by TRCA, STEP and its partners for the Low Impact Development Stormwater Management Practice Inspection and Maintenance Guide[11].

The 6 page document prompts users to fill out details previously mentioned above on this page in other sections about various zones associated with [[[Vegetated Filter strips]] and Soil amendments 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).

Example of a plan drawing and appropriate construction specifications for Vegetated filter strips designed and implemented within Toronto's, municipal bounds (City of Toronto, 2021)[12]

load PDF


  1. TRCA. 2013. Fact Sheet - LID Manual Master Reference List: Vegetated Filter Strips.
  2. Greater Vancouver regional District (GVRD). 2005. Stormwater Source Control Design Guidelines 2005. Lanarc Consultants Ltd., Kerr Wood Leidal Associates Ltd. and Goya Ngan. April 2005.
  3. STEP. 2016. Low Impact Development Stormwater Management Practice Inspection and Maintenance Guide.
  4. METROPOLITAN AREA PLANNING COUNCIL (MAPC). 2010. Fact Sheet: Grass Filter Strips. Witten by: Kit Un. 5 February 2010. Accessed 12 July 2022.
  5. METROPOLITAN AREA PLANNING COUNCIL (MAPC) and 495/MetroWest Partnership. 2017.Massachusetts Low impact Development Toolkit. Fact Sheet #3: Grass Filter Strips. Accessed 12 July 2022.
  6. 6.0 6.1 Chesapeake Stormwater Network (CSN). 2013. Bioretention Illustrated: A Visual Guide for Constructing, Inspecting, Maintaining and Verifying the Bioretention Practice. CSN Technical Bulletin No. 10. Version 2.0. Ellicott City, MD.
  7. Washington State Department of Transportation. 2018. Vegetated Filter Strips: Low-impact development fact sheet. Authored by: Maria Cahill, Derek Godwin, and Jenna Tilt. Published June 2018. Accessed: 14 July 2022.
  8. The Anxious Gardner. n.d. Gallery. Accessed 14 July 2022.
  9. Minnesota Pollution Control Agency. 2014. When it rains, it flows. 6 May 2014. Accessed 156 July 2022.
  10. Toronto and Region Conservation Authority (TRCA). 2016. Low Impact Development Stormwater Management Practice Inspection and Maintenance Guide. Prepared by the Sustainable Technologies Evaluation Program. Vaughan, Ontario.
  11. STEP. 2016. Low Impact Development Stormwater Management Practice Inspection and Maintenance Guide.
  12. City of Toronto. 2021. Construction Specifications and Drawings for Green Infrastructure. T-850.151 Vegetated Filter Strip – Section. Developed by: Engineering and Construction Services.