Inspection and Maintenance: Enhanced Swales

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Inspection & Maintenance Guidance of enhanced swales, which are a vegetated stormwater best management practices that contains gently sloping open channels featuring a parabolic or trapezoidal cross-section and check dams, designed to both convey and treat stormwater runoff temporarily before entering the storm system (TRCA, 2016[1])

Overview[edit]

Enhanced swales are gently sloping vegetated open channels featuring a parabolic or trapezoidal cross-section and check dams, designed to convey and treat stormwater runoff (i.e., rainwater or snowmelt from roofs or pavements). The grading, Check dams and vegetation spreads out and slows down the flow of water, allowing suspended sediment and floatables (e.g., trash, natural debris, oil and grease) to settle out. A portion of the flowing water soaks into the soil and replenishes groundwater or is taken up by plant roots and evaporated 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 grasses or a mixture of flood tolerant, erosion resistant vegetation and stone. They do not feature filter media soil and sub-drains like bioretention or bioswales do. Water not ponded behind check dams or absorbed by the planting bed is conveyed to an adjacent drainage system (e.g., municipal storm sewer or other BMP) at the lowest downstream point by an outlet structure (e.g., ditch inlet catchbasin, culvert). Key components of this feature are described in further detail below.

Properly functioning enhanced swales 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, enhanced swales provide aesthetic value as attractive landscaped features.

Key components of Enhanced swales to pay close attention to are the:

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 storm drain inlet sediment control measure (sediment retention barrier) used at the bottom of an enhanced swale to limit excess suspended sediment from entering the storm drain outlet at the end of the feature. This type of barrier would generally be in place during construction activity and would be reviewed during construction inspections to ensure its operating efficiently and excess sediment is removed routinely (Source: ESC Guide, 2019[2]


The image above shows a simulated storm event testing indicator for an enhanced swale taking place at the Social Housing Landscapes project located in London, England. The simulation event occurs by calculating the amount of expected rainfall to occur during a 1-in-100 year event by adding water to the feature with a large municipal watering truck during a 1-hr. event at the same expected magnitude during a natural event to ensure the LID BMP is effectively conveying an filtering stormwater in a real-life scenario (Source: Connop and Nash, 2019)[3]
Visual Indicators Framework - Enhanced Swales

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
Pretreatment sediment accumulation x x x
Inlet erosion x x
Perimeter
BMP dimensions x x x
Side slope erosion x x
Surface ponding area x x x
Filter Bed
Standing water x x x
Trash x x
Filter bed erosion x x
Filter bed sediment accumulation x x x
Surface ponding depth x x x
Filter bed surface sinking x x x
Check dams x x x x
Planting Area
Vegetation cover x x x x
Vegetation condition x x
Vegetation composition x x x
Outlet
Overflow outlet obstruction x x x x


Testing Indicators Framework - Enhanced Swales

Component

Indicators

Construction Inspection

Assumption Inspection

Routine Operation Inspection

Verification Inspection
Testing Indicators
Soil characterization testing x x (x)
Sediment accumulation testing x x x x
Surface infiltration rate testing x (x)
Natural or simulated storm event testing x (x)
Note: (x) denotes indicators to be used for Performance Verification inspections only (i.e., not for Maintenance Verification inspections)
The diagram above depicts the use of an enhanced swale as a temporary detention basin during construction. This practice is avoided at all costs as it can lead to subgrade clogging and compaction, but in the case where LIDs must be used for construction stormwater detention due to site constraints, protection measures can be applied to prevent accumulated sediment from migrating into the subgrade when an LID is first being developed. This method would have to be outlined on the inspection sheet specifically to ensure construction inspection tasks include addressing its condition through all three sequences (Source: TRCA, 2019)[2]


Construction Inspection Tasks[edit]

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. Prior to hand-off points in the construction sequence when the contractor is 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


A rudimentary way for determining topsoil depth of an LID BMP and determining that it is acceptable for the practice (Source: Vidacycle, 2020)[4]
Enhanced 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 landscaping 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
Landscaping – after final grading, prior to planting Topsoil depth, degree of compaction and surface elevations at inlets and outlets 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[edit]

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 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:

Enhanced Swales: 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.
Ponding and sediment accumulation on the CDA is visible indicating runoff is not freely entering the BMP and that the pavement has not been swept recently.).
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
The grass filter strip pretreatment is free of sediment, trash and debris. (Source: Abbey and Associates).
Sediment and debris has accumulated in the forebay and is preventing stormwater from flowing into the BMP
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;
  • 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.
There are no obstructions at the inlet and stormwater can freely flow into the BMP.
The overflow outlet elevation and maximum surface ponding area closely match what was specified in the final design.
Accumulated sediment and vegetation is preventing stormwater from entering the BMP. Sediment on the pavement surface in front of the inlet indicates ponding is occurring.
The elevation of the overflow outlet is higher than what was specified in the design, producing a much larger surface ponding area than intended which could produce standing water for prolonged periods and cause vegetation to die off.
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 3:1 (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 the maximum surface ponding depth behind check dams meets design specifications;
  • Check for side slope erosion/damage from vehicular/foot traffic.
The footprint area of the BMP does not significantly deviate from the final design and should not negatively affect its stormwater management treatment performance.
The footprint area of the BMP is significantly smaller than what was specified in the final design of this example and differ greater than the recommended SWM criteria requirements (>10%), due to half the width having been paved over.
Filter Bed

Linearly-oriented, gently sloping area (between 0.5 and 4% slope) where runoff is filtered and conveyed; parabolic or trapezoidal cross-section, lined with 20 to 30 cm of planting soil and covered with deep rooting perennial grasses or a mixture of vegetation and stone.

  • 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;
  • Mow grasses to maintain height of > 10 cm;
  • For sod or turf grass vegetation cover, aerate and dethatch annually to maintain soil permeability and dense grass cover;
  • 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 behind check dams exceeds 48 hours.
The filter bed has retained its original grading without any sharp depressions that would indicate surface bed sinking.
TThe maximum surface ponding depth behind check dams matches what was specified in the final design. (Source: Mark M. Holeman, Inc., 2015)[5]
Clear evidence of bed sinking is visible, creating a preferential ponding area where vegetation has died off.
The maximum ponding depth of the swale is significantly deeper than intended as the elevation of the check dam or overflow outlet is too high. (Source: Stiffler, 2012[6])
Vegetation

Deep rooting perennial grasses or a mixture of wildflowers and shrubs, tolerant to both wet and dry conditions and salt; 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 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 by the third growing season;
  • Do not apply chemical fertilizers.
The planted portion of the swale is well covered with dense, attractive vegetation which helps to maintain its stormwater treatment function and aesthetic value.
Major portions of the swale surface contains dead or dying vegetation which reduces its aesthetic value and could be negatively affecting its stormwater treatment function.
Check Dams

Structures constructed of a non-erosive material, such as suitably sized aggregate, wood, gabions, riprap, stone or concrete; used to slow runoff water. Can be employed in practices such as bioswales and enhanced grass swales. Constructed across a drainage ditch, swale, or channel to lower the speed of concentrated flows for a certain design range of storm events and to promote infiltration.

  • Remove accumulated sediment by rake/shovel.
  • Check for signs of oil or grease contamination (e.g. sheen on surface of water when sediment is submerged).
  • If oil or grease contamination is suspected, submit a sediment sample for contaminant testing by an accredited laboratory to determine the proper disposal method. Install check dams where specified in the final design.
  • Assess the CDA for changes in land cover or point sources of sediment.
  • Inspect and remove sediment from pretreatment devices.
  • If problems persist, consider adding pretreatment devices or increasing frequency of routine maintenance.
The check dams are visible and continue to help retain sediment and spread the flow of water across the BMP surface.
Sediment has accumulated on the upstream side of the check dam and is affecting its function. (Source: Tennessee EPSC).
  1. TRCA. 2016. Fact Sheet - Inspection and Maintenance of Stormwater Best Management Practices: Enhanced Swales. https://sustainabletechnologies.ca/app/uploads/2018/02/Enhanced-Swales-Fact-Sheet.pdf
  2. 2.0 2.1 Toronto and Region Conservation Authority (TRCA). 2019. Erosion and Sediment Control Guideline for Urban Construction. Toronto and Region Conservation Authority, Vaughan, Ontario. https://sustainabletechnologies.ca/app/uploads/2020/01/ESC-Guide-for-Urban-Construction_FINAL.pdf
  3. Connop S. and Nash, C. 2019. A Storm in a Bioswale: Breaking Down Barriers to Nature-Based Solutions. The Nature of Cities. 16 December 2019 Accessed: 4 July 2022. https://www.thenatureofcities.com/2019/12/16/a-storm-in-a-bioswale-breaking-down-barriers-to-nature-based-solutions/
  4. Vidacycle. 2020. Soil Monitoring Guide: Other Soil Tests. Accessed 4 July 2022. https://soils.vidacycle.com/soil-tests/
  5. Mark M. Holeman, Inc. 2015. What is a Bio-Swale? Authored by Rick Blankenship. 25 September 2015. Accessed 5 July 2022. http://www.holemanlandscape.com/2015/09/25/what-is-a-bio-swale/
  6. Stiffler, L. 2012.RAIN GARDEN REALITY CHECK: Comparing LID to conventional system failures. Authored by: Eric De Place. 18 April 2012. Sightline Institute. Sustainable Living Series. Accessed 5 July 2022. https://www.sightline.org/2012/04/18/rain-garden-reality-check/