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:

  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/