Inspection and Maintenance: Enhanced Swales

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)

Overview

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:

• Inlets
• Vegetated Slopes
• 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 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

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)

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)

Construction Inspection Tasks

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)

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

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:

• 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
• Oregon State University Stormwater Solutions, 2013, Field Guide: Maintaining Rain Gardens, Swales and Stormwater Planters, Corvallis, OR.
• 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.

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)	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)
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 bioretention cell is completely covered with dense, attractive vegetation which helps to maintain its stormwater treatment function and aesthetic value.	A larger portion of the bioretention cell has no vegetation cover which reduces its aesthetic value and could be negatively affecting its stormwater treatment function. Much less than 80% of the planting area is covered by living vegetation requiring immediate maintenance and reseeding.

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 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.

Toronto and Region Conservation Authority

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.

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 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

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.

Box like underground concrete structure with openings in curb and gutter designed to collect runoff from streets and pavement.

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.

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

Stormwater Management

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.

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.

Vegetated open channel, with check dams; designed to convey, treat and attenuate stormwater runoff.

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.

(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).

Water ponded on the ground surface.

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.

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

The temporary storage of stormwater to control discharge rates, and allow for sedimentation.

Ground depression acting as a flow control and water treatment structure, that is normally dry.

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.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

Water ponded on the ground surface.

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

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.

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.