Green Infrastructure: Swales can be an important tool for retention and detention of stormwater runoff. Depending on design and construction, swales may provide additional benefits, including cleaner air, carbon sequestration, improved biological habitat, and aesthetic value. See the section Green Infrastructure for stormwater management.

The most frequently cited maintenance concern for wet swales is that they provide a breeding ground for mosquitoes. Common operational problems include:

  • blockage by debris and vegetation;
  • sediment accumulates in the swale, reducing the storage volume;
  • slope stabilizing vegetation is lost; and
  • invasive plants out-compete native vegetation

Design phase maintenance

Implicit in the design guidance is the fact that many design elements of filtering systems can minimize the maintenance burden and maintain pollutant removal efficiency. Key examples include:

  • limiting drainage area;
  • providing easy site access (REQUIRED);
  • providing pretreatment (RECOMMENDED); and
  • utilizing native plantings (see Plants for Stormwater Design).

Wet swales can be designed, constructed and maintained to minimize the likelihood of being desirable habitat for mosquito populations. Designs that incorporate constant inflows and outflows, habitat for natural predators, and constant permanent pool elevations limit the conditions typical of mosquito breeding habitat (see section on mosquito control). For more information on design information for wet swales, link here.

Construction phase maintenance

Proper construction methods and sequencing play a significant role in reducing problems with operation and maintenance (O&M). Inspections during construction are needed to ensure that the wet swale practice is built in accordance with the approved design standards and specifications. Detailed inspection checklists should be used that include sign-offs by qualified individuals at critical stages of construction, to ensure that the contractor’s interpretation of the plan is acceptable to the professional designer. An example construction phase inspection checklist is provided below.

Wet swale construction inspection checklist.
Link to this table
To access an Excel version of form (for field use), click here.

Site Status:
Construction Sequence Satisfactory / Unsatisfactory Comments
1. Pre-Construction
Pre-construction meeting
Runoff diverted (Note type of bypass)
Facility area cleared
Project benchmark near site
Facility location staked out
Temporary erosion and sediment protection properly installed
2. Excavation
Size, location, and inverts per plans
Side slopes stable
Lateral slopes completely level
Longitudinal slopes within design range
Groundwater/bedrock verified
Stockpile location not adjacent to excavation area and stabilized with vegetation and/ or silt fence
Verify stockpile is not eroding
3. Structural Components
Outlets installed pre plans
Pretreatment devices installed per plans
Soil bed composition and texture conforms to specifications
Inlets installed per plans
4. Vegetation
For native wet swales, plants and materials ordered 6 months prior to construction
For native wet swales, construction planned to allow for adequate planting and establishment of plant community
Complies with planting specs
Topsoil complies with specs in composition and placement
Soil properly stabilized for permanent erosion control
5. Final Inspection
Dimensions per plans
Check dams operational
Pre-treatment operational
Inlet/outlet operational
Effective stand of vegetation stabilized per specifications
Construction generated sediments removed
Contributing watershed stabilized before flow is diverted to the practice
Actions to be taken:

Post-construction operation and maintenance

Proper maintenance is critical to the successful operation of a wet swale. Without regular maintenance, wet swales can fill in with sediment and lose important vegetation. This can lead to a reduction or elimination of pollutant removal capacity. Warning: A maintenance plan clarifying maintenance responsibility is REQUIRED. Effective long-term operation of filtration practices necessitates a dedicated and routine maintenance schedule with clear guidelines and schedules. Proper maintenance will not only increase the expected lifespan of the facility but will improve aesthetics and property value.

Inspection and maintenance planning

A maintenance plan clarifying maintenance responsibilities is REQUIRED. Effective long-term operation of wet swales necessitates a dedicated and routine maintenance schedule with clear guidelines and schedules. Proper maintenance will not only increase the expected lifespan of the facility but will improve aesthetics and property value. Some important post-construction considerations are provided below along with RECOMMENDED maintenance standards.

  • A site-specific O&M plan that includes the following considerations should be prepared by the designer prior to putting the stormwater practice into operation:
    • Inspection checklist
    • Routine maintenance checklist (see below)
    • Operating instructions for any outlet components
    • Vegetation maintenance schedule (see item 2 in checklist below and section below)

Wet swale operation and maintenance checklist.
Link to this table
To access an Excel version of form (for field use), click here.

Site Status:
Maintenance Item Satisfactory / Unsatisfactory Comments
1. Debris Cleanout (Monthly)
Contributing areas clean of litter and vegetative debris
Filtration or infiltration facility clean
Inlets and outlets clear
2. Vegetation (Monthly)
Vegetation maintenance complies with O&M plan
Vegetation meets performance standards (including control of specified invasive species)
No evidence of erosion
Maintenance of adequate water depths for desired wetland plant species
Have sediment accumulations reduced wet swale volume significantly or are plants “choked” with sediment
3. Inundated Portion of Swale (Monthly)
Floating or floatable debris removal required
Visible pollution
Eutrophication level of the wet swale
No evidence of erosion
4. Sediment Deposition (Annual)
Area clean of sediment
Contributing drainage area stabilized and free of erosion
Winter accumulation of sand removed each spring
5. Outlet/Overflow Spillway (Annual, After Major Storms)
Good condition, no need for repair
No evidence of erosion
No evidence of any blockages
No evidence of structural deterioration
6. Other (Monthly)
Encroachment on easement area (if applicable)
Complaints from residents (if applicable)
Any public hazards (specify)
Actions to be taken:

  • A legally binding and enforceable maintenance agreement should be executed between the practice owner and the local review authority to ensure the following:
    • Sediment should be cleaned out of any sedimentation chamber when it accumulates to a depth equal to ½ the total depth to the outlet, or when greater than 1.5 feet, whichever is less. The sediment chamber outlet devices should be cleaned/repaired when drawdown times exceed 36 hours. Trash and debris should be removed as necessary; and
    • Silt/sediment should be removed from the swale bottom when the accumulation exceeds one inch.
  • Adequate access must be provided for inspection, maintenance and landscaping upkeep, including appropriate equipment and vehicles.
  • Wet swales generally should not be used as dedicated snow storage areas, but can be with the following considerations.
    • Snow storage should not occur in areas designated as potential stormwater hotspots for road salt. NOTE: Chloride will not be attenuated in filtration BMPs such as wet swales.
    • When used for snow storage, or if used to treat parking lot runoff, the BMP area should be planted with salt tolerant and non-woody plant species.
    • Practices should always be inspected for sand build-up on the surface following the spring melt event.
  • General maintenance activities and schedule are provided below.

Summary of typical maintenance regime

The list below highlights the assumed maintenance regime for a wet swale.

  • First year after planting
    • Adequate water is crucial to plant survival and temporary irrigation may be needed unless rainfall is adequate until plants mature
    • Inspect after significant rain events (e.g. ½ inch)
  • As needed
    • Prune and weed to maintain appearance
    • Remove trash and debris
    • Mow filter strip/grass channel (if present)
    • Replace vegetation whenever percent cover of acceptable vegetation falls below 90 percent or project specific performance requirements are not met. If vegetation suffers for no apparent reason, consult with horticulturist and/or test soil as needed
  • Semi-annually
    • Inspect inflow and pretreatment systems for clogging (off-line systems) and remove any sediment
    • Inspect filter strip/grass channel for erosion or gullying. Sod as necessary
    • Herbaceous vegetation, trees and shrubs should be inspected to evaluate their health and replanted as appropriate to meet project goals
    • Remove any dead or severely diseased vegetation
  • Annually in fall
    • Inspect and remove any sediment and debris build-up in pretreatment areas
    • Inspect inflow points and wet swale bottom for buildup of road sand associated with spring melt period, remove as necessary, and replant areas that have been impacted by sand/salt build up
  • Annually in spring
    • Cut back and remove previous year’s plant material and remove accumulated leaves if needed (or conduct controlled burn where appropriate)

Estimated hours to perform maintenance activities

All estimated hours listed below would be to perform maintenance on a wet swale system approximately 1,000 square feet in size that has adequate pretreatment and where seed and/or live plants have been installed appropriately.

  • Plant Establishment Period (First two years)
    • Monthly weeding – 12 visits (6 per year) at 1 hour per visit
    • Vegetation replacement – 1 overseeding or replanting effort, 2 hours (assuming 10 percent warrants replacement)
    • Spring cleanup (cut back of previous years vegetation) – 2 cleanups (1 per year) at 2 hours each
    • Erosion, sediment, and pretreatment cleanout – 2 cleanouts (1 per year) at 1 hour each (assuming vacuum truck clean-out of any sump catch basins)
  • Regular Maintenance (After first two years)
    • Bi-monthly (every other month) weeding – 3 visits per year at 1 hour per visit
    • Vegetation replacement – 1 overseeding or replanting effort per year on average, 1 hour (assuming 5 percent warrants replacement)
    • Spring cleanup (cut back of previous years vegetation) – 1 per year at 2 hours
    • Erosion, sediment, and pretreatment cleanout – 2 hours per year on average (assuming vacuum truck clean-out of any sump catch basins once per year, and at least one bi-yearly (every other year) sediment removal from the bottom of the swale)

Erosion protection and sediment monitoring, removal, and disposal

Regular inspection of not only the BMP but also the immediate surrounding catchment area is necessary to ensure a long lifespan of the water quality improvement feature. Erosion should be identified as soon as possible to avoid the contribution of significant sediment to the BMP.

Pretreatment devices need to be maintained for long-term functionality of the entire BMP. Accumulated sediment in filter strips, rock diaphragms, water quality sump catch basins, or any pretreatment features will need to be inspected yearly. Timing of cleaning of these features is dependent on their design and sediment storage capabilities. In watersheds with erosion or high sediment loadings, the frequency of clean out will likely be increased. A vacuum truck is typically used for sediment removal. It is possible that any sediment removed from pretreatment devices or from the bottom of a dry swale may contain high levels of pollutants. All sediments, similar to those retrieved from a stormwater pond during dredging, may be subjected to the MPCA’s guidance for reuse and disposal.

If a grassed filter strip is used as pretreatment, they should be mowed as frequently as a typical lawn. Native vegetated filter strips can be maintained less frequently, such as once per year (e.g., mow and remove cut material or prescribed burn). Depending on the contributing watershed, grassed BMPs may also need to be swept before mowing. All grassed BMPs should be swept annually with a stiff bristle broom or equal to remove thatch and winter sand. The University of Minnesota’s Sustainable Urban Landscape Series website provides guidance for turf maintenance, including mowing heights.

Seeding, planting, and landscaping maintenance

Plant selection during the design process is essential to limit the amount of maintenance required. It is also critical to identify who will be maintaining the BMP in perpetuity and to design the plantings or seedings accordingly. The decision to install containerized plants or to seed will dictate the appearance of the BMP for years to come. Inundated areas are typically planted with live plant material such as plugs (as opposed to seed); however, it may be feasible to vegetate these areas using seed if the practice is constructed off-line and the seed is able to grow sufficiently prior to inundation. If the BMP is designed to be seeded with an appropriate native plant based seed mix, it is essential the owner have trained staff or the ability to hire specialized management professionals. Seedings can provide plant diversity and dense coverage that helps maintain drawdown rates, but landscape management professionals that have not been trained to identify and appropriately manage weeds within the seeding may inadvertently allow the BMP to become infested and the designed plant diversity be lost. The following are minimum requirements for seed establishment and plant coverage.

  • At least 50 percent of specified vegetation cover at end of the first growing season, not including REQUIRED cover crop
  • At least 90 percent of specified vegetation cover at end of the third growing season, not including REQUIRED cover crop
  • Supplement seeding/plantings to meet project specifications if cover requirements are not met
  • Tailor percent coverage requirements to project goals and vegetation. For example, percent cover required for turf after one growing season would likely be 100 percent, whereas it would be lower for other vegetation types.

For information on plant selection, link here.

For proper nutrient control, swales must not be fertilized unless a soil test from a certified lab indicates nutrient deficiency. If this is the case, apply the minimum rate of appropriate nutrients to provide a suitable environment for vegetation establishment while also minimizing the mobilization (and loss) of nutrients to downstream receiving waters. Irrigation may be needed during establishment, depending on soils, precipitation, and if stormwater flows are kept off-line during establishment.

Weeding is especially important during the plant establishment period, when vegetation cover is not 100 percent yet. Some weeding will always be needed. It is also important to budget for some plant replacement (at least 5 to 10 percent of the original plantings or seedings) during the first few years in case some of the plants or seed that were originally installed don’t become vigorous. It is highly recommended that the install contractor be responsible for a plant warranty period. Typically, plant warranty periods can be 60 days or up to one year from preliminary acceptance through final inspections. If budget allows, installing larger plants (#1 container vs. 4” pot) during construction can decrease replacement rates if properly cared for during the establishment period.

Weeding in years after initial establishment should be targeted and thorough. Total eradication of aggressive weeds at each maintenance visit will ultimately reduce the overall effort required to keep the BMP weed free. Mulch is generally not recommended for use in swales since flowing water typically washes it downstream; however, mulch may be appropriate in planting beds or around individual trees on upper sideslopes and adjacent areas.

Rubbish and trash removal will likely be needed more frequently than in the adjacent landscape. Trash removal is important for prevention of mosquitoes and for the overall appearance of the BMP.

Sustainable service life

The service life of swales depends upon the pollutant of concern.

Infiltration rate service life before clogging

Infiltration is not a primary function of wet swales.

Nitrogen reduction

Nitrate is generally less than one-third of the total nitrogen in urban stormwater runoff. Denitrification is a bacterial reaction occurring under anaerobic conditions that may occur in swales that pond water for extended periods of time. Denitrification converts nitrate in stormwater to nitrogen gas, requiring a source of organic matter. Denitrification occurs under anoxic conditions where carbon is supplied via rooted plants via sediments comprised of decomposing organic material. Denitrification is also controlled by temperature with colder temperatures limiting microbial processing of nitrogen is limited. (Erickson, Weiss and Gulliver, 2013).

Wet swales have an internal water storage (i.e., inundation) zone. If this zone is deep enough and flow rates are low enough, soluble nitrogen will be removed through denitrification, a microbially-mediated process that occurs only under anoxic conditions. Denitrification requires organic matter as a carbon source, which is supplied by decaying root matter and mulch. Particulate bound nitrogen in stormwater runoff will typically be removed through sedimentation. Lastly, plants uptake nitrogen since it is essential for plant growth. All of these processes are self-sustaining with routine maintenance, and the nitrogen reduction service life of a wet swale should be very long. In very shallow or high flow wet swales (i.e., oxygenated systems), denitrification is not an important process, and leaching of nitrate may occur. In systems having soils with a high organic matter content, organic nitrogen can be converted to nitrate, resulting in loss of nitrogen through leaching (Liging and Davis, 2014).

Phosphorus reduction

Phosphorus removal in wet swales is achieved primarily through sorption of phosphorus to trapped sediments. Therefore, it is beneficial to intermittently remove sediment (with its attached phosphorus) from the bottom of wet swales. Sediment should be disposed in an acceptable manner (e.g., landfill).

Heavy metals retention

Metals are typically retained in wet swale systems (including wet swales) through sedimentation and adsorption processes. Therefore, it is beneficial to intermittently remove sediment (with its attached metals) from the bottom of wet swales. Sediment should be disposed in an acceptable manner (e.g., landfill). Since there are a finite amount of sorption sites for metals in a particular soil/media, there will be a finite service life for the removal of dissolved metals. Morgan et al. (2011) investigated cadmium, copper, and zinc removal and retention with batch and column experiments. Using synthetic stormwater at typical stormwater concentrations, they found that 6 inches of filter media composed of 30 percent compost and 70 percent sand will last 95 years until breakthrough (i.e., when the effluent concentration is 10 percent of the influent concentration). They also found that increasing compost from 0 percent to 10 percent more than doubles the expected lifespan for 10 percent breakthrough in 6 inches of filter media for retainage of cadmium and zinc. Using accelerated dosing laboratory experiments, Hatt et al. (2011) found that breakthrough of Zn was observed after 2000 pore volumes, but did not observe breakthrough for Cd, Cu, and Pb after 15 years of synthetic stormwater passed through the media. However, concentrations of Cd, Cu, and Pb on soil/media particles exceeded human and/or ecological health levels, which could have an impact on disposal if the soil/media needed replacement. Since the majority of metals retainage occurs in the upper 2 to 4 inches of the soil/media (Li and Davis, 2008), long-term metals capture may only require rejuvenation of the upper portion of the media. If concentrations of metals in runoff are anticipated to be elevated, wet swale design should include soil amendments as indicated above.

Polycyclic aromatic hydrocarbons (PAHs) reduction

Accumulation of polycyclic aromatic hydrocarbons (PAHs) in sediments has been found to be so high in some stormwater retention ponds that disposal costs for the dredging spoils were prohibitively high. Research has shown that rain gardens, on the other hand, are “a viable solution for sustainable petroleum hydrocarbon removal from stormwater, and that vegetation can enhance overall performance and stimulate biodegradation.” (Lefevre et al., 2012). Given that wet swales provide some of the same functions as stormwater retention ponds (i.e., inundated portions) and rain gardens (i.e., higher sideslopes), it would be expected they provide some PAH management. However, swales performance in PAH management has not been the focus of any identified studies.

Typical maintenance problems and activities

The following table summarizes common maintenance concerns, suggested actions, and recommended maintenance schedule.

Typical maintenance problems and activities for wet swales
Link to this table

Inspection Focus Common Maintenance Problems Maintenance Activity Recommended Maintenance Schedule
Drainage Area Erosion of catchment area contributing significant amount of sediment In case of severely reduced drawdown time, scrape bottom of basin and remove sediment. Disc or otherwise aerate/scarify basin bottom. De-thatch if basin bottom is turf grass. Restore original design cross section or revise section to increase infiltration rate and restore with vegetation as necessary. Bi‐monthly April through October
Site Erosion Scouring at inlets Correct earthwork to promote non‐erosive flows that are evenly distributed As necessary
Unexpected flow paths into practice Correct earthwork to eliminate unexpected drainage or created additional stable inlets as necessary As necessary
Vegetation Severe weed establishment Limit the ability for noxious weed establishment by properly mowing, mulching or timely herbicide or hand weeding. Refer to the MDA Noxious Weed List Bi‐monthly April through October
Vegetative cover Add seed/plants to maintain ≥95% vegetative cover. Bi‐monthly April through October
Pretreatment Pretreatment screens or sumps reach capacity Remove sediment and oil/grease from pretreatment devices/structures Minimum yearly or as per manufacturer's recommendations
Vegetative filter strip failure Reduce height of vegetative filter strip that may be limiting in‐flow. Re‐establish vegetation to prevent erosion. Leave practice off‐line until full reestablishment. Mow grass filter strips monthly. Restore as necessary

Maintenance agreements

A Maintenance Agreement is a legally binding agreement between two parties, and is defined as ”a nonpossessory right to use and/or enter onto the real property of another without possessing it.“ Maintenance Agreements are often required for the issuance of a permit for construction of a stormwater management feature and are written and approved by legal counsel. Maintenance Agreements are often similar to Construction Easements. A Maintenance Agreement is required for one party to define and enforce maintenance by another party. The Agreement also defines site access and maintenance of any features or infrastructure if the property owner fails to perform the required maintenance. Maintenance Agreements are commonly established for a defined period such as five years for a residential site or 10 to 20 years for a commercial/governmental site after construction of the filtration practice. Maintenance agreements often define the types of inspection and maintenance that would be required for that filtration practice and what the timing and duration of the inspections and maintenance may be. Essential inspection and maintenance activities include but are not limited to sediment removal, erosion monitoring and correction, and vegetative maintenance and weeding. If maintenance is required to be performed due to failure of the site owner to properly maintain the filtration practices, payment or reimbursement terms of the maintenance work are defined in the Agreement. Below is an example list of maintenance standards from an actual Maintenance Agreement.

  1. Live plantings and seeding areas shall be watered as necessary to achieve performance standards.
  2. Weeding and vegetation management (e.g., mowing, spot spraying) shall be conducted as necessary to achieve performance standards.
  3. Dead plant material, garbage, and other debris shall be removed from the swale at least annually.
  4. Silt/sediment should be removed from the swale bottom when the accumulation exceeds one inch.
  5. Side slopes must be inspected for erosion and the formation of rills or gullies at least annually, and erosion problems must be corrected immediately.
  6. If properly planned, designed, constructed, and maintained (including protected from sediment and compaction and incorporating sufficient pretreatment), a wet swale is likely to retain its effectiveness for well over 20 years. After that time, inspection will reveal whether interventions are warranted.

In some project areas, a drainage easement may be required. Having an easement provides a mechanism for enforcement of maintenance agreements to help ensure swales are maintained and functioning. Drainage easements also require that the land use not be altered in the future. Drainage easements exist in perpetuity and are required property deed amendment to be passed down to all future property owners. As defined by the Maintenance Agreement, the landowner should agree to provide notification immediately upon any change of the legal status or ownership of the property. Copies of all duly executed property transfer documents should be submitted as soon as a property transfer is made final.

Maintenance inspection reports

The maintenance inspection report for dry swale with check dams can be used with some modifications (link here). The contents of the inspection form are provided below. For another source of information on visual indicators, see Chesapeake Stormwater visual indicators form.

Maintenance Inspection Report for Dry Swale with Check Dams and Stormwater Step Pool. Can be used for wet swales with exceptions, as noted in footnotes.

Date: ____________________________________________________________________

Inspector Name/Address/Phone Number: _______________________________________

Site Address: ______________________________________________________________

Owner Name/Address/Phone Number: _________________________________________

Drainage Area Stabilization (Inspect after large storms for first two years, Inspect yearly in spring or after large storms after first two years)

  • Erosion control/planting/seeding necessary: __________________________________________________
  • Mowing, pruning and debris removal necessary: _______________________________________________
  • Observations:

______________________________________________________________________________________ ______________________________________________________________________________________

Inlets & Pretreatment Structures (Inspect in Spring and Fall)

  • Repair needed: _________________________________________________________________________
  • Debris & sediment removal required: _______________________________________________________
  • Erosion evident: _________________________________________________________________________
  • Water by-passing inlet: ___________________________________________________________________
  • Vegetation control necessary: _____________________________________________________________
  • Observations:

______________________________________________________________________________________ ______________________________________________________________________________________

Swale (Inspect after large storms for first two years, Inspect yearly in spring or after large storms after first two years)

  • Condition of infiltration area1: ______________________________________________________________
  • Condition of check dams: _________________________________________________________________
  • Surface erosion evident: __________________________________________________________________
  • Debris/sediment removal required: _________________________________________________________
  • Adequate drawdown/standing water2: _______________________________________________________
  • Weeding and pruning necessary: ___________________________________________________________
  • Mulch replacement necessary3: _____________________________________________________________
  • Observations:

______________________________________________________________________________________ ______________________________________________________________________________________

Outlet/Emergency Overflow (Inspect in Spring and Fall)

  • Overflow type: _________________________________________________________________________
  • Debris/sediment removal required: _________________________________________________________
  • Repair needed: _________________________________________________________________________
  • Observations:

______________________________________________________________________________________ ______________________________________________________________________________________

1For wet swale, check condition of inundated area
2For wet swale with check dam, drawdown applies to the water elevation at the botton of weir
3Not applicable for wet swale

Related pages

This page was last modified on 21 May 2018, at 08:42.

Minnesota Pollution Control Agency | 651-296-6300, 800-657-3864 | Assistance | Web site policy