The most frequently cited maintenance concern for wet swales is that they provide a breeding ground for mosquitoes. Common operational problems include:
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:
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.
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.
|Satisfactory / Unsatisfactory
|Runoff diverted (Note type of bypass)
|Facility area cleared
|Project benchmark near site
|Facility location staked out
|Temporary erosion and sediment protection properly installed
|Size, location, and inverts per plans
|Side slopes stable
|Lateral slopes completely level
|Longitudinal slopes within design range
|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
|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
|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:
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.
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.
|Satisfactory / Unsatisfactory
|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
|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:
The list below highlights the assumed maintenance regime for a wet swale.
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.
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.
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.
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.
The service life of swales depends upon the pollutant of concern.
Infiltration is not a primary function of wet swales.
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 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).
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.
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.
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
|Common Maintenance Problems
|Recommended Maintenance Schedule
|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
|Scouring at inlets
|Correct earthwork to promote non‐erosive flows that are evenly distributed
|Unexpected flow paths into practice
|Correct earthwork to eliminate unexpected drainage or created additional stable inlets as necessary
|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
|Add seed/plants to maintain ≥95% vegetative cover.
|Bi‐monthly April through October
|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
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.
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.
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.
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)
Inlets & Pretreatment Structures (Inspect in Spring and Fall)
Swale (Inspect after large storms for first two years, Inspect yearly in spring or after large storms after first two years)
Outlet/Emergency Overflow (Inspect in Spring and Fall)
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