Wet Swale
A wet swale is a vegetated open channel conveyance system specifically designed to treat stormwater runoff within a series of shallow pools. Unlike other open channel systems, a wet swale is excavated to intersect the seasonal high groundwater table, which maintains saturated soil conditions and supports a dense stand of wetland vegetation. As stormwater flows through the channel, it is slowed by check dams and vegetation, allowing pollutants to be removed through gravitational settling and biological uptake by the plants and microbial communities in the soil.
The system functions as a linear wetland, providing reliable pollutant removal in a configuration well-suited for treating runoff from roads and highways. By combining conveyance with water quality treatment, wet swales offer an efficient use of land in transportation corridors. They are a more robust treatment practice compared to a simple Grass Channel and offer a different treatment mechanism than an infiltrating Dry Swale. The constant presence of standing water makes them a linear alternative to a Stormwater Wetland.
Applicability
Wet swales are most effective when site conditions align with their unique hydraulic and ecological requirements. Their linear form makes them ideal for placement along property edges, road corridors, and in utility rights-of-way. Proper siting is critical to their performance and longevity.
Drainage Area and Slope
A single wet swale is best suited for treating small drainage areas, typically five acres or less. In larger catchments, flow velocities can become too high, leading to erosion and reducing treatment effectiveness. The longitudinal slope of the channel is a critical design constraint; it must be 4% or less, with slopes between 1% and 2% being ideal for promoting slow, non-erosive flow and allowing for effective treatment.
Soils and Water Table
A defining characteristic of a wet swale is its interaction with groundwater. The bottom of the swale must be excavated deep enough to intercept the seasonal high water table, ensuring the channel remains saturated for long periods. This hydrological connection is essential for supporting the required wetland vegetation. Unlike dry swales, which require permeable soils for infiltration, wet swales can be constructed in a wider range of soil types, including those with lower permeability, since exfiltration is not the primary removal pathway.
Regional and Land Use Suitability
Wet swales can be implemented in most regions. In cold climates, they can provide a location for snow storage, but roadside applications require the selection of salt-tolerant wetland vegetation. In arid or semi-arid climates, their reliance on groundwater can make them more sustainable than grass channels that require irrigation.
Because they feature shallow standing water, wet swales are not recommended for residential settings. The permanent pools can be perceived as a nuisance and may create potential mosquito breeding habitat if not properly designed and maintained with healthy predator populations.
Stormwater Hotspots and Retrofits
Wet swales should not be used to treat runoff from designated stormwater hotspots, such as gas stations or vehicle maintenance areas. The direct connection to the water table creates a risk of groundwater contamination from the higher pollutant loads. However, wet swales are an excellent option for retrofitting existing drainage ditches in areas with a high water table, transforming a simple conveyance channel into a functional water quality treatment system.
Design Criteria
The design of a wet swale must balance the need to convey storm flows safely with the goal of maximizing pollutant removal. This involves careful consideration of the channel’s geometry, flow control features, and vegetation.
Feasibility and Pretreatment
Before detailed design, confirm that the site has a sufficiently high water table and a longitudinal slope of less than 4%. Pretreatment is essential to prevent the main channel from being clogged with coarse sediment. For concentrated inflows, a small forebay or sediment trap should be installed at the head of the swale. For lateral sheet flow entering from the sides, a pea gravel diaphragm—a shallow trench filled with gravel running parallel to the channel—can effectively trap coarse particles before they enter the vegetated area. Pretreatment should be designed to capture sediment from the first 0.1 inches of runoff per impervious acre.
Conveyance and Geometry
A wet swale should have a trapezoidal or parabolic cross-section with gentle side slopes, typically 3H:1V or flatter. Flatter slopes are more stable, easier to maintain, and provide a wider surface area for runoff to spread across as it enters the channel. The channel bottom width should be between two and eight feet to ensure adequate filtering area while preventing the formation of braided, low-flow channels.
The design must accommodate multiple storm events. The channel should be sized to convey the 2-year storm at a non-erosive velocity. It must also be able to safely pass the 10-year storm with at least six inches of freeboard to prevent overtopping.
Treatment Volume and Flow Control
Treatment is achieved by temporarily storing the water quality volume (WQv) in a series of shallow pools created by check dams. These check dams, often made of earth, timber, or rock, are placed at intervals along the channel to slow flow and create the desired ponding. The maximum ponding depth for the WQv should be about 12 inches in the middle of a pool and no more than 18 inches directly behind a check dam. The system should be designed so that this ponded water is displaced by subsequent flows, rather than drawing down within a specific timeframe.
Landscaping
A dense, healthy stand of vegetation is the primary engine of pollutant removal in a wet swale. The landscape plan must specify emergent wetland plants that can tolerate both saturated soil conditions and periods of inundation. Species such as rushes, sedges, and reeds are common choices. Turf grass is not appropriate for the bottom of a wet swale. Plant selection should be based on local climate, soil conditions, and the expected hydrologic regime.
Pollutant Removal
Monitoring data for open channel systems show effective removal for total suspended solids (TSS) and metals. Performance for nutrients is more variable. Wet swales can provide good nitrogen removal through denitrification in the saturated soils but may export soluble phosphorus under certain anaerobic conditions. Data from a national study (Winer, 2000) on water quality swales, which includes wet swales, indicates the following performance.
| Pollutant | Pollutant Removal (%) |
|---|---|
| Total Suspended Solids (TSS) | 81 ± 14 |
| Total Phosphorus (TP) | 34 ± 33 |
| Nitrogen (NOx) | 31 ± 49 |
| Metals (Cu, Pb, Zn) | 42 – 71 |
| Bacteria | -25 |
The negative removal rate for bacteria suggests that swales may sometimes act as a source, potentially due to the warm, moist environment supporting bacterial growth or from external sources like pet waste. These figures represent an average across different swale designs; actual performance will vary by site. More recent and comprehensive data can be explored in the national pollutant removal database.
Construction and Cost Considerations
Construction costs for wet swales are moderate compared to other stormwater practices. Costs are higher than for a simple grass channel due to the need for deeper excavation, installation of check dams, and the use of specialized wetland plantings. However, they are typically less expensive than dry swales, which require imported engineered soil media and a perforated pipe underdrain system.
Key construction steps include excavation to the correct grade, installation of check dams and pretreatment features, and planting of wetland vegetation. It is critical to establish the specified vegetation quickly to prevent erosion and ensure the system functions as designed. Using temporary erosion control measures like matting may be necessary until the plants are fully established.
During construction, careful grade control is paramount. Even minor deviations from the design elevations can disrupt the intended hydraulic profile, leading to areas that are too dry to support wetland plants or too deep, creating stagnant open water.
Maintenance
Proper maintenance is essential for the long-term performance of a wet swale. Unlike grass channels, the wetland vegetation in the swale bottom should not be mowed. Maintenance focuses on ensuring hydraulic function and plant health.
| Activity | Schedule |
|---|---|
| Inspect for erosion, sediment accumulation, and vegetation health. | Semi-annually for the first year, then annually. |
| Remove trash and debris from the entire swale, especially the forebay. | Annually, or as needed. |
| Inspect check dams for structural integrity and correct any damage. | Annually. |
| Manage invasive plant species and replant bare areas with appropriate wetland vegetation. | Annually. |
| Remove accumulated sediment from the forebay and channel bottom when it reduces capacity by 25% of the original design volume. | As needed (typically every 5 to 10 years). |
Limitations
While effective, wet swales have several limitations that may restrict their use.
- They require a site with a high seasonal water table, which is not always available.
- The presence of standing water makes them unsuitable for most residential areas due to concerns about aesthetics and mosquitoes.
- They are not appropriate for treating runoff from stormwater hotspots due to the risk of groundwater contamination.
- They are limited to treating relatively small drainage areas (under 5 acres).
- Under certain conditions, they can become a net exporter of soluble phosphorus and bacteria.
- A dense and specific palette of wetland vegetation is required for proper function, which may be difficult to establish or maintain.
Frequently Asked Questions
What is the main difference between a wet swale and a dry swale?
The primary difference is their relationship with the water table. A wet swale is designed to intersect the groundwater, creating a saturated, wetland environment. It treats water through settling and biological uptake. A dry swale is built above the water table and uses an engineered soil media and underdrain system to filter runoff, promoting infiltration and filtration. The vegetation types are also different: wetland plants for wet swales and drought-tolerant grasses for dry swales.
Why can’t wet swales be used in residential areas?
Wet swales maintain shallow pools of standing water, which are often viewed as a nuisance by homeowners. These pools can create boggy conditions and may become a breeding ground for mosquitoes if not properly designed to support predator populations like dragonflies and amphibians. Due to these concerns, their application is generally limited to commercial, industrial, or transportation corridors where public access is restricted.
What kind of vegetation is used in a wet swale?
Wet swales require emergent wetland vegetation that thrives in saturated soil and can tolerate periodic inundation. This includes species like cattails, bulrushes, sedges, reeds, and pickerelweed. Conventional turfgrass is not suitable for the bottom of a wet swale as it will not survive the constantly wet conditions. The specific plant palette should be chosen based on local climate, hydrology, and sun exposure to ensure a robust, self-sustaining plant community.
How large of an area can a single wet swale treat?
A single wet swale is typically designed to treat a small drainage area, generally five acres or less. For larger areas, the flow velocity and volume of runoff can become too great, leading to erosion within the channel and reducing the contact time needed for effective water quality treatment. In larger developments, multiple swales may be used to treat different sub-catchments of the site.
Are wet swales effective at removing nutrients?
Performance is mixed. The anaerobic conditions in the saturated soils of a wet swale are ideal for denitrification, leading to good removal of nitrogen. However, these same conditions can cause phosphorus that was previously bound to sediment to be released in a soluble, bioavailable form. As a result, wet swales can sometimes become net exporters of soluble phosphorus, a factor that must be considered in nutrient-sensitive watersheds.
What is the role of check dams in a wet swale?
Check dams are small barriers placed at intervals along the length of the swale. Their purpose is to slow the flow of water and create a series of shallow pools. This ponding increases the time runoff spends in the swale, allowing more sediment to settle out and providing greater opportunity for biological uptake of nutrients by the wetland plants. They are essential for transforming a simple conveyance channel into an effective treatment system.
Can a wet swale handle large storm events?
Yes, but its primary purpose is water quality treatment, not flood control. A wet swale is designed with the capacity to safely convey larger storms, such as the 10-year storm, without overtopping or causing erosion. However, the features that provide treatment, like check dams and dense vegetation, are optimized for the smaller, more frequent storms that carry the majority of the annual pollutant load.
Is a wet swale the same as a stormwater wetland?
They share similar treatment mechanisms but differ in form and scale. A wet swale is a linear, channelized practice, essentially a long, narrow wetland. A stormwater wetland is typically a larger, basin-shaped system with more complex microtopography, including deep pools, shallow marshes, and semi-wet zones. While both use wetland vegetation and saturated soils, the swale is primarily a conveyance feature adapted for treatment, whereas the wetland is a destination treatment facility.
What maintenance is required for the vegetation?
Unlike a grass swale, the bottom of a wet swale is not mowed. Vegetation maintenance focuses on ensuring the health of the desired wetland plant community. This involves periodic inspections to identify and remove invasive species, which can outcompete the native plantings and degrade the swale’s function. In the early years, some replanting may be necessary in areas where vegetation has failed to establish.
Are there design calculators for wet swales?
While a specific tool for wet swales is not provided, the general hydraulic and geometric principles can be explored using resources like the dry swale design calculator. Designers can adapt the channel geometry, slope, and flow calculations from standard open channel design methods (e.g., Manning’s equation) and then incorporate the specific features of a wet swale, such as check dams and saturated soil conditions, to meet treatment objectives.
How do I choose between different open channel practices?
The choice between a wet swale, dry swale, or grass channel depends on site constraints and treatment goals. Key factors include water table depth, soil type, available slope, and the target pollutants. A wet swale is ideal for sites with a high water table, while a dry swale is better for sites with deep water tables and permeable soils. A BMP selector tool can help guide the selection process by comparing the suitability of different practices.