Dry Swale
A dry swale is a vegetated open channel specifically engineered to treat and convey stormwater runoff. Unlike a conventional drainage ditch or simple grass channel, a dry swale incorporates a prepared filter bed of sand and soil mix over an underdrain system. As stormwater flows into the swale, it ponds temporarily on the surface before filtering down through the engineered soil media. The treated water is then collected by the perforated underdrain pipe and discharged to the storm drain system. This design ensures the swale drains completely between storm events, preventing standing water issues.
The primary function of a dry swale is water quality improvement. The filtration process effectively removes pollutants, including suspended solids, heavy metals, and some nutrients. The linear nature of swales makes them particularly well-suited for treating runoff from roads, highways, and residential developments. They can be integrated into site landscaping as an attractive, vegetated feature. While related to other open channel practices, the dry swale’s engineered components provide more reliable pollutant removal than a basic Grass Channel and avoid the permanent pool of water found in a Wet Swale.
Applicability
Dry swales are a versatile stormwater practice, but their successful implementation depends on site-specific conditions. Proper siting is essential for performance and longevity.
Drainage Area and Slope
A single dry swale is best suited for treating small drainage areas, typically five acres or less. For larger areas, the flow velocity and volume can become too great, risking erosion and overwhelming the treatment capacity. The longitudinal slope of the swale is a critical factor; to ensure slow flow velocities and allow for effective treatment, slopes should be 4% or less. Steeper slopes can cause erosion and prevent adequate contact time with the filter media.
Soils and Groundwater
Because a dry swale is constructed with an imported, engineered soil bed, it can be used in areas with less permeable native soils. The key constraint related to subsurface conditions is the depth to the seasonal high groundwater table. To ensure the filter bed can drain properly and to prevent interaction with groundwater, the bottom of the swale’s underdrain system should be at least two feet above the seasonal high water table.
Regional and Site Suitability
Dry swales can be adapted for most regions. In cold climates, they can double as snow storage areas, though salt-tolerant vegetation such as creeping bentgrass should be specified for roadside applications. In arid climates, drought-tolerant grasses like buffalo grass are necessary, and designers must weigh the water quality benefits against the need for irrigation to maintain vegetation.
While not typically used in ultra-urban areas where runoff is conveyed in pipes, dry swales are an excellent option for stormwater retrofits. Existing drainage ditches can be excavated and rebuilt as dry swales to add significant water quality treatment. Their ability to handle contaminated runoff also makes them suitable for stormwater hotspots, such as gas stations or commercial parking lots. The underdrain system prevents infiltrated hotspot runoff from reaching and contaminating local groundwater.
Design Criteria
The design of a dry swale integrates principles of open channel conveyance with soil filtration. Key criteria ensure the practice is effective for water quality treatment while safely passing larger storm flows.
Feasibility and Geometry
A dry swale must be designed with a longitudinal slope between 1% and 4%. The channel cross-section should be trapezoidal or parabolic with side slopes no steeper than 2:1, and preferably 3:1 or flatter. Flatter side slopes maximize the wetted perimeter, which helps slow flow velocities, and facilitates mowing and maintenance. The bottom width of the channel should be between two and eight feet. A width less than two feet may not provide enough treatment area, while a width greater than eight feet can lead to braiding and the formation of erosive low-flow channels.
Conveyance and Ponding
The swale must be designed to handle multiple storm events. For the water quality storm, runoff should pond to a maximum depth of one foot in the middle of a swale cell and 18 inches at the downstream end (e.g., behind a check dam). The entire water quality volume (WQv) must drain through the soil media within 48 hours. The channel should also be designed to convey the 2-year storm at a non-erosive velocity and safely pass the 10-year storm with at least six inches of freeboard.
Pretreatment
Pretreatment is essential to prevent the filter bed from clogging with coarse sediment. This is typically achieved with a small forebay at the swale inlet or by installing check dams at pipe inlets and driveway crossings. For sheet flow entering along the sides of the swale, a pea gravel diaphragm—a small trench filled with gravel running parallel to the channel—can be used to trap sediment and spread flows evenly.
Treatment Media and Underdrain
The core of the dry swale is its treatment system. The design requires the excavation of native soil and replacement with a 30-inch deep engineered soil bed. This media is typically a mix of sand, topsoil, and organic matter that provides both high permeability and pollutant removal capacity. Beneath the soil bed, an underdrain system is installed. This consists of a perforated pipe, typically 4 to 6 inches in diameter, embedded in a layer of clean gravel. The underdrain collects the filtered stormwater and conveys it to the downstream storm drain network. The combination of a properly specified soil mix and a functional underdrain is what allows the swale to meet the 48-hour drawdown requirement. For assistance with sizing and specifications, engineers can use a dry swale design calculator.
Check dams are small, permeable barriers made of stone or other materials placed across the swale. They create a series of shallow treatment cells, slowing the flow of water, promoting settling of particles, and increasing the volume of water that can be stored and treated within the swale.
Pollutant Removal
Monitoring data indicates that dry swales provide effective removal for several common stormwater pollutants. Performance is highest for pollutants associated with particulate matter, such as total suspended solids (TSS) and heavy metals. Removal of soluble pollutants like nutrients is more moderate. The data below, derived from a national database of stormwater practice performance, summarizes the expected pollutant removal efficiencies for water quality swales, which includes the dry swale design.
| Pollutant | Removal Efficiency (%) |
|---|---|
| Total Suspended Solids (TSS) | 81 ± 14 |
| Total Phosphorus (TP) | 34 ± 33 |
| Nitrogen (as NOx) | 31 ± 49 |
| Metals (Cd, Cu, Pb, Zn) | 42 to 71 |
| Bacteria | -25 |
| Source: Adapted from Winer, 2000. Negative value for bacteria indicates export. More recent studies can be found in the comprehensive pollutant removal database. | |
The high TSS removal is a key benefit of the filtration process. However, like many vegetated systems, swales can sometimes be a source of bacteria, potentially due to resident wildlife or the warm, moist soil environment. The wide variability in nutrient removal highlights the influence of design, vegetation, and soil media composition on performance.
Construction and Cost Considerations
Proper construction is critical to the long-term function of a dry swale. Key steps include careful grading to achieve the correct slopes, protection of the site from compaction by heavy equipment, installation of the underdrain and filter media in lifts, and immediate stabilization with vegetation. The use of erosion control matting is often necessary until the grass cover is fully established.
Cost data for dry swales can be estimated based on their similarity to bioretention systems. Early estimates suggest a construction cost of approximately $5.50 per cubic foot of water quality volume treated (Brown and Schueler, 1997). This is significantly more than a simple grass channel due to the costs of excavation, imported soil media, and the underdrain system. However, it is generally less expensive than concrete-intensive structural practices and can reduce the need for traditional curb and gutter infrastructure, offering potential cost savings on a site-wide basis.
Maintenance
Dry swales require regular maintenance to preserve their hydraulic function and pollutant removal capacity. A routine schedule of inspection and basic upkeep can prevent more costly rehabilitation efforts. Key activities include mowing, sediment removal, and ensuring the filter bed drains properly.
| Activity | Schedule |
|---|---|
| Inspect for erosion, sediment buildup, and health of vegetation. Check for clogging at inlets and in the pea gravel diaphragm. | Annually (semi-annually for the first year) |
| Remove trash and debris from the swale and pretreatment areas. | Annually, or as needed |
| Mow grass to maintain a height of 3 to 4 inches. Avoid scalping. | As needed during growing season |
| Remove sediment from the forebay and swale bottom when it accumulates to 25% of the original design volume. | As needed (infrequent) |
| Rototill or cultivate the surface of the soil bed if the swale fails to drain within 48 hours. | As needed (infrequent) |
| Replant bare areas or replace vegetation if the original cover has not been successfully established. | As needed |
Limitations
While effective, dry swales have certain limitations. Their reliance on vegetation means that a dense, healthy grass cover is required for proper function; they will not perform well until the vegetation is fully established. They are not intended to provide flood control or significant channel protection for downstream areas, as their storage volume is relatively small. If a site has multiple constraints or goals, a BMP selector tool can help identify the most appropriate practice or combination of practices.
The practice is also restricted to sites with slopes of 4% or less and a minimum 2-foot separation from the groundwater table. Finally, as noted in the performance data, swales can sometimes export bacteria, which may be a concern in watersheds with bacteria-related water quality impairments.
Frequently Asked Questions
What is the main difference between a dry swale and a grass channel?
The key difference is the engineered soil media and underdrain system in a dry swale. A grass channel is a simple vegetated channel that relies on native soils for infiltration and treats runoff primarily through vegetative filtering as water flows over the surface. A dry swale is a filtration practice, capturing the water quality volume and filtering it through a 30-inch-deep sand/soil mix before discharging it via an underdrain. This provides more reliable and effective pollutant removal.
How does a dry swale treat stormwater?
A dry swale treats stormwater through several mechanisms. The primary process is filtration, where water passes through the engineered soil media, physically straining out suspended solids and associated pollutants. Adsorption occurs as pollutants like metals and phosphorus bind to soil particles. Biological processes, including nutrient uptake by plants and microbial activity in the soil, also contribute to pollutant removal. The swale’s vegetation and check dams slow runoff velocity, allowing heavier particles to settle out before filtration begins.
What is the typical drainage area for a dry swale?
A single dry swale is generally designed to treat a small drainage area, typically five acres or less. This limitation ensures that runoff velocities and volumes do not become erosive or exceed the swale’s capacity to store and treat the water quality volume. For larger sites, multiple swales can be used in series or parallel, or the runoff can be divided among several smaller drainage areas, each served by its own dry swale.
Can a dry swale be used on steep slopes?
No, dry swales are not suitable for steep slopes. The recommended maximum longitudinal slope is 4%. On slopes greater than this, water velocity increases, which can cause erosion within the channel, prevent effective pollutant removal, and make it difficult to establish and maintain vegetation. On moderately sloped sites, check dams are essential to create flat ponding areas and slow the flow of water, allowing for proper treatment to occur.
How long should water pond in a dry swale?
A dry swale is designed to drain completely between storm events. The maximum allowable ponding time for the water quality volume is 48 hours after a storm ends. This drawdown time ensures that the soil filter bed remains aerobic, which is crucial for healthy vegetation and effective pollutant removal processes. It also prevents the swale from becoming a nuisance or a potential mosquito breeding habitat. Proper drainage is achieved through the combination of a permeable soil mix and a functional underdrain.
Is a dry swale suitable for treating runoff from a gas station?
Yes, a dry swale is an excellent choice for treating runoff from stormwater hotspots like gas stations, vehicle maintenance areas, and commercial parking lots. The engineered soil media is effective at trapping hydrocarbons and heavy metals. Critically, the underdrain system collects and conveys the filtered water, preventing the direct infiltration of highly concentrated pollutants into the underlying soil and groundwater. This containment feature makes it a safer option for hotspots than practices that rely solely on infiltration.
What is an underdrain and why is it necessary in a dry swale?
An underdrain is a perforated pipe installed in a gravel layer at the bottom of the dry swale’s soil filter bed. Its purpose is to collect the stormwater after it has filtered through the soil media and convey it to a discharge point, such as a storm drain inlet. The underdrain is essential for ensuring the swale dewaters within the required 48-hour timeframe, especially on sites with poorly draining native soils. This rapid drainage keeps the filter bed from becoming saturated and anaerobic.
How is a dry swale different from a bioretention cell?
A dry swale and a bioretention cell operate on the same filtration principle but differ in their form and application. A dry swale is a linear, channel-like practice designed to both treat and convey stormwater. A Bioretention cell, often called a rain garden, is typically a shallow, landscaped basin or depression. While both use an engineered soil media and often an underdrain, the dry swale’s primary application is along roadsides and property boundaries, whereas bioretention is more commonly used in parking lot islands and landscape areas.
What kind of maintenance does a dry swale require?
Routine maintenance for a dry swale includes regular mowing to a height of 3 to 4 inches, removing trash and debris, and inspecting for signs of erosion or sediment buildup. The pretreatment forebay or check dams should be cleaned out periodically as sediment accumulates. Less frequently, the surface of the soil bed may need to be cultivated if it becomes clogged and drainage slows. A dense, healthy vegetative cover is crucial, so any bare spots should be re-seeded promptly.
Do dry swales provide flood control?
No, dry swales are not designed for flood control. They are primarily a water quality practice, sized to capture and treat a specific volume of runoff from smaller, more frequent storms. While they provide some minor peak flow reduction through temporary storage, they do not have the capacity to manage large storm events. To meet flood control or channel protection requirements, a dry swale must be used in conjunction with a separate detention facility, such as a detention pond or underground storage system.