Filter Strip
A filter strip is a vegetated area designed to treat sheet flow from adjacent impervious or managed turf areas. Also known as vegetated filter strips or grassed filters, these practices function by slowing runoff velocities, filtering sediment and other pollutants, and allowing for some infiltration into the underlying soil. The primary mechanism involves passing runoff over a uniform, gently sloping surface of dense turf.
Originally an agricultural practice for treating field runoff, the filter strip has been adapted for use in urban and suburban environments. For a filter strip to function effectively, runoff must be maintained as shallow, even sheet flow across the entire width of the strip. The key challenge in urban settings is preventing runoff from concentrating into channels or rills, which can short-circuit the treatment process and render the practice ineffective. When designed correctly, particularly with enhancements like a pervious berm, filter strips can serve as a reliable pretreatment device or a standalone water quality practice.
Applicability
Filter strips are best suited for treating runoff from small, linear impervious surfaces such as roads, highways, and small parking lots, or from pervious surfaces like managed turf. They are an ideal pretreatment component for other stormwater practices and can be integrated into the outer zone of a stream buffer. The suitability of a filter strip depends heavily on site topography and available space. The interactive BMP selector tool can help determine if a filter strip is appropriate for a specific site’s constraints.
Regional Suitability
Filter strips can be implemented in most regions. In arid or semi-arid climates, designers must specify drought-tolerant grasses to minimize irrigation needs. In cold climates, they provide a convenient area for snow storage and treatment of meltwater. If used for this purpose, the selected vegetation must be salt-tolerant, and the maintenance plan should include periodic removal of accumulated sand.
Ultra-Urban and Retrofit Applications
Due to their relatively large land requirement, filter strips are often impractical in ultra-urban areas where space is limited. Similarly, they are generally not a preferred option for stormwater retrofits unless sufficient undeveloped land is available adjacent to the target drainage area.
Stormwater Hotspots
Filter strips are not recommended for treating runoff from stormwater hotspots, which are land uses that generate highly contaminated runoff (e.g., vehicle service stations). The infiltration component of a filter strip creates a potential pathway for high concentrations of pollutants to contaminate groundwater.
Cold Water Streams
Because filter strips do not pond water on the surface for extended periods, they do not contribute to the warming of stormwater runoff. This makes them a suitable management practice in watersheds that drain to cold-water streams sensitive to thermal impacts.
Design Criteria
While appearing simple, the successful performance of a filter strip hinges on careful attention to design details that promote sheet flow and provide adequate contact time for treatment.
Feasibility
- Drainage Area: The primary design constraint is not the total drainage area but the length of the contributing flow path. To maintain sheet flow, the maximum overland flow path length contributing to a strip should be 75 feet for impervious surfaces and 150 feet for pervious surfaces.
- Slope: The slope of the filter strip itself should be between 2% and 6%. Slopes steeper than 6% can cause high velocities and concentrated flow, while slopes flatter than 2% may lead to surface ponding and soggy conditions.
- Soils and Groundwater: The underlying soils should have some infiltration capacity and not have a high clay content. A vertical separation of at least two to four feet between the bottom of the practice and the seasonally high groundwater table is required to prevent groundwater contamination and ensure the filter strip can dry out between storms.
Conveyance and Pretreatment
The entire design must be oriented around establishing and maintaining sheet flow. A level spreader or a pea gravel diaphragm is essential at the top of the slope where runoff enters the strip. This feature distributes flow evenly across the strip’s width and provides pretreatment by settling out coarse sediment particles before they can clog the vegetation.
Treatment Volume and Geometry
A standard filter strip provides pretreatment but may not receive credit for meeting water quality volume (WQv) requirements. To function as a credited treatment practice, a filter strip must be enhanced with a pervious berm at the toe of the slope.
- Length: The filter strip should have a minimum length of 25 feet, measured in the direction of flow, to provide adequate contact time for treatment.
- Pervious Berm: A small berm constructed of sand and gravel at the downstream end of the strip creates a shallow ponding area. This temporarily detains runoff, allowing for increased infiltration and filtration. The storage volume behind the berm should be designed to hold the target water quality volume. Runoff then gradually flows through or over the berm. This design shares principles with a Sand Filter.
- Flow Bypass: Filter strips are designed for water quality treatment and not for flood control. An offline design or a bypass system must be included to safely convey larger storm events (e.g., the 10-year storm) around the strip to a stabilized outlet.
Landscaping
A dense, robust vegetative cover is critical for performance. The landscaping plan should specify grass species capable of withstanding both periodic inundation and drought conditions, as well as the expected runoff velocities. The contributing drainage area must be fully stabilized before runoff is directed to the filter strip. The design of vegetated practices like filter strips and Bioretention areas requires careful plant selection for long-term success.
Pollutant Removal
The pollutant removal performance of filter strips can be highly variable and is strongly dependent on maintaining sheet flow. Research from agricultural settings has shown high removal rates, but translating these results to an urban environment is challenging due to differences in runoff characteristics and pollutant loads. One study of an urban filter strip treating parking lot runoff demonstrated that performance improved significantly with greater length.
The data below highlights the variability, with the shorter 75-foot strip showing poor or even negative removal for certain pollutants, likely due to scour or release of previously trapped materials. The longer 150-foot strip performed much more effectively. For a broader view of BMP performance, consult the comprehensive pollutant removal database.
| Pollutant | Removal Rate (%) 75-Foot Filter Strip |
Removal Rate (%) 150-Foot Filter Strip |
|---|---|---|
| Total Suspended Solids (TSS) | 54 | 84 |
| Nitrogen Oxides (NOx) | -27 | 20 |
| Total Phosphorus (TP) | -25 | 40 |
| Extractable Lead | -16 | 50 |
| Extractable Zinc | 47 | 55 |
| Source: Yu et al., 1993 | ||
Construction and Cost Considerations
Construction costs for filter strips are primarily associated with fine grading and vegetation establishment. The cost for seed or sod can range from approximately $13,000 to $30,000 per acre. In many development scenarios, this area would have been landscaped anyway, so the incremental cost may only be for the specific grading and installation of features like the gravel diaphragm and pervious berm. The design of these features can be explored using tools like the filtering practice design calculator.
The true cost of a filter strip is often the opportunity cost of the land it occupies. The practice requires a relatively large footprint, often equal to the impervious area it treats, which can make it cost-prohibitive where land values are high. Annual maintenance costs are comparatively low, estimated at around $350 per acre per year, and can often be integrated into existing landscape maintenance contracts.
Maintenance
Regular maintenance is critical to ensure a filter strip functions as designed, with the primary goal being the prevention of concentrated flow. Maintenance activities are similar to those for other vegetated practices, such as a Grass Channel.
| Activity | Schedule |
|---|---|
| Inspect the pea gravel diaphragm or level spreader for sediment buildup and clogging. Remove accumulated sediment as needed. | Annually |
| Inspect the vegetated surface for signs of erosion, such as rills and gullies. Repair and re-seed or sod any bare areas. | Annually (Semi-annually for the first year) |
| Ensure dense grass cover has been established. If not, replace with an alternative species better suited to site conditions. | Semi-annually for the first year |
| Mow grass to maintain a dense turf with a height of 3 to 4 inches. | As needed during growing season |
| Remove sediment that accumulates at the toe of the slope or behind the pervious berm when it reduces storage capacity. | As needed |
Limitations
Filter strips have several notable limitations that designers must consider. Their performance is highly sensitive to proper grading and the maintenance of sheet flow; minor construction flaws can lead to channelization and complete failure of the practice. They consume a large amount of land, making them unsuitable for dense urban sites.
Furthermore, there is limited monitoring data confirming high pollutant removal rates in urban settings, and performance can be inconsistent. If slopes are too flat or soils are compacted, filter strips can become waterlogged, creating soggy areas and potential mosquito breeding habitat. They are also not suitable for treating large drainage areas or providing flood control.
Frequently Asked Questions
What is the primary function of a filter strip?
A filter strip’s main purpose is to treat stormwater runoff by slowing it down and filtering it through dense vegetation. This process removes sediment and associated pollutants. They are most effective when used as a pretreatment device for another stormwater practice or for treating runoff from small, linear impervious surfaces like roads. To receive credit for full water quality treatment, they typically require design enhancements such as a pervious berm to increase detention time.
Why is maintaining sheet flow so important for a filter strip?
Maintaining sheet flow—a shallow, even flow across the entire surface—is critical for a filter strip’s success. It maximizes the contact area and time between the runoff and the vegetation, which is necessary for effective filtering and infiltration. If runoff concentrates into small channels or rills, it moves too quickly, bypasses the filtering media, and can cause erosion within the strip, rendering the practice ineffective for pollutant removal.
How does a filter strip handle runoff from large storms?
Filter strips are designed to manage the smaller, more frequent storms that account for the majority of annual pollutant loads, not large flood events. They must be designed as “offline” systems. This means that an engineered bypass, such as a swale or storm drain inlet, must be included to safely convey runoff from larger storms (e.g., the 10-year storm event) around the filter strip to prevent erosion and washout of the vegetated area.
What is the minimum recommended length for a filter strip?
For a filter strip to provide meaningful water quality treatment, it should have a minimum flow path length of 25 feet. However, research indicates that performance, particularly for dissolved pollutants, improves with greater length. Studies have shown that a 150-foot strip provides significantly better pollutant removal than a 75-foot strip. The optimal length depends on the target pollutants, soil type, and slope, but a longer strip is almost always more effective.
What causes a filter strip to fail?
The most common cause of failure is the development of concentrated flow, which prevents treatment and erodes the strip. This is often caused by improper grading during construction, lack of a level spreader at the inflow point, or sediment accumulation that blocks uniform flow. Other failure modes include inadequate vegetative cover, soil compaction that prevents infiltration, and slopes that are either too steep (causing high velocity) or too flat (causing prolonged ponding).
How does a pervious berm improve a filter strip’s performance?
Adding a pervious berm, which is a low dam made of sand and gravel at the downstream end of the strip, significantly enhances performance. The berm creates a shallow ponding area that temporarily detains the water quality volume (WQv). This detention increases the contact time for filtration through the vegetation and allows for more infiltration into the soil. This design feature elevates the filter strip from a simple pretreatment device to a practice that can be credited with meeting regulatory water quality treatment requirements.
Are filter strips a good choice for dense urban sites?
No, filter strips are generally not a good choice for dense, ultra-urban environments. Their primary limitation is their large land requirement, which is often equal to or greater than the impervious area they are designed to treat. In areas where land is scarce and expensive, other more compact stormwater practices like bioretention cells or sand filters are far more feasible and cost-effective. Filter strips are better suited to suburban or lower-density settings with available open space.
What is a pea gravel diaphragm and why is it needed?
A pea gravel diaphragm is a shallow trench filled with pea gravel installed along the top edge of a filter strip, where runoff enters. Its purpose is twofold. First, it acts as a level spreader, forcing incoming runoff to spread evenly across the entire width of the strip to ensure sheet flow. Second, it serves as a pretreatment device, allowing coarse sediment particles to settle out within the gravel before they can smother the grass or clog the filter strip surface.
Can a filter strip be used to treat runoff from a large parking lot?
A single filter strip is generally not suitable for treating an entire large parking lot. The maximum contributing flow path length to a strip should not exceed 75 feet for impervious surfaces. On a large lot, runoff will concentrate long before reaching the edge. However, filter strips can be used effectively within a large parking lot design by breaking the lot into smaller drainage areas. For example, strips can be placed along the edges of parking bays to treat runoff from a limited number of spaces.
What kind of vegetation should be used in a filter strip?
The ideal vegetation is a dense, soil-binding turf grass that can tolerate both drought and periodic inundation. The specific species should be selected based on regional climate and local conditions. The grass must be resilient enough to withstand the expected velocity of runoff without eroding. In cold climates where road salt is used, salt-tolerant species are necessary. In arid regions, drought-tolerant grasses should be chosen to avoid the need for extensive irrigation.