Grass Channel

A grass channel is a vegetated open channel designed to both convey and treat stormwater runoff. These systems are an engineered improvement over traditional drainage ditches, incorporating specific design features like gentle slopes and controlled flow velocities to enhance water quality. As stormwater flows through the channel, pollutants are removed through filtration by the dense turf grass, sediment deposition, and some infiltration into the underlying soil. Grass channels are often used for pretreatment before runoff enters another stormwater practice or as the primary treatment for small drainage areas like roadsides and residential lots.

The primary treatment mechanism relies on maximizing the contact time between runoff and the vegetation. Unlike many other stormwater practices that are sized to capture a specific volume of water, a grass channel is typically sized based on a peak flow rate from a water quality storm. The design goal is to slow the flow, ensuring a minimum residence time (e.g., ten minutes) to allow for filtering and settling. While they are the simplest form of vegetated open channel, more complex variations include the dry swale, which adds an engineered soil media, and the wet swale, which maintains a permanent pool of water.

Applicability

Grass channels are a versatile practice suitable for many development situations, particularly in residential and transportation settings where their linear form fits naturally into the landscape. However, site conditions must be appropriate for them to function effectively.

Drainage Area

Individual grass channels are best suited for small drainage areas, typically five acres or less. For larger contributing areas, flow velocities and volumes become too high to prevent erosion and provide effective water quality treatment within a reasonably sized channel.

Slope and Topography

The longitudinal slope of a grass channel is critical to its performance. Slopes should be relatively flat, ideally between 1% and 2%, and must not exceed 4%. Steeper slopes generate high velocities that can cause erosion, scour the channel, and prevent pollutants from settling or being filtered by the vegetation.

Soils and Groundwater

Grass channels can be constructed on most soil types. The bottom of the channel should be separated from the seasonal high groundwater table by at least two feet to ensure the root zone remains unsaturated and to prevent potential groundwater contamination.

Regional Suitability

In cold climates, grass channels can be used for snow storage and treatment, though salt-tolerant grass species are required. In arid or semi-arid regions, the need for irrigation to maintain a dense vegetative cover must be weighed against water conservation goals; using drought-tolerant grasses is essential.

Ultra-Urban and Hotspot Sites

Grass channels are generally not feasible in ultra-urban landscapes where space is limited and runoff is managed in piped systems. They are also not recommended for treating runoff from stormwater hotspots, such as gas stations or industrial sites. Their reliance on infiltration creates a risk of mobilizing high concentrations of pollutants into the groundwater.

Retrofit Potential

One of the most common and effective applications of grass channels is retrofitting existing drainage ditches. Traditional ditches can be regraded to meet the required slope and cross-section criteria, transforming a simple conveyance system into a functional treatment practice. Check dams can also be added to older ditches to slow flow and capture sediment.

Design Criteria

The design of a grass channel focuses on creating conditions for non-erosive flow and sufficient contact time for treatment during frequent storm events, while also safely conveying larger flows.

Feasibility Criteria

• Drainage Area: Less than 5 acres.
• Longitudinal Slope: 1% to 4%. Slopes less than 1% may have drainage problems, while slopes greater than 4% result in erosive velocities.
• Groundwater Separation: Minimum 2 feet from the channel bottom to the seasonal high water table.

Conveyance and Geometry

• Cross-Section: A trapezoidal or parabolic shape is recommended.
• Bottom Width: Between 2 and 8 feet. A minimum width provides an adequate filtering surface, while a maximum width prevents the formation of braided, low-flow channels.
• Side Slopes: Should be 3:1 (H:V) or flatter to promote sheet flow into the channel and for ease of mowing. Side slopes should never be steeper than 2:1.
• Flow Velocity: The channel must be designed to handle the peak velocity from the 2-year storm in a non-erosive manner.
• Capacity: The channel must safely convey the 10-year storm event with at least 6 inches of freeboard.

Pretreatment

Pretreatment is necessary to prevent sediment from overwhelming the channel. For concentrated inflows, a small forebay or plunge pool should be used. For lateral sheet flow entering from the side, a pea gravel diaphragm—a small trench filled with gravel running parallel to the channel—can effectively trap coarse sediment before it enters the vegetated area. This design element is also common in a filter strip.

Treatment Design

The grass channel is a rate-based practice, not a volume-based one. The design objective is to achieve a minimum hydraulic residence time of 10 minutes for the peak flow of the water quality design storm. Using Manning’s equation, the designer manipulates the channel’s cross-section and slope to ensure the flow velocity during this event does not exceed 1.0 foot per second. More complex swale systems with storage components can be analyzed using tools like a swale design calculator.

Landscaping

A dense, erosion-resistant vegetative cover is the most important component of a grass channel. Turf-forming grasses that can withstand periodic inundation and flow velocities are ideal. During construction, the channel must be stabilized immediately with seed and erosion control blankets or with sod. For roadside applications in northern climates, salt-tolerant species like creeping bentgrass should be specified. In arid regions, drought-tolerant species such as buffalo grass are more appropriate.

Pollutant Removal

Monitoring data for grass channels shows effective removal for total suspended solids (TSS) and moderate removal for some other pollutants. Performance is highly dependent on achieving the design flow velocity and maintaining a dense stand of grass. Compared to more complex systems like dry and wet swales, pollutant removal is generally lower. The data suggests grass channels can be a source of bacteria, possibly due to resident animal waste or conditions in the soil that favor bacterial growth.

Source: Adapted from Winer, 2000. Data represents multiple swale types, including grass channels. For more detailed information, consult the full pollutant removal database.

Construction and Cost Considerations

The construction cost of a grass channel is low compared to most other structural stormwater practices. An early study estimated costs at approximately $0.25 per square foot (SWRPC, 1991), though a more realistic estimate including design and contingencies would be higher. Costs are primarily associated with grading and vegetation establishment.

Proper construction sequencing is critical. The channel should be graded and stabilized early in the construction process to prevent erosion. It is essential to achieve the design grades and cross-section accurately, as even minor deviations can compromise performance. The vegetation must be fully established before stormwater is directed to the channel.

Maintenance

Maintenance is focused on keeping the vegetation healthy and the channel free of debris and sediment. A regular schedule of inspection and routine tasks ensures the channel continues to function as designed.

Limitations

While effective in the right context, grass channels have several limitations. Their performance is highly sensitive to slope and vegetative cover; if not properly designed or maintained, they can provide very little pollutant removal. They are not effective at removing bacteria and offer minimal groundwater recharge, channel protection, or flood control benefits. Due to their space requirements and unsuitability for hotspot runoff, their application in dense urban areas is limited. For sites with these constraints, an engineer may need to use a BMP selector tool to identify more appropriate practices.

Frequently Asked Questions