Grassed Channel

From the Massachusetts Stormwater Handbook

Image of a grassed channel


Grassed Channels are treatment systems with a longer hydraulic residence time than drainage channels. The removal mechanisms are sedimentation and gravity separation, rather than filtration. To receive TSS credit, a sediment forebay or equivalent must be provided for pretreatment. Note that the sediment forebay does not receive a separate TSS removal credit.

Ability to Meet Massachusetts Stormwater Management Standards

2 - Peak FlowN/A
3 - RechargeNo infiltration credit
4 - TSS Removal50% TSS with adequate pretreatment
5 - Higher Pollutant LoadingN/A
6 - Discharges near or to Critical AreasNot suitable for vernal pools or bathing beaches. At other critical areas, may be used as a pretreatment device.
7 - RedevelopmentTypically not suited for retrofits.


  • Provides pretreatment if used as the first part of a treatment train.
  • Open drainage system aids maintenance
  • Accepts sheet or pipe flow
  • Compatible with LID design measures.
  • Little or no entrapment hazard for amphibians or other small animals


  • Short retention time does not allow for full gravity separation
  • Limited biofiltration provided by grass lining.
  • Cannot alone achieve 80% TSS removal
  • Must be designed carefully to achieve low flow rates for Water Quality Volume purposes (<1.0 fps)
  • Mosquito control considerations

Pollutant Removal Efficiencies

  • Total Suspended Solids (TSS)    50%1 for Regulatory Purposes (47%)2
  • Total phosphorus (TP)    -121%2
  • Total Nitrogen    Insufficient Data
  • Metals (copper, lead, zinc, cadmium)    Insufficient Data
  • Pathogens (coliform, e. coli)    Insufficient Data
1 Atlanta Regional Commission et al, 2001, Georgia Stormwater Manual, Volume 2, Section 3-3-2,

2 International Stormwater Database, based on MassDEP analysis of raw influent & effluent values reported in 2005.


Remove sediment from forebayAnnually
Remove sediment from grass channelAnnually
MowOnce a month during growing season
Repair areas of erosion and revegetateAs needed, but no less than once a year

Special Features

Reduces volume and rate of runoff.

Grass Channels

Grass channels convey and treat stormwater. Grass channels were referred to as biofilter swales in the 1996 MassDEP/CZM Stormwater Handbook, based on the nomenclature coined by the Center for Watershed Protection (CWP). The CWP is now referring to biofilter swales as grass channels – so MassDEP is adopting the same name as the CWP to minimize confusion.

Properly designed grass channels are ideal when used adjacent to roadways or parking lots, where runoff from the impervious surfaces can be directed to the channel via sheet flow. Runoff can also be piped to the channel. If piped, locate the sediment forebay at the pipe outlet and include a check dam separating the forebay from the channel. For sheet flow, use a vegetated filter strip on a gentle slope or a pea gravel diaphragm. Make the longitudinal slope as flat as possible. This increases the Hydraulic Residence Time (HRT) and allows gravity separation of solids and maximizes sediment removal. Install check dams to further increase the HRT.

Review of the International Stormwater Database, updated in 2005, indicates lower TSS removal when compared to similar treatment practices (dry water quality swales, wet water quality swales, and bioretention areas). The information in the International Stormwater Database indicates grass channels are likely to export phosphorus (hence the negative removal efficiency cited above). Grass channels are not a practice suitable for treating stormwater that discharges to waters impaired by phosphorus or for waters where phosphorus TMDLs have been established.

Differences from dry water quality swales, wet water quality swales, bioretention cells, and drainage channels: Dry water quality swales contain a specific soil media mix and underdrain, providing greater treatment than grass channels. Wet water quality swales are designed with a permanent wet channel, whereas grass channels must be designed to completely drain between storms. Bioretention areas, including rain gardens, are designed solely as a treatment practice, and not for conveyance. Lastly, drainage channels act solely as a conveyance, in contrast to properly designed grass channels where runoff flow is deliberately lagged to provide treatment.

Design Considerations


Water Quality Volume

Design grass channels to maximize contact with vegetation and soil surface to promote greater gravity separation of solids during the storm associated with the water quality event (either ½ inch or 1-inch runoff). Design the channel such that the velocity does not exceed 1 foot per second during the 24-hour storm associated with the water quality event.
Do not allow the water depth during the storm associated with the water quality event to exceed 4 inches (for design purposes). Make sure the selected design storm provides at least 9 minutes of HRT within the channel. Increasing the HRT beyond 9 minutes increases the likelihood of achieving the 50% TSS removal efficiency. Adding meanders to the swale increases its length and may increase the HRT.
2-year and 10-year conveyance capacity: Design grass channels to convey both the 2-year and 10-year 24-hour storms. Provide a minimum of 1-foot freeboard above the 10-year storm. Make sure that the runoff velocities during the 2-year 24-hour storm do not cause erosion problems. Channel Length: Length depends on design factors to achieve the minimum 9-minute residence time for the storm associated with the water quality event.

Channel Crossings

In residential settings, driveways will cross over the channel, typically via culverts (pre-cast concrete, PVC, or corrugated metal pipe).


Grass channels may be constructed from most parent soils, unless the soils are highly impermeable. Soils must be able to support a dense grass growth. MassDEP recommends sandy loams, with an organic content of 10 to 20%, and no more than 20% clay. Highly impermeable soils, such as clays, are not suitable for grass channels, because they do not support dense grass stands. Similarly, gravelly and coarse soils may not be suitable due to their lower moisture retention capability, leading to potential die-back of the grass lining during the summer when the inter-event period between storms is longer than during other times of the year. Grasses: The grasses serve to stabilize the channel, and promote conditions suitable for sedimentation, such as offering resistance to flow, which reduces water velocities and turbulence. Select a grass height of 6 inches or less. Grasses over that height tend to flatten when water flows over them, inhibiting sedimentation. Select grasses that produce a fine, uniform and dense cover that can withstand varying moisture conditions. Regularly mow the channel to ensure that the grass height does not exceed 6 inches. Select grasses that are salt tolerant to withstand winter deicing of roadways. In the spring, replant any areas where grasses died off due to deicing. (Franklin 2002 and Knoxville 2003 provide recommendations for the best grass species.)

Pea Gravel Diaphragm

Use clean bank-run gravel, conforming to ASTM D 448, varying in size from 1/8 inch to 3/8 inch (No. 6 stone).

Outlet Protection

Must be used at discharge points to prevent scour downstream of the outlet. Construction Considerations: Stabilize the channel after it is shaped before permanent turf is established, using natural or synthetic blankets. Never allow grass channels to receive construction period runoff.

Site Constraints

A proponent may not be able to install a grass channel swale because of:
  • High groundwater;
  • Presence of utilities; or
  • Other site conditions that limit depth of excavation because of stability.



Maintenance access must be designed as part of the grass channel. If located adjacent to a roadway, make the maintenance access at least 15 feet wide, which can also be combined with a breakdown lane along a highway or on- street parking along a residential street. When combined with on-street parking, post signs prohibiting parking when the swale is to be inspected and cleaned. Do not use travel lanes along highways and streets as the required maintenance access.


Set the mower blades no lower than 3 to 4 inches above the ground. Do not mow beneath the depth of the design flow during the storm associated with the water quality event (e.g., if the design flow is no more than 4 inches, do not cut the grass shorter than 4 inches). Mow on an as-needed basis during the growing season so that the grass height does not exceed 6 inches. Inspection: Inspect semi-annually the first year, and at least once a year thereafter. Inspect the grass for growth and the side slopes for signs of erosion and formation of rills and gullies. Plant an alternative grass species if the original grass cover is not successfully established. If grass growth is impaired by winter road salt or other deicer use, re-establish the grass in the spring. Trash/Debris Removal: Remove accumulated trash and debris prior to mowing.

Sediment Removal

Check on a yearly basis and clean as needed. Use hand methods (i.e., a person with a shovel) when cleaning to minimize disturbance to vegetation and underlying soils. Sediment build-up in the grass channel reduces its capacity to treat and convey the water quality event, 2-year and 10-year 24-hour storm.


Atlanta Regional Commission et al, 2001, Georgia Stormwater Management Manual, Volume 2, Section 3-3-2, Grass Channel,

Center for Watershed Protection, undated, Stormwater Management Fact Sheet: Grass Channel, (accessed October 23, 2007)

Shanti R. Colwell, Richard R. Horner, Derek B. Booth, 2000, Characterization of Performance Predictors and Evaluation of Mowing Practices in Biofiltration Swales,

Franklin, City of, 2002, PTP-05, Biofilters: Swales and Strips,

Idaho Department of Environmental Quality, 2005, Storm Water Best Management Practices Catalog, BMP 1, Biofiltration Swale (Vegetated Swale).

International Stormwater BMP Data Base, 2005 Knoxville, City of, 2003, ST-05, Filter Strips and Swales,

Minton, G., 2002, Stormwater Treatment, Resource Planning Associates, Seattle, WA, p. 174