This practice promotes grass swales as an alternative to curbs and gutters along residential streets. Curbs and gutters are designed to quickly convey runoff from the street to the stormdrain and, ultimately, to a local receiving water. Consequently, they provide little or no removal of stormwater pollutants. Indeed, curbs often act as traps where deposited pollutants remain until the next storm washes them away. Many communities require curbs and gutters as standard elements of road sections. In fact, many communities discourage the use of grass swales. Revisions to current local road and drainage regulations are needed to promote greater use of grass swales along residential streets. The stormwater management and pollutant removal benefits of grass swales are documented in detail in the
Grassed Swales fact sheet.
The use of engineered swales in place of curbs and gutters should be encouraged in low- and medium-density residential zones where soils, slope and housing density permit. However, eliminating curbs and gutters is generally not feasible for streets with high traffic volume or extensive on-street parking demand (i.e., commercial and industrial roads). Nor is it a viable option in arid and semi-arid climates where grass cannot grow without irrigation.
Siting and Design Conditions
A series of site factors must be evaluated to determine whether a grass swale is a viable replacement for curbs and gutters at a particular site.
Contributing drainage area. Most individual swales cannot accept runoff from more than 5 acres of contributing drainage area. Typically, they serve 1-2 acres each.
Slope. Swales generally require a minimum slope of 1 percent and a maximum slope of 5 percent.
Soils. The effectiveness of swales is greatest when the underlying soils are permeable (hydrologic soil groups A and B). The swale may need more engineering if soils are less permeable.
Water Table. For most designs, swales should be avoided if the seasonally high water table is within 2 feet of the proposed bottom of the swale.
Development Density. The use of swales is often difficult when development density becomes more intense than four dwelling units per acre, simply because the number of driveway culverts increases to the point where the swale essentially becomes a broken-pipe system. Typically, grass swales are designed with a capacity to handle the peak flow rate from a 10-year storm, and fall below erosive velocities for a 2-year storm.
A number of real and perceived limitations hinder the use of grass swales as an alternative to curb and gutters:
- Snowplow operations can be more difficult without a defined road edge. However, on the plus side, roadside swales increase snow storage at the road edge, and smaller snowplows may be adequate.
- The pavement edge along the swale can experience more cracking and structural failure, increasing maintenance costs. The potential for pavement failure at the road/grass interface can be alleviated by "hardening" the interface with grass pavers or geo-synthetics placed beneath the grass. Other options include placing a low-rising concrete strip along the pavement edge.
- The shoulder and open channel will require more maintenance. In reality, maintenance requirements for grass channels are generally comparable to those of curb and gutter systems. The major requirements involve turf mowing, debris removal, and periodic inspections.
- Some grass swales can have standing water, which make them difficult to mow, and can cause nuisance problems such as odors, discoloration, and mosquitoes. In reality, grass channels are not designed to retain water for any appreciable period of time.
Other concerns involve fears about utility installation and worries that the grass edge along the pavement will be torn up by traffic and parking. While utilities will need to be installed below the paved road surface instead of the right of way, most other concerns can frequently be alleviated through the careful design and integration of the open channels along the residential street. (Consult the
Grassed Swales fact sheet for details on design variations that can reduce these problems.)
The major maintenance requirement for grass swales is mowing during the growing season, a task usually performed by homeowners. In addition, sediment deposits may need to be removed from the bottom of the swale every ten years or so, and the swale may need to be tilled and re-seeded periodically. Occasionally, erosion of swale side slopes may need to be stabilized. The overall maintenance burden of grass swales is low in relation to other stormwater practices, and it is usually within the competence of the individual homeowner. The only major maintenance problem that might arise pertains to "problem" swales that have standing water and are too wet to mow. This particular problem is often alleviated by ammending the soil with rocks and well-drained soils to promote drainage.
Removal of curbs and gutters decreases the peak flow discharge to receiving waters. Futhermore, under the proper design conditions, grass swales can be effective in removing pollutants from urban stormwater (Schueler, 1996). More information on the pollutant removal capability of various grass swale designs can be found in the
Grassed Swales fact sheet.
Engineered swales are a much less expensive option for stormwater conveyance than the curb and gutter systems they replace. Curbs and gutters and the associated underground storm sewers have been documented to cost as much as $36 per linear foot, which is roughly twice the cost of a grass swale (Schueler, 1995, and CWP, 1998). Consequently, when curbs and gutters are eliminated, the cost savings can be considerable.
Center for Watershed Protection (CWP). 1998. Better Site Design: A Handbook for Changing Development Rules in Your Community. Center for Watershed Protection, Ellicott City, MD.
Schueler, T.R. 1995. Site Planning for Urban Stream Protection. Metropolitan Washington Council of Governments. Washington, DC.
Schueler, T.R. 1996. "Ditches or Biological Filters? Classifying the Pollutant Removal Performance of Open Channels." Watershed Protection Techniques 2(2) pp. 379-83.
Claytor, R.A., and T.R. Schueler. 1997. Design of Stormwater Filtering Systems. Center for Watershed Protection, Ellicott City, MD.