Stormwater runoff from residential, commercial, industrial and agricultural land uses contains pollutants that can contaminate water bodies. Stormwater runoff from impervious surfaces also can increase runoff velocities and contribute to streambank erosion. Swales and buffer strips are a type of stormwater treatment composed of vegetation and a porous subsoil medium. Buffer strips are vegetated areas adjacent to a waterway that prohibit stormwater runoff from flowing directly into a water body.
The vegetation catches pollutants carried by stormwater, decreases the rate of flow and volume of runoff, and stabilizes the soil on the shoreline or bank, lessening erosion caused by runoff. A swale is a long, vegetated depression often used as a water conveyance system which is also designed to infiltrate water and remove sediment and pollutants from runoff. A swale, therefore, assists in recharging ground water and managing stormwater runoff quantity and quality. This fact sheet provides guidance on the design, implementation and maintenance of vegetated swales and buffer strips and on programs to educate the public and decision makers about installing them.
Properly designed swales and buffers protect and separate a stream, lake or wetland from future disturbance or encroachment and sustain the integrity of stream ecosystem habitats. Maintaining a buffer or swale upstream of surface waters reduces pollutant impacts from sediment, phosphorus, nitrogen and high temperature waters. Additional benefits include ground water recharge, channel protection, erosion control, aquatic and terrestrial riparian habitat, flood control and recreational and educational opportunities.
Swales and buffer, when constructed properly, can remove a series of pollutants from stormwater runoff. Water quality removal rates for are variable and depend on a number of factors including slope, width, and vegetation. The Minnesota Stormwater Manual contains relevant data on pollutant removal efficiencies.
===Program Development & Implementation===\ Programs designed to facilitate the installation of swales and buffers for water quality protection take into account both the mechanism of implementation and the ideal design characteristics for maximum effectiveness. For additional information on developing a buffer ordinance see the Establishing a Buffer Ordinance fact sheet. The following elements can be implemented in a vegetated swale and buffer initiative.
Awareness campaigns inform businesses, developers, and property owners of the benefits of vegetated swales and buffers. Efforts can contribute to encouraging individuals and organizations to implement buffers on a voluntary basis, educating individuals on proper installation and maintenance. Suggested educational methods include:
Brochures Develop informative brochures, and guidance for specific audiences such as businesses, contractors, developers, homeowners associations and garden clubs.
Signage at municipal installations Locate signage at parks, government buildings, and parking lots identifying swales and buffers and explaining their functions and benefits.
Workshops and seminars Workshops and seminars can be used to provide needed technical assistance to homeowners, property owners and citizens for establishing a swale or buffer on their property.
Vegetated swales are linear, channel-like surface depressions that can be utilized as conveyance to direct stormwater away from or around a structure, for treatment to remove pollutants from stormwater, to promote infiltration of runoff into the ground and as volume control for stormwater runoff. Vegetated swales can also be landscaped to provide an aesthetic appeal and provide natural habitat within an urban setting.
Design considerations The size of the swale including length, width, and depth is dependent upon soil conditions, ground water level, the area discharging stormwater, the amount of impervious area discharging stormwater, and the topography of the contributing area. The recommendations below are from Chapter 12 of the Minnesota Stormwater Manual.
Vegetation Vegetation can range from tall plants and grasses to a short turf grass depending upon the desired application of the swale. Any vegetation used should be water tolerant. Native vegetation is preferred with its ability to uptake water and filter pollutants like phosphorus and sediment. Roots of native vegetation grow deep to stabilize the soil and promote infiltration, and native vegetation does not require irrigation after the first year of establishment.
Review of a cross section of peer-reviewed studies identifies the following buffer characteristics that are most effective at filtering runoff and treating stormwater.
Width The determination of buffer widths on individual wetlands could be based on the following minimum guidelines:
Use the high end of the range for sensitive water bodies, steep slopes and surrounding land uses that could adversely impact the water body. Add buffer width to off-set the adverse impacts of slope, poor soils, human land use pressures, or to add extra protection for sensitive aquatic organisms or wildlife. Flexibility in application of buffer width requirements can be achieved through buffer width averaging, variances and conservation easements. Overall, the U.S. Fish and Wildlife Service, the MNDNR and the Center for Watershed Protection agree that bigger is better in terms of many factors including water quality treatment, erosion control and habitat.
Vegetation A vegetative mix of trees, shrubs and groundcover are recommended to provide several layers of protection. Consider native prairie planting for the groundcover portion of any buffer. The deep roots, hardiness, aesthetic appeal, unique habitat character and filtering ability all make prairies an ideal vegetative ecosystem for a conservation buffer. Trees and shrubs can also be used as a vegetative transition from the water body. Trees and shrubs can provide for enhanced infiltration and nutrient uptake while stabilizing soil and dissipating rainfall. Specific plantings will depend upon your application and area within the state. Take care to assure that vegetation and grading of any buffer area enables runoff to occur as sheet flow rather than forming channels and rills.
Structure The Center for Watershed Protection recommends designing a structure that includes the following elements: