Aquatic Buffers Fact Sheet: Buffer Zones
An aquatic buffer is an area along a shoreline, wetland, or stream where development is restricted or prohibited. The primary function of aquatic buffers is to physically protect and separate a stream, lake or wetland from future disturbance or encroachment. If properly designed, buffers can provide stormwater management and act as a right-of-way during floods, sustaining the integrity of stream ecosystems and habitats. Technically aquatic buffers are one type of conservation area, function as an integral part of the aquatic ecosystem, and can also function as part of an urban forest.
The three types of buffers are water pollution hazard setbacks, vegetated buffers, and engineered buffers. Water pollution hazard setbacks are areas that may create a potential pollution hazard to the waterway. By providing setbacks from these areas in the form of a buffer, potential pollution can be avoided. Vegetated buffers are any number of natural areas that exist to divide land uses or provide landscape relief. Engineered buffers are areas specifically designed to treat stormwater before it enters into a stream, shore or wetland.
An effective stream buffer design should be based on 10 practical performance criteria that govern how a buffer will be sized, delineated, managed, and crossed (for more information see The Architecture of Urban Stream Buffers, Article 39 in The Practice of Watershed Protection).
In general, a minimum base width of at least 100 feet is recommended to provide adequate stream protection. The three zone buffer system is an effective technique for establishing a buffer, consisting of an inner, middle, and outer zones. The zones are distinguished by function, width, vegetative target, and allowable uses. Figure 1 shows the three zones and each of their characteristics.
For optimal stormwater treatment, the following buffer designs are recommended. The buffer should be composed of three lateral zones: a stormwater depression area that leads to a grass filter strip that in turn leads to a forested buffer. The stormwater depression is designed to capture and store stormwater during smaller storm events, and bypass larger stormflows directly into a channel. The captured runoff within the stormwater depression can then be spread across a grass filter designed for sheetflow conditions for the water quality storm. The grass filter then discharges into a wider forest buffer designed to have zero discharge of surface runoff to the stream (i.e, full infiltration of sheetflow).
There are several perceived impediments associated with the implementation of a buffer program, that may include:
An effective buffer management plan should include establishment, management and distinctions of allowable and unallowable uses in the buffer zones. Buffers boundaries should be well defined and visible before, during and after construction. Without clear signs or markers defining the buffer, boundaries become invisible to local governments, contractors, and residents (see Invisibility of Stream and Wetland Buffers in the Field, Article 41 in The Practice of Watershed Protection). Buffers designed to capture stormwater runoff from urban areas will require more maintenance if the first zone is designated as a bioretention or other engineered depression area.
The pollutant removal effectiveness of buffers depends on the design of the buffer and while water pollution hazard setbacks are designed to prevent possible contamination from neighboring land uses, they are not designed for pollutant removal during a storm. With vegetated buffers, some pollutant removal studies have shown that they range in effectiveness (Table 1). Proper design of buffers can help increase the pollutant removal from stormwater runoff (Table 2).
|Table 1. Pollutant Removal Rates (%) in Buffer Zones|
|Reference||Buffer Vegetation||Buffer Width (meters)||Pollutant|
|Dillaha et al.1989||Grass||4.6||63||57||50|
|Magette et al. 1987||Grass||4.6||72||41||17|
|Schwer and Clausen 1989||Grass||26||89||78||76|
|Lowrance et al. 1983||Native hardwood forest||20 - 40||-||23||-|
|Doyle et al. 1977||Grass||1.5||-||8||57|
|Barker and Young 1984||Grass||79||-||-||99|
|Lowrance et al. 1984||Forested||-||-||30-42||85|
|Overman and Schanze 1985||Grass||-||81||39||67|
|Young et al. 1980||Grass||27.4||-||88||87|
|Table 2. Factors Affecting Buffer Pollutant Removal Performance|
|Factors that Enhance Performance||Factors that Reduce Performance|
|Slopes less than 5%||Slopes greater than 5%|
|Contributing flow lengths < 150 ft.||Overland flow paths over 300 feet|
|Water table close to surface||Groundwater far below surface|
|Check dams/ level spreaders||Contact times less than 5 minutes|
|Permeable, but not sandy soils||Compacted soils|
|Growing season||Non-growing season|
|Long length of buffer or swale||Buffers less than 10 feet|
|Organic matter, humus, or mulch layer||Snowmelt conditions, ice cover|
|Small runoff events||Runoff events > 2 year event.|
|Entry runoff velocity less than 1.5 ft/sec||Entry runoff velocity more than 5 ft/sec|
|Swales that are routinely mowed||Sediment buildup at top of swale|
|Poorly drained soils, deep roots||Trees with shallow root systems|
|Dense grass cover, six inches tall||Tall grass, sparse vegetative cover|
Several studies have documented the increase of property values in areas adjacent to buffers. The real costs of instituting a buffer program for local government involve the extra staff and training time to conduct plan reviews, provide technical assistance, field delineation, construction and ongoing buffer education programs. To implement a stream buffer program, a community will need to adopt an ordinance, develop technical criteria, and invest in additional staff resources and training. The adoption of a buffer program also requires an investment in training for the plan reviewer and the consultant alike. Manuals, workshops, seminars and direct technical assistance are needed to explain the new requirements to all the players in the land development business. Lastly, buffers need to be maintained and resources should include systematic inspection of the buffer network before and after construction and work to increase resident awareness about buffers.
One way to relieve some of the significant financial hardships for developers is to provide flexibility through buffer averaging. Buffer averaging allows developers to narrow the buffer width at some points if the average width of the buffer and the overall buffer area meet the minimum criteria.
Barker, J.C. and Young, B.A. 1984. Evaluation of a Vegetative Filter for Dairy Wastewater in Southern Appalachia, Water Resource Research Institute, North Carolina State University, Raleigh, N.C.
Dillaha, T.A., Renear, R.B., Mostaghimi, S. and Lee, D. 1989. "Vegetative Filter Strips for Agricultural Nonpoint Source Pollution Control." Transactions of the American Society of Agricultural Engineers, 32(2):513-519.
Doyle, R.C., Stanton, G.C., and Wolf, D.C. 1977. Effectiveness of Forest and Grass Buffer Filters in Improving the Water Quality of Manure Polluted Runoff. American Society of Agricultural Engineers Paper No. 77-2501. St. Joseph, MI.
Heraty, M. 1993. Riparian Buffer Programs: A Guide to Developing and Implementing a Riparian Buffer Program as an Urban Stormwater Best Management Practice. Metropolitan Washington Council of Governments. Produced for US EPA Office of Wetlands, Oceans, and Watershed. Washington, DC.
Lowrance, R.R., Todd, R.L., and Asmussen, L.E. 1983. "Waterborne Nutrient Budgets for the Riparian Zone of an Agricultural Watershed." Agriculture, Ecosystems, and Environment, 10:371-384.
Lowrance, R.R., Todd, R.L., Fail, J., Hendrickson, O., Leonard, R., and Asmussen, L.E. 1984. "Riparian Forests as Nutrient Filters in Agricultural Watersheds." Bioscience, 34:374-377.
Magette, W.L., Brinsfield, R.B., Palmer, R.E., Wood, J.D., Dillaha, T.A. and Reneau, R.B. 1987. "Vegetated Filter Strips for Agriculture Runoff Treatment." United States Environmental Protection Agency Region III, Report #CBP/TRS 2/87-003314-01.
Overman, A.R. and Schanze, T. 1985. "Runoff Water Quality from Wastewater Irrigation." Transaction of the American Society of Agricultural Engineers, 28:1535-1538.
Peterjohn, W.T. and Correll, D.L. 1984. "Nutrient Dynamics in an Agricultural Watershed: Observations in the Role of the Riparian Forest." Ecology, 65(5):1466-1475.
Schueler, Thomas R. Site Planning for Urban Stream Protection. Metropolitan Washington Council of Governments. Washington, D.C. 1995.
Schwer, C.B. and Clausen, J.C. 1989. "Vegetative Filter Treatment of Dairy Milkhouse Wastewater." Journal of Environmental Quality, 18:446-451.