Buffer Zones: What They Are and How Stream Buffers Are Designed
A buffer zone is a strip of land kept free of development or disturbance to separate and protect one land use from another. In watershed management practice, an aquatic buffer is the protected area along a shoreline, wetland, or stream where development is restricted or prohibited. The primary function of these buffers is to physically protect and separate a water body from future disturbance or encroachment. They are a critical component of sustainable site design, serving multiple roles beyond simple separation.
Properly designed aquatic buffers provide significant stormwater management benefits, act as a right-of-way during floods, and help sustain the integrity of stream ecosystems and habitats. They filter pollutants, reduce runoff velocity, and promote infiltration. By preserving native vegetation, buffers also contribute to wildlife habitat, create corridors for animal movement, and can function as an integral part of a community’s urban forest. They are a type of conservation area that is fundamental to the health of the aquatic ecosystem it protects.
Three Types of Buffers
Aquatic buffers are generally categorized into three functional types, each with a different primary purpose. A comprehensive buffer program may incorporate elements of all three to achieve a range of water resource protection goals.
Water Pollution Hazard Setbacks
This type of buffer is designed to create a simple separation distance between a water resource and a potential pollution source. The setback prevents accidental spills or chronic low-level contamination from directly entering a stream, wetland, or lake. Common examples include setbacks from septic systems, fuel storage tanks, or areas where pesticides and fertilizers are stored or applied.
Vegetated Buffers
Vegetated buffers are natural or managed areas of vegetation, typically forest or dense grass, that divide land uses or provide landscape relief. In a stormwater context, their primary role is to intercept and slow surface runoff, allowing sediment and associated pollutants to settle out. The vegetation also takes up nutrients and promotes infiltration of runoff into the soil.
Engineered Buffers
Engineered buffers are specifically designed and constructed to treat stormwater before it enters a water body. These systems often incorporate features like level spreaders to convert concentrated flow to sheet flow, or they may include bioretention areas or other best management practices (BMPs) within the buffer footprint. The goal is to maximize pollutant removal and volume reduction for a calculated water quality storm event.
The Three-Zone Buffer System
A widely adopted model for effective stream protection is the three-zone buffer system, which recommends a minimum total base width of at least 100 feet from the stream bank. This system divides the buffer into distinct zones, each with its own width, vegetative target, and set of allowable uses. This tiered approach provides robust protection for the stream channel while allowing for some compatible human activity in the outer areas.
The following table, based on guidance from the Center for Watershed Protection, outlines the characteristics of each zone in a typical three-zone system.
| Zone | Typical Width | Vegetative Target | Allowable Uses |
|---|---|---|---|
| Streamside (Inner) Zone | Minimum 25 ft from each stream bank | Mature, undisturbed riparian forest | Very restricted: flood control structures, utility rights-of-way, footpaths at designated crossings |
| Middle Zone | 50 ft or more, extended for steep slopes, the 100-year floodplain, and adjacent wetlands | Managed forest, some clearing may be allowed | Limited: some recreational uses (trails, bike paths), stormwater BMPs, some managed forestry |
| Outer Zone | 25 ft setback from the middle zone to the nearest permanent structure | Forest, turf, or lawn encouraged | Unrestricted residential uses: lawn, garden, compost piles, and most other yard activities |
For optimal stormwater treatment, a buffer can be engineered as a treatment train. Runoff from an adjacent developed area first enters a stormwater depression area designed to capture and store runoff from smaller storms while bypassing larger flows. The captured runoff is then spread evenly across a grass filter strip designed to maintain sheetflow. This strip discharges to a wider forested buffer designed to achieve zero surface discharge to the stream, meaning it fully infiltrates the treated sheetflow. While buffers provide critical protection, it is important to recognize that the volume of runoff is driven by the upstream impervious fraction, a factor that can be estimated using tools like the Simple Method runoff calculator.
Ten Performance Criteria for Buffer Design
An effective and defensible stream buffer ordinance or design should be based on a clear set of performance criteria. These criteria govern how a buffer is sized, delineated, managed, and crossed, ensuring its functions are maintained over the long term. The following ten criteria provide a framework for a comprehensive buffer program.
- Minimum total buffer width: Establish a science-based minimum width (e.g., 100 feet) to serve as the foundation of the buffer.
- Three-zone system: Delineate streamside, middle, and outer zones to tailor protection and allowable uses.
- Mature forest as the vegetative target: Prioritize the establishment and preservation of a native, mature forest community, especially in the inner zones.
- Conditions for expansion or contraction: Define criteria for increasing buffer width (e.g., steep slopes, critical habitats) or allowing narrowing (e.g., buffer averaging).
- Physical delineation requirements: Mandate that buffer boundaries be clearly surveyed, staked, and marked in the field before, during, and after construction.
- Conditions for crossings: Specify strict criteria for the number, design, and construction of any necessary road, utility, or trail crossings.
- Integrating stormwater management: Define how and where stormwater BMPs may be located within the buffer, typically in the outer or middle zones.
- Buffer limit review: Establish a clear process for reviewing and approving proposed buffer boundaries and encroachments during site plan review.
- Education, inspection and enforcement: Develop programs for educating landowners and enforcing buffer restrictions over the long term.
- Buffer flexibility: Incorporate mechanisms like buffer averaging or variance procedures to address unique site constraints without compromising overall function.
How Well Do Buffers Remove Pollutants?
The effectiveness of vegetated buffers in removing pollutants from stormwater runoff varies widely depending on buffer width, vegetation type, soil characteristics, and the nature of the incoming runoff. It is important to distinguish their function from that of water pollution hazard setbacks, which are designed for physical separation to prevent contamination, not for active pollutant removal during a storm event. Numerous studies have measured the performance of grass and forest buffers, with removal rates often increasing with buffer width.
The pollutant removal rates from several key studies on vegetated buffers are summarized in the table below.
| Study | Vegetation | Width (m) | TSS (%) | TP (%) | TN (%) |
|---|---|---|---|---|---|
| Dillaha et al., 1989 | Grass | 4.6 | 63 | 57 | 50 |
| Dillaha et al., 1989 | Grass | 9.1 | 78 | 74 | 67 |
| Magette et al., 1987 | Grass | 4.6 | 72 | 41 | 17 |
| Magette et al., 1987 | Grass | 9.2 | 86 | 53 | 51 |
| 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 |
Buffer performance is not guaranteed and depends heavily on site-specific factors. The following table contrasts conditions that tend to enhance pollutant removal with those that reduce it.
| 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 |
Maintenance and Management
Effective long-term protection requires a clear buffer management plan. This plan should specify allowable and unallowable uses within each buffer zone and outline maintenance responsibilities. If a buffer is intended to treat runoff, the plan must include provisions for inspecting and maintaining engineered components, such as removing sediment from forebays or mowing grass filter strips to maintain dense vegetative cover. For naturally vegetated buffers, management often focuses on controlling invasive species and ensuring the vegetative target is met over time.
Unmarked buffer boundaries are frequently ignored during and after construction. Fencing, permanent monuments, or highly visible signage are critical to prevent accidental clearing, grading, or dumping within the protected area.
One of the most critical elements of buffer management is ensuring the boundaries are clearly defined and visible in the field. Before construction begins, buffer limits should be surveyed and staked. During construction, high-visibility fencing is essential to prevent encroachment by equipment and material storage. After construction, permanent markers or signs should be installed to keep the boundaries visible to residents, contractors, and municipal staff. The legal protection of these areas can be further strengthened through the use of conservation easements.
Implementation Barriers and Local Code Notes
Implementing a local buffer program can face several perceived impediments. A primary concern is the potential loss of developable land, which can create opposition from the development community. Other common concerns include requirements for public access on private land, fears that natural buffers will harbor nuisance species like ticks or snakes, and the additional demands a program places on local government staff for plan review and enforcement.
Successfully launching a buffer program requires a community to adopt a clear ordinance, develop supporting technical criteria, and invest in staff resources and training. This includes training for municipal plan reviewers as well as outreach to consultants and developers to explain the new requirements. The integration of buffer requirements into a broader framework of watershed-based zoning can improve consistency and effectiveness. Despite the initial costs, several studies have documented that property values often increase in areas adjacent to protected buffers and stream corridors.
To address concerns about financial hardship or unusual site constraints, ordinances often include flexibility provisions. The most common of these is buffer averaging, which allows a developer to narrow the buffer width at certain points along the stream, provided that the overall buffer area and average width still meet the minimum requirements. This can help accommodate a site plan without compromising the buffer’s overall protective function.
Frequently Asked Questions
What does buffer zone mean?
A buffer zone is a strip of land intentionally kept free of development or significant disturbance. Its purpose is to separate and protect one land use from another. In environmental and watershed planning, an aquatic buffer is the protected vegetated area along a stream, wetland, or shoreline where development is restricted or prohibited to protect water quality, habitat, and stream stability.
What is a riparian buffer?
A riparian buffer is a specific type of aquatic buffer located along the banks of a river or stream. The term “riparian” refers to the area of land adjacent to a stream that is directly influenced by the presence of water. These buffers are typically forested and are critical for stabilizing stream banks, shading the stream to keep water temperatures cool for aquatic life, filtering pollutants from runoff, and providing food and habitat for both aquatic and terrestrial wildlife.
How wide should a stream buffer be?
The ideal width for a stream buffer depends on its specific goals, site conditions like slope, and local regulations. However, a common science-based recommendation for general stream protection is a minimum total base width of at least 100 feet. Wider buffers are often required to protect sensitive habitats, steep slopes, or highly erodible soils. Some functions, like significant nitrogen removal, may require buffers that are considerably wider.
What are the three buffer zones?
The three-zone buffer system is a model that divides a stream buffer into distinct management areas. The inner “Streamside Zone” is closest to the stream and is the most protected, typically an undisturbed forest. The “Middle Zone” allows for some managed use, like trails or stormwater facilities. The “Outer Zone” is a setback from the middle zone to structures and allows for more residential uses, like lawns and gardens. This tiered approach provides core protection while accommodating some human activity.
What can a homeowner do in a buffer zone?
Allowable activities depend on the specific buffer zone and local ordinances. In the highly restricted Streamside Zone, very little is permitted beyond preservation. The Middle Zone may allow for passive recreation like walking trails, some tree management, and potentially stormwater BMPs. The Outer Zone is the most flexible, typically permitting lawns, gardens, compost piles, and other common residential yard activities, as it functions primarily as a setback from permanent structures to the more sensitive inner zones.
Do buffers remove pollutants?
Yes, vegetated and engineered buffers can be effective at removing pollutants from stormwater runoff, but their performance varies greatly. They work by slowing runoff, which allows sediment and attached pollutants to settle out. Vegetation and soil microbes can also absorb and break down nutrients like nitrogen and phosphorus. Effectiveness depends on factors like buffer width, vegetation density, soil type, slope, and whether runoff enters as distributed sheet flow or concentrated channelized flow.
What makes a buffer fail?
A buffer’s protective functions can fail for several reasons. The most common cause is concentrated flow, where runoff cuts channels through the buffer instead of flowing across it as a shallow sheet. This “short-circuits” the filtering process. Other causes of failure include inadequate width for the site conditions, steep slopes that cause high-velocity runoff, compacted soils that prevent infiltration, lack of dense vegetation, and encroachment from clearing, grading, or building within the designated buffer boundaries.
What is buffer averaging?
Buffer averaging is a flexibility mechanism used in some local ordinances. It allows the buffer to be narrower than the standard minimum width at some points along a property, as long as it is made wider at other points. To be approved, the average width of the entire buffer must meet the minimum requirement, and the total square footage of the buffer must be equal to or greater than what the standard fixed-width buffer would have provided. It is a tool to accommodate unique site constraints without a net loss of protected buffer area.
Are buffer zones required by law?
The requirement for a buffer zone depends entirely on the jurisdiction. There is no single national law in the U.S. that mandates buffers for all water bodies. However, they are frequently required by local (city or county) zoning, subdivision, or stormwater ordinances. State environmental regulations may also require buffers, particularly for sensitive waters, trout streams, or coastal areas. In some cases, federal programs like the Clean Water Act can lead to buffer requirements as part of a watershed plan to address pollution (TMDLs).