Vegetated buffers, commonly called Vegetated Natural Buffers (VNBs) to emphasize the need for natural areas (not constructed areas), consist of vegetation suitable for sediment removal, nutrient uptake, and soil stabilization. These areas are set aside between developed areas and a receiving water or wetland for stormwater treatment purposes. They can also be used as a pre-filter for other BMPs.
Under certain conditions, VNBs are an effective best management practice for control of stormwater pollutants in overland flow by providing opportunities for:
- Filtration
- Deposition
- Infiltration
- Absorption
- Adsorption
- Decomposition
- Volatilization
VNBs are most commonly used as an alternative to, or in combination with, swale/berm systems installed between residential or commercial backyards and receiving waters.
Potential impacts to adjacent wetlands and upland natural areas are reduced because fill is not required to establish grades that direct stormwater flow from the back of the lot toward the front for collection in the primary stormwater system.
Additional impacts are potentially reduced since VNB strips can:
- Serve as wildlife corridors
- Reduce noise
- Reduce the potential for siltation into receiving waters
A schematic of a buffer is:
Vegetated natural buffers are not intended to be the primary stormwater management system for residential developments.
They are most commonly used to treat rear-lot portions of development that cannot be feasibly routed to the primary system serving roads and lot fronts.
Use of a VNB in combination with other BMPs is only allowed if the applicant demonstrates there are no practical alternatives for those portions of the project.
Important:
The VNB area must equal the contributing or catchment area (CA in the figure).
Nitrogen Removal Research
Research for nitrogen removal was conducted by the National Risk Management Research Laboratory and published as:
Riparian Buffer Width, Vegetative Cover, and Nitrogen Removal Effectiveness: A Review of Current Science and Regulations
EPA/600/R-05/118, October 2005.
Conditions for nitrogen removal were established. Infiltration of runoff water was noted as one way natural areas can reduce discharge mass and water volume.
There are two removal mechanisms:
- Lateral seepage flow
- Overland flow
Linear interpolation between buffer widths is used in BMPFast.
Recognized Percent Pollutant Removal
Lateral Seepage Flow Through Buffer (Maximum VNB Slope 2%)
| Buffer Width (ft) | TSS (%) | TN(%) | TP(%) |
| 10 | 50 | 50 | 25 |
| 25 | 75 | 60 | 40 |
| 50 | 90 | 65 | 55 |
| 100 | 95 | 75 | 60 |
| 350 | 95 | 90 | 65 |
Lateral Seepage Flow Through Buffer (Maximum VNB Slope 2%)
| Buffer Width (ft) | TSS (%) | TN(%) | TP(%) |
| 10 | 25 | 10 | 5 |
| 25 | 50 | 15 | 10 |
| 50 | 90 | 25 | 15 |
| 100 | 95 | 45 | 25 |
| 350 | 95 | 60 | 65 |
Input Data
Design guidelines for natural buffers are found in:
Bentrup, G. (2008). Conservation Buffers: Design Guidelines for Buffers, Corridors, and Greenways.
Gen. Tech. Rep. SRS-109. Asheville, NC: USDA Forest Service, Southern Research Station. 110 p.
These designs are typically not normal geometric shapes such as squares or rectangles. Therefore, VNB areas must be converted to an equivalent rectangular area defined by width and length.
Feeder areas may direct runoff into the natural area, such as constructed setbacks in residential development. This feeder width is entered separately as input data.
Storage and Soil Capacity
Storage depth for capture efficiency is limited to 1 foot.
Soil storage capacity depends on soil type:
- Sandy soil example: 0.4 inch/inch
- Hydrologic Class C and D soils: values as low as 0.16 inch/inch
Example input data are shown below.
