GUIDELINES / 1.0 Water Quality
1.19 Buffer Width Design Tool for Surface Runoff
At any given site, the level of pollutant removal from surface runoff depends primarily on buffer width. The graph and tables on the following pages can be used to estimate a buffer width that will achieve a desired level of pollutant removal.
The tool is designed to quickly generate estimates of design width for a broad range of site conditions. Adjustments are made for land slope, soil texture, field size, and soil surface condition. The tool can be used for sediment, sediment-bound pollutants, and dissolved pollutants.
This tool was developed specifically for agricultural runoff but can be applied in a more general way to other land uses as well.
For more information on how this tool was developed, refer to the Frequently Asked Questions section at the end of this guideline.
Buffer Width Design Tool for Surface Runoff
Buffer Width Graph
The seven lines in the Buffer Width Graph represent seven different site conditions (shown in Table A) that describe the typical range of agricultural sites. The lines divide up the full range of possible pollutant removal levels into convenient increments. Use of this graph amounts to selecting one line that is most appropriate for conditions at a given site.
How to Use the Buffer Width Design Tool
1. From Table A, identify a reference line number for conditions that mostly closely resembles one�s site.
2. Using Table B, select a line number that is higher or lower than the reference line number depending on how one�s site conditions and pollutant type differ from those of the reference line. To do so:
- Add up the pluses and minuses to get the total adjustment.
- Add the total adjustment number to the reference line number. The result is the appropriate line number to use for determining a buffer design width at one�s site.
3. Identify the desired level of pollutant removal, then using the appropriate line in the graph, estimate the corresponding buffer width that will achieve that level.
Examples Using the Buffer Width Design Tool
The tables below illustrate two examples using the Buffer Width Design Tool. In example one, the final reference line after adjustments is 4 while in example two, the final reference line is 1. The dashed lines on the graph below demonstrate how to obtain a buffer design width for the two examples at two particular desired trapping efficiencies.
Buffer Width Design Tool: Frequently Asked Questions
How was the tool developed?
The tool was developed using a complex mathematical model of buffer processes called VSFMOD (Vegetative Filter Strip Model). It computes runoff loads of water and sediment from agricultural fields and their deposition and infiltration within buffers. Using the model, trapping efficiencies for sediment and water were estimated for a range of buffer widths and different combinations of slope, soil texture, field C-factor, and field length that are common in agricultural fields. Other site conditions were held constant (see table below). For more information, refer to Dosskey et al. (2008).
What are the limitations of this tool?
This tool does not account for long-term sediment accumulation or long-term fate of dissolved pollutants. These limitations should remind users that the estimated trapping efficiencies are only rough estimates and may decrease over time. By reducing the number of site variables, the tool becomes simpler to use but less accurate than the full VSFMOD model.
Can other site factors be accounted for in the design tool?
Yes, any site condition that would double or halve the field runoff load should dictate an adjustment of one line below and one line above the initial reference line, respectively. To account for different size design storms, a 3.6 inch per hour and 1.5 inch per hour storm would roughly double or halve, respectively, the runoff load compared to the 2.4 inch per hour storm used to generate the reference lines.
Buffer Width Design Tool: Frequently Asked Questions
What about extremely narrow buffers less than 15 feet?
Narrow buffers less than 15 feet can be effective for sediment removal in some locations. These will be locations that closely resemble conditions for lines 5, 6, and 7 (relatively lower slopes, smaller runoff areas, and permeable soils).
What if the tool shows that buffers are not particularly effective for my site conditions?
In some cases, the buffer width estimated to achieve a desired level of trapping efficiency may exceed what a landowner is willing to set aside for a buffer. These situations call for alternative or additional conservation practices to reduce runoff load, a first step in enhancing the effectiveness any conservation buffer system (see below and 1.1).
What if runoff flow is not uniform?
Non-uniform flow into a buffer in effect increases the runoff load into the portion of the buffer that has contact with the flow, reducing the effectiveness of the buffer. Select a lower line to estimate buffer width for the effective area (see 1.7 and 1.8). Grass barriers can help in spreading out concentrated flow and increase the effective area (see 1.21).
What about trees and shrubs?
The Buffer Width Design Tool was developed for dense grass as the buffer vegetation type. Trees and shrubs can be a part of the buffer zone without changing its effectiveness as long as dense groundcover (plants and debris) is present to provide roughness and flow resistance (see 1.20).