19 – Guide to Vegetated Filter Strips
Vegetated Filter Strips — Design & Analysis
Vegetated Filter Strips: Definition, Design, and BMPFast Analysis
BMPFast Training Series | Stormwater Nonpoint Source Pollutant Removal | Florida Department of Environmental Protection
Contents
1. VFS Definition & Purpose
Topic 1 · Source Slides: 1, 2, 3
A Vegetated Filter Strip (VFS) is a constructed, vegetated area positioned between a pollution source and a receiving water body. Unlike a naturally occurring riparian or vegetated buffer, a VFS is deliberately engineered to intercept and treat stormwater runoff carrying nonpoint source pollutants — primarily total nitrogen (TN) and total phosphorus (TP) — before that runoff enters surface water.
Core Definition
A VFS is a constructed vegetated area — not a natural buffer — designed to remove nonpoint source pollutants from stormwater runoff before discharge to a surface water body.
What Makes a VFS Distinct from a Natural Buffer?
Natural vegetated buffers exist along waterways and provide incidental water quality benefits. A VFS, by contrast, is a purposefully sited and sized engineered practice. Its performance can be calculated, credited, and permitted. This distinction matters for regulatory programs such as Environmental Resource Permits (ERP) and Basin Management Action Plans (BMAP).
Catchment Area Accounting
A fundamental rule governs VFS design: the contributing area (the land that generates runoff flowing into the VFS) plus the VFS area itself must together equal the total catchment area. This mass-balance requirement ensures that all runoff from the catchment is accounted for and routed correctly through the treatment system.
Area Accounting Rule
Catchment Area = Contributing Area + VFS Area
All land within the catchment boundary must be assigned to one of these two categories. Runoff from the contributing area flows into and through the VFS before reaching the receiving water body.
Pollutants Removed
- Total Nitrogen (TN) — via filtration, adsorption, and biological uptake in vegetated media
- Total Phosphorus (TP) — via adsorption in BAM (Biosorption Activated Media) layers and plant uptake
- Sediment and associated particulate pollutants — via settling and filtration as sheet flow passes through vegetation
Discharge Pathway
After treatment through the VFS, discharge is directed to a surface water body — not to groundwater via an underdrain. This distinguishes the VFS from bioretention cells with underdrains, which discharge to subsurface conveyance.
2. VFS Applications & Design
Topic 2 · Source Slides: 4, 5
Vegetated filter strips are applicable across a wide range of developed land uses where impervious or semi-impervious surfaces generate sheet flow. Their relatively simple construction and modest footprint make them suitable for retrofitting existing sites as well as incorporating into new development.
Typical Land Use Contexts
- Roads and rights-of-way — VFS placed parallel to roadway edge to capture pavement runoff
- Parking lots — VFS at lot perimeter intercepts sheet flow before it reaches curb and gutter systems
- Residential areas — VFS along property or subdivision edges treats lawn and driveway runoff
Slope Requirements
Slope Criterion
Minimum slope: 2% | Maximum slope: 20%
Slopes below 2% risk ponding and anaerobic conditions. Slopes above 20% generate excessive velocities that erode the filter media and reduce contact time, degrading pollutant removal efficiency.
Width Requirements
Width Criterion
Minimum width: 15 feet per filter strip | Maximum width: 30 feet (based on available design data)
Widths beyond 30 feet are not supported by the current BMPFast performance curves; designs should remain within this range to maintain model validity.
BAM Media for Nutrient Removal
Biosorption Activated Media (BAM) is incorporated into the VFS filter layer specifically to enhance phosphorus and nitrogen removal beyond what vegetation alone provides. BAM is a manufactured media engineered with high adsorptive capacity for dissolved nutrients, and it is recognized by Florida’s stormwater rules as a qualifying enhancement for credit purposes.
Drainage Configuration
No Underdrain
A VFS does not use an underdrain pipe. Treated water percolates laterally and vertically through the filter media and discharges to an adjacent ditch or swale, which then conveys flow to the receiving surface water body. This is a critical distinction from bioretention with underdrain.
3. Example Project Setup
Topic 3 · Source Slides: 6
The following worked example illustrates a complete VFS design for a small roadway site in the Tallahassee area. This example is used throughout the BMPFast training to demonstrate data entry, model configuration, and results interpretation.
Site Characteristics
| Parameter | Value | Notes |
|---|---|---|
| Land use | Roadway | Impervious pavement generating sheet flow |
| Location | Tallahassee area, Florida | North Florida rainfall regime |
| Total site area | 2.0 acres | Verified as 1,013 ft × 86 ft |
| Directly Connected Impervious Area (DCIA) | 70% | High impervious fraction typical of roadway corridor |
| Site slope | 4% | Within allowable 2%–20% range |
Media and Storage Properties
BAM Media Specification
Media depth: 1 foot | Storage capacity: 0.38 inch/inch (volumetric water content at capacity)
This means the BAM layer can hold 0.38 inches of water per inch of media depth, or 0.38 inches total across the 1-foot layer — used by BMPFast to calculate available storage volume.
VFS Hydrologic Parameters
- VFS storage capacity: 6.6 inches of runoff — the volume of stormwater the VFS can absorb before overflow occurs
- Curve Number (CN): 60 — reflects the vegetated, partially permeable surface of the VFS itself; used by BMPFast to model VFS runoff response
Dimensional Verification
Area Check
1,013 ft × 86 ft = 87,118 ft²
87,118 ÷ 43,560 ft²/acre = 2.0 acres ✓
Always verify that input dimensions produce the stated total area before proceeding with BMPFast data entry.
ERP Performance Standard
The Environmental Resource Permit (ERP) performance standard for this project requires:
- 55% TN removal — the VFS must remove at least 55% of the total nitrogen load generated by the contributing area
- 80% TP removal — the VFS must remove at least 80% of the total phosphorus load generated by the contributing area
BMPFast output will be compared against these standards to confirm the design meets permit requirements.
4. BMPFast Data Entry & Output
Topic 4 · Source Slides: 7, 8, 9
BMPFast is Florida’s stormwater BMP performance calculation tool used to estimate average annual pollutant mass removal for permitted stormwater practices. This section walks through the complete data entry workflow for the example VFS project and presents the resulting output values.
Data Entry Categories
BMPFast organizes input data into three main categories. All three must be completed before the model can generate output:
- Site & watershed data — project location, rainfall zone, land use type, total catchment area, and DCIA percentage
- Treatment data / physical dimensions — VFS dimensions (length, width, slope), BAM media depth, storage capacity, and curve number
- Configuration data — runoff routing method, BMP type selection (VFS with BAM), and any series/parallel BMP arrangements
Step-by-Step Data Entry Sequence
Select project location/rainfall zone. Enter total catchment area (2.0 acres) and DCIA (70%). Identify land use as roadway.
Input VFS length (1,013 ft), width (86 ft), and slope (4%). Enter BAM media depth (1 ft) and storage capacity (0.38 in/in). Set Curve Number to 60.
Select BMP type: Vegetated Filter Strip with BAM. Set routing to discharge to adjacent ditch (surface water pathway, no underdrain). Confirm no additional BMP in series.
Execute the calculation. Review TN and TP removal values in pounds per year. Compare against ERP performance standards.
Save the completed project as a .BMPT file for inclusion with permit applications, BMAP submittals, or future re-analysis.
Model Output Results
BMPFast Output — Example VFS Project
TN removal: 7.64 pounds per year | TP removal: 0.789 pounds per year
| Pollutant | BMPFast Removal (lbs/yr) | ERP Standard | Standard Met? |
|---|---|---|---|
| Total Nitrogen (TN) | 7.64 | 55% removal | Verify % against load calculation |
| Total Phosphorus (TP) | 0.789 | 80% removal | Verify % against load calculation |
Saving the Project File
File Format
Save the completed BMPFast analysis as a .BMPT project file. This binary file preserves all input parameters and output results. Attach the .BMPT file (or a printed PDF export) to ERP applications, BMAP credit documentation, and stormwater management reports.
5. Key Takeaways
Topic 5 · Source Slides: 10, 11
The following points summarize the essential concepts from this module. These are the core facts a practitioner must understand before designing, permitting, or crediting a vegetated filter strip in Florida.
Design and Function
Takeaway 1
A VFS removes pollutants from stormwater runoff before discharge to a surface water body. It is not a passive buffer — it is a constructed, engineered treatment practice with documented performance.
Takeaway 2
The catchment area must equal the contributing area plus the VFS area. This mass-balance check is mandatory before entering data into BMPFast. Failure to verify this will produce incorrect load and removal estimates.
BMPFast Analysis
Takeaway 3
BMPFast calculates average annual pollutant mass removal in pounds per year for TN and TP. These values are the standard currency for ERP permit compliance and BMAP load reduction credit reporting.
Takeaway 4
Results must be verified against ERP performance standards. For the example project: 55% TN removal and 80% TP removal. BMPFast output must meet or exceed these thresholds for the design to be approvable.
Permitting and Credit
Takeaway 5
VFS pollutant removal is assignable as credit under BMAP programs. The computed annual mass removal values (lbs TN/yr, lbs TP/yr) can be reported to the appropriate water management district as part of a required load reduction schedule.
Takeaway 6
Always save the BMPFast analysis as a .BMPT file. The saved project file provides a defensible, reproducible record of all design inputs and modeled performance. Include it with permit applications and BMAP reporting packages.
Summary Statement
A properly designed and modeled VFS with BAM media provides a permittable, creditable method of removing TN and TP from roadway and parking lot runoff. BMPFast ties the physical design to a defensible annual performance number that satisfies both ERP and BMAP program requirements.
Quick Reference — VFS Definition
- Constructed vegetated area between source and water body
- Removes TN and TP from nonpoint source runoff
- Discharge to surface water — no underdrain
- Catchment = Contributing Area + VFS Area
Quick Reference — Design Limits
- Slope: 2% minimum, 20% maximum
- Width: 15 ft minimum, 30 ft maximum
- Media: BAM mix for nutrient removal
- No underdrain; percolates to adjacent ditch
Quick Reference — Example Site
- 2.0-acre roadway, Tallahassee, FL
- DCIA 70%, slope 4%
- BAM 1 ft deep, 0.38 in/in storage
- VFS storage 6.6 in; CN = 60
- 1,013 ft × 86 ft = 2.0 acres ✓
Quick Reference — BMPFast Output
- TN removal: 7.64 lbs/year
- TP removal: 0.789 lbs/year
- ERP standard: 55% TN, 80% TP
- Save project as .BMPT file
Quick Reference — BMPFast Inputs
- Site & watershed data (area, DCIA, land use)
- Physical dimensions & media properties
- Configuration data (routing, BMP type)
- Verify area before entry
Quick Reference — Key Takeaways
- VFS removes pollutants before surface water discharge
- Area accounting is mandatory pre-entry check
- BMPFast = average annual lbs/yr removal
- Credit assignable to BMAP programs
- Save .BMPT for permit documentation
BMPFast Training Series · Florida DEP Stormwater Program