13.5 BMP T1050 - Compost-Amended Vegetated Filter Strip (CAVFS)
CAVFS are a variation of the vegetated filter strip that includes soil amendments. The soil amendments improve infiltration characteristics, increase surface roughness, and improve plant sustainability.
CAVFS can be used to help ensure compliance with Minimum Requirement 5 - Onsite Stormwater Management and Minimum Requirement 6 - Stormwater Treatment. CAVFS provide basic and enhanced treatment.
13.5.3 Design Criteria
Comply with all criteria and standards in Modeling Your Best Management Practices, Design Criteria for All Stormwater Treatment and Flow Control BMPs, Constructing Your Best Management Practices and Accessing and Maintaining Your Best Management Practices as applicable to the project in addition to criteria within each BMP. Where criteria or standards conflict, utilize the criteria and standards contained within the specific BMP.
See Figure 4 - 19: Compost Amended Vegetated Filter Strip (CAVFS).
CAVFS shall only receive sheet flow.
Use curb cuts ≥ 12 inch wide and 1 inch above the filter strip inlet.
CAVFS shall be designed to treat 91% of the influent runoff file through the soil profile. Use the CAVFS element of an Ecology approved continuous simulation model, assuming a 15-minute timestep, to ensure this requirement is met.
The following apply to the required compost-amended soil section:
Have an initial saturated hydraulic conductivity less than 12 inches/hour.
Have a minimum long-term hydraulic conductivity less than 1.0 inches/hour per ASTM D2434 (Standard Test Method for Permeability of Granular Soils) at 85% compaction per ASTM D1557 (Standard Test Method for Laboratory Compaction Characteristics of Soil Using Modified Effort).
Have a minimum organic contact of 5% by dry weight per ASTM Designation D2974 (Standard Test Method for Moisture, Ash, and Organic Matter of Peat and Other Organic Soils).
Have a clay content less than 5%.
Have a pH between 5.5 and 7.0.
Be uniform and free of stones, stumps, roots, or material larger than 2 inches.
The compost component shall comply with A900 - Compost.
The soil component shall be loamy sand (USDA Soil Textural Classification).
The final soil mixture should be tested prior to installation for fertility, micronutrient analysis and organic material content.
Choose one of the following methods in order to obtain a soil mix that meets the criteria above:
Presumptive Approach: Place and rototill 1.75 inches of composted material into 6.25 inches of soil to achieve a total amended depth of 9.5 inches, for a settled depth of 8 inches. Water or roll compact soil to 85% maximum. Plant the CAVFS.
Custom Approach: Place and rototill the calculated amount of composted material into a depth of soil needed to achieve 8 inches of settled soil at 5% organic content. Water or roll to compact soil to 85% maximum. Plant the CAVFS.
Generally, the required soil mix can be achieved with 60-65% loamy sand mixed with 25-30% compost or a 30% sandy loam, 30% coarse sand, 30% compost blend.
13.5.4 Compost Amended Vegetated Filter Strips for Minimum Requirement 5 - Onsite Stormwater Management
CAVFS may be used to help meet the mitigation requirements of Minimum Requirement 5 - Onsite Stormwater Management for both the List Approach and LID Performance Standard Approach for pedestrian and multi-use sidewalks, trails, and walkways. CAVFS are not an approved option for other surface types.
13.5.4.2 Infeasibility Criteria
The following infeasibility criteria describe conditions that make CAVFS infeasible when applying The List Approach for compliance with Minimum Requirement 5 - Onsite Stormwater Management.
The infeasibility criteria shall also be used to determine the appropriateness of installing CAVFS on a project site. Project proponents may be allowed to install CAVFS even if considered infeasible, unless otherwise stated within the specific infeasibility criteria, provided documentation is included that substantiates the design. Documentation may be required by a Washington State Licensed Professional Engineer or other appropriate professional depending on the project conditions.
If the design criteria cannot be met.
If the setback criteria cannot be met. Setback criteria are generally required by Tacoma Municipal Code, Uniform Building Code, the Tacoma Pierce County Health Department, or other state regulations. Where a conflict between setback requirements occurs, the City shall require compliance with the most stringent of the setback requirements from the various codes and regulations. The following are the minimum setback requirements required by this manual.
The CAVFS shall be at least 10 feet from any building structure and at least 5 feet from any other structure or property line. If necessary, setbacks shall be increased from the minimum 10 feet in order to maintain a 1H:1V side slope for future excavation and maintenance.
The CAVFS shall not be located in critical areas, critical area buffers, streams, or lakes.
The CAVFS shall be setback a minimum of 50 feet from the top of any slope greater than 15% and from geologically hazardous areas. The dispersion area may be sited within 50 feet of a slope greater than 15% and/or geologically hazardous area if a geotechnical analysis performed by a Washington State Licensed Professional Engineer or Washington State Licensed Professional Geologist determines that there will be no negative impacts to the slopes and/or geologically hazardous area caused by the location of the dispersion area.
Setbacks from Onsite Sewage Systems shall conform to WAC 246-272A-0210.
If there are any conflicts with any of the following competing needs criteria:
Requirements of the following federal or state laws, rules, and standards:
Historic Preservation Laws and Archaeology Laws as listed at: https://dahp.wa.gov/project-review/preservation-laws
Federal Superfund or Washington State Model Toxics Control Act
Federal Aviation Administration requirements for airports
American with Disabilities Act
When found to be in conflict with special zoning district design criteria adopted and being implemented pursuant to a community planning process.
Public health and safety standards
Transportation regulations to maintain the option for future expansion or multi-modal use of public rights-of-way.
City of Tacoma Critical Area Ordinances that provides protection of tree species or other critical areas.
If the use of the CAVFS is likely to create flooding or erosion problems. A report from a Washington State Licensed Professional Engineer or Washington State Licensed Professional Geologist with appropriate expertise must be included if citing flooding or erosion concerns as the infeasibility criteria.
Where the sidewalk, walkway, or trail exceeds a lateral slope of 5% or a longitudinal slope of 8%.
The following infeasibility criteria are based upon subsurface characteristics and require a soils report to determine infeasibility. See Appendix B - Appendix B Soils Reports for Soils Report Requirements.
Field testing indicates potential CAVFS locations have a measured (initial) native soil saturated hydraulic conductivity less than 0.3 inches/hour.
CAVFS installed for the purposes of Minimum Requirement #5 shall only be constructed where infiltration is feasible.
If the measured soil infiltration rate is less than 0.3 inches/hour, CAVFS cannot be used to meet The List Approach or Low Impact Development (LID) Performance Standard.
Where the minimum vertical separation between the lowest elevation of the CAVFS and the seasonal high groundwater elevation, bedrock, or other impermeable layer is 1 foot or less.
13.5.4.3 Subsurface Characterization
A soils report is required to design a CAVFS. A soils report is also required if citing that a CAVFS is not feasible due to subsurface characteristics. See Appendix B - Appendix B Soils Reports requirements.
13.5.4.4 Sizing Criteria for Meeting Minimum Requirement 5 - Onsite Stormwater Management – The List Approach
For sidewalks, walkways, and trails less than 10 feet and where the measured soil infiltration rate is 0.3 inches/hour or greater:
Sidewalks, walkways, and trails greater than 10 feet or where the measured soil infiltration rate is less than 0.3 inches/hour:
Size the CAVFS utilizing the Sizing Criteria for meeting Minimum Requirement 5 - Onsite Stormwater Management – Low Impact Development (LID) Performance Standard.
13.5.4.5 Sizing Criteria for Meeting Minimum Requirement 5 - Onsite Stormwater Management – Low Impact Development (LID) Performance Standard
Utilize the CAVFS Element of an Ecology approved continuous simulation model, assuming a 15-minute timestep, to ensure the LID Performance Standard is met.
13.5.4.6 Additional Design Criteria when Utilizing CAVFS for Minimum Requirement 5 - Onsite Stormwater Management
In addition to meeting the design criteria in Section 13.5.3 - Design Criteria, the following criteria apply to CAVFS that will be used to meet Minimum Requirement 5 - Onsite Stormwater Management:
The sidewalk, walkway, or trail shall have a lateral slope greater than 1% and less than 5%.
The sidewalk, walkway, or trail shall have a longitudinal slope less than 8%.
The CAVFS shall have a minimum slope of 2%.
The CAVFS shall have a maximum slope of 25%.
Note: A report from a Washington State Licensed Professional Engineer or Washington State Licensed Professional Geologist is required for slopes over 15%.
Figure 4 - 19: Compost Amended Vegetated Filter Strip (CAVFS)