9.1 BMP L614 - Full Dispersion
This BMP allows for "fully dispersing" runoff from impervious surfaces and cleared areas of project sites into areas preserved as forest, native vegetation, or cleared area.
This BMP can be used to ensure compliance with Minimum Requirement 5 - Onsite Stormwater Management, Minimum Requirement 6 - Stormwater Treatment and/or Minimum Requirement 7 - Flow Control. Sites that can fully disperse are not required to provide additional stormwater treatment or flow control.
The area of the project site that is using full dispersion must be sited to allow stormwater from the impervious (or cleared) surface to fully disperse into the preserved dispersion area.
The dispersion area may be a previously cleared area that has been replanted in accordance with native vegetation landscape specifications described within this BMP.
Situate the dispersion area to minimize the clearing of existing forest cover, to maximize the preservation of wetlands, and to buffer stream corridors.
A covenant and easement agreement or separate recorded tract of land will be required to protect the dispersion area.
Show the dispersion area on all maps and plans and clearly mark the area during clearing and construction.
Retain all trees within the dispersion area (except as allowed in the bullet below for passive recreation), aside from approved timber harvest activities regulated under WAC Title 222, except for Class IV General Forest Practices that are conversions from timberland to other uses, and the removal of dangerous or diseased trees.
The dispersion area may be used for passive recreation with related recreational facilities, including pedestrian and bicycle trails, nature viewing area, fishing and camping areas, and other similar activities that do not require permanent structures, provided that cleared areas and areas of compacted soil associated with these areas and facilities do not exceed eight percent of the preserved area.
The dispersion area may contain utilities and utility easements, but not on-site sewage disposal systems such as septic tanks or septic fields.
The dispersion of stormwater must not create flooding or erosion impacts.
The BMP is infeasible if any of the following criteria are met:
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 setbacks required by this manual.
The flow spreader and dispersion area 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 dispersion area shall not be located in critical areas, critical area buffers, streams, or lakes.
The dispersion area 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 Onstie 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 dispersion of stormwater 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.
9.1.4 Elements of a Full Dispersion System
The area of the project site that applies full dispersion consists of the following elements:
An impervious (or cleared area). This is the area that requires mitigation.
A flow spreader. Stormwater from the impervious (or cleared) area may need to be routed through a flow spreader depending on the layout and type of impervious surface, as further described below.
A dispersion area. This area defines the limits of the Full Dispersion BMP. The impervious or cleared area must disperse into the preserved dispersion area.
The dispersion area must be forest, native vegetation, or a cleared area.
The dispersion area is not allowed in critical area buffers unless approved through a Critical Area Permit.
The dispersion area 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 caused by the location of the dispersion area.
A flowpath through the dispersion area. The length of the flowpath from the impervious (or cleared) area through the dispersion area varies based on the project layout and type of impervious surface. Regardless of the layout and type of impervious surface, the flowpath must meet the following criteria:
The slope of the flowpath must be less than 15% for any 20' reach of the flowpath. Slopes up to 20% are allowed where flow spreaders are located upstream of the dispersion area and at sites where vegetation can be established.
The flowpaths from adjacent flow spreaders must be sufficiently spaced to prevent overlap of flows in the flowpath areas.
9.1.5 Design Criteria for Residential Projects
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.
9.1.5.1 Full Dispersion for Impervious Surfaces in Residential Projects
Impervious surfaces within residential projects may be "fully dispersed" if they are within a TDA that is less than 10% impervious. If the TDA has more than 10% impervious area, the design may still fully disperse up to 10% of the TDA's area. The impervious area beyond the 10% cannot drain to the dispersion area and those areas are subject to Minimum Requirement 6 - Stormwater Treatment and Minimum Requirement 7 - Flow Control (if thresholds are met) and the areas must be considered when determining if those requirements apply to a project.
The lawn and landscaped areas adjacent to the impervious area being mitigated (for example a lawn associated with a single family home) may be dispersed into the dispersion area. Those lawn and landscaped areas must comply with BMP L613 - Post-Construction Soil Quality and Depth.
The dispersion area must be preserved as forest or native vegetation.
The dispersion area shall have a minimum area 6.5 times the area of the impervious surface draining to it.
The flowpath from the impervious surface through the area preserved as forest or native vegetation must be at least 100 feet in length.
The slope of the flowpath must be less than 15% for any 20' reach of the flowpath. Slopes up to 20% are allowed where flow spreaders are located upstream of the dispersion area and at sites where vegetation can be established.
The flowpaths from adjacent flow spreaders must be sufficiently spaced to prevent overlap of flows in the flowpath areas.
The following additional guidance must be followed dependent on the type of impervious surface in the residential project.
Full dispersion from roof surfaces: Stormwater from roof surfaces must either:
Provide dispersion BMPs per BMP L603 - Downspout Dispersion prior to stormwater entering the dispersion area.
Combine the roof runoff with the road runoff and follow the guidance for full dispersion from roadway surfaces below.
Full dispersion from driveway surfaces: Stormwater from driveway surfaces must either:
Provide dispersion BMPs per BMP L611 - Concentrated Flow Dispersion and BMP L612 - Sheet Flow Dispersion prior to stormwater entering the dispersion area.
Combine the driveway runoff with the road runoff and follow the guidance for full dispersion from roadway surfaces below.
Full dispersion from roadway surfaces: Stormwater from roadway surfaces shall comply with all the following requirements:
Design the road section to minimize collection and concentration of roadway runoff. Use sheet flow over roadway fill slopes (i.e., where roadway subgrade is above adjacent right-of-way) wherever possible to avoid concentration.
When it is necessary to collect and concentrate runoff from the roadway and adjacent upstream areas (e.g., in a ditch on a cut slope), concentrated flows shall be incrementally discharged from the ditch via cross culverts or at the ends of cut sections. These incremental discharges of newly concentrated flows shall not exceed 0.5 cfs at any one discharge location from a ditch for the 100-year return period flowrate. Where flows at a particular ditch discharge location were already concentrated under existing site conditions (e.g., in a natural channel that crosses the roadway alignment), the 0.5 cfs limit would be in addition to the existing concentrated peak flows.
Ditch discharge locations with up to 0.2 cfs discharge for the 100-year return period flowrate shall use rock pads or dispersion trenches to disperse flows. Ditch discharge locations with between 0.2 and 0.5 cfs discharge for the 100-year return period flowrate shall use only dispersion trenches to disperse flows. See A400 - Outfall Systems for design criteria.
Be designed to accept surface flows (free discharge) from a pipe, culvert, or ditch end.
Be aligned perpendicular to the flowpath.
Have a minimum 2 foot by 2 foot cross section.
Be filled with ¾” to 1.5” washed rock.
Have a level notched grade board.
Have a minimum spacing of 50 feet between centerlines of other dispersion trenches.
Manifolds may be used to split flows up to 2 cfs for the 100-year peak flow between a maximum of 4 trenches.
9.1.5.2 Full Dispersion for Cleared Areas in Residential Projects
Stormwater from cleared areas of residential projects that are comprised of bare soil, non-native landscaping, lawn, and/or pasture is fully dispersed if it meets all of the following criteria:
Cleared areas must comply with BMP L613 - Post-Construction Soil Quality and Depth.
The dispersion area must be preserved as forest or native vegetation.
The flowpath through the cleared area (and leading to the dispersion area) must be 25 feet or less.
The minimum flowpath from the cleared area through the dispersion areas must be 25 feet.
If the cleared area is greater than 25 feet:
Add an additional foot of flowpath length for every three feet of width area (up to a maximum width of 250 feet).
The slope of the flowpath must be less than 15% for any 20' reach of the flowpath. Slopes up to 20% are allowed where flow spreaders are located upstream of the dispersion area and at sites where vegetation can be established.
The flowpaths from adjacent flow spreaders must be sufficiently spaced to prevent overlap of flows in the flowpath areas.
The topography of the cleared area must be such that stormwater will not concentrate prior to discharge to the dispersion area.
The width of the dispersion area must equal the width of the cleared area. The maximum width of the cleared area is 250 feet.
9.1.6 Design Criteria for Road-Related Projects
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.
9.1.6.1 Full Dispersion by Sheet Flow from Uncollected, Unconcentrated Stormwater into the Dispersion Area
The stormwater shall be considered fully dispersed if it meets all of the following criteria.
The minimum flowpath lengths shall be based upon the native soil type of the dispersion area as follows:
Outwash Soils (Type A- Sand and Sandy Gravels and Some Type B – Loamy Sands)
Must have an initial saturated hydraulic conductivity rate of 4 inches per hour or greater. The saturated hydraulic conductivity must be based on an in-situ infiltration method as identified in Appendix A - Appendix A Determining the Design Infiltration Rate.
Provide a 10 foot vegetated flowpath for up to 20 feet of impervious surface flowpath. Provide an additional 0.25 foot of vegetated flowpath for every additional 1 foot of impervious flowpath.
Other Soils (Type C and D and Type B not meeting the criteria above)
Provide a 6.5 foot vegetated flowpath for every 1 foot of flowpath length across the impervious surface.
The minimum flowpath length through the dispersion area shall be 100 feet.
The dispersion area must be legally preserved as forest or native vegetation.
The slope of the flowpath must be less than 15% for any 20' reach of the flowpath. Slopes up to 20% are allowed where flow spreaders are located upstream of the dispersion area and at sites where vegetation can be established.
The flowpaths from adjacent flow spreaders must be sufficiently spaced to prevent overlap of flows in the flowpath areas.
The minimum depth to the average annual maximum groundwater depth shall be 3 feet.
The flowpath through any impervious surface leading to the dispersion area shall be less than 75 feet.
The flowpath through any pervious area leading to the dispersion area shall be less than 150 feet. Pervious surface flowpaths include those upgradient road side slopes that run onto the road and down gradient road side slopes that precede the dispersion area.
Lateral slope of impervious contributing area shall be less than 8%.
Road side slopes shall be less than 25%. Road side slopes do not count as part of the dispersion area unless native vegetation is reestablished and slopes are less than 15%.
Road shoulders that are paved or graveled to withstand occasional vehicle loading are considered impervious surfaces.
Longitudinal slope of road shall be less than 5%.
The width of the dispersion area should be equivalent to the width of impervious surface sheet flowing into it.
Average longitudinal (parallel to road) slope of dispersion area shall be less than 15%.
Average lateral slope of dispersion area shall be less than 15%.
9.1.6.2 Full Dispersion of Channelized (Collected and Re-dispersed) Stormwater into the Dispersion Area
The stormwater from road-related projects that is collected and re-dispersed is fully dispersed if it meet all of the following criteria.
The minimum flowpath length shall be based upon the native soil type of the dispersion area as follows:
Outwash Soils (Type A- Sand and Sandy Gravels and Some Type B. Loamy Sands)
Must have an initial saturated hydraulic conductivity rate of 4 inches per hour or greater. The saturated hydraulic conductivity must be based on an in-situ infiltration method as identified in Appendix A - Appendix A Determining the Design Infiltration Rate.
Provide a dispersion area of at least ½ of the impervious contributing area.
Other Soils (Type C and D and Type B not meeting the criteria above)
Provide a 6.5 foot vegetated flowpath for every 1 foot of flowpath length across the impervious surface. The minimum flowpath length through the dispersion area shall be 100 feet.
The minimum depth to the average annual maximum groundwater depth shall be 3 feet.
Channelized flow shall be redispersed to produce the longest possible flowpath.
Flows shall be evenly dispersed across the vegetated flowpath.
Ditch discharge locations with up to 0.2 cfs discharge for the 100-year return period flowrate shall use rock pads or dispersion trenches to disperse flows. Ditch discharge locations with between 0.2 and 0.5 cfs discharge for the 100-year return period flowrate shall use only dispersion trenches to disperse flows. See A400 - Outfall Systems for design criteria.
Be designed to accept surface flows (free discharge) from a pipe, culvert, or ditch end.
Be aligned perpendicular to the flowpath.
Have a minimum 2 foot by 2 foot cross section.
Be filled with ¾" to 1.5" washed rock.
Have a level notched grade board.
Have a minimum spacing of 50 feet between centerlines of other dispersion trenches.
Manifolds may be used to split flows up to 2 cfs for the 100-year peak flow between a maximum of 4 trenches.
The impervious surface area is limited to onsite (associated with roads only) flows.
The width of the dispersion area shall be equivalent to the length of the road from which stormwater is collected.
Average longitudinal and lateral slopes of the dispersion areas shall be less than or equal to 8%.
The slopes of any flowpath segment shall be less than 15% for any 20-foot reach of the flowpath segment.
The dispersion area must be legally preserved as forest or native vegetation.
The slope of the flowpath must less than 15% for any 20' reach of the flowpath. Slopes up to 20% are allowed where flow spreaders are located upstream of the dispersion area and at sites where vegetation can be established.
The flowpaths from adjacent flow spreaders must be sufficiently spaced to prevent overlap of flows in the flowpath areas.
9.1.6.3 Full Dispersion by Engineered Dispersion
Stormwater from road-related projects is fully dispersed if it meets all of the following criteria:
The minimum flowpath lengths shall be based upon the native soil type of the dispersion area as follows.
Outwash Soils (Type A- Sand and Sandy Gravels and Some Type B . Loamy Sands)
Must have an initial saturated hydraulic conductivity rate of 4 inches per hour or greater. The saturated hydraulic conductivity must be based on an in-situ infiltration method as identified in Appendix A - Appendix A Determining the Design Infiltration Rate.
Soils must be compost amended in accordance with BMP L613 - Post-Construction Soil Quality and Depth.
Provide a 10 foot vegetated flowpath for up to 20 feet of impervious surface flowpath. Provide an additional 0.25 foot of vegetated flowpath for every additional 1 foot of impervious flowpath.
Other Soils (Type C and D and Type B not meeting the criteria above)
Soils must be compost amended in accordance with BMP L613 - Post-Construction Soil Quality and Depth.
The dispersion area must be 6.5 times the area of the surface(s) draining to it.
The minimum depth to the average annual maximum groundwater depth shall be 3 feet.
Flows shall be evenly dispersed across the vegetated flowpath.
Average longitudinal and lateral slopes of the dispersion areas shall be less than or equal to 15%.
The dispersion area shall be planted with native trees and shrubs where possible.
A covenant and easement agreement or separate recorded tract of land will be required to protect the dispersion area.
9.1.7 Native Vegetation Landscape Specifications
The following design criteria may be used in areas where previously developed surfaces are being converted to a native vegetation area for purposes of meeting full dispersion requirements.
Conversion of a developed surface to native vegetation landscape requires the removal of impervious surface, decompaction of soils, and the planting of native trees, shrubs, and groundcover in compost-amended soil. The following criteria must be met:
Existing impervious surface and any underlying base course shall be completely removed from the conversion area(s).
Underlying soil shall be scarified to a depth of 18 inches.
A minimum of 4 inches of well-decomposed compost shall be tilled into the scarified soils. The finished surface should be gently undulating and lightly compacted.
Plant the area of native vegetation with native species trees, shrubs, and groundcover. Select species appropriate for the site's shade and moisture conditions and in accordance with the following requirements:
Trees: Plant a minimum of two tree species, one species shall be an evergreen. Space trees at the recommendation of a professional landscaper or according to other City of Tacoma landscape codes as applicable. Where frequent watering is not practical, bare-root stock may be substituted at variable spacing from 10 to 12 feet on center. Bare root stock types shall be 1-1, 2-1, P-1 and P-2. Live stakes at 4 feet on center may be substituted for willow and red-osier dogwood in wet areas.
Shrubs: Plant a minimum of two species of shrubs. Space plants to cover landscaped area, except where trees are present. Where frequent watering is not practical, bareroot stock may be substituted at variable spacing from 4 to 6 feet on center. Bare root stock types shall be 1-1, 2-1, P-1 and P-2.
Groundcover: Plant a minimum of two species of ground cover. Space plants to cover all remaining bare soils.
Increase the number of native species if possible based upon native vegetation area size.
Place at least 4 inches of mulch between plants for weed control.
Water plantings as necessary to ensure proper plant growth.
A minimum of 90% plant survival is required after 3 years.
Conversion of an area under cultivation to native vegetation requires elimination of non-native cultivated plants, grasses, and weeds before planting and continuously during native vegetation establishment. The following criteria must be met:
Till the soil to a minimum depth of 18 inches.
Be a minimum of 8 inches having an organic content of 6-12%, OR
Place a 4 inch layer of compost on the surface before planting, OR
Till 4 inches of clean wood chips into the soil (only upon recommendation of a landscape architect or forester)
Plant the area of native vegetation with native species trees, shrubs, and ground cover. Select species appropriate for the sites shade and moisture conditions and in accordance with the following requirements:
Trees: Plant a minimum of two tree species, one species shall be an evergreen. Space trees at the recommendation of a landscape professional or according to other City of Tacoma landscape codes as applicable. Where frequent watering is not practical, bare-root stock may be substituted at variable spacing from 10 to 12 feet on center. Bare root stock types shall be 1-1, 2-1, P-1 and P-2. Live stakes at 4 feet on center may be substituted for willow and red-osier dogwood in wet areas.
Shrubs: Plant a minimum of two species of shrubs. Space plants to cover landscaped area, except where trees are present. Where frequent watering is not practical, bareroot stock may be substituted at variable spacing from 4 to 6 feet on center. Bare root stock types shall be 1-1, 2-1, P-1 and P-2.
Groundcover: Plant a minimum of two species of ground cover. Space plants to cover all remaining bare soils.
Increase the number of native species if possible based upon native vegetation area size.
Place at least 4 inches of mulch or compost between plants for weed control.
Water plantings as necessary to ensure proper plant growth.
A minimum of 90% plant survival is required after 3 years
9.1.8 Full Dispersion Modeling
Sites that can fully disperse are not required to provide stormwater treatment or flow control facilities. No modeling is necessary.