9.3 BMP L611 - Concentrated Flow Dispersion
Dispersion of concentrated flows from driveways or other pavement through a vegetated pervious area attenuates peak flows by slowing stormwater.
Any situation where concentrated flow can be dispersed through vegetation.
This BMP can be utilized to ensure compliance with Minimum Requirement 5 - Onsite Stormwater Management.
The BMP can be used solely to mitigate for other hard surfaces if utilizing the List Approach or may be used in combination with other BMPs to satisfy the LID Performance Standard.
This BMP may also be used to help ensure compliance with Minimum Requirement 7 - Flow Control and Minimum Requirement 8 - Wetlands Protection.
Where compliance with Minimum Requirements is not required, where feasible, this BMP is recommended to be used to help protect receiving waterbodies from the effects of stormwater.
9.3.3 Infeasibility Criteria for Concentrated Flow Dispersion
The following infeasibility criteria describe conditions that make concentrated flow dispersion infeasible when applying The List Approach for compliance with Minimum Requirement 5 - Onsite Stormwater Management. If any of the infeasibility criteria or design criteria cannot be met, the BMP is considered infeasible.
The infeasibility criteria shall also be used to determine the appropriateness of installing concentrated flow dispersion on a project site. Project proponents may be allowed to install concentrated flow disperison 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.
Concentrated flow dispersion is considered infeasible if any of the following criteria are met:
If the design criteria below cannot be met.
The setback criteria below cannot be met. Setback criteria are generally required by the Tacoma Municipal Code, Uniform Building Code requirements, the Tacoma Pierce County Health Department, or other state regulations. Where a conflict between setback occurs, the City shall require compliance with the most stringent of the setback requirements from various codes/regulations. The following are the minimum setbacks required per this manual.
At least 10 feet from any building and at least 5 feet from any property line or structure. If necessary, setbacks shall be increased from the minimum 5 feet in order to maintain a 1:1 side slope for future excavation and maintenance.
A minimum of 50 feet from the top of any slope greater than 15% and from geologically hazardous areas. The dispersion system 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 areas caused by the dispersion system.
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 erosion or flooding of downstream properties might occur as the result of using concentrated flow dispersion. A report prepared by a Washington State Licensed Professional Engineer or Washington State Licensed Professional Geologist with supporting documentation is needed to substantiate this infeasibility criteria.
9.3.4 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.
• Figure 4 - 11: Concentrated Flow Driveway Dispersion (also called Green Stormwater Infrastructure - Figure 009 - available at https://www.cityoftacoma.org/government/city_departments/public_works/engineering/standard_plans_and_g_i_s_typical_details) shows design criteria for concentrated flow dispersion from driveways.
A maximum of 700 square feet of impervious area may drain to each concentrated flow dispersion BMP.
Maintain a minimum vegetated flowpath of 25 feet between the discharge location and any property line, structure, slope greater than 15%, surface water, or other hard surface.
If the vegetated flowpath is 25-50 feet, a dispersion trench must be used prior to discharge over the vegetated flowpath.
If the vegetated flowpath is 50 feet or more, a dispersion trench or a rock pad may be used prior to discharge over the vegetated flowpath.
Asphalt or concrete berms used to concentrate flow shall be 2-4” in height and 6” in width.
Trench drains shall be modular slotted drain units with a minimum width of 4 inches.
Place a dispersion trench per BMP L603a - Downspout Dispersion Trenches or a pad of clean crushed rock or 4" cobbles per WSDOT Standard Specification 9-03.11(2) - Streambed Cobbles that is 2 feet wide by 3 feet long by 6 inches deep at each discharge location.
9.3.5 Concentrated Flow Dispersion Modeling
If the runoff is dispersed according to the requirements of this section into an undisturbed native landscape area or an area amended to meet BMP L613 - Post-Construction Soil Quality and Depth, model the connected area using the lateral flow element to send the impervious area onto the lawn/landscaped area that will be used for dispersion.
Where multiple concentrated flow dispersion BMPs will be used for a project, to allow for the model schematic to be manageable, the following modeling options may be used:
When a pad of crushed rock or dispersion trenches are used per the guidance above, and the length of the vegetated flowpath is at least 50 feet, the impervious area may be modeled as a landscaped area.
When dispersion trenches are used and the length of vegetated flowpath is 25-50 feet, the impervious area may be modeled as 50% landscaped and 50% impervious.
Figure 4 - 11: Concentrated Flow Driveway Dispersion