Rain gardens are non-engineered, shallow, landscaped depressions with compost-amended soils and adapted plants. Rain gardens temporarily store stormwater from adjacent areas. Stormwater passes through the amended soil profile and into the native soil beneath. Stormwater that exceeds the rain garden is designed to overflow to an adjacent stormwater system.
This BMP can be utilized to ensure compliance with Minimum Requirement 5 - Onsite Stormwater Management.
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.
The following infeasibility criteria describe conditions that make rain gardens 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 rain gardens on a project site. Project proponents may be allowed to install a rain garden 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 upon the project conditions.
Setback distances are measured from the bottom edge of the rain garden soil mix.
A site characterization study must be completed in order to determine if the following infeasibility criteria apply and to determine the appropriateness of installing a rain garden.
The following infeasibility criteria are based on conditions such as topography and distances to predetermined boundaries. Citation of the following do not need site-specific written recommendations from a Washington State Licensed Professional Engineer or Washington State Licensed Professional Geologist though some criteria may require professional services to determine if the infeasibility criteria apply.
Within 10 feet of a building structure.
Within 5 feet of any other structure or property line.
Within 50 feet from the top of any slope greater than 20% and geologically hazardous areas. The rain garden may be sited within 50 feet of a slope greater than 20% 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 rain garden.
Where land for the rain garden is within an area designated as an erosion hazard or landslide hazard.
Within 100 feet of an underground storage tank and connecting underground pipes when the capacity of the tank and pipe system is greater than 1100 gallons.
Within 10 feet of an underground storage tank and connecting underground pipes when the capacity of the tank and pipe system is 1100 gallons or less. (As used in this criteria, an underground storage tank means any tank used to store petroleum products, chemicals, or liquid hazardous wastes of which 10% or more of the storage volume (including volume in the connecting piping system) is beneath the ground surface.)
Within 100 feet of a closed or active landfill.
Within 100 feet of a drinking water well, or a spring used for drinking water supply.
Within 10 feet of small on-site sewage disposal drainfield, including reserve areas, and greywater reuse systems. For setbacks from a “large on-site sewage disposal system”, see WAC Chapter 246-272B.
Where the site cannot be reasonably designed to locate rain garden on slopes less than 8%.
Where they are not compatible with the surrounding stormwater system (e.g., surfaces drain to an existing stormwater system whose elevation precludes proper connection to the bioretention facility).
For properties with known soil or groundwater contamination:
Within 100 feet of an area known to have deep soil contamination;
Where groundwater modeling indicates infiltration will likely increase or change the direction of the migration of pollutants in the groundwater.
Wherever surface soils can be found to be contaminated unless those soils are removed within 10 horizontal feet from the infiltration area;
Any area where these facilities are prohibited by an approved cleanup plan under the state Model Toxics Control Act or Federal Superfund Law, or an environmental covenant under Chapter 64.70 RCW.
Where rain gardens are constructed with imported compost material within ¼ mile of the rain garden. The restriction does not apply to construction within ¼ mile of Wapato Lake.
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.
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.
Where the minimum vertical separation between the lowest elevation of the rain garden soil mix and the seasonal high groundwater elevation, bedrock or other impermeable layer is 1 foot or less.
Where the field testing indicates potential rain garden locations have a measured (initial) native soil saturated hydraulic conductivity less than 0.3 inches/hour.
Rain gardens installed for the purposes of Minimum Requirement #5: Onsite Stormwater Management shall only be constructed where infiltration is feasible.
If the measured soil infiltration rate is less than 0.30 inches/hour, rain gardens cannot be used to meet The List Approach.
The following infeasibility criteria require evaluation of site specific conditions and a written recommendation from an appropriate Washington State Licensed Professional (e.g., Professional Engineer, Professional Geologist, Professional Hydrogeologist)
Where the only area available for siting would threaten the safety or reliability of preexisting underground utilities, preexisting underground storage tanks, preexisting structures, or preexisting road or parking lot surfaces.
Where the only area available for siting does not allow for a safe overflow pathway to the City stormwater system or a private stormwater system.
Where professional geotechnical evaluation recommends infiltration not be used due to reasonable concerns about erosion, slope failure, or down gradient flooding.
Within an area whose groundwater drains into an erosion hazard or landslide hazard area.
Where infiltrating water would threaten existing below grade basements.
Where infiltrating water would threaten shoreline structures such as bulkheads.
Where there is lack of usable space onsite for rain gardens at redevelopment sites.
For work within the public right of way, for public road projects, where there is insufficient space.
11.1.4 Subsurface Characterization
A soils report is required to design a rain garden. A soils report is also required if citing that a rain garden is not feasible due to subsurface characteristics. See - Appendix B Soils Reports for Soils Report requirements.
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.
Project design shall consider the following for siting the rain garden.
Utility conflicts: See Volume 5 for utility separation requirements.
Transportation safety: The design configuration and selected plant types should provide adequate sight distances, clear zones, and appropriate setbacks for roadway applications in accordance with City of Tacoma Design Standards.
Impacts of surrounding activities: Human activity influences the location of the facility in the development. For example, locate rain gardens away from traveled areas on individual lots to prevent soil compaction and damage to vegetation or provide elevated or bermed pathways in areas where foot traffic is inevitable. Provide barriers, such as wheel stops, to restrict vehicle access in roadside applications.
Visual buffering: Rain gardens can be used to buffer structures from roads, enhance privacy among residences, and for an aesthetic site feature.
11.1.5.2 Flow Entrance/Presettling
Use one of the following types of flow entrances (other alternatives may be considered on a case-by-case basis):
Dispersed across a landscaped area.
Dispersed through an open swale with plants and decorative rock. For slopes greater than 2%, add rock check dams every 5 to 10 feet to slow water flow.
Place a rock pad where stormwater enters the rain garden from a swale or pipe. It is recommended to use washed round rock that is a minimum of 2 inches in diameter. Rock pad should be 4”thick and 2 feet wide and extend 2 feet.
Avoid the use of angular rock or quarry spalls as sediment removal is difficult.
Per Joint Administrative Policy and Procedure Directive No. 2021-02-001, Environmental Services/Site Development Group may require flow dispersion for rain gardens depending upon type of inlet design and size of the facility.
Do not place plants directly in the entrance flowpath as they can restrict or concentrate flows.
Install flow diversion and erosion control measures to protect the rain garden from sedimentation until the upstream area is stabilized.
The ponding depth shall be 6", minimum and 18” maximum.
The minimum freeboard measured from the maximum ponding water surface elevation to the top of the facility shall be 2” for contributing areas less than 1,000 square feet and 6” for contributing areas 1,000 square feet or greater.
If berming is used to achieve the minimum top elevation, maximum slope on berm shall be 2H:1V, and minimum top width of design berm shall be 1 foot. Berm shall be a material which is water tight. Imported soil may be necessary to ensure berm does not fail. Berm shall be tightly packed during construction.
Provide an overflow pathway lined with a 4” thick washed rock pad. Washed rock shall be a minimum of 2 inches in diameter. Extend overflow 4 feet past rain garden edge.
Overflow shall not be directed to structures, neighboring properties, or over sidewalks.
Overflow shall not cause damage to downstream properties or receiving waters.
Minimum depth of rain garden mix shall be 12 inches.
The compost component shall meet the specifications in A900 - Compost.
Obtain a rain garden soil mix by one of the following methods.
Method 1: Excavate and replace existing soil with a rain garden soil mix.
Use this method if existing soils are poor quality.
If clay content is greater than 5%.
In gravel soils as plant growth will be inhibited.
A rain garden soil mix typically contains about 60% sand content and 40% compost by volume.
Method 2: Excavate and amend existing soil
Use 1/3 compost to every 2/3 of existing soil.
Use this method when existing soils are moderately good to good quality soils.
Do not use this method if clay content is greater than 5%. Use Method 1.
Use this method if the existing soils are good quality.
Do not use this method if the clay content is greater than 5%. Use Method 1.
Use this option only if the infiltration rate is 1 inch per hour or greater.
Amend soil by excavating to the ponding depth plus 3 inches. Spread 3 inches of compost and till to depth of 4 to 5 inches to fully incorporate compost component.
Do not use underdrains in rain gardens.
Submit a planting plan showing the type (species), location and size of each plant.
Plants must be tolerant of summer drought, ponding fluctuation, and saturated soil conditions.
Rain gardens have three planting zones. Zone 1 is bottom area of the rain garden, which is frequently wet during the rainy season. Zone 2 includes the side slopes, which occasionally become wet during rain events. Zone 3 includes the area around the perimeter of the rain garden, starting above the top surface of ponding elevation, which has drier soil.
See A1000 - Vegetation for plants appropriate for rain gardens. For rain gardens that will be publically maintained, plants must be chosen from the list and approved by Environmental Services/Site Development Group in accordance with Joint Administrative Policy and Procedure Directive No. 2021-02-001. Additional information on planting zones and appropriate plants for each zone can be obtained through the Rain Garden Handbook for Western Washington, available at https://extension.wsu.edu/raingarden/homeowner-resources/.
In general, the following guidelines should be used when considering which plants to use:
At least 50% (by quantity) should be evergreen plants. Leaf fall can reduce the function of the facility.
Do not leave large areas of the soil unplanted/uncovered. Exposed soil can cause erosion and reduce the function of the facility. Plant types can be overlapped (tree canopy can overlap shrubs and groundcover) to reduce the area of exposed soil.
Provide a variety of plant types with various rooting structures. Plant variety encourages good soil health. Typical plant types used in rain garden construction include:
Emergents: Rushes, grasses, and sedges have shallow, fibrous roots that remain close to the soil surface. Planting solely with shallow rooted plants can cause thick root mats to form decreasing the infiltrative ability of the soil.
Woody shrubs and trees: Woody shrubs and trees have a mixture of shallow fibrous roots and deeper structural roots that can penetrate deeper into the soil and increase soil porosity over time.
Groundcover: Groundcover tends to form dense masses of vegetation low to the ground surface. Low, dense vegetation can be effective at consolidating soil in areas otherwise prone to erosion such as the side slopes of the facility (Zone 2).
Herbaceous perennials: Herbaceous perennials do not have woody plant parts and the above ground growth typically dies back in the winter.
Roots must not damage underground infrastructure.
Consider adjacent plant communities and avoid potential invasive species.
Consider aesthetics, rain gardens should blend into surrounding landscapes.
Irrigation may be required until plants are fully established and in the summer months.
Rain garden facilities should be designed with a mulch layer. Properly selected mulch material reduces weed establishment, regulates soil temperatures and moisture, and adds organic matter to the soil.
Mulch should be free of weed seeds, soil, roots, and other material that is not trunk or branch wood and bark. Mulch shall not include grass clippings, mineral aggregate, pure bark, or beauty bark. Mulch should be coarse mulch.
Wood chip mulch composed of shredded or chipped hardwood or softwood, depth 2-3 inches. Additional rain garden depth will be needed to ensure appropriate ponding and freeboard.
A dense groundcover can be used as an alternative to mulch although mulch should be used until the dense groundcover is established.
The top of the ponded surface area below the overflow shall be at least 5% of the total area draining to it.
The maximum side slopes allowed shall be 2:1.
The following table provides the minimum top of ponded surface area, minimum bottom pond area, and minimum top of berm area based upon side slope and contributing area. The applicant shall determine the geometry of their rain garden based upon contributing square footage, minimum sizing criteria and preferred side slope. The applicant shall submit the proposed ponding area, bottom ponding area and top of berm area to Environmental Services/Site Development Group for review.
11.1.6 General Construction Criteria
Do not install media or excavate rain garden during soil saturation periods.
Excavation and soil placement should be done from equipment operating adjacent to the facility – no heavy equipment should be operated in the facility.
If equipment must be operated within the facility for excavation, use lightweight, low ground pressure equipment and scarify the base to reduce compaction upon completion. Do not use equipment on top of rain garden soil mix.
Do not use fully excavated rain garden for erosion and sediment control during construction.
Clogged soil and silt shall be removed during excavation to finished bottom grade prior to installing rain garden soil mix.
Scarify sides and bottom to roughen where equipment may have compacted soil.
Ensure the rain garden is protected from erosion and sedimentation until all contributory areas are fully stabilized.
If sedimentation occurs within the rain garden, excavate the area a minimum of 12 inches below final grade to remove sediment and replace media, mulch, and plants as necessary.
No flow credits are allowed for rain gardens and therefore rain gardens should not be included as BMPs that mitigate for stormwater for flow control purposes in the model. Areas contributing to the rain garden and the rain garden itself shall be modeled as impervious surface area.