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Chapter 6
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Streetscape Design Guidelines
6.4 Green Stormwater Infrastructure

6.4.1 Green Stormwater Infrastructure (GSI) in the Right-of-Way
6.4.2 Bioretention Design Guidance
6.4.3 Permeable Pavement Design Guidance

  6.4.4 Compost Amended soils Design Guidance
6.4.5 Approval and Permit Procedures
6.4.6 Maintenance Responsibilities
Figures for ROW IM 6.4
6.4 Overview

The purpose of this chapter is to provide guidance to applicants required or interested in Green Stormwater Infrastructure (GSI) and/or Natural Drainage System (NDS) designs as part of their right-of-way improvement project. The project may incorporate such elements as bioretention or permeable pavement as a result of Stormwater Code compliance, Seattle Green Factor requirements, or simply to improve the environment around them. The information is intended to help lower the barriers for implementation of Green Stormwater Infrastructure approaches in the street right-of-way.

Seattle Stormwater and Drainage Control Code requirements, including information on the requirement to use Green Stormwater Infrastructure to the Maximum Extent Feasible, provided in Ch. 4.17.  
6.4.1 Green Stormwater Infrastructure (GSI) in the Right-of-Way

GSI can be used as part of partial street improvements or full right-of-way retrofit.  Partial street improvements usually include improvements to sidewalk and planting strip areas. Full right-of-way improvements are less common and involve sidewalks, planting strips and full roadway width reconstruction.

It is sometimes challenging to construct surface stormwater elements while meeting the multiple other needs within the City’s rights-of-way, including addressing public safety and public mobility needs. The information provided in this section is intended to provide applicants with designs that have undergone review from the various City departments involved in street improvement permitting. All sites are different so the project’s engineer may need to modify the details provided to address local conditions.

If GSI approach is being considered, the must obtain 60% Complete SIP approval through an SDOT design guidance meeting with City staff to discuss possible site constraints, transportation needs, soil conditions, design and accessibility issues, and maintenance responsibilities.


6.4.1a  GSI as part of Partial Street Improvements

GSI constructed as part of partial street improvements include bioretention in the planting strip area and permeable pavement sidewalks.  After determining the approach appropriate for the given site, see the design sections below for bioretention (Ch. 6.4.2) and permeable pavement (Ch. 6.4.3).  Setbacks for compost amended soils are provided in Ch. 4.17.2  

6.4.1b  GSI as part of Full Right-of-Way Reconstruction

Implementation of large scale GSI techniques as part of full right-of-way reconstruction is sometimes used by Seattle Public Utilities (SPU) or desired by applicants for the aesthetic benefits in combination with the stormwater function and traffic calming.  SPU’s projects where full rights-of-ways were reconfigured to achieve stormwater retrofit goals are called Natural Drainage Systems; these projects are described in detail in SPUs green infrastructure website.

Low volume streets: The primary function of residential access streets is to provide access to neighborhood land uses and connections to higher level traffic streets, such as arterials. Residential access streets typically have lower traffic volumes, lower speeds and lower volumes of trucks and buses than arterial streets.  At this time the City will only consider full street right-of-way NDS concepts on residential streets and low volume collector streets.

Sufficient Right of Way width: The right-of-way width must be at least 56 feet.

Adjacent Land Use: Due to the competing space needs for high density areas, NDS designs are encouraged only in Single Family or LR1 zoning areas.

Locations with existing informal drainage: Seattle’s creek watersheds are largely served by informal drainage (e.g., ditch and culvert systems) or no formal system at all.  Full right-of-way NDS improvements provide an opportunity to enhance the existing informal system. The City encourages the use of NDS as a means of providing transportation and pedestrian improvement to these areas as a way to protect our receiving water bodies.

If the above criteria are met and your project is considering proposing a full right-of-way NDS design the applicant is encouraged to participate in an SDOT design guidance meeting.  Figures 6-11 through 6-13 contain NDS conceptual design details the City has used on previous projects to assist in the design and review process.

Figure 6.10 Evaluation of NDS Full Street Concept as a part of Full Street Improvements, is available to help determine if NDS Full Street Concept is feasible for a particular street.
6.4.2 Bioretention Design Guidance

Bioretention cell designs may vary based on design goals and site conditions.  Generally all the cells include: surface grading, and soil and plant complexes to manage stormwater.  Factors influencing the design include: native soils, longitudinal and cross slopes, presence or absence of curbs, and space availability.  Design information on the various types of cells is provided in the Chapter 4 of the Stormwater Manual.  The information provided here is specific to placing bioretention within the street ROW.  Additional guidance is provided for surface grading features commonly used in combination with bioretention.  To aid the designer, the City has compiled the following Bioretention details:

Bioretention, general applications

Typical details for site reconfiguration with Bioretention

The above details are generic in nature.  They must be modified to match your site.  CADD drawings of these files are available at the SPU GSI website.

Additional design guidance for bioretention systems is provided below:

  • Bioretention plants for systems in the right-of-way installed for stormwater code compliance or Seattle Green Factor requirements shall use plants identified in the Seattle Green Factor list.
  • Provide for sediment pre-settling if drainage area exceeds 2,000 SF. 
  • Longitudinal Slope: Grade elevation through the length of the cell should be flat or gradual (<2% slope). 
  • Soils: The soils for use in bioretention systems should be designed for Stormwater Code compliance using the City's Bioretention Soil Specification. If designing for 2-inch ponding then the underlying soil may be amended with compost per Volume 3 of Stormwater Code Director’s Rule rather than importing bioretention soil
Additional drainage design guidance is provided on Seattle Public Utilities website.  Information includes minimum bioretention area requirements for bioretention being installed for stormwater compliance and design phase reviewers checklist.
6.4.3 Permeable Pavements as part of Partial or Full Street Improvements

Permeable pavement is a paving system which allows the rainfall to percolate into an underlying soil or aggregate storage reservoir, where stormwater is stored and infiltrated to underlying subgrade, or removed by an overflow drainage system.  Permeable pavements can be used to achieve City of Seattle Stormwater Code Flow Control and GSI to the MEF Core Requirements.

At this time, permeable pavements are limited to non-street surfaces, such as sidewalks, and planting strips.

Permeable pavement systems can either be designed as permeable pavement facilities or permeable pavement surfaces.  Permeable facilities are designed to infiltrate surface water run-on for up to three times the square footage of the permeable pavement area and have a thicker aggregate discharge subbase layer for increased water storage.  Permeable pavement surfaces are designed only for the rainfall that falls directly on the permeable pavement; with no run-on.  Permeable pavement systems for stormwater code compliance can be designed for 100 percent impervious area credit or 50 percent impervious area credit depending on the type of facility and longitudinal slope of the proposed installation location.

Refer to Stormwater Manual Vol. 3, Stormwater Flow Control and Water Quality Treatment Technical Requirements Manual for detailed stormwater calculations and design guidance for permeable pavement facilities and permeable pavement surfaces.  The Stormwater Manual should be used in conjunction with this section of the ROWIM and the Standard Specification and Plans for the design of permeable paving within the right of way.  With respect to structural and maintenance concerns, any permeable pavement system proposed for use in the street ROW must be on listed as accepted in the Permeable Pavement CAM 2215.

The following are categories of permeable pavement systems that can be incorporated in the street ROW, as a walking surface:

  • Permeable Cement Concrete: The permeable cement concrete mixture omits the fines to create stable air pockets encased within it.  Depending upon the mix design, permeable cement concrete can have a rougher surface than conventional cement.  Construction of Permeable Cement Concrete shall follow the requirements for Permeable Pavement Construction per Section 5-06 of the Standard Specifications. 

The following are categories of permeable pavement systems that can be incorporated in the street ROW, outside the limits of the designated walking surface, i.e. in the planting strip:

  • Permeable Asphalt Concrete: Permeable asphalt concrete is open-graded asphalt with reduced fines and stable air pockets encased within it that allow water to drain to the base below. Aggregate binders and additives can be added to increase durability. Like conventional concrete it is laid with traditional asphalt paving equipment.
  • Permeable Cement Concrete: Permeable cement concrete is similar to permeable asphalt concrete in that the mixture omits the fines to create stable air pockets encased within it. Depending upon the mix design, permeable cement concrete can have a rougher surface than conventional cement.
  • Interlocking Concrete Pavers: Interlocking concrete paver blocks themselves are not always permeable, but they are typically installed with gaps between them to allow stormwater to infiltrate into the subsurface. The gaps, typically 10 percent of the surface area, are filled with a permeable material, usually small clean stone.
  • Open-Celled Paving Grid with Vegetation: Open-celled paving grids consist of a rigid grid composed of concrete or a durable plastic that is filled with a mix of sand, gravel, and topsoil for planting vegetation. The cells can be planted with a variety of grasses or low-growing groundcovers.  The support base and the ring walls prevent soil compaction and reduce rutting and erosion by supporting the weight of traffic and concentrated loads.
  • Open-Celled Paving Grid with Gravel: The same open-celled grid structure is employed but the voids in the rings are filled with a mix of gravel.

To aid the designer, the City has compiled the following Permeable Pavement Design details for project designers to evaluate, modify and incorporate into their Street Improvement Plans:

  • Permeable Pavement Sidewalk (Figure 6-23)
  • Permeable Pavement Facility, Sidewalk (Figure 6-24)
  • Permeable Pavement Facility, Sidewalk, Check Dam, Interceptor (Figure 6-25)
  • Permeable Pavement Facility in Planting Strip (Figure 6-26)
6.4.4 Compost / Amended Soils

See ROW IM Ch. 4.17

6.4.5 Approval and Permit Procedures

If you are interested in using bioretention or permeable pavement within the street right-of-way, you must get a Street Improvement Permit

6.4.6 Maintenance Responsibilities

As with other types of street improvements, adjacent property owners are responsible for maintaining sidewalks, driveways and parking pads as well as landscaping in the street right-of-way.   However, the City also has a maintenance role in bioretention and permeable pavement installed in the right-of-way for stormwater code compliance which is described below.  The following sections provide general maintenance guidelines specific to bioretention landscaping and permeable pavements.

6.4.6a Maintenance for Bioretention  

In bioretention cells healthy plants and soils break down pollutants through natural processes. Maintaining healthy plant and soil communities is a critical part of the system functioning.

Establishment of plantings takes approximately 3 years.

Year 1: Plants are working very hard below the ground to develop new roots. Appropriate soil moisture will make the difference between success and failure during the first year. Plants need watering, a minimum of once per week for shady areas and twice per week for sunny areas, throughout the first summer. Pruning should be limited to the removal of damaged limbs, since plants and trees need maximum foliage to generate energy to develop new roots.

Year 2: Plants will begin to put on new growth and continue to develop root systems. Soil moisture is less critical than during the first year, so watering can be done less frequently. Weeding will be necessary. Pruning is still discouraged, except to remove damaged or dead limbs.

Year 3 and beyond: Successfully established plantings will flourish in the third year. Weeding will continue to be necessary but as the planted areas mature the abundance of weeds decrease.  Periodic trimming, thinning and pruning of plantings and trees will be necessary to ensure that the sidewalk or the swale edge is not completely obscured. This is particularly important on narrow rights-of-way so that pedestrians, bicyclists and drivers are aware of the change in elevation between the roadway and the swale. 

Natural lawn and garden care only:  Pesticide use is not allowed in the City‘s drainage system including bioretention systems.  Refer to the City’s Natural Lawn and Garden Care website for tips regarding smart water and pesticide use.

Plant replacement during establishment:  Adequate plant coverage is necessary to guard against soil erosion. Ideally the original planting will include a wide selection of species spaced to provide more than adequate coverage.  If there is adequate coverage of the swale soils, not all plants that fail to thrive will need to be replaced.  If patches of bare soil emerge, plantings should be replaced.  If groups of plants are lost, a different species may need to be considered.

Sediment Removal:  Minimizing sediment accumulation in the bioretention system is critical to allowing the infiltration of stormwater through the bioretention soil.  Sediment is likely to accumulate where concentrated flows enter the bioretention cells; removing the sediment accumulated at those locations is an easy way to help ensure the long term success of the system.  If sediment accumulates within the bioretention cells; that debris should be hand removed.

Maintenance of the mulch layer: is important for both moisture retention and weed control.  Spring and Fall are excellent times to mulch and prune trees and shrubs where needed.

The City’s Role:  Once bioretention installed for stormwater code compliance within the right-of-way is accepted by the City, SPU will supplement the maintenance after the first year of plant establishment.  Bioretention design requirements for stormwater code compliance are documented in the Stormwater Manual.  Additionally, the minimum size of a bioretention landscaped area for stormwater code compliance is 500-square feet.  The applicant is solely responsible for the first year of plant establishment and the required plant establishment reporting refer to Ch. 4.17.2 for plant establishment requirements.  Systems that are accepted by are entered into SPU’s long term maintenance program.  SPU maintenance focuses on function, not aesthetics and includes removal of noxious weeds and rehabilitating the system should the ponded area hold water for more than 72-hours past the end of a rain event.  Systems that are not installed for stormwater code compliance, as well as code facilities not passing City inspection and therefore not taken over for maintenance by the City are solely the maintenance responsibility of the adjacent property owner.  Maintenance responsibilities are defined in Appendix D of the Stormwater Code Technical Manual Volume Three
6.4.6b Maintenance for All Permeable Pavements

Best Management Practices for the on-going maintenance of permeable pavement can be found in the Appendix D of the Stormwater Code Technical Manual Volume Three.  If permeable pavement installation meets the requirements set forth in CAM 2215, and installations passes SDOT acceptance, SDOT will assume maintenance responsibilities.

The City’s Role:  Once permeable pavement installed for stormwater code compliance within the right-of-way is accepted by the City, SDOT will accept all maintenance responsibilities.  Permeable pavement design requirements for stormwater code compliance are documented in the Stormwater Manual.  Additionally, SDOT requires that a minimum area of permeable pavement be met prior to accepting ownership and maintenance.  Refer to CAM 2215 for the acceptance thresholds.    Systems that are accepted by are entered into SDOT’s long term maintenance program.  Systems that are not installed for stormwater code compliance, as well as code facilities not passing City inspection and therefore not taken over for maintenance by the City are solely the maintenance responsibility of the adjacent property owner.  Maintenance responsibilities are defined in Appendix D of the Stormwater Code Technical Manual Volume Three
6.4.6c. Restoring Damages to GSI from Construction or Vehicular Damage

Bioretention of Permeable pavement impacted negatively due to construction activities, utility cuts, vehicular accidents, or damage from oil spills, fertilizers, or other harmful substances must be properly repaired to restore the systems.  Restoration of bioretention usually includes plant removal, bioretention soil excavation and replacement to original depth, mulch replacement, and replacement of landscaping to equal or better than the original design. 

continue to section 6.5»   
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Detailed Table of Contents
Chapter 6
Streetscape Design Guidelines
6.1 Street Design Concept Plans
6.2 Green Streets
6.3 Neighborhood Based Plans
6.4 Green Stormwater Infrastructure
6.5 Traffic Calming
6.6 Contact Information
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