The purpose of this chapter is to provide guidance to applicants that are 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 swales or permeable pavement as a result of Stormwater Code compliance, Green Factor area requirements, or simply to improve the environment around them. The Information is intended to help encourage the use, and lower the barriers for implementation of Green Stormwater Infrastructure approaches in the street right-of-way.

Natural Drainage Systems (NDS) are street rights of way designed to use planted swales adjacent to sidewalk or roadway pavement to do the work of pipes, by capturing stormwater and letting it soak into the ground and/or be filtered by vegetation. NDS attempts to mimic the natural system that existed before development which has significantly increased runoff and its associated pollutants.

With urban growth, trees and vegetation have been replaced with impermeable or disturbed surfaces such as rooftops, roadways, and construction sites. Consequently, stormwater flows across these surfaces and collect pollutants generated by our urban activities, such as landscaping, transportation, and business. The increased stormwater flow and pollutants have the potential to negatively impact our environment.

The following concepts identify benefits associated with implementing NDS along the pavement edge instead of constructing traditional curb, gutter and storm drains:

• Stormwater flow control is provided by reducing impervious surfaces (using reduced roadway width and, permeable pavements) and bioretention and biofiltration swales which temporarily detain and infiltrate rainwater.
• In the bioretention and biofiltration swales, healthy plants and soils break down pollutants through natural processes. Water quality treatment is achieved through biofiltration from the plants and soils when they capture and break down pollutants flowing from the adjacent surfaces.
• Increased street-side landscaping beautifies and adds value to the neighborhood.
• Traffic calming from narrower and/or gradually curving roadways that still allow for sufficient emergency vehicle access.

Full Street Improvements

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. If the right-of-way is narrower than 56 feet, NDS Full Street concept may be feasible if project schedule can allow adequate time to develop design with City. Otherwise, consider Green Stormwater Infrastructure for sidewalk drainage only.

Adjacent Land Use: Due to the competing space needs for high density areas, NDS designs are encouraged only in Single Family or L1 zoning areas. NDS Full Street concept maybe feasible at different zoning density if project schedule can allow adequate time to develop design with City. Otherwise, consider Green Stormwater Infrastructure for sidewalk drainage only.

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) and NDS improvements provide an opportunity to enhance the existing informal system. The City encourages 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 street NDS design the applicant is encouraged to participate in an early design guidance meeting. To assist in the design review process, we recommend the applicant consider using one or more of the design concept s in the NDS conceptual design details (Figures 6-11 through 6-13) the City has used on previous projects.

Figure 6.10 . Evaluation of NDS Full Street Concept as a part of Full Street Improvements.

Sidewalk Improvements

The cumulative effective of new impervious area created by sidewalks and pathways across the City can have significant effects on our stormwater systems and receiving water bodies. Use of green stormwater infrastructure techniques such as bioretention and permeable pavements to the maximum extent feasible can help mitigate the impacts. See the Green Infrastructure BMP Flow Chart for Sidewalks (Figure 6-14) to help establish what alternatives are most suitable to your project site.

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.

There are a few guiding principles for the site engineer to consider when designing a Natural Drainage System (NDS) project in the street right-of-way:

Minimize impermeable surfaces : Avoid paving or compacting soils where it is not necessary. This could mean a narrower driveway, a smaller parking area, or use of alternative materials which reintroduce water back into the soil.

Gentle slopes to slow the flow : When stormwater moves slowly through a system more pollutants are filtered and more water infiltrates or evaporates. A pipe is designed to move water at top velocities. Re-grading a ditch with gentler side slopes and a moderately sloping, broad channel allows water to percolate into the soil. Channel bottoms slopes can be made gentler by adding “steps.” Generally sites with longitudinal slopes greater than 8 percent are unlikely candidates for bioretention swale elements.

Stormwater quantity can be reduced by amending soils and adding vegetation:

• Amended soils : Adding organic compost or mulch to soil improves its ability to support plants and absorb stormwater. Healthy soil is the backbone of natural drainage systems. Refer to the Stormwater Manual for more information on compost amended soils.
• Adding vegetation : Trees and vegetation catch rainfall before it reaches the ground reducing the amount of stormwater runoff. Native shrubs, perennials and small trees planted in natural drainage systems slow the movement of stormwater, encourage infiltration and provide bio-remediation of pollutants. The planting palette used on previous NDS project is provided on SPU’s NDS website.

Not all NDS options are appropriate for all project types. NDS options may be the only alternatives available for some ditches in creek watersheds (see Section 6.4.5c below). Minimum requirements defined in the Land Use Code, the design criteria in Chapter 4 Design Criteria of this Manual, and any applicable City of Seattle Standard Plans and Specifications apply to NDS improvements in the street right-of-way. Deviations from the design criteria presented in this Manual may be allowed in some cases, where minimum requirements can not be achieved due to site constraints.

Swale designs vary based on design goals and site conditions. Swales may include bioretention swales, biofiltration swales or conveyance swales. Generally all the swales 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 swales is provided in the Stormwater Manual. The information pro vided here is specific to placing swales within the street ROW. To aid the designer, the City has compiled the following Natural Drainage Swale details:

• Bioretention Swale (Figure 6-15)
• Bioretention Swale with Underdrain (Figure 6-16)
• Conveyance Swale (Figure 6-17)
• Curb Drain Cut Opening for Swale (Figure 6-18)
• Concrete Inlet, Channel and Grate (Figure 6-19)
• Log Weir (Figure 6-20)
• Curb Extensions (Figure 6-21, 6-21A and 6-21B)
• Tree Planting within Bioretention Swale (Figure 6-22)

Additional design guidance is provided below:

• Longitudinal Slope: Grade elevation through the length of the swale should be flat or gradual.
• Soils: The soils for use in bioretention systems should be designed for Stormwater Code compliance using the City's Bioretention Soil Specification and mus t be used upstream.
• Culverts/Pipes: Minimum 10-inch diameter culvert should be used with a minimum slope of 1.0%. A trash rack should be placed on upstream end of all culvert inlets receiving flow from 10-acres or greater. Use a minimum opening of 4-inches.

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 reduce impermeable surfaces and can be used to achieve City of Seattle water quality requirements and flow control requirements.

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

The surface layer of a permeable pavement system is the wearing course. Categories of wearing courses include:

• 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 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.

Any permeable pavement wearing course proposed for use in the street ROW must be on the Permeable Pavement Wearing Course Approved list for City ROW Applications (this is under development and will be provided in August 2008).

Permeable Cement Concrete, which has been approved for use in the street ROW, must use the Standard Specification for Permeable Pavement Construction (this is under development and will be provided in August 2008).

Permeable pavement systems for stormwater code compliance can be designed as a facility for 100 percent impervious area credit, or as a surface for 50 percent impervious area credit. Design information on these two types of permeable pavement systems are provided in the Stormwater Manual. The information provided here is specific to placing permeable pavement within the street ROW. 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)

The City does not permit the filling of a ditch if that ditch functions as part of the City’s informal drainage system in the street right-of-way and is located within a creek watershed. Creek watersheds are identified on the City GIS system, and on the Seattle Creek Watersheds map. The SDOT Director may approve a requested exception per the Street Use Code (SMC 15.04.112) if the Director determines that the modification is likely to be equally protective of public health, safety and welfare, the environment, or public and private property. If the proposed modification is not equally protective, the Director may approve a requested exception if substantial reasons are documented such as:

• An emergency situation necessitates approval of the exception;
• A reasonable use of the adjacent property is not possible unless the exception is approved; or harm or threat of harm to public health, safety and welfare, the environment, or public and private property is at risk unless the exception is approved.

If you are interested in using NDS for street improvements, you must get a Street Improvement Major permit:

• Apply for a permit at the SDOT Street Use Counter.
• Site plan that meets the Standard Drafting Requirements.
• Site plan must show the following features:
• Existing structures in the street right-of-way such as culvert invert elevations, edges of any pavement including roads and driveways, and locations of manholes, fire hydrants, light poles, water main and meters, significant trees, etc.
• Locations of new plant material, rocks, hardscaping, or other materials.
• Indicate flow direction in the swale.
• Cross-section drawn to scale (1 inch=5 feet or 1 inched=10 feet scale suggested) that shows maximum side slopes and maximum swale depth (as measured from the edge of the road).

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. The following sections provide general maintenance guidelines specific to natural drainage landscaping and permeable pavements.

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: Successfully established plantings will flourish in the third year. Expect larger shrubs to require some pruning for clearance along the sidewalk. Pruning should be limited to encourage the natural growth habit of the plant.

Natural lawn and garden care only: Pesticide use is not allowed in the City‘s drainage system including natural drainage 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.

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.

Permeable pavements and pavers require some additional maintenance to keep them functioning properly:

Inspect for surface material that may clog the pavement: Inspect the project upon completion to remove any fine material that has accumulated on the surface. Conduct periodic visual inspections to determine if surfaces are clogged with vegetation or fine soils. Clogged surfaces should be corrected immediately.

Periodic vacuum sweeping or pressure washing: Permeable concrete and permeable asphalt surfaces should be swept with a high-efficiency or vacuum sweeper twice per year, preferably, once in the autumn after leaf fall, and again in early spring. High pressure hosing could substitute for sweeping or supplement sweeping if material appears clogged. For gravel pave or unit pavers, replace gravel if clogging occurs.

Surface sealing is NOT allowed: Permeable paving or pavers will no longer function if the surface is sealed with a sealant application or other uncontrolled use.

Replenish aggregate: Replenish paver aggregate material as needed.

Maintain public safety: With NDS systems that have highly technical designs, it is critical to identify responsibility for utility providers, SPU and property owners on how the pavement or area will be maintained and re-established to assure public safety.