The purpose of this chapter is to provide guidance to applicants that are interested in considering Natural Drainage System (NDS) designs as part of their development project. NDS designs can improve community livability and creek health and be accomplished consistent with public safety, utility functions, transportation uses and other environmental priorities for Seattle’s street rights-of-way.

NDS design is an alternative approach to a typical curb and gutter street improvement with underground drainage and detention systems. An NDS uses swales, landscaping and permeable pavements to accomplish the following:

• reduce the amount of impermeable surface in the street right-of-way;
• filter pollutants from surface water through soil and plants; and,
• slows the flow of water to improve habitat for fish and other wildlife in Seattle’s urban creeks.

When you live in a creek watershed, the rain that falls in your neighborhood flows through pipes and ditches to the creek as stormwater. In Seattle, creeks are fragile ecosystems, home to salmon and other wildlife. At one time stormwater infiltrated the soils of meadows and wetlands, and collected on the surface of trees and vegetation. It was released slowly throughout the year to streams, lakes and estuaries.

With urban growth the trees and vegetation have been replaced with impermeable or disturbed surfaces such as rooftops, roadways, and construction sites. Consequently stormwater flows across these surfaces collecting pollutants generated by our urban activities, such as landscaping, transportation, and business. Instead of reaching creeks slowly, the water flows quickly and in great volumes to Seattle 's creeks. This causes erosion of stream banks, increased flooding, and degradation of water quality and aquatic habitat.

Seattle has implemented a pilot NDS Program as one tool to mitigate these impacts. The program includes City-funded capital projects that retrofit residential streets in creek watersheds to improve stormwater management and provide other neighborhood amenities such as landscaping. Get more information on the City’s Natural Drainage System Capital Program.

Based on the success of these capital projects, Seattle Department of Transportation and Seattle Public Utilities have compiled the design guidance in this chapter to assist developers with projects in creek watersheds that want to include NDS techniques as part of their projects.

Residential access streets in creek watersheds: 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.

Locations with existing informal drainage: Seattle’s creek watersheds [ 500 kb ] 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. Refer to Figure 6-9: City of Seattle Creek Basin [ 500 kb ].

Areas with existing ditches: The Natural Drainage System (NDS) guidance in this chapter may be used to implement street improvements in locations that have a ditch and culvert system. Since 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, NDS options are encouraged as an alternative to ditch filling. Exceptions to the ditch filling moratorium policy include culvert installations that are necessary to implement driveway permits, required street improvements, City-approved street improvements, or City capital improvements.

The SDOT Director may approve a requested exception per the Street Use Code (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.

The design guidance in this chapter typically applies to street improvements using Natural Drainage System (NDS) elements, and may need to be modified to address local conditions.

The design guidance presented below should be considered once the applicant and the City have jointly determined that a NDS approach is appropriate for the project. Keep in mind that NDS projects need to be designed in a manner which ensures protection of the public’s safety and addresses the mobility needs of the traveling public. If an NDS approach is being considered, the applicant is encouraged to participate in an early design guidance meeting with City staff to discuss possible site constraints, transportation needs, soil conditions, design and accessibility issues, and maintenance responsibilities.

Note: 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.

There are a few guiding principles to consider when designing an 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 can be made gentler by adding “steps.”

Amend soil and add vegetation to reduce stormwater quantity:

• Amend 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 Standard Specification 7-21 for more information.
• Add 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.

NDS may be accomplished as a street improvement or in conjunction with a construction permit. Not all NDS options are appropriate for all project types. Figure 6-10: Natural Drainage System Menu of Options [ 535 kb ] identifies appropriate NDS options based on the permit type.

1. Swales

Swales replace traditional planting strips with linear rain gardens. Swales can vary in depth from six inches to 24 inches and can be planted with grass, perennials, shrubs and small trees. Consider the following guidance for swale design:

2. Soils and Planting

The success of a natural drainage system has much to do with building healthy soils that can readily absorb surface water, and selecting appropriate plants. Consider the following guidance for soil and plant selection:

3. Sidewalks

If sidewalks are required for your development permit, consider using alternative materials such as permeable pavement or unit pavers.

4. Driveways

The location and design of driveways has a significant influence on the success of an NDS. Consider the following design guidance for driveways located within the street right-of-way when a NDS is being used:

 5. Parking

The location and design of on-street parking has a significant influence on the success of a natural drainage system. Consider the following design guidance on parking space size, location and materials:

The design of the adjacent roadway must meet minimum City of Seattle design criteria. Consider the following design guidance for altering or adding roadways in creek watersheds:

7. Permeable Paving

Permeable pavements are surfaces that allow water to pass through voids in the paving material and/or between paving units while providing a stable, load-bearing surface. Permeable pavements reduce impermeable surfaces and can be used to achieve City of Seattle water quality requirements and credit toward flow control requirements.

An important component to permeable pavements is the reservoir base course, which provides stability for load-bearing surfaces and underground storage for runoff. There are three main categories of permeable pavements that meet Seattle ’s permeable pavement standards:

Permeable concrete or permeable asphalt pavement: Is similar to standard pavement, but the fine material (sand and finer) is reduced or eliminated in the mix. As a result, channels form between the aggregate in the pavement surface and allow water to infiltrate.

Plastic grid systems: Come in rolls that are covered with soil and grass or gravel. The grid sections interlock and are pinned in place.

Interlocking pavers: Include cast-in-place or modular pre-cast blocks. The cast-in-place systems are reinforced concrete made with reusable forms. Pre-cast systems are either high-strength Portland cement concrete or plastic blocks. Both systems have wide joints or openings that can be filled with soil and grass or gravel.

Refer to Figure 6-11: Permeable Pavement Comparison Guide [ 5 MB ] for more information on these materials.

Permeable pavements—use within the street right-of-way

At this time, permeable pavements are limited to non-street surfaces, such as sidewalks, driveways and parking pads. The following table outlines which materials are allowed for each street right-of-way component.

Permeable pavements—pathways only

If you are interested in using NDS guidance for street improvements, the following steps may be required in addition to the standard permit applications process:

• Apply for a permit at the SDOT Street Use Counter.

• Site plan drawn to scale (1 inch=20 feet scale suggested) that must include the area in the street right-of-way between parcel boundaries and 3 feet beyond, and from the street right-of-way boundary to the centerline of the road.
• 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).

• SPU, SDOT Traffic Operations and Street Use sections will review the application, and conduct a site visit if necessary prior to approving the plan.
• If approved, the applicant must pay the permit fee and pick up the completed permit at the Street Use counter.
• Following the completion of construction, a final site inspection will be conducted by a City of Seattle inspector.

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.

Trimming, thinning and pruning: Once plants are established and thriving, periodic trimming, thinning and pruning of plantings and trees may be necessary to ensure that 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.

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.

Utility cut repair: Utility cuts should be backfilled with the same aggregate base used under the permeable paving to allow continued conveyance of stormwater through the base, and to prevent migration of fines from the standard base aggregate to the more open graded permeable base material. Small utility cuts can be repaired with conventional asphalt or concrete if small batches of permeable material are not available or are too expensive. Replace broken pavers or grid segments where three or more adjacent rings are broken or damaged.

Replenish aggregate: Replenish paver aggregate material in surface 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 maintain public safety.

These maintenance responsibilities are critical to the successful functioning of an NDS and will be included as a project permit condition.