Seattle, Washington

Incorporating Water Quality Features into the Right-of-Way

Broadview Green Grid Roadside Gardens at 2nd Avenue NW (Source: Seattle Public Utilities)

The City of Seattle's Natural Drainage System (NDS) approach to stormwater retrofitting focuses on the use of highly visible, landscaped street edges to manage stormwater from road surfaces, as well as narrower streets and porous sidewalks that reduce impervious surface area (Seattle Public Utilities, 2007). The system is designed to improve subsurface infiltration of stormwater, allowing natural biological and chemical processes within the soil environment to remove pollutants. NDS is part of a broader approach to urban planning known as green infrastructure, which emphasizes more environmentally sound urban planning and development. In essence, green infrastructure involves design elements that employ natural or innovative processes or design schemes that serve as viable urban development alternatives. Several different NDS designs have been implemented in the City of Seattle, and all are considered effective from a stormwater management standpoint. In addition, all are economically advantageous when compared to traditional stormwater systems (Seattle Public Utilities, no date). NDS projects also meet the approval of public citizens who view the retrofits as amenities with great aesthetic appeal.


The Seattle Public Utilities' (SPU) NDS program, started in 1999, focuses on addressing a key water quality concern: stormwater runoff. Traditional stormwater management entails shunting rain water into an underground grid of pipes with direct outfalls to neighboring waterbodies. This system results in the conveyance of contaminated runoff, which pollutes waterways, scours stream banks, and causes other forms of erosion.

The NDS program is designed to mitigate the effects of stormwater pollution and erosion by implementating unconventional stormwater management structures and practices. The main goal of this innovative program is to increase stormwater retention time and slowing stormwater flow by increasing pervious surface area along street edges. Doing so allows the infiltration of stormwater into the soil, where a natural water purification processes take place. These installations also ensure slower recharge of local waterbodies via groundwater, reducing or eilminating harmful streamside erosion. SPU has projected that NDS installations are at least 25 percent less expensive than traditional stormwater systems due to decreased build and infrastructure maintenance costs.

SPU has implemented five projects to date, varying in size and scope, but all with the intended goal of protecting the integrity of local waterbodies. The projects use alterations to streets and street right-of-ways that decrease impervious surface area, slow stormwater flows, and increase infiltration.

Street Edge Alternatives (SEA Streets) Project—This was SPU's pilot study into the practicality and efficacy of the NDS program. A single residential block was retrofitted, reducing impervious surface area by 11 percent by constructing narrowed, curvilinear streets and moving parking areas to the street side. Vegetated swales and rain gardens were created to increase runoff detention time. Monitoring efforts by the SPU have demonstrated a 98 percent reduction of stormwater flow for a 2-year storm event.
SEA Street Project vegetated swales (Source: Seattle Public Utilities)
110th Cascade Project—Four blocks of a traditional ditch-and-culvert system waere replaced by a cascading pool design with shallow pools in series, which reduce flow velocity, retain stormwater, and remove pollutants. This design is ideal for this location as the street in question is steep and had high stormwater flow volumes. Construction was completed in December of 2002. Since that time, SPU has been monitoring the effectiveness of the system to evaluate its performance.
110th Cascade Project weir wall separating swale cells (Source: Seattle Public Utilities)
Broadview Green Grid Project—An NDS installation is being implemented on 15 city blocks to reduce the quantity, speed, and pollutant load of stormwater, from approximately 32 acres, to Piper's Creek and Puget Sound. The project entails the construction of vegetated swales, cascading pools, small wetland ponds, landscaped rain gardens, and a reduction of overall paved area. Construction began in summer of 2003 and will be completed in the next few years.
Broadview Green Grid Vegetated Swale (Source: Seattle Public Utilities)

Pinehurst Green Grid Project—Twelve city blocks were retrofitted to include new sidewalks, roadways, and extensive landscaping to enhance the visual appeal of the neighborhood and improve stormwater management to protect the integrity of Thornton Creek. Construction was completed in fall of 2006.

High Point Project—This is the largest NDS to be implemented in the City and will be constructed in a high density residential area that offers low-income housing in partnership with the Seattle Housing Authority. In order to treat 10 percent of the water comprising the Longfellow Creek watershed, vegetated swales will be used to retain and cleanse runoff, while secondary ponds will be constructed to handle stormwater overflow resulting from prolonged or high-intensity precipitation events.

NDS Components

Component Description Function
Vegetated Swales Vegetated swales are built roadside using specific soil and planting schemes with stormwater management in mind. Soils are designed and implemented to achieve adequate infiltration, while still maintaining integrity and strength to prevent slope failures. Soils must also be of a proper growth medium. Plantings generally consist of groundcover and shrub strata. Vegetated swales intercept stormwater, slowing flow velocity and improving water quality via retention and infiltration. The vegetation serves to not only stabilize the soil and prevent erosion, but it also functions as a physical flow-barrier and promotes evapotranspiration.
Narrower Streets Traditional street widths are reduced to reduce impervious surface. Narrower streets reduce the ammount of impervious surface (by up to 11 percent in some projects), while still allowing for two-way traffic and street-side parking. Narrower streets also require less paving.
Porous Sidewalks Sidewalks are made of a more porous concrete mixture than traditional sidewalks that are highly impervious. Porous sidewalks have increased pore space in the concrete mixture, which allows for stormwater infiltration and reduced runoff volume.
Cascading Pools A series of shallow pools linked by culverts that allow runoff to flow down a gradient from one pool to another. This design helps ensure proper infiltration in areas with moderately steep slopes and high stormwater volumes. The shallow pools also reduce stormwater velocity as water is retained by each subsequent pool. In general, cascading pool designs incorporate extensive vegetation.
Curvilinear Street Design Instead of using a traditional straight street design, roads are built in a curvilinear fashion. Curvilinear roads help to ensure that runoff leaves the road surface more rapidly and before gaining velocity, which reduces erosion. This road design can be especially useful in areas with steeper gradients and can slow traffic.
Rain Gardens Rain gardens are beds of vegetation planted near roads, parking lots, and other impervious areas with a large propensity for run-off. Rain gardens function similarly to vegetated swales, but without a centralized ditch to pool water. They are effective at slowing runoff and infiltrating the first flush of runoff—they are not designed to retain high stormwater volumes.

Drivers for Green Approach

SEA Street Project curvilinear 2nd Avenue (Source: Seattle Public Utilities)

SPU recognized that that the normalization of hydrologic conditions to pre-development levels is a key component of stream restoration efforts and concluded that infrastructure-based solutions alone would not restore aquatic habitat. This understanding, in conjunction with increasingly stringent water quality regulations (which require the monitoring of pollutants in stormwater) led to SPU's decision to begin implementing "greener" stormwater management projects.

Addressing the quality of stormwater and its impact on local waterbodies was a major component of SPU's push towards NDSs. In addition to improving the water quality of stormwater by removing contaminants, NDSs also enable greater groundwater recharge and significantly reduce the likelihood of flash flooding. SPU expects that the performance of NDSs will improve over time as plants mature and soil is stabilized, leading to an increase in overall filtration and retention.

Factors for Success

Plants growing on green walls as part of the 110th Cascade Project (Source: Seattle Public Utilities)

SPU's NDS program has received much praise and has generated worldwide attention from a variety of audiences. SPU strove to create a presentable package that met the interests of policy makers, regulators, and public citizens. The program did face numerous challenges, however, especially from the city's emergency and transportation departments who questioned the system's safety, integrity, and applicability. To address these concerns, SPU worked with emergency and transportation personnel to ensure that new road designs met their needs. Also, risks such as standing water and slope failure were addressed with design features and careful site selection.

In many ways, the NDS projects speak for themselves as they afford measurable advantages over traditional stormwater systems. By relying on natural ecological processes to retain and cleanse runoff, NDSs cost less to install than traditional engineered stormwater infrastructure. The systems also mitigate pollution to surficial waterbodies, reducing environmental control costs in the long run. Offering substantial aesthetic improvements over their traditional grey infrastructure counterparts, NDSs also generate broad public appeal and may even increase the property value of retrofitted neighborhoods.

Members of the local community were involved in all stages of NDS planning and construction, and public outreach programs were initiated to educate citizens on the importance and benefits of the project. SPU emphasized environmental and cost savings, and the attractive designs, which incorporated plants and other landscape features, helped ensure that the program met public approval. SPU plans to continue enlisting the help of homeowners in the maintenance of road-side vegetation to reduce the City's maintenance costs. Continued success of the program relies on monitoring efforts to determine the effectiveness and long-term viability of the installations.

Additional Information

    The Natural Drainage Systems Program Website This website was developed by the Seattle Public Utilities to provide an overview of NDS as well as specifically highlighting successful NDS projects.

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    "City Gets Creative with Storm Water: Grass Swales Narrower Streets Help Reduce Runoff" This Seattle Post-Intelligencer article by Lisa Stiffler discusses the City of Seattle's NDS effort.

    Harvard Report on Conservation Innovation (PDF, 502K) Fall 2004.
    Report contains an article by James N. Levitt and Lydia K. Bergen entitled "Using Nature's Plumbing to Restore Aquatic Ecosystems: The City of Seattle's Natural Drainage System" which highlights Seattle's NDS program.

    NDS Cost Analysis (PDF, 82K) Cost analysis created by Seattle Public Utilities that compares the costs of constructing NDSs with the cost of traditional drainage systems.

    NDS Soil Mixes (PDF, 15K) Seattle Public Utilities has created a guide to preparing and implementing proper soil mixes for a NDS.

    Broadview Green Grid Plant List (PDF, 105K) Provides a list of plant species used in the Broadview Green Grid Project. Species are categorized as being coniferous or deciduous, with stratum clarified. Each species has specifics regarding mature height, spread, and speed of growth.


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