*4.1. Seattle: The Pioneer*

Seattle is an innovator in green streets in the US. The city is located in a populated region surrounded by a fragile and delicate estuary. The Puget Sound estuary, a rich ecosystem, faced continuous degradation that reduced the salmon population and diminished recreational appeal. Even before the EPA identified the problem, scientists in Seattle found that stormwater from urbanized areas highly contributed to that degradation [63]. Yet, back in 1995, the effects of watershed urbanization on streams around the US were well documented [64]. But at the time (in 1995), nothing concrete for green street implementation was available besides timid sentences in plans suggesting the idea of front-end solutions able to mimic pre-development conditions instead of end-of-pipe solutions.

Although there were earlier experiments with GSI in Davis, California, and Prince George's County, Maryland, Seattle has the merit of initiating the first green street pilot project in the US. Called the Street Edge Alternative (SEA street) project, it involved the complete reconstruction of the street and its drainage system. The roadway was narrowed from 7.62 m to 4.27 m to create a meandering road surrounded by permeable green zones, including 100 new evergreens and 1100 shrubs, in the remaining 18.28 m of the ROW [65]. The emblematic curb-free design allows water to drain into swales along the street edges. The project, completed in the spring of 2001, was designed to decrease the quantity of stormwater discharged into Pipers Creek [66]. Besides meeting the Clean Water Act's water quality requirements, decreasing stormwater quantities discharged into creeks was one of the main goals for the Seattle area, in order to prevent channel erosion and to enhance reduced salmon reproduction rates.

Pilot projects like SEA streets provided real performance data, which served for the planning and design of future green streets. Results were overwhelmingly good in hydrological, acceptance, and even financial terms. The hydrological performance estimation of SEA street was too conservative by far when designed. The SEA street could fully attenuate up to 19 mm of precipitation and has prevented the discharge of all dry season flow (10% of the yearly rainfall) and 99% of the wet season and overall runoff. The SEA street drainage performance increased in time, withholding more water from discharge as time went by [67,68].

The ROW allocation principles in Seattle are very progressive for the US and likely in other latitudes, as they explicitly consider the fulfillment of many functions in the street. The Right-of-Way Improvement Manual is Seattle's reference for street design and states that "they [the ROW] must safely accommodate multiple modes of travel, offer universal access around and through the city, provide access to private property, enhance a place's character, protect environmental resources, and allow for the delivery of utility services" [69]. This manual contains the necessary information to design streets with GSI measures (hereafter referred to as green street, even though in Seattle the meaning of the term green street is different). This manual proposes two modalities of green streets: GI can be used as part of partial street improvements or as complete ROW retrofit. Partial street improvements usually include improvements to sidewalk and planting strip areas (Figure 1). Full ROW improvements involve sidewalks, planting strips and full roadway width reconstruction [70].

The procedure used to select the street to be intervened was not particularly strict. Seattle Public Utilities, the entity responsible for green streets implementation, wants projects in places that serve their stormwater objectives. The location of the projects within the city is determined with the drainage system and the sewer basins (combined, partially separated, or separated) in mind [71]. Maps with the drainage basins show the zones of combined sewers, which are the preferred places for infiltrating stormwater. Once the zones had been selected for the pilot projects, with the protection of a specific creek as the objective, a neighborhood was selected; low traffic streets are the preferred location for such projects. However, since few projects have actually been completed in Seattle, it is not clear how a specific street is selected. The green streets coordinator at Seattle Public Utilities suggests some criteria for selecting the location of the block: first, that the type of soil on the block can infiltrate the water; second, that there is space available (opportunity to narrow the roadway or replace parking); and finally, that the community accepts the implementation [72]. The Right-of-Way Improvement Manual enumerates the following factors to be considered when designing each GSI: native soil permeability, longitudinal and cross slopes, presence or absence of curbs, and space availability [70].

Once the zone and street are selected, engineers then provide the street designer with actual sizes of the GSI to be provided. According to the water practice director at a specialized consulting firm in Seattle, the GSI is sized for minimum goals; for example, to retain 90% of the runoff or a fixed amount (at least the first 19.05 mm) [73]. It includes the determination of the catchment area of each of these GSIs and, depending on the expected probable runoff for a given return period, the GSI facility designed. In practice, the manual also provides design standards for different types of GSI: conveyance swale, curb extension, tree planting within bioretention swale, and bioretention biofiltration cell with or without underdrain. Following this, the manual directs the designer to other codes and other technical documents for the sizing and design of hydraulic elements.

To complement the street design story in the city, Seattle has a Complete Street policy (ordinance No. 122386). The way in which both complete and green streets have been harmonized is through a street design checklist that gives extra points for GSI according to the Seattle green streets coordinator [72]. But not every Complete Street project will necessarily have GI. Both the lack of space on busy streets and the Seattle Department of Transportation (DOT) budgetary priorities have brought green streets to be an exceptional outcome, not the norm. In main and arterial streets, where pavement and safety priorities are concentrated, space is scarce. An official at Seattle Department of Planning and Development, explains that attempting to force the inclusion of GSI within Complete Streets redesigns could result in subnormal designs, for example, "narrower than standard bike lanes, sidewalks" [74]. In addition, the usage of permeable pavements and other alternative stormwater infrastructure is expensive. Seattle's green streets coordinator states that the DOT has not had the money for stormwater managemen<sup>t</sup> facilities within the ROW (bioretention and permeable pavements) or for their maintenance [72].

### *4.2. Portland: Pioneer and Fruitful Implementer*

Portland is a progressive city that has been at the forefront of many topics, and street design is one of them. Recurrently, plans and manuals reaffirm the importance of having streets beyond cars. The famous tram system and extensive bike network prove this strong tendency towards multimodal mobility. This multimodal approach existed in Portland prior to any Complete Streets policy or other influence. Already in 2002, the metropolitan regional governmen<sup>t</sup> for the Portland area (METRO) published Creating Livable Streets, a guideline that explicitly acknowledged the incompleteness of the conventional mobilityaccessibility street classification system of the American Association of State Highway and Transportation Officials (AASHTO) [75]. It recalled the importance of considering all modes, 15 years before the city adopted its Complete Street policy in 2012. In Portland, balancing the multiple interests that use the ROW is important [76], and those interests can go beyond transportation.

Portland's development of green streets began early in the 2000s and has more than 1200 operating GSI within its streets. Like any other city with combined sewer systems, in 1987 Portland faced federal requirements to manage stormwater. However, in this case, the city defined its stormwater strategy after a litigation process with an environmentalist advocacy group that pushed for definite solutions. As a result, in 1991, the city of Portland entered into an agreemen<sup>t</sup> with the Oregon Department of Environmental Quality (EPA's designated authority) to control 99% of the combined sewer overflows in 20 years. At that time, traditional solutions were evaluated and the problem was approached by means of the construction of three big underground pipes to collect the mixed water during wet seasons to be stored and later conveyed to the wastewater treatment plants. However, due to technical and budgetary reasons, the biggest pipe was downsized, requiring alternative solutions. After the success of rainwater infiltration in the roof disconnection program and experimentation with green roofs and other GI, the city started a green street pilot from 2003 to 2007 that included several street interventions. Green streets emerged as this alternative to fulfill the requirements of the agreement. A Cross-Bureau Task Force, a program launched in 2005, led to the enactment of the green street policy in 2007 (passed by resolution) [77]. The program was a temporal effort aimed at creating a programmatic approach, which would make GSI possible in street projects wherever feasible [76].

Portland issued many different plans and manuals to support the development of green initiatives: Green Spaces Master Plan [78], the Best Management Practices Manual [79], Creating Livable Streets [75], Green Streets Handbook [80], and the Stormwater Management Manual (SWMM) (first in 1999 and updated in 2003, 2008, 2014, and 2016). Out of this set of documents, two deserve special mention. The 2002 Creating Livable Streets manual was a visionary document published by METRO that provides general and descriptive design guidelines of GSI in the ROW. However, this idea was never developed or updated with qualitative and technical information. In contrast, in the 2003 version of the SWMM, a set of green street design minimums was defined. The SWMM is fundamental because it contains the stepwise procedure used to design green streets first introduced in the 2008 version. In 2006, the Drainage Manual was updated (to replace the 1991 Sewer Design Manual) and it currently considers green infrastructures as a legal element for treating stormwater. The SWMM, issued by the Bureau of Environmental Services BES (not by the DOT), became the primary reference manual for managing stormwater from public and private areas, and designing water quality facilities and storage structures for managing stormwater flows [77].

The green streets design comprises nine steps summarized in Table 2. The process has evolved and the manual currently explains, in a clear but technical fashion, a complex and comprehensive design process. It begins by evaluating the condition of the project site, which is derived from a regional or citywide analysis. Step 3 considers an analysis of the GSI project in the current stormwater urban system, which implies revising at all scales (from the city scale to the street scale). Steps 4 and 5 are dedicated to details of the local street design. Several steps (e.g., 2, 7, or 8) represent procedural or legal requirements. Many steps direct us to other documents, for example, to the Sewer and Drainage Facilities Design Manual, to appendixes with technical information, and to calculators (worksheets).

Like Seattle, the Portland case has no clearly defined procedure for selecting locations of projects within the city. Nevertheless, there are some criteria. The first is the soil's infiltration capacity at the desired location. For example in Portland, most projects are on the eastern side of the Willamette River where soils are very forgiving, compared with the eastern part. A Green Street Program official at the Bureau of Environmental Services (BES) in Portland, was interviewed for this study. She stated that another criterion for selecting a street for GSI is sewer network capacity. Congested sewers produce backflows in basements during heavy rain events. By modeling the sewer, points where the sewer is congested can be identified, and GSI installations upstream from congested segments within the sewer basin prevent runoff from entering the sewer (Figure 2). An example of this is the "Tabor to the river program."

**Table 2.** Steps in the design and permit process for green stormwater infrastructures (GSI). Source: Bureau of Environmental Services [81].
