**3. Results**

### *3.1. Potential Benefits of Green Street*

Thirty-six items in an exhaustive list of benefits (subcategory) were identified from the first-round analysis derived from the objectives and definitions in the manuals. Five themes (top category) were identified as higher level criteria upon summarizing and grouping subcategories (Figure 1): Stormwater managemen<sup>t</sup> (at its source), environmental preservation and improvement, social improvement, transportation enhancement, and economic e fficiency. Among the identified benefits grouped under the five themes, significant benefits were found based on the frequency mentioned in the manuals, in particular, stormwater quality improvement (1), stormwater volume reduction (2), stormwater infiltration (3), stormwater capture and store (4), basement alleviation and flood managemen<sup>t</sup> (5), utilization of soils and vegetation (7), connection to landscape areas and natural areas (9), wildlife habitat and natural pathway creation (10), urban heat reduction (11), neighborhood beautification (21), pedestrian experience and safety enhancement (27), urban street design and function improvement (30), and cost effective solution for stormwater managemen<sup>t</sup> (34). As stated, benefits related to stormwater managemen<sup>t</sup> were the most frequently mentioned elements in the manuals. The overall elements are not limited solely to stormwater treatment but also include a range of environmental, social, and economic aspects, indicating that green streets are clearly expected to provide a number of benefits in addition to stormwater treatment.


**Figure 1.** List of potential benefits derived from the selected manuals.

### *3.2. Green Street Typology: Form-Based Approach*

Many municipalities that are adopting green streets describe their design, specifications, and applications in a set of manuals, standards, municipal plans, and online databases: Philadelphia, PA has the Green Streets Design Manual and the green street information provided on the Philadelphia Water Department website; New York published green street information in its NYC Green Infrastructure Plan, along with design standards from the city's Parks Department and Environmental Protection Agency; Portland, OR's green street information is provided under their Sustainable Stormwater Management initiative; and Seattle, WA, defines green streets in its Right-of-Way Improvement Manual, which also covers its Green Stormwater Infrastructure approach.

Those design standards published by various municipal agencies provide technical designs for the green street stormwater treatment systems that generally include items such as the sizes of the di fferent types of stormwater treatment facilities involved, inlet designs, plant selection, soil combinations, and other related technical and design specifications. In order to deliver the potential benefits that are frequently discussed in the manuals, these items need to be integrated with contexts of streets where any given green street projects are applied.

It is important to note that each of the manuals and design standards selected for the research has adopted various types of green street practices that they refer to under slightly di fferent terms (Table 4). Thus, the author first categorized stormwater treatment measures applied to green street projects depending on where they are located within the right of way and their structural shapes based on the manuals. This approach to categorization was possible because each manual actually shares quite similar types of green street facilities, despite the various ways they were referred to. Then, a list of green street practice types and typologies were created to help develop a better understanding of current practices and techniques and thus facilitate communication e fficiently among designers and planners based on the di fferent facility types in the design manuals and case study projects (Table A1). The form-based categorization for deriving typologies was taken into account for defining di fferent types of green street practices integrated into street network systems (Figure 2). Here, the types are divided first according to whether the practice is applied at an intersection, midblock, or both (a combination), then subdivided based on whether the facility was located within the sidewalk or extended outward from the curb towards the roadway. After the di fferent shapes of facilities have been listed, permeable paving, runnels (open/closed), cisterns (above/underground), fountains, street furniture, and structures are then categorized as special applications of green street practices based on constructed green street projects studied.


**Table 4.** Green street typology and terms from selected manuals.

**Figure 2.** Form-based green street typology.

The green areas in Figure 2 indicate where green street stormwater facilities are located. In these areas, stormwater runoff is treated with plants and soils. An overflow and underdrain system with perforated pipes should be considered as well. At a street intersection, stormwater treatment facilities can be implemented on all four corners. They can be located within the curb (1.A-1. *cell type*) or extended from the curb (1.B-1, 2. *extended curb*: a type, b type). One of the benefits of *extended curb type* is that it can provide safer pedestrian crossing by reducing the crossing distance and introducing a traffic calming effect. If the stormwater treatment structure is a *cell type* (normally a singular facility) but has a more organic shape than the usual rectangular shape, it is categorized as a *pocket rain garden* (1.A-2, 1.B-2). These are typically found in isolated areas such as gore points (normally a triangular shape) and areas close to intersections rather than the midblock of streets. The *pocket rain garden* can be conveniently implemented in a smaller area but may be less effective in treating stormwater runoff compared to the *cell type* or the *swale type* in the midblock. It can also serve as a small community park with additional design features such as benches and shade structures. The *island type* (I.C-1) can be a focal point in the area to promote a better image of a city with beautification while accepting and treating stormwater runoff from the surrounding roads.

In the midblock area of a street, if a facility is small and repeated in one or more blocks, it is referred to as a *cell type* (2.A-1, 2), which can be divided into two forms: A *planter* form or a *natural* form. The walls of the *planter* form are made of durable materials such as concrete and are often rectangular in shape. The walls of the *natural* form are constructed with more raw materials such as soils and stones and have more natural/organic shapes than the *planter* form. The *cell type* can be accepted better by the users since its form is similar to the conventional street planters. Each cell can be connected with an overflow system for optimized stormwater runoff treatment. It can then provide continuous planting bed for providing greenness and community beautification. The *cell type* can also provide better access utilizing spaces in between planters, such as pedestrian movement from the sidewalk to street parking. If the stormwater treatment facility is linear and long, it is labelled a *swale type* facility (2.A-3, 4). These can again be subdivided into a *planter* form and a *natural* form depending on the materials used and the type of construction as mentioned above. The *swale type* can be more efficient in treating stormwater runoff since it has more contact area with stormwater than the *cell type*, however, it may obstruct pedestrian movement on the street because it lacks breaks along the linear facility. Similar to the *cell type* in the midblock, the *covered cell type* (2.A-5) appears in the conventional form of a planter. *Extended curb* (2.B-1) and *median types* (2.C-1) are also implemented in the midblock and can provide safer pedestrian crossing as making the crossing distance shorter.

The *combination types* (3.A, 3.B, 3.C) are composed of types in the intersection and types in the midblock. The *applied type* (3.C) suggests an integration of any available public open spaces with green street stormwater facilities. The integration can enhance the experience in a community by improving walkability, providing gathering spaces, effective stormwater treatment with connected system, and more. *Other applications* (4.A, 4.B, 4.C, 4.D, 4.E, 4.F) are additional design features that can be integrated within green streets for providing multifaceted benefits for users.

These typologies can be understood as a form-based implementation tool that can be systematically referred by designers and planners during the green street design processes. The implementation tool emphasizes on the importance of addressing multiple benefits while treating stormwater efficiently. Table 5 is provided to help better understanding of green street typology developed in this paper with more descriptions and constructed green street project samples for each typology.

When planning and designing green streets, it is important to note that street elements (e.g., roads, sidewalks, structures, furniture, buildings, and users) also need to be considered along with the application of stormwater treatment facilities (e.g., planter type, swale type) because street's contexts differ from one another while current manuals tend to focus primarily on the stormwater facilities instead. It is not easy to standardize design applications that can deliver certain benefits since every site has its own unique conditions, and an approach that may be suitable for one location could potentially inhibit some of the multiple potential green street benefits in another location. Thus, typologies in this study are an important tool for communicating among planners and the public: They consider standardized designs but also offer flexibility concerning adaptive design application and the considerations of the different conditions and needs of each site.

