Introduction
The SW 12th Avenue Green Street project, located adjacent to Portland State University in downtown Portland, is unique to Portland and the United States in the way the pedestrian zone of this street has been transformed to sustainably manage street stormwater runoff. As part of the City of Portland’s commitment to promote a more natural approach to urban stormwater management, this “green street” project converts the previously underutilized landscape area between the sidewalk and street curb into a series of landscaped stormwater planters designed to capture, slow, cleanse, and infiltrate street runoff. Built in the summer of 2005, this street retrofit project demonstrates how both new and existing streets in downtown or highly urbanized areas can be designed to provide direct environmental benefits and be aesthetically integrated into the urban streetscape. Though this green street project maintains a strong functional component, it is the ability of the landscaped stormwater planters to be integrated into the urban fabric that has the design community, developers, policy makers, and local citizens excited about the SW 12th Avenue Green Street.
How does the 12th Avenue Green Street Work?
The 12th Avenue Green Street project essentially disconnects the street’s stormwater runoff from the storm drain system that feeds directly into the Willamette River and manages it on-site using a landscape approach. Stormwater runoff from 8,000 square feet of SW 12th Avenue flows downhill along the existing curb until it reaches the first of four stormwater planters. A 12-inch curb cut channels the street runoff into the first stormwater planter. Once inside the planter, the water is allowed to collect until the water level reaches a depth of 6 inches. The landscape system within each planter allows the water to infiltrate in the soil at a rate of 4 inches per hour. If a rain event is intense enough, water will exit through the planter’s second curb cut, flow back out into the street and eventually enter the next downstream stormwater planter. Depending on how intense a particular storm is, runoff will continue its downhill “dance” from planter to planter until all of the stormwater planters are at capacity. Once exceeding capacity, the water exits the last stormwater planter and enters the existing storm drain system. With the new stormwater facilities now in place, nearly all of SW 12th Avenue’s annual street runoff, estimated at 180,000 gallons, is managed by its landscape system. In fact, a simulated flow test has shown that stormwater planters at SW 12th Avenue have the ability to reduce the runoff intensity of 25-year storm events by at least 70 percent. Where communities struggle with ever-increasing impervious areas and degraded water quality, these simple landscape approaches can have a measurable positive impact.
Design Solutions for Multiple Uses
The paramount design challenge for retrofitting SW 12th Avenue Green Street was finding sufficient space to locate the stormwater planters while minimizing conflict with other streetscape elements. It was difficult to integrate pedestrians, on-street parking, street trees, landscaping, street lighting, signage, and stormwater planters within the 8-foot wide zone of space. It was quickly realized that for the stormwater planters to work in tandem with on-street parking that the design must focus on strong pedestrian circulation and connection from the sidewalk to the parking zone. Because of this, multiple design strategies were employed. A 3-foot wide parking egress zone was dedicated for people to access their vehicles without competing with the stormwater planters. Furthermore, perpendicular pathways were located between each stormwater planter so that a pedestrian would not have to walk very far to access their cars or the sidewalk. The design also calls for a 4-inch curb exposure at each planter to help indicate to the pedestrian that there is a drop in grade. Each curb cut that allows the street runoff to enter the stormwater planters has an ADA accessible grate to allow for unencumbered pedestrian flow along the parking egress zone.
Special attention was also given to the landscape component to the SW 12th Avenue Green Street. The native grooved rush (Juncus patens) planted within each stormwater planter is the workhorse for stormwater management. The upright growth structure of Juncus patens slows down water flow and captures pollutants, while its deep penetrating roots work well for water absorption. The Juncus plants were installed deliberately in rows 18-inches on center to allow space for a leaf rake to easily remove any accumulation of sediment and debris commonly found in urban conditions. The design also boldly locates the project’s street trees right in the middle of each stormwater planter. Tupelo Tree (Nyssa sylvatica) was chosen because of its tolerance to both wet and dry conditions and its beautiful fall color.
Special Design Considerations and Project Goals
Several design elements used at SW 12th Avenue have helped with the success of this project. One is the elegant and detailed treatment of the curb cuts that allow the stormwater to enter the landscaped planters. Because the focus of this project is on water, it made sense to expend extra detail and design attention by choosing an ornamental trench grate that covers each curb cut. Also, sand-set tumbled concrete unit pavers were utilized in all of the project’s pathways so that there was a clear physical and aesthetic separation from the sidewalk zone. Lastly, a landscape buffer was added on the outside of each planter’s sidewalls in order to further “soften” the look of the stormwater planters as well as define where the access paths are located.
The design of the SW 12th Avenue Green Street has met three important goals: 1) it is low-cost in its design and execution; 2) it benefits the environment and embodies community livability; and 3) it provides a model for other jurisdictions in addressing important national and local stormwater regulations. These stormwater planters are well integrated into the urban streetscape and bring natural hydrologic functions back into the City. The retrofit of SW 12th Avenue Green Street with landscaped stormwater planters cost approximately $30,000 to construct.
Community Partnerships
The success of innovative stormwater projects like the SW 12th Avenue Green Street is dependent on community partnerships. Representatives from Portland State University were active participants in the project’s design process. Communication with Portland State University representatives continues to this day to determine the overall success of the project from the neighborhood perspective. In addition, many students and professors within the urban planning and environmental studies departments at Portland State University are excited to use this project in their own research.
In a unique partnership, the city and Portland State University have agreed to share responsibilities in maintaining the new stormwater facilities at SW 12th Avenue. To further engage the community, a small interpretative sign has also been placed at the project site to describe how the stormwater facilities function, as well as how to find more information on sustainable stormwater management practices. Despite the fact that the SW 12th Avenue Green Street has not even reached its first anniversary, the project has caught the attention of visitors from all over the United States, Europe, and Asia. The aesthetic appeal and intrigue of the new stormwater facilities has created a community asset that promotes both environmental stewardship and education at the neighborhood level within the urban core of the city.
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Project Resources |
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Stormwater Planter Construction:
City of Portland, Department of Transportation
Landscape Installation and Maintenance:
City of Portland, Department of Parks and Recreation |
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Plant Nursery:
Northwest Ornamental Grasses
Plant Nursery:
Cascadian Nurseries
Ornamental Trench Grates
Urban Accessories |
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