SW 12th Avenue Green Street Project 

SW 12th Avenue between SW Montgomery and SW Mill 

Portland, Oregon 

PROJECT SUMMARY 

Project Type: Stormwater retrofit of an existing downtown street – demonstration project 

Technology: A treatment train of four consecutive street stormwater planters 

Major Benefits: • The planters capture runoff from approximately 7,500 square feet of paved 

surfaces. They treat and infiltrate most of the runoff they receive, providing 

volume and flow control and water quality benefits. 

• Runoff is managed onsite, instead of entering the storm drain system that feeds 

directly into the Willamette River. 

• The planters improve the existing urban streetscape by adding attractive 

greenspace. 

• The planters are designed to safely accommodate pedestrians, on-street parking, 

and vehicle access. 

 Cost: The total project construction cost, including project management (but not design), was 

$38,850. Of this, construction of the stormwater planters cost $34,850, or $4.65 per 

square foot of impervious area managed. The remaining $4,000 was required for 

ancillary street and sidewalk repairs and landscaping adjacent to the planters; these 

costs may not be needed for other similar projects. 

Constructed: May-June 2005 

Maintenance The City of Portland will maintain the facilities. 

Features 

• This was Portland’s first Green Street 

project to transform existing landscape to 

street stormwater planters that manage street 

runoff and safely maintain on-street parking. 

The design provides water quality treatment 

The award-winning design successfully 

• 

and maximizes infiltration of the runoff. 

Each facility can pond about 7 inches of 

stormwater runoff and retain it for 

infiltration. 

Stormwater Street Planters at SW 12th and Montgomery • Portland, Oregon 

integrates stormwater management into an 

urban environment. 

1
 BACKGROUND 

This Green Street project converted a previously underused landscape area between the sidewalk 

e 

ote 

ow do the Street Stormwater Planters Work? 

Stormwater runoff from the street flows downhill along the existing curb until it reaches the 

A 12-inch-wide trench drain channels the street runoff into the first stormwater planter. The 

Stormwater is allowed to pond to a depth of 7 inches before infiltrating through the soil at a 

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The adjacent sidewalk slopes toward the planters, and sidewalk runoff enters the planters 

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, the 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. Although th

project has a strong functional component, it is the integration of the landscaped stormwater 

planters into the urban environment that has gained the interest of the design community, 

developers, policy makers, and local citizens in the City of Portland’s commitment to prom

natural systems to manage urban stormwater runoff. 

H

• 

first of four consecutive stormwater planters. 

• 

trench drain moves the water under the vehicle step-out area and into the facility. The runoff 

is directed over a concrete pad, where sediment and debris are deposited for easy removal. 

• 

rate of approximately 4 inches per hour. During large storm events, water may enter the 

planter at a rate faster than the soil can infiltrate, resulting in a ponding depth greater than

inches. In that case, the runoff exits a second curb cut, flows back into the street, and enters 

the second (downhill) planter. This process continues for the third and fourth planters. If the

fourth planter ponds to capacity, it overflows to the existing storm system. 

• 

through curb cuts. 

2
3
SITE SELECTION CRITERIA 

• Traffic Impacts: The project was not expected to have any traffic impacts. 

• Stormwater Catchment Area: The size of the catchment area, approximately 7,500 square 

feet, was considered fairly representative of conditions in the surrounding area. 

• Utility Conflicts: An existing gas service line to the adjacent building was the only subsurface 

utility that intersected any of the stormwater planters (the third one). The existing shut-off 

valve was located and preserved with a plastic standpipe for easy access. The existing street 

lighting remained in place. 

• Loss of Parking Spaces: The project did not affect existing on-street parking. 

• Street Slope: The moderate street slope (2 percent) was suitable for the project. 

• Suitability for Monitoring: The configuration of the local combined sewer allowed for 

placement of a flow monitor. There is also a rain gage near the project to measure rainfall 

events. 

• Soil Infiltration Rates: Specialized infiltration tests were not required at the site. (See 

“Geotechnical Evaluation,” below.) 

• Available Space: The existing underused landscape area was 8 feet wide from face of curb to 

sidewalk edge. This allowed for 3 feet of flat area for parking egress and 5 feet for the 

stormwater planter, including 6-inch-wide perimeter curbing around the planters. 

STORMWATER CAPACITY AND SYSTEM CONFIGURATION 

Stormwater Management Goals 

The stormwater management goal was to maximize the capture, treatment, and infiltration of 

street runoff, while providing a visual amenity for the neighborhood. 

Geotechnical Evaluation 

An infiltration test was not required before construction 

because adequate documentation already existed 

concerning the well-draining characteristics of the local 

soils. The Natural Resources Conservation Service soil 

survey for Multnomah County classifies the soils as 

51C-Urban Land and well-drained Multnomah soils. 

The surface horizon typically is dark brown loam about 

16 inches thick. Soil below this depth is gravelly sand to 

a depth of approximately 60 inches or more. 

Inlet to Stormwater Street Planter 

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System Configuration 

Catchment Area (street, driveway, sidewalk): 

• 7,500 square feet 

Planter Details (applies to each of the four facilities): 

• Dimensions: 

− Length: 18 feet 

− Width: 4 feet (not including 6-inch-wide perimeter curb) 

− Depth: Approximately 13 inches from sidewalk grade to finish grade of planter 

• Maximum ponding depth: Approximately 7 inches 

• 6-inch-wide perimeter curb around each planter: 4 inches high 

• Total landscape area: 72 square feet 

• A 3-foot-wide parking egress zone made of sand-set concrete pavers provides for vehicle 

access.

• A 2-foot-wide landscape buffer at each end of the planters directs people safely around the 

facilities. 

Overflow: 

• Overflow from each planter exits through a second curb cut back into the street. Final 

overflow exits through a curb cut in the last planter and enters the existing storm inlet in the 

street. 

Additional Information: 

• A design modification placed asphalt berms (1 inch high) on the downhill side of each curb 

cut to help runoff make the 90-degree turn into the planter. 

• No rock sub-base was used underneath the planters. 

• An 18-inch-wide concrete pad at each planter’s uphill curb cut dissipates flow and collects 

sediment and debris. 

Landscaping 

The facilities were excavated throughout to 24 inches below grade and backfilled in 6- to 9-inch 

lifts with a three-way mix of sand, topsoil, and compost. The mix was tilled into the native soil 

and spread to create a flat cross section. (See illustration on page 3.) 

 

Planting internal and external to the facilities 

tructure 

ot 

e 

e 

Each facility was densely planted with Juncus patens and a 

Nyssa sylvatica tree. Both types of vegetation appear 

tolerant of the wet and dry soil conditions. The stiff s

of the J. patens helps slow the passage of water, and the ro

structure helps infiltrate water into and through the soil. The

evergreen characteristic of the J. patens also helps minimiz

weed growth. A row of J. patens was planted next to th

concrete pad in each facility to hold back sediment and 

debris and keep it from entering the facility. 

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The plants were installed at a density greater than required by the City’s Stormwater 

Management Manual. This was done to reduce maintenance requirements (weeding, watering, 

etc.) and to create an aesthetically appealing landscape quickly. 

The types of vegetation external and adjacent to the facilities were selected for their drought 

tolerance, low maintenance, evergreen foliage, and short stature; the typical mature height of the 

plants is approximately 24 inches. Plants included Nandina domestica ‘Moon bay’; Liriope 

muscari ‘lilac beauty’; and Polystichum munitum (under the existing trees). 

PROJECT COSTS 

Street planter construction and landscaping costs for the project totaled $38,850, including 

project and construction management and ancillary construction costs (but not including design). 

Stormwater Street Planters under construction: Grading (far left); Formed concrete walls (center); Forming the concrete trench and 

sediment collection pad (far right) 

• Curb Extension Construction 

The core construction activities cost $29,950, or approximately 77 percent of the total project 

cost. This included sawcutting and removing existing landscaping and hardscape, 

excavation, concrete curb installation, sand-set concrete unit pavers installation, curb cuts 

and ornamental grate installation, soil import and preparation, and final grading. 

• Landscape Construction 

The landscape construction activities cost $4,900, or approximately 13 percent of the total 

project cost. This included fine grading, plant procurement, plant material, and pea gravel 

mulch installation. 

• Ancillary Construction Activities 

Miscellaneous street and sidewalk repair work cost $4,000, or approximately 10 percent of 

the total project cost. This included re-landscaping of a small area just south of the facilities, 

under existing trees, for visual continuity. These ancillary construction and landscaping tasks 

contributed to higher project costs. 

As Green Street projects become an accepted practice, it is likely that design and project 

management costs will decline. 

6
MAINTENANCE AND MONITORING 

Maintenance 

Sediment collecting behind the juncus on the 

concrete pad

Maintenance of the facilities includes hand weeding (no 

chemicals are allowed), plant trimming, plant replacement, 

and debris and sediment removal. 

Portland Parks and Recreation staff maintained and handwatered the planters and adjacent landscaping during the 2-

year plant establishment period. There is no permanent 

irrigation system. 

City staff removed sediment from the planters four or five 

times during the first 2 years. A landslide upland of the 

facilities in winter 2006 and debris from adjacent fruiting 

trees contributed to heavier-than-average sediment loading during that time. 

he maintenance frequency for these types of facilities depends on the site. City staff will 

ear; 

onitoring 

he City will monitor the facilities for hydraulic performance, maintenance requirements, metalT

monitor long-term maintenance needs. Maintenance visits will occur at least four times a y

additional visits will occur if needed because of sediment accumulation. 

 

M

T

and petroleum-based constituents, and the success of the vegetation. Because of the difficulty of 

collecting stormwater samples, water quality monitoring will not occur. 

SUCCESSES AND LESSONS LEARNED 

• Where communities struggle with ever-increasing impervious areas and degraded water 

A flow test at this site indicated that the stormwater planters are effective in reducing the 

d 

The final project design resulted from several discussions among design and engineering 

ll 

quality, these simple landscape approaches can have a measurable positive impact. 

• 

peak flow of a 25-year storm event by at least 70 percent. (This value is site dependent an

may not be the same at a different location.) 

• 

staff from various bureaus (Transportation, Maintenance, and Environmental Services). A

bureaus agreed that the design would not pose safety issues for pedestrians or for people 

getting in and out of vehicles. 

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• In the first 2 years of operation, the first two planters filled often with sediment and debris (to 

a depth of 1-3 inches). City staff removed the sediment by hand with a shovel and rake. The 

facilities appear to manage much more than the drainage area that was assumed during 

design. The inlet just upstream of the facility and another inlet at the base of an overpass 

frequently clog with leaves and other debris, resulting in significant additional flow and 

resultant higher levels of sediment. 

• It is difficult to force curb runoff to turn 90 degrees 

into curb openings during heavy storm flows. The 

momentum of the runoff becomes more problematic 

as the slope of the street increases. Small asphalt 

berms have been installed to encourage flow to enter 

the facilities and appear to be working well. Other 

options can include angled entries, gutter 

depressions, or other methods to ease the transition 

of flow from the curb into the facility. 

Asphalt berm at facility inlet to help d

stormwater flow into the stormwat

irect 

er planter

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