SE Ankeny Green Street Project 

SE Ankeny between SE 56th and SE 57th avenues 

Portland, Oregon 

PROJECT SUMMARY 

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

Technology: Stormwater curb extensions 

Major Benefits: • Two stormwater curb extensions capture runoff from 7,300 square feet of paved 

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

and flow control and water quality benefits. 

• By managing runoff onsite, the project decreases the amount of stormwater that 

enters the city’s combined sewer system and helps protect residents from sewer 

backups. 

• The facilities are attractive extensions of the adjacent landscaping. 

 Cost: The cost for construction and project management was $11,946. The unit cost was 

$1.64 per square foot of impervious area managed. 

Constructed: May/June 2004 

Maintenance The City of Portland maintains the facilities with assistance from adjacent residents. 

Features 

• The project converted about 495 square feet of pavement to a vegetated system for 

stormwater management. 

• The curb extension on the north side of the street is smaller than its companion unit on the 

south side of the street. Existing residential driveways dictated the asymmetrical design, 

• The project demonstrates one of the simplest types of Green Street retrofits. The existing 

street curb was left intact, and no modifications were made to the stormwater collection 

system. 

• The design integrates well with its surroundings; low evergreen plantings enhance 

landscaped areas on the adjacent residential properties. 

• Neighbors help maintain the two facilities, providing weeding and seasonal watering during 

the plant establishment period. 

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BACKGROUND 

The Bureau of Environmental Services (BES) constructed the project in 2004 as part of an 

evaluation of techniques for managing street runoff. Urban stormwater runoff, if not managed 

properly, can pollute rivers and streams and contribute to combined sewer overflows (CSOs). It 

can also cause sewer backups during large storms. 

Stormwater curb extensions are a sustainable practice for managing runoff from existing streets. 

These natural systems capture and filter runoff and allow it to soak into the ground. They also 

add green space and enhance the urban environment. 

 

South Curb Extension

North Curb Extension

SE Ankeny Street 

SE Ankeny Green Street Plan View 

SITE SELECTION CRITERIA 

• Traffic Impacts: The low-traffic residential setting was ideal for a demonstration project. 

The addition of two 6.5-foot-wide curb extensions created an acceptable queuing 

configuration on the 28-foot-wide street. 

• Stormwater Catchment Areas: The size of the catchment area, 7,300 square feet, is 

considered representative of catchment areas in the surrounding neighborhood. 

• Utility Conflicts: An existing water line was the only subsurface utility within the project 

area. It did not present a constraint because it was not directly under the facility. 

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• Loss of Parking Spaces: The project transformed three on-street parking spaces to 

stormwater curb extensions. Property owners agreed in advance of the project that this was 

acceptable. 

• Street Slope: The street slope is approximately 1 percent from east to west. 

• Soil Infiltration Rates: Soil infiltration rates were not tested at the site before construction. 

(See “Geotechnical Evaluation,” below.) Staff conducted a flow test in 2006; the lowest 

draw-down rate over the course of the 5-hour test was 1.8 inches per hour. 

• Space Available for the Facilities: The north extension is half the size of the south 

extension because of the short curb length adjacent to residents’ driveways. 

Compilation photo of Ankeny Green Street looking west. 

STORMWATER CAPACITY AND SYSTEM CONFIGURATION 

Stormwater Management Goals 

The goal was to maximize the capture, treatment, and infiltration of street runoff, while 

providing a visual amenity for the neighborhood and reducing the risk of sewer backups. 

Geotechnical Evaluation 

An infiltration test was not required before construction because adequate documentation already 

existed concerning local soil drainage. The Natural Resources Conservation Service soil survey 

for Multnomah County classifies the soils as 51A-Urban Land and well-drained Multnomah 

soils. The surface horizon typically is dark brown silt loam about 25 inches thick. Soil below this 

depth is gravelly silt loam and gravelly sand to a depth of approximately 60 inches. 

System Configuration 

Catchment Area (street and driveways): 

• North curb extension: 2,500 square feet 

• South curb extension: 4,800 square feet 

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Street Slope: 

• Approximately 1% 

Earthen check dam in South facility 

Southern Curb Extension

Facility Dimensions: 

• Length: 60 feet 

• Width: 6 feet (does not include 0.5-inch wide 

curb) 

• Total vegetated area: 330 square feet 

• Maximum ponding depth at center: 6 inches 

• Radius of curved portion of the extension: 10 

feet 

Surface Storage Capacity: 

• 165 cubic feet 

Overflow: 

• Overflow exits through a curb notch at the downhill (west) end of the facility, draining to the 

combined sewer via an existing street inlet. No modifications were made to the inlet. 

Check Dams: 

• Originally, three check dams created four separate compartments for retaining and infiltrating 

runoff. The design was subsequently modified to include only one check dam, with two 

compartments. (See “Successes and Lessons Learned” below.) 

Northern Curb Extension

Facility Dimensions: 

• Length: 30 feet 

• Width: 6 feet (does not include 0.5-inch wide 

curb) 

• Total vegetated area: 165 square feet 

• Maximum ponding depth at center: 6 inches 

• Radius of curved portion of the extension: 10 feet 

• 

Surface Storage Capacity: 

• 83 cubic feet 

Overflow: 

• Overflow exits through a curb notch at the 

downhill (west) end of the facility, draining to the combined sewer via an existing street 

inlet. No modifications were made to the inlet. 

Stormwater retained in North curb extension facility 

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Check Dams: 

• No check dams were necessary because of the short length of the curb extension and shallow 

slope.

Additional Information:

• The configuration of the curb extensions in the southern facility is similar to a swale with 

side slopes and check dams. 

• The two extensions were excavated to 18 inches below grade. The native soil was tilled 

before importing 12 inches of a three-way mix of topsoil, sand, and compost and tilling it 

into the native soil. The soil mix was acquired from the City of Portland’s Sunderland 

Recycling Yard. 

• There is no gravel trench underneath the facility (a common feature of some designs) because 

the goal was flow control and water quality treatment rather than complete disposal. 

• Check dam composition is compacted clay covered with pea gravel and river rock to 

minimize erosion. (See “Successes and Lessons Learned” below.) 

• The first compartment (forebay) is concrete to allow for easy removal of street sediment and 

debris. About 3 to 4 inches of sediment can accumulate in the forebay. 

Landscaping 

Plants were selected for their drought tolerance, 

evergreen foliage, and typical short stature. These 

characteristics help minimize maintenance and safety 

concerns. The most common plants are native to the 

Pacific Northwest, including rushes and broadleaf 

evergreen shrubs. Rushes are the dominant plant in the 

bottom of the facilities because they thrive in the 

variable moisture conditions and their stiff structure 

helps slow the passage of water. Some non-native plants 

were included, primarily to provide seasonal color 

accents. 

The plants were installed approximately 8 inches apart, a density greater than required by the 

city’s Stormwater Management Manual. This was done to reduce maintenance requirements 

(weeding and watering) and to create an aesthetically appealing landscape quickly. 

Plants include: 

Scientific Name Common Name 

Actostaphyllos uva-ursi ‘Vancouver Jade’ 

Mahonia repens 

Helictotrichon sempervirens 

Juncus patens*

Vancouver jade kinnickinnick 

Creeping Oregon grape 

Tall blue oat grass 

California grey rush or grooved rush 

* Juncus patens has a tendency to grow taller than the recommended 24-inch height and therefore requires pruning 

at least once per year. 

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PROJECT COST 

The final project cost, excluding design, was $11,946 for both curb extensions, including 

construction and project management. 

• Curb Extension Construction 

Construction cost $9,626, or 81% of the total project cost. This included sawcutting and 

removal of existing asphalt, excavation, concrete curb installation, soil import and 

preparation, grading, and safety painting and reflectors on the curbs. 

• Landscape Construction 

Landscape construction cost $2,320, or 19% of the total construction cost. This included 

check dam construction, fine grading, plant procurement, plant installation, and mulch cover. 

MAINTENANCE AND MONITORING 

Maintenance 

Adjacent property owners allowed the city to use their outside hose bibs to help water the curb 

extensions during the 2-year plant establishment period. The city will continue long-term 

maintenance, including hand weeding (no chemical applications are allowed), plant trimming, 

plant replacement if needed, and removal of leaves, debris, and sediment. These activities will be 

performed approximately four times annually. (As is typical, more visits for irrigation were 

required during the plant establishment period). There is no permanent irrigation system, since 

irrigation is not needed once the plants are established. 

Water entering the south facility during a 

flow test. 

Monitoring 

BES conducted a flow test in early 2006 to assess the 

hydraulic performance of the facilities. Results indicated an 

infiltration rate of 1.8 inches per hour. Monitoring results 

were instrumental in identifying necessary design 

modifications to improve the facilities’ function. (See 

“Successes and Lessons Learned” below.) 

SUCCESSES AND LESSONS LEARNED 

• Effectiveness: The facilities are proving effective in reducing peak flows that can cause 

sewer backups and in retaining stormwater volumes to reduce combined sewer overflow 

events. (Visit the Sustainable Stormwater Management website for monitoring reports). 

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Forebay Asphalt berm 

• Check dams: Given the minimal longitudinal slope of the 

street and results of the flow test, it was determined that the 

south curb extension needs only one check dam to retain 

runoff. Rather than adding to the effectiveness of the facility, 

the other two check dams were essentially minimizing storage 

capacity. The slope of the street determines the number of 

check dams needed. 

Grade is very important when leveling check dams. If check 

dams are too high, runoff spills over the side of the facility into 

the street; if they are too low, stormwater is not sufficiently 

detained for infiltration. 

• Design Modifications: Curb cuts on the sides of the facilities were filled in following 

construction. Because of the relatively flat grade of the street, a curb cut located just 

upstream of a check dam served more as an outlet for flow out of the facility than as an inlet. 

Curb cuts should be located just downstream of a check dam and above the normal ponding 

elevation. A small asphalt berm was placed adjacent to the first inlet to direct water into the 

facility. 

• Concrete Forebay: A small, 2-inch concrete lip was originally placed behind the slab 

forebay of each facility to keep sediment from migrating into the landscaped area. Sediment 

is not desired in the planting area because it is difficult to remove and can reduce stormwater 

storage capacity. The concrete lip created ponding water, however. A row of Juncus proved 

to be an effective and less costly method to minimize sediment migration, while also 

allowing unencumbered stormwater flow. The concrete slab forebay at each inflow point (at 

the east end of each facility) works well for easy sediment removal. 

• Landscaping: Planting the Juncus patens 8 inches apart proved to be too dense. This species 

tends to grow vigorously in this environment, reaching 24 inches or more and requiring 

trimming so it does not inhibit the growth of other plants and obscure the curb. This density 

did, however, minimize weeding during the first year. Cost savings could be obtained with 

less density and fewer plants. The city is researching alternative lower-growing Juncus

species. 

Vigorous growing Juncus patens (dark green plant. 

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