## 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 wa ter 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 facili ties 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 st reet 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 surr oundings; low evergreen plantings enhance landscaped areas on the adjacen t residential properties.

• Neighbors help maintain the two facilities, providing weeding and seasonal watering during the plant establishment period.

1BACKGROUND

The Bureau of Environmental Se rvices (BES) construc ted the project in 2004 as part of an evaluation of techniques for mana ging 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 bac kups during large storms.

Stormwater curb extensions are a sustainable pr actice for managing runoff from existing streets. These natural systems capture and filter runoff a nd allow it to soak into the ground. They also add green space and enhance the urban environment.

South Curb Extension North C urb E xtension

SE Ankeny Street

SE Ankeny Green Street Plan View

SITE SELECTION CRITERIA

• Traffic Impacts : The low-traffic residential setting wa s ideal for a demonstration project. The addition of two 6.5-foot-wide curb ex tensions 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 becau se it was not directly under the facility.

2• Loss of Parking Spaces: The project transformed thr ee on-street parking spaces to stormwater curb extensions. Property owners agr eed in advance of the project that this was acceptable.

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

• Soil Infiltration Rates: Soil infiltration rates were not test ed 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 th e 5-hour test was 1.8 inches per hour.

• Space Available for the Facilities: The north extension is ha lf the size of the south extension because of the s hort 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, treatmen t, and infiltration of street runoff, while providing a visual amenity for th e neighborhood and reducing the risk of sewer backups.

Geotechnical Evaluation

An infiltration test was not required before c onstruction because adequate documentation already existed concerning local soil drainage. The Natura l 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 sa nd 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

3Street 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 separa te compartments for retaining and infiltrating runoff. The design was subseque ntly modified to include on ly 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

4Check Dams :

• No check dams were necessary because of the s hort 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 inch es 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 undern eath the facility (a common f eature of some designs) because the goal was flow control and water quality tr eatment rather than complete disposal.

• Check dam composition is compacted clay cove red 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 sedime nt can accumulate in the forebay.

Landscaping

Plants were selected fo r their drought tolerance, evergreen foliage, and typi cal short stature. These characteristics help minimize maintenance and safety concerns. The most common pl ants are native to the Pacific Northwest, includ ing rushes and broadleaf evergreen shrubs. Rushes are the dominant plant in the bottom of the facilities b ecause 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 aes thetically 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.

5PROJECT COST

The final project cost, excl uding design, was $11,946 for both curb extensions, including construction and project management.

• Curb Extension Construction Construction cost $9,626, or 81% of the tota l project cost. This included sawcutting and removal of existing asphalt, excavation, concrete curb installation, soil import and preparation, grading, and safety pain ting and reflectors on the curbs.

• Landscape Construction Landscape construction cost $2,320, or 19% of th e total construction cost. This included check dam construction, fine grading, plant procur ement, 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 chemi cal applications are allowed), plant trimming, plant replacement if needed, and re moval of leaves, debris, and sedi ment. These activities will be performed approximately four times annually. (As is typical, more visi ts 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 fac ilities. Results indicated an infiltration rate of 1.8 inches per hour. Monitoring results were instrumental in id entifying 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).

6Forebay 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 stre et 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 fo r flow out of the facil ity 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 creat ed 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 conc rete slab forebay at each inflow point (at the east end of each facility) works we ll 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 environmen t, reaching 24 inches or more and requiring trimming so it does not inhibit th e growth of other plants and obscure the curb. This density did, however, minimize weeding during the first year. Cost savings co uld 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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