NE Siskiyou Green Street Project 

 NE Siskiyou Street between NE 35th Pl. and NE 36th Ave., Portland, Oregon 

Project Summary

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

Technology: A pair of stormwater curb extensions 

Major Benefits: • The extensions capture runoff from approximately 9,300 sq. ft. of paved surfaces, treating and 

infiltrating a large proportion of the runoff (see Flow Test Report1

). 

• The curb extensions converted about 590 square feet of pavement to landscape. 

• They are attractive additions to the neighborhood, improve the urban environment, and increase 

pedestrian safety at the intersection.

 Cost: The total project cost, including management, design, and construction was $20,000 of which $3,000 is

attributed to ancillary street and sidewalk repairs costs that may not be needed for other similar projects. 

Total cost for the stormwater curb extensions only was $17,000 or $1.83 per square foot of impervious area 

managed.

Constructed: City crews constructed the extensions in two weeks in October 2003. 

Maintenance Portland Parks and Recreation will maintain the facilities during the two-year establishment period (until 

October 2005); the source of long-term maintenance services is to be determined. 

Features 

NE Siskiyou Green Street Project Report: Completed April 2005 

March 2005

• This was Portland’s first Green Street project to use 

landscaped stormwater curb extensions to manage street 

runoff. 

• The design provides water quality treatment but also 

maximizes infiltration of the runoff. Each facility has 

four compartments separated by check dams to promote 

infiltration. 

• 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. The 

low evergreen plantings blend with landscape areas on 

the adjacent residential properties. 

• The adjacent neighbors have played a major role in 

maintaining the two facilities, providing seasonal 

watering and weeding. 

 

1

 Flow Test Report for the Siskiyou Curb Extension, October 2004; BES 
 Background 

The Environmental Services Sustainable Management Team undertook the project in 2003 as part of 

an on-going evaluation of techniques for managing runoff from streets. Urban stormwater runoff 

pollutes rivers and streams and contributes to combined sewer overflows (CSOs) to the Willamette 

River. It can also cause localized backups of the combined sewer during large storms. 

Stormwater curb extensions hold particular promise as a sustainable practice for managing runoff from

existing streets. These natural systems capture and filter runoff and allow it to infiltrate into the 

ground. They are an alternative to traditional stormwater sumps for managing street runoff. In addition 

to treating and disposing of runoff, they integrate well with existing neighborhood vegetation and 

generally improve the urban environment. Portland has constructed many curb extensions over the 

years to improve pedestrian safety; this new version of the curb extension provides many additional 

benefits. 

The project on Siskiyou was the first retrofit to an existing street in Portland. Environmental Services 

built a second set of curb extensions on Ankeny Street (at SE 56th) in 2004, and by 2005 both the City 

and private developers were implementing a number of similar projects. These types of facilities can 

be constructed in a variety of configurations, as retrofits or as part of new development, and are 

referred to alternatively as bump-out swales, pocket swales, or street stormwater planters. 

South Curb Extension 

North Curb Extension

NE Siskiyou Street 

NE Siskiyou Green Street Plan View

NE Siskiyou Green Street Project Report: Completed April 2005 
Site Selection Criteria 

• Traffic Impacts: City traffic engineers considered the low-traffic residential setting ideal for a 

demonstration project. The street is 28 feet wide. The addition of two 7-foot wide curb extensions 

created an acceptable queuing configuration. 

• Stormwater Catchment Areas: The size of the catchment, a little over 9,000 sq. ft., was 

considered fairly representative of conditions in the surrounding neighborhood. 

• Utility Conflicts: Water lines were the only subsurface utilities within the project area and did not 

present obstacles. See a detailed discussion under “System Configuration.” 

• Loss of Parking Spaces: The project did not eliminate on-street parking. Adjacent property 

owners can park in front of their houses on SE 35th Place. 

• Street Slope: The moderate street slope (2%) was suitable for a first test of the technology. 

• 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. 

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

Capacity and System Configuration” for details. 

• Space Available for the Facilities: The space available for the curb extensions – length of curb 

unbroken by driveways or near a fire hydrant – is considered representative of conditions in other 

areas. 

Stormwater Capacity and System Configuration 

Stormwater Management Goals

The objective was to maximize the capture, 

treatment, and infiltration of street runoff while 

providing a visual amenity for the 

neighborhood and improved pedestrian safety. 

Geotechnical Evaluation

An infiltration test was not required before 

construction. Adequate documentation of 

characteristics of the local soils already existed. 

The Natural Resources Conservation Service 

(NRCS) 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” thick. Soil below this depth is gravelly silt 

loam and gravelly sand to a depth of 

approximately 60”. 

March 2005 

NE Siskiyou Green Street Project Report: Completed April 2005 
 NE Siskiyou Green Street (Cross Section) 

NE Siskiyou Green Street Project Report: Completed April 2005 
System Configuration

With few exceptions, the two curb extensions are identical in 

configuration. 

Catchment Areas (pavement, driveways): 

• North curb extension: 3,000 square feet 

• South curb extension: 6,300 square feet 

Street Slope: Approximately 2%

Facility Dimensions (applies to each facility):

• Length: 60 feet; width: 7 feet 

• Total area: 275 square feet 

• Depth at curb: 6 inches 

• Depth at center: 12 inches 

• Maximum ponding depth: 7 inches at center 

Internal Storage Volume: 120 cubic feet 

Overflow: Overflow exits through a curb cut at the west end 

of the facility, draining to the combined sewer via the 

street gutter and the existing street inlet. No 

modifications were made to the inlet.

Check Dams: Each curb extension has three checkdams, with 

four separate compartments for ponding runoff and 

slowing its passage through the facility. 

Additional Information: 

• The asphalt crown on Siskiyou Street is north of the 

center of the street, a large factor in the difference in 

catchment sizes. 

• The gravel subgrade under the asphalt on Siskiyou 

Street is about 4 inches thick. 

• The two extensions were excavated to a depth of 14 

inches below grade. The native soil was tilled prior to 

importing 8 inches of soil mix and then the material 

was tilled a second time. There is not a gravel trench 

underlying the facility (a common feature of some 

designs). 

• Excavation in the north extension intersected the top 

of a gravel-filled trench containing a water utility 

line. The trench runs the length of the middle of the 

facility, about 3 feet from the pre-existing curb. 

Excavation of the southern extension did not intersect 

the adjacent water utility trench, which runs 

underneath the new curb (about seven feet off the preexisting curb). 

• The check dams were constructed of compacted clay 

and covered with pea gravel and river rock to 

minimize erosion. See “Success and Lessons 

Learned” for details. 

• The first compartment (forebay) in each curb 

extension has the same configuration and planting regime as the other compartments. 

NE Siskiyou Green Street Project Report: Completed April 2005 
Landscaping

Curb cuts: inflow point (top), side inlet (bottom)

The facilities were excavated throughout to 14 inches 

below grade and backfilled with a 3-way mix of sand, 

topsoil, and compost. The mix was tilled into the native 

soil and spread to create a shallow parabolic cross section 

(see illustration). 

The plants were selected for their drought tolerance, 

evergreen foliage, and short stature. The typical mature 

height of the plants is less than two feet. These 

characteristics were a priority in order to minimize

maintenance and address safety concerns. The most 

common plants, including sedges, rushes, ferns, and 

broadleaf evergreen shrubs, are native species. The 

planting plan also included some non-native plants, 

primarily to provide seasonal color accents. Rushes are 

the dominant plant in the lowest portions of the two 

facilities. Their stiff structure helps slow the passage of 

water and they thrive in the variable moisture conditions. 

There is no permanent irrigation system in the curb 

extensions. 

The plants were installed at a density greater than 

required by the Environmental Services Stormwater 

Management Manual in order to reduce weeding and 

other maintenance requirements and to quickly create an 

aesthetically appealing landscape. 

BOTANICAL NAME COMMON NAME PNW Native

SHRUBS

Euonymus japonica "Microphylla" Boxleaf Euonymus no

Mahonia repens Creeping Oregon Grape yes

Polyshtichum munitum Swordfern yes

PERENNIALS

Helictotrichon sempervirens Blue Oat Grass no

Deschamsia caespitosa "Northern Lights" Variegated Tufted Hair Grass native cultivar

Iris spp. Iris Bulbs no

Narcissus spp. Daffodil bulbs no

 BASIN PLANTINGS

Carex testacea New Zealand Orange Sedge no

Juncus patens California Grey Rush yes

NE Siskiyou Green Street Plant List 

NE Siskiyou Green Street Project Report: Completed April 2005 
Project Costs

The final project cost was $20,000 for design, management, construction activities, and ancillary 

sidewalk repairs. 

I. Budget Elements

1. Construction Management and Overhead 

Project and construction management cost $4,500 or 23% of the total. 

2. Construction Activities (Curb Extensions Only) 

Curb extension construction and landscaping cost $12,500 or 62% of the total budget. 

• Curb Extension Construction 

The core construction activities cost $8,250 or 41% of the total project cost. This included 

sawcutting and removing existing asphalt, excavation, concrete curb installation, soil import 

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

• Landscape Construction 

Landscape construction cost $4,250 or 21% of the total project costs. This included check 

dam construction, fine grading, plant procurement, plant material, and mulch installation. 

3. Ancillary Construction Activities

Miscellaneous street and sidewalk repair work cost $3,000 or 15% of the total. 

II. Budget Discussion

It’s likely that design and project management costs will decline for curb extension projects as they 

become more routine. The project included one-time costs such as the development of outreach materials 

and standard drawings. Ancillary construction tasks – repair to the adjacent pavement as well as the 

sidewalk – may have also made this project more expensive than subsequent projects. 

The south planter; fall 2004 South curb extension, Spring 2004

NE Siskiyou Green Street Project Report: Completed April 2005 
Maintenance and Monitoring 

Maintenance

Adjacent property owners have voluntarily modified their 

home irrigation systems to help water the curb extensions. In 

the long term, the City is responsible for providing any 

irrigation required. Given the shady location of the curb 

extensions, irrigation after the 2-year startup period should be 

minimal.

Portland Parks and Recreation will maintain the curb 

extensions for two years after construction, ending October 

2005. Maintenance includes hand weeding (non-chemical 

applications), trimming plants, plant replacement, and major 

leaf and debris removal as needed. Environmental Services 

will re-evaluate maintenance requirements in December 2005 

and make arrangements for long-term maintenance services. 

Monitoring

Environmental Services will monitor hydraulic performance, maintenance requirements, the success of 

the planting regime, and comments from neighborhood residents. Environmental Services conducted a 

first flow test in summer 2004. See Flow Test Report: Siskiyou Curb Extensions, August 4th, 2004 

online at www.portlandonline.com/shared/cfm/image.cfm?id=63097. 

Successes and Lessons Learned 

• The data suggest that the curb extensions on Siskiyou are capturing and infiltrating a large 

proportion of the runoff that drains to them. 

• The plants grew vigorously during the first year and little weeding was required. 

• In the first year the vegetated forebays filled twice with sediment and debris (to a depth of 4-6 

inches). City staff removed the sediment by hand with a shovel and rakes. The cleanings typically 

required about 30 minutes per forebay. 

• The earthen checkdams are susceptible to erosion during large storms, as observed during the hose 

test in August 2004. A more substantial structural design should be employed in the future. 

NE Siskiyou Green Street Project Report: Completed April 2005