## 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 approxi mately 7,500 square feet of paved surfaces. They treat and infiltrate most of the runoff they receive, providing volume and flow control an d 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 urba n streetscape by adding attractive
greenspace.

• The planters are designed to safely accomm odate 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 stormw ater 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 a nd 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 p arking.

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 Planter s at SW 12th and Montgomery • Portland, Ore g on integrates stormwater management into an urban environment.

1B ACKGROUND

T his Green Street project conve rted a previously underused lands cape area between the sidewalk

e

ote

ow do the Street Stormw ater Planters Work?

Stormwater runoff from the st reet flows downhill along the ex isting 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

7

The adjacent sidewalk slopes toward the planters, and sidewalk runoff enters the planters and street curb into a series of landscaped stormwater planters de signed 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 aes thetically integrated into the urban streetscape. Although th project has a strong functional component, it is the integration of the landscaped stormwater planters into the urba n 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 ur ban 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. Du ring large storm events, water may enter the planter at a rate faster than the soil can inf iltrate, resulting in a ponding depth greater than inches. In that case, the runoff exits a second cu rb cut, flows back into the street, and enters the second (downhill) planter. This process conti nues for the third and fourth planters. If the fourth planter ponds to capacity, it overf lows to the existing storm system.

• through curb cuts.

23SITE 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 st andpipe 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 proj ect 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 wa s 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, includi ng 6-inch-wide perimeter curb ing around the planters.

STORMWATER CAPACITY AND SYSTEM CONFIGURATION

Stormwater Management Goals

The stormwater management goal was to maximi ze the capture, treatment , and infiltration of street runoff, while providing a vi sual amenity for the neighborhood.

Geotechnical Evaluation

An infiltration test was not required before construction because adequate documentation already existed concerning the well-draining ch aracteristics 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 4System 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 sidewa lk 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 sa nd-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 seco nd curb cut back into the street. Final overflow exits through a curb cut in the last plan ter 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 inch es below grade and backf illed 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.

5The plants were installed at a density greater than require d by the City’s Stormwater Management Manual. This was done to redu ce maintenance requirements (weeding, watering, etc.) and to create an aestheti cally appealing landscape quickly.

The types of vegetation external and adjacent to the facilities were selected for their drought tolerance, low maintenance, evergr een foliage, and short stature; th e 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 rem oving existing landscaping and hardscape, excavation, concrete curb insta llation, sand-set concre te unit pavers installation, curb cuts and ornamental grate installation, soil im port and preparation, and final grading.

• Landscape Construction The landscape construction activities cost $4,90 0, or approximately 13 percent of the total project cost. This included fine grading, plan t procurement, plant material, and pea gravel mulch installation.

• Ancillary Construction Activities Miscellaneous street and sidewalk repair wo rk cost $4,000, or approximately 10 percent of the total project cost. This included re-landscapin g of a small area just so uth of the facilities, under existing trees, for visual continuity. Th ese ancillary constructi on and landscaping tasks contributed to highe r project costs.

As Green Street projects become an accepted practice, it is likely that design and project management costs will decline.

6MAINTENANCE AND MONITORING

Maintenance

Sediment collecting behind the juncus on the concrete p ad Maintenance of the faciliti es includes hand weeding (no chemicals are allowed), plan t 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 heavie r-than-average sediment lo ading d uring that time.

he maintenance frequency for these types of f acilities depends on the site. City staff will
ear;

onitoring

he City will monitor the facilities for hydrauli c performance, maintenanc e requirements, metalT monitor long-term maintenance needs. Maintenan ce visits will occur at least four times a y additional visits will occur if neededbecause 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.

S UCCESSES AND LESSONS LEARNED

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

A flow test at this site indicated that the st ormwater planters are e ffective in reducing the d

The final project design resulted from seve ral discussions among de sign 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, Main tenance, 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.

7• In the first 2 years of operation, the first two plan ters 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 th at was assumed during design. The inlet just upstream of the facility and another inlet at the base of an overpass frequently clog with leaves a nd other debris, resulting in si gnificant 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 b ecomes 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.

A sphalt berm at facility inlet to help d stormwater flow into the stormwat irect er planter

8