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• An Eight-Step Approach to Implementing Stormwater Retrofits

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Photo: Ashforth Pacific Inc.
Planters at Liberty Centre

Liberty Centre Parking Garage
When it comes to urban stormwater retrofitting, every little bit counts.

In the case of Liberty Centre Parking Garage in Portland, OR, that little bit consists of two planters squeezed between two exterior walls of the parking garage and the sidewalks. They make up just 5% of the drainage catchment area of the 36,000-square-foot parking deck, but they can infiltrate almost all of the stormwater from a two-year storm.

The parking garage project is part of a program implemented by the city’s Bureau of Environmental Services, which offered grants and technical support to retrofit commercial properties to reduce the amount of stormwater flowing into the city’s combined sewer system. Stormwater causes the sewers to overflow, which results in polluted runoff spilling into creeks and the Willamette River.

“The owner of the parking garage stepped forward and was willing to do something innovative on the property,” says Henry Stevens, an environmental specialist with the Bureau of Environmental Services. “It really was their project, which we contributed to.”

With its setting in a highly urban area along busy sidewalks, it was the only project of its kind. It’s also different from many other projects with planters next to buildings, Stevens says, which usually have a hard bottom and porous pipe beneath about 18 inches of soil. The Liberty Centre planters have soil floors, so water infiltrates directly into the ground.

The parking garage’s reduction in stormwater volume and its use of native vegetation have qualified it for LEED (Leadership in Energy and Environmental Design) certification.

Photo: Ashforth Pacific Inc. Planters at Liberty Centre

The goals of the project, which was carried out between 2001 and 2003, were to manage as much stormwater runoff as possible, Stevens says. It would both reduce the volume of stormwater flowing into the combined sewers and improve the quality of water flowing into the Willamette River.

The Project. The parking garage was built in 1996. In its original design, pipes carried stormwater from the parking deck to four storm drains inside the parking garage. Now, they carry it to the two planters, which run the length of the two walls. Half of the stormwater infiltrates into the planter along the eastern wall, and the other half runs into the planter along the western wall. The excess flows into the city’s combined sewer system.

One of the biggest challenges was changing the piping inside the parking garage. “It was a fairly substantial piece of work,” Stevens says.

The pipes were easy to access, because they hang from the ceiling on the main floor. But the project added more than 150 feet of iron pipe, extending the original piping through the two walls to scuppers that drain into the planters. The configuration of the city’s storm drains determined the location of the scuppers. In addition, the project redirected the four interior storm drains to the exterior.

When the parking garage was built, the foundation walls were protected from moisture by a waterproof seal, drainage panels that included a fabric cover to limit the passage of sediment, and pervious pipes along the bottom of the foundations’ edges, which drain into the combined sewer.

The planter on the eastern side is 175 feet long and about 6.6 feet wide. The planter on the western side is slightly larger and more sloped, with greater stormwater capacity. The drainage protection system was raised 6 inches—higher than the level that stormwater can pool on the planters’ surfaces—to protect the two walls. The floors and the sides of the planters slope toward the sidewalk, and the floors slope slightly to the south, like a swale, to further protect the walls.

Photo: Seattle Public Utilities A vegetated swale, part of Seattle's Broadview Green Grid Project

“We had enough experience with soils to think they’d drain pretty well,” Stevens says. Approximately 6 inches of soil was excavated, amended with compost, and replaced. The planters were then filled with drainage rock to a depth of 12 inches. Each planter has four retention dams to help retain and infiltrate the runoff.

Most of the existing vegetation along both sides of the building was removed, including 10 red maple trees, which were deemed unlikely to survive in the new environment. Although the soil typically drains in three hours, it’s wetter than before, Stevens says.

The owners chose dense plantings of mostly native vegetation, including alder saplings, grasses, sedges, and rushes. These help treat the runoff and promote stormwater uptake, and they are low maintenance. They get little irrigation and need no chemicals.

“Non-natives can make sense, for example, if there’s heavy foot traffic,” Stevens says. “The alders have done really well.”

Runoff enters each planter from two decorative scuppers: one at the north end and one at the midpoint. The slope beneath the scuppers is lined with impervious polyethylene fabric and covered with river rock to reduce erosion.

The planters absorb and infiltrate at least 2 inches of stormwater per hour. At their southern ends, overflow standpipes 11 inches above the floors send overflow to the combined sewer. This is unusual since the city usually does not allow private stormwater facilities to be connected to public ones.

Funding. The garage’s owner received a $30,000 grant from the Bureau of Environmental Services. “We welcomed the participation of the company,” Stevens says. “They designed and built the project.” The total cost was $75,530, which includes design, construction, and management. That breaks down to $2.08 per square foot of catchment area.

According to the city’s Web site, 44% of the budget, or $33,015, went to construction. The highest construction expenses were for plumbing. Another 33% of the budget, or $24,140, went to landscaping. At almost $11 per square foot, this was especially high. Most of the landscaping costs were to remove and replace the maple trees, plant dense vegetation, and, because of the time of year, use plugs instead of seeds.

Ashforth Pacific, which owns the building, is responsible for all of the maintenance.

Results. Logistically, it’s been unfeasible to monitor the project, Stevens says, but it’s performing very well. Although it wasn’t designed to provide complete onsite disposal for all storm events, the planters rarely, if ever, overflow.

“This is getting to be a common sight in Portland,” he says. “It looks like landscape, but it’s designed to manage runoff.”

Photo: Barr Engineering Stormwater flowing into a rain garden in Burnsville, MN

Photo: Barr Engineering Homeowners were motivated by the idea of having a rain garden in their front yard and wanted to be part of improved water resources.

Burnsville Rain Gardens
Stormwater used to flow down Rushmore Drive in Burnsville, MN, right into Crystal Lake. The amount of phosphorus it carried was causing algae blooms in the lake, which affected recreation. Today, though, the suburban street is the site of a demonstration project where 17 of the 25 homes have rain gardens that capture and infiltrate runoff before it reaches the lake.

“The project was initiated by the city,” says Kurt Leuthold, a civil engineer with Barr Engineering, who did the engineering on the project. There wasn’t enough space for traditional stormwater ponds in the neighborhood, which was built in the 1980s, but Rushmore Drive has a gentle topography, sandy soils, and 15-foot rights of way that provide plenty of space for the rain gardens.

Construction of the project, which has a drainage area of 5.3 acres, began in 2003 and was completed in 2004. Monitoring of the stormwater runoff from the neighborhood started in 2001 and continued through 2005, and there are plans to resume the monitoring. Compared to a similar neighborhood nearby without rain gardens, the project has reduced runoff into the lake by 90%.

The Project. Once a survey showed that no underground utilities had to be moved, Leuthold and landscape architect Fred Rozumalski began creating grading plans and designing the gardens to maximize infiltration.

“Since it’s a retrofit, we did the best we could with the area that was available,” Leuthold says.

At some locations, they used limestone retaining walls for the grade change between the gardens and the lawns behind them, and gradual slopes to wrap around the sides of the gardens. A narrow strip of lawn in front of the gardens traps sediment from the street. To allow stormwater to reach the gardens from the street, they replaced 10-foot lengths of curb with curb cuts.

The gardens were made as large as possible given the existing conditions. This resulted in rainwater gardens that can accommodate 0.9 inch of runoff from the tributary impervious surface.

In 2003, the contractor excavated the native sandy topsoil and mixed it with sandy topsoil and compost 8 inches deep. Residents planted their own gardens with help from Leuthold, Rozumalski, and the city staff. This helped keep costs down, but there was another reason as well. “We like to include people in the process,” Leuthold says. “It gives them an intimate understanding of what’s there.”

When the planting was finished, the contractor mulched the gardens with shredded hardwood mulch. The curb cuts weren’t built until 2004, to allow time for the plants to become established.

Residents. There was a fairly significant educational component to the project, Leuthold says. “When you tell people their garden would take runoff from the street, the initial reaction is ‘No.’”

City staff and consultants first contacted residents by mail and telephone, letting them know how rain gardens would improve the lake’s clarity, then followed up with neighborhood meetings. Ultimately, more than 80% of the homeowners wanted to participate. Many were motivated by the idea of having a garden in their front yard, Leuthold says, but many also wanted to be part of improved water resources in the city.

City staff and consultants then went door to door to talk about the specifics of each garden. They discussed the palette of plants and gave homeowners a choice of three basic styles: native wildflowers, cultivated perennials, and/or shrubs. All but one chose low-maintenance perennials and shrubs.

The homeowners maintain their own gardens. “It’s a reflection of the people who live there,” Leuthold says. “These people are very proud and happy with their gardens. They see them as an amenity and, therefore, take good care of them.”

Funding. The cost to design and construct the rain gardens was $147,000. The total cost per garden was approximately $7,500, about $500 of which was for the plants. The city of Burnsville funded $30,000, and the Metropolitan Council provided the rest. The Metropolitan Council also pays for most of the monitoring.

Results. The project is different from most rain garden projects, which tend to be more spread out, Leuthold says. “The real impact is when you cluster these gardens together.”

The monitoring shows excellent results. In nearly all cases, the gardens have been able to infiltrate and treat at least 0.9 inch of stormwater runoff. Most gardens have dried within three or four hours, and there haven’t been any adverse effects from ice buildup in the winter. Leuthold credits the city and Barr Engineering for the success of the project, which hasn’t gone unnoticed. Many low-impact-development bus tours go through the area, he says.

“I’m hoping this continues,” he says. “The demand for rain gardens is constantly increasing. They’re very effective at improving water quality, and they’re an asset and an amenity for the homeowners.”

Broadview Green Grid Project
Contrary to popular belief, Seattle doesn’t receive an excessive amount of annual rainfall. However, the 36 inches it does receive falls on slopes of dense glacial fill and impermeable urban surfaces. Stormwater is causing the familiar problems: polluted runoff, eroded stream channels, and impaired wildlife habitat.

Photo: Seattle Public Utilities Swale cells in Seattle's 107th Street, an east-west street

Photo: Seattle Public Utilities The 107th cascade before planting

In 1999, Seattle Public Utilities (SPU) began its Natural Drainage System program. It focuses on increasing pervious areas along street edges by redesigning existing streets and installing landscaping that infiltrates stormwater efficiently.

In 2003, SPU started its third project in the program, the Broadview Green Grid Project, in partnership with the Seattle department of transportation, and completed it in 2004. The project covers approximately 32 acres, almost an entire sub-basin of Piper’s Creek, which leads to the Puget Sound.

“We have our goals for our creek watersheds,” says Tracy Tackett, a civil engineer who runs SPU’s low-impact-development programs. “We wanted to do a full sub-basin to measure the reduction in stormwater runoff.”

The goals of the project were to move stormwater off of roads and properties, slow it down, and allow it to infiltrate before it reached Piper’s Creek. This would recharge the groundwater and sustain the creek during the dry summer months, as well as reduce erosion in the creek and the amount of pollutants—oil, grease, heavy metals, pet waste, sediments, fertilizers, and pesticides—emptying into it.

The Project. The project encompasses 15 blocks of residential property. The vast amount of stormwater coming from upstream is considered public water, so the entire project took place on public land: across the width of the streets and the easements on both sides, for a total width of about 60 feet, says Tackett, who started as lead designer of the project, became program manager, then moved on to funding.

One of the challenges that the SPU team faced was working in the street rights of way, which residents, pedestrians, traffic, and utilities all share. Collaboration and negotiation are crucial in resolving the conflicting priorities, she says.

Before committing to the project, SPU surveyed the residents to ensure that they would support it. SPU held community meetings about the project and the design concept, and later, the designers toured the blocks with residents and an arborist, who discussed the health of their trees.

“It’s in their front yards,” Tackett says. “We wanted them to be engaged.”

The roadway redesign affected only three north-south streets, which slope down to the west. They began as straight, 25-foot-wide roadways with two-way traffic and large parking areas on both sides. They’ve been narrowed to about 19 to 20 feet wide, Tackett says, and they meander slightly, slowing the runoff and guiding it off the road. Every street has two-way traffic, one parking lane, and room for emergency vehicles, which are designed for urban areas. Some streets have a sidewalk.

The narrow, winding streets are a bonus for residents. “Traffic is going to be moving very slowly,” Tackett says. “We don’t want residential streets to be cut-through streets.”

The easements on both the north-south and the east-west streets have some traditional drainage features, such as culverts and catch basins, as well as swales, bioretention cells (called “rain gardens” when they’re on private land, Tackett says), and cascades.

“I have a strong preference for living systems,” she says. The soil maintains infiltration, and may even increase it over time as worms and tree roots create flow paths for stormwater. Bacteria in the soil help break down pollutants like motor oil. There’s also less maintenance needed.

The type of design was based on the slope of street: The steeper the street, the more grade control is used.

The east-west streets have very steep downhill slopes. The swales, which are along only one side of the streets, are giant “swale cells.” They’re divided by concrete weirs, each with a notch to control the flow of water. The weirs act as a series of steps that slow stormwater as it flows down into the swales. Rock walls line one side of these swales.

The north-south streets, which have cross slopes to a maximum of approximately eight degrees, have 20-foot easements with swales along both sides of the streets. Rock walls line one side of the swales to maximize their area. The bioretention cells are on flatter ground and aren’t designed to retain the high volumes of stormwater that the swales do.

Before planting, the team tested the soil infiltration rates by digging test pits. “Test pits 4 by 4 by 4 onsite give a better idea of soil infiltration rates than soil borings,” Tackett says.

Photo: Seattle Public Utilities Swale and curves on Phinney Avenue, a north-south street

The team dug two pits per block, and even then found soil variability during construction. On north-south streets, they found that they needed to add compost and engineered soil to maximize absorption and filtration.

All of the features are landscaped with native plants, whose roots help stabilize the soil, absorb runoff, and remove pollutants. Smaller trees and shrubs were chosen that wouldn’t outgrow the easements, as well as grasses, sedges, and rushes in dense groups and wetland plants in lower, moister areas.

Most of the swales are designed to infiltrate half an inch of stormwater per hour and all stormwater within three days, Tackett says. There is never more than 12 inches of standing water while it’s raining. Any water that doesn’t infiltrate flows into a pool where it’s treated and detained before continuing into the downstream stormwater network.

Costs. SPU funded the entire project, which cost $5 million, Tackett says, with 11% of the costs for the preliminary engineering, 21% for the design phase, and 68% for construction. On one hand, costs for planning and communication were higher than in the average project because the concept is still new. On the other hand, maintenance costs are lower.

Homeowners maintain the landscape. Most of SPU’s maintenance costs consist of keeping the landscape mulched. Sedimentation structures, which accumulate pollutants attached to dirt and particles, are cleaned out once a year. “If the swale isn’t too high, it seems there aren’t too many issues with sediment,” she says.

According to SPU projections, natural drainage systems are costing at least 25% less than its traditional stormwater systems because of decreased building and maintenance costs. They also offer aesthetic improvements that traditional systems do not.

The Future. Although Seattle’s right-of-way improvements manual has a chapter on natural drainage systems, meeting existing guidelines was a challenge, Tackett says.

“We’re updating the chapter to have a lot more content on how to do natural drainage. One of the main targets is to provide enough information in the manual and codes so other people can do this, and it doesn’t all have to be city funded. We want to one day make this a standard.”