Making Rain Gardens Work

July 22, 2014

Time is the test that reveals how well a rain garden is working. Time tells whether the rain garden’s design and construction are as effective as they were projected to be. Portland, OR, has a number of rain gardens that meet the test of time, including the one at Glencoe Elementary School.

“We certainly spent a good amount of time designing it–but even with all the preparation you don’t know exactly how it will turn out,” says Tim Kurtz of Portland’s Bureau of Environmental Services (BES). “We invested in monitoring and have done monitoring fairly consistently since. We have it sized the right way, and we’ve kept up with the maintenance.”

Like many such projects, the Glencoe rain garden and nearby bioswale were designed and installed in response to complaints from the neighborhood residents. After heavy storms, the city sewer system’s pipes backed up and residents found sewage in their basements.

The pipes in the Mt. Tabor section of southeast Portland were laid in 1937. As the population grew and stormwater volume increased because of more impervious surface, the combined sanitary and storm sewer system was no longer large enough to function adequately.

RainWise rain garden in the Ballard neighborhood of Seattle. Garden design by Jan Satterthwaite, ASLA, Vireo Design Studio

Portland’s BES decided that green infrastructure–a rain garden and a bioswale–would offer an economical and effective solution to managing runoff and reducing combined sewer overflows (CSOs) in the area. For the project BES formed a partnership with Portland Public Schools and the elementary school.

Infiltration rate plays a critical part in whether or not a rain garden manages runoff successfully. Determining that rate requires taking the time to test and learn about the soil on the site.

Staff members from the BES Materials Testing Laboratory (MTL) began that phase of the project in September 2001. They used augers to a depth of more than 10 feet and also took grab samples at various depths. The soil samples taken were then evaluated with both a mechanical grader and a hydrometer.

The top 5 feet of soil was described by MTL staff as “medium stiff silt” with “low to very low permeability,” based on grain-size distribution. At 4 feet below grade, the samples had a silt/clay fraction of more than 85%.

Below that level, the soil became increasingly more sandy. MTL staff found “sandy silts” and “silty sands” at 8 to 10 feet below grade.

In April 2003, MTL staffers conducted an infiltration test at 3 feet below grade, using a double-ring infiltrometer. The result was far from desirable: They did not see any evidence of infiltration.

“It’s pretty rare for us to get a zero, but it did teach us that there are so many variables,” explains Kurtz. “We were using a double-ring infiltrometer, the most conservative, and water can move sideways, not just down.”

But the knowledge that the soil at the site becomes sandier with depth suggested that the site might be acceptable for the rain garden. MTL staffers conducted a second infiltrometer test in the nearby alley and obtained a much more promising result.

The test at the second location yielded infiltration rates of more than 5 inches per hour. Although the soils were similar at the two locations, those at the site in the alley appeared to penetrate the sandier horizon lower in the soil profile. BES staff also had conducted informal infiltration tests (simple pit tests) that suggested adequate infiltration rates.

Glencoe rain garden, view looking southeast
The rain garden and bioswale were installed in response to local flooding.

The 2,000-square-foot rain garden and swale project was designed to handle runoff from a 25-year storm event. The two adjoining streets measure 25,000 square feet. The school’s parking lot is another 10,000 square feet, and the portion of its driveway that is included adds about 1,000 square feet.

Two major factors constrained the design. The first was the maximum ponding depth: for public safety–especially with children nearby–the depth was limited to 6 to 8 inches under most conditions. The second constraint was installing plantings no taller than about 2 feet. Although that condition meant a much smaller list of plants from which to choose, it was considered essential to create a visually open rain garden and to minimize maintenance, which is done by the owner, BES.

The forebay was planted with rushes (gallon containers, 18 inches on center). The rushes and pea gravel mulch do a good job at controlling erosion there. The main section of the rain garden contains mostly slough sledge (gallon containers, 18 inches on center). A mix of native and non-native species including salal, kinnikinnick, Oregon grape, dwarf arctic blue willow, and lavender were planted in the upland areas.

Kurtz says that plant selection is a factor in any rain garden’s success. “Absolutely–right plant, right place. We do include non-native or less [commonly] native that can thrive in Portland’s dry summers and wet winters.”

He adds, “We still use junkus patens, a rush, but it’s a plant that will grow bigger next to a sidewalk. We’ve moved to carex, a sedge, because it’s lower growing and doesn’t need trimming.”

There is no permanent irrigation system. The plantings have not been watered since the summer of 2005.

The site of the rain garden was a grassy area before construction. The facility sits on native soils. There is no subsurface rock trench or gallery under the facility. The native soils were ripped and tilled; they were not otherwise modified or amended.

The maximum excavation depth was approximately 3 feet below grade. The concrete walls are 2 feet deep (top to bottom), with a 12-inch exposure.

The bottom of the rain garden is entirely flat. The planted edges have a slope of 4:1. The forebay accepts runoff from the street and overflow from the swale alongside the parking lot through a 12-inch pipe.

A weir controls the depth of ponding in the forebay of the rain garden. When 2 inches of ponding accumulates, runoff passes into the main compartment. This main section covers about 1,450 square feet. There, runoff ponds to 5.5 inches before overflowing across a second weir into the drainfield.

Three rock-filled trenches, each 12 inches deep, lie across the main compartment. They were installed to help spread flows evenly across the bottom of the rain garden and to promote infiltration.

More than 10 years after its construction in late summer 2003, the Glencoe School rain garden is actually performing better than expected. Most significantly, not even one basement backup has been reported since the project was completed.

The plants and soils in the facility are performing well, soaking up 85% of the annual volume of runoff. The annual peak flow has been reduced by 80%. By design, any pollutants from the street runoff are concentrated at the rain garden’s forebay where the water first enters. Monitoring continues to be sure this remains as such.

“We could have put in sediment control manholes, a little touch of gray infrastructure,” says Kurtz. “When we cleared the forebay last fall [2013] we had to remove some of the plants [for access] and replace them.”

Thanks to a carefully designed and thoroughly tested and monitored rain garden, neighborhood residents have dry clean basements and an attractive natural space to enjoy. For the teachers and students at Glencoe Elementary School, the rain garden offers more outdoor classroom and butterfly garden learning experiences.

“Sometimes rain gardens drain slowly right after construction,” notes Kurtz, “but after time, as the plants’ roots grow and worms bore around, the soil is loosened and the facility is working much better.”

Portland is encouraging the use of more rain gardens. Tabor to the River is an ongoing program in southeast Portland that will last for about a dozen more years. Its purpose is to lessen the frequency of CSOs that result in backups of sewage in homeowners’ basements and sewage discharges from outlet pipes into the Willamette River.

“Our goal is to have 500 rain gardens in [the area served by] Tabor to the River,” says Emily Hauth, sustainable stormwater manager at BES.

Working in one area, BES partnered with 33 homes and businesses to have rain gardens installed on these private properties. The purpose was to infiltrate runoff from the roofs, which totaled 45,000 square feet. These rain gardens infiltrate nearly one million gallons of stormwater each year that would otherwise enter the combined sewer system pipes, contributing to basement sewage backups and sewer overflows to the Willamette River.

The projects are part of the public-private partnerships happening in the Tabor to the River area. In Tabor to the River, green infrastructure like rain gardens and green streets are helping save sewer ratepayers $63 million as the city improves the old sewer system.

Other interesting and well-functioning rain garden projects in Portland include those at the Mt. Tabor Middle School and Café Au Play. The latter is a coffee shop geared to parents and their young children, with a strong connection to the community. Although it’s located on private property, it infiltrates stormwater from adjacent public streets.

Rain gardens are also popular in Seattle. Some rain gardens there that at least five years old and all still working well, says Doug Hutchinson, stormwater monitoring lead at Seattle Public Utilities. He cites Street Edge Alternatives (SEA Street), 110th Cascade Project, Broadview Green Grid, High Point Natural Drainage System, and Pinehurst Green Grid.

Having a rain garden work well is “all about good geotechnological investigation and site-specific, too,” says Hutchinson.

The rain gardens also serve as outdoor classrooms.

Knowing the types of soil and how well they infiltrate is important, especially in locations such as Seattle, which has a glacial till interspersed with other types of soil. In such cases, “you really do have to look at every specific site,” warns Hutchinson. “There’s such micro-variation and subsurface variation.”

In some parts of Seattle, he says, “a finger of glacial till runs down one block, but in another block the soil is a mixture of sand and cobblestone that would infiltrate very well.”

Monitoring a rain garden’s performance provides insight and ways to improve future projects. “The most effective monitoring is a combination,” explains Hutchinson. “Continuous monitoring, the traditional way, relies on a natural storm event, with flow meters downstream.”

He also likes to use “controlled flow testing, simulating a design storm. Then you have a rain event with a specific pattern, a specific depth, a specific duration.”

Hutchinson says that underdrains “are only put in when we have to,” but they’re “what you need when you don’t have satisfactory infiltration rates in the native soils.” He adds, “Underdrains with an orifice-restriction system–an 8-inch-diameter pipe capped with a 0.8-inch diameter orifice–are an effective way to retain stormwater, to really delay the water. With the delay, we get the capacity we’re looking for.”

Hutchinson likes to install flat, flush rain gardens to prevent surface ponding. “A gravel river channel moving down the center allows the water to not exit, by backing up. The homeowner doesn’t see standing water, and we don’t see a quick hit to our system.”

For rain gardens in Seattle, he says, “our perfect goal is no surface water in the rain garden 24 hours after the storm event is over.”

In the voluntary RainWise program, Seattle Public Utilities (SPU) and King County Wastewater Treatment Division partner with property owners to design and build rain gardens, cisterns, or both on private properties. The property owner retains ownership at all times and is responsible for maintenance. (Seattle’s Roadside Rain Garden program is a separate program. In that program, the city installs rain gardens on public rights of way and does all maintenance work required.)

Credit: Portland BES.
Brim full at the end of the storm event

Director Bob Spencer says that RainWise has been popular with homeowners. After some pilot rain gardens in 2010 proved as successful as expected, the project opened to other homeowners in Ballard, an older neighborhood in north Seattle with a combined sewer system (CSS). “Since then, we’ve added other basins at irregular intervals,” says Spencer.

By fall 2013, RainWise was operating in 17 basins around Seattle, adding southeast and southwest neighborhoods. That means there are 44,000 eligible properties whose owners could choose to participate. RainWise is scheduled to be funded through 2017.

Seattle homeowners can check online to see if their property is eligible for RainWise. Typing in an address also gives GIS information on whether the site has any conditions, such as too steep a slope or leaking underground storage tanks, that would make it unwise to install a rain garden. If so, a cistern might be suitable.

“We’ve done about 390 installations. They’re keeping five and a half million gallons of stormwater out of the system on an annual basis,” says Spencer.

“We’re concentrating, really targeting outreach, to historically underserved communities. We’re working with our community partners to get people involved who have not been involved before,” he adds.

RainWise requires that rain gardens or cisterns be large enough to mitigate a minimum of 400 square feet of impervious roof. If the yard has the best-infiltrating soil, the rain garden can be as small as 28 square feet. Plants can be a mixture of deciduous and evergreen. Native plants are suggested, but not required.

RainWise is set up to involve the property owner from the start. “The homeowner has to pick the contractor and work with the contractor through a pretty involved process,” says Spencer.

The property owner must comply with several requirements, including “giving the city access to inspect it. If they sell their homes, they have to notify us and they have to keep the rain garden or cistern in place for five years,” says Spencer.

“It’s a 10-page rebate form they have to fill out, so they’ve got some skin in the game,” he notes.

It takes about six to eight weeks for property owners to receive their rebate checks from SPU for most of the cost of the project. They also have to sign a form stating that they have observed their rain garden or cistern and that it is working as it is supposed to.

As for maintenance responsibilities, the licensed installers teach homeowners how to care for their new rain gardens and cisterns. SPU also keeps in touch with property owners in the spring and fall by postcards and e-mail to remind them of the seasonal tasks that need to be done.

“We send them a postcard saying “˜It’s time to weed; make sure you have three inches of mulch.’ For the cistern owners the card says “˜Make sure the inlets and outlets are not blocked and the screens are clean,'” says Spencer.

Contractors are required to put down 3 inches of mulch. Homeowners must replenish this once a year to a depth of 3 to 4 inches, weed, and water the rain gardens for the first one or two summers.

Contractors also have requirements to fulfill. “We have two inspections the contractor has to perform: pre- and post-construction,” says Spencer. “We want to be sure that all the ducks are in a row.”

Contractors are also required to perform and sign off on an infiltration test for each potential rain garden site. “They know they’re on the hook if it doesn’t work,” says Spencer.

Preventing CSOs in Seattle neighborhoods with CSSs still requires a combination of gray and green infrastructure. Detention tanks that are underground or partially underground have been, or will be, constructed in the various basins with CSOs problems.

“Most of the heavy lifting is done by the detention facilities, but with RainWise citizens can participate with us and be part of the solution,” explains Spencer.

He adds, “What’s neat about it is the dual benefit. The homeowner gets a cistern to water the landscaping or a rain garden to keep the basement from flooding, and the public gets the function of keeping stormwater out of the system.”

As for skeptics who think rain gardens don’t work, Spencer says, “The proof is in the pudding. Mine work. Nobody’s called me up and said “˜My rain garden’s not working.'”

Kurtz is also enthusiastic about rain gardens. “They definitely perform even better than I expected. As engineers, we’re taught to be conservative, so I thought they would be variable. But in general, they’ve all worked well. Performance has remained consistent over time for infiltration with these facilities.”

Portland is known for being an innovator and pioneer for its use of green infrastructure in stormwater projects. The city now has 1,400 Green Streets, the oldest ones about 15 years old.

“Green infrastructure can last for a long period of time. We have a few projects that are more than 20 years old and working fine. In the next few years we’ll have much more older green infrastructure,” says Kurtz. “My confidence has remained high, but I’m also watching carefully.”

For communities that hesitate about installing green infrastructure, Kurtz advises, “Just do a few small pilot projects. If they’re wrong, you can do something else. For us, it really was the Downspout Disconnect program in the 1990s that was really successful. That gave us the ability to do pilots.”

“Green infrastructure is not as cut and dried as gray,” says Hutchinson. “It makes sense to start small with a pilot and figure out what works. The feedback loop from using a pilot [before a full project] is important.”

He mentions another reason to manage stormwater through green strategies: “There can be a truly significant financial savings in using green infrastructure over gray. We’re also choosing it because it’s a great tool. It offers other amenities, too.”

Citing a Portland project, Kurtz adds, “The Tabor to the River project [designed to keep stormwater from southeast Portland out of the Willamette River] is very substantially cheaper using a green and gray mix instead of just gray.”

“When you do cost evaluations of green stormwater infrastructure versus gray, it doesn’t always pencil out,” says Hutchinson. “Say it’s $5 million to build storage tanks and $5.3 million to build rain gardens; tanks would be the logical choice.”

He adds, “But when you look at all the other benefits to green infrastructure, that’s often the better choice. It allows you to put in other amenities, things the neighborhood wants.”

About the Author

Margaret Buranen

Margaret Buranen writes on the environment and business.

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