For more than a decade, the Watershed Manage-ment Division of the City of Gresham, Ore., looked for ways to deal with runoff from an 800-acre industrial and commercial site.
This untreated water flowed from parking lots and streets directly into the Columbia Slough. The slough consists of 60 miles of waterways, wetlands and slow-moving channels. It feeds the Columbia River, and provides salmon-rearing habitat and a favorite place for canoers and kayakers.
The ultimate solution was to build an engineered wetland. Several factors converged to make the Columbia Slough Regional Stormwater Treatment Facility happen. They included a donation of valuable acreage by a major corporation, an innovative design to capture and clean the water,and an open, community-based planning process.
Construction began in June 2006 and ended on time and on budget in July 2007. Stormwater utility fees covered the project cost of $2.9 million. Steve Fancher, division manager, says the project demonstrated that building wetlands can be a viable way to solve water-treatment problems naturally.
“The primary goal of the wetland was to protect our rivers, and our secondary goal was to help protect the groundwater, which is one of our sources of drinking water,” says Fancher. The wetland also provides a sanctuary for wildlife and serves as an environmental education center with guided tours and signage along the nearby Gresham Fairview trail. It covers a small section of a 40-mile loop for bicycling, walking and hiking.
Meeting a mandate
Gresham is Oregon’s fourth-largest city and a fast-growing place, where the population has risen from 90,000 in the 2000 census to 100,000 in 2008. The city covers 22.2 square miles.
Gresham has grown in part because it is close to Portland, an appealing place to live for its culture and many fine restaurants. Gresham is also popular for its proximity to Mount Hood, which attracts skiers, hikers and snowboarders. It also appeals to retirees and young people alike, with real estate prices more affordable than in Portland.
The area’s water resources are important to local residents and visitors. As a sensitive waterway, the Columbia Slough had an extensive list of total maximum daily loading (TMDL) limits on pollutants, including PCBs, DDT, bacteria and mercury. “We were mandated by the state to come up with a plan to address the TMDLs and reduce pollution,” Fancher says.
Annual rainfall dictated the size and configuration of the Columbia Slough treatment facility. Gresham averages 37 inches of rainfall annually, most of it from October through March. Fancher notes that rain in the region usually comes as misty, steady drizzles, rather than in downpours that can overload treatment facilities.
The Columbia Slough facility is designed to capture runoff from the first inch of rainfall. In fact, more than 80 percent of the annual rainfall is treated, including the “first flush,” which contains most of the oil and dirt from parking lots and streets.
Finding a partner
“Large-scale stormwater retrofit opportunities are extremely rare in most developed cities like Gresham,” Fancher says. A public-private partnership between the city and the Boeing Co. enabled the wetland to be built. Boeing donated to the city 13 acres of unused industrial land next to an existing manufacturing plant.
The property was perfect for a wastewater treatment facility and close to the 800-acre site that was the source of problem runoff. “We saw the opportunity with 13 acres of land that begged to be used,” says Tom McCausland, a senior engineer for the city. “You need space to build a natural, functioning facility.”
Previously, the land was mainly pasture with some trees. To develop the wetlands, contractors excavated to create a series of wetland terraces and a pond to which the runoff water was diverted. In addition, the city planted some 8,500 native riparian plants and trees from local nurseries.
“We wanted to treat the runoff naturally,” says McCausland. “Wet-land plants capture and break down the pollutants and the grasses filter them out. We’re also storing water in the pond so that sediments and pollutants drop out, and then when the sediment is dry, we pick it up and dispose of it properly.”
Making it work
A series of pipes and a concrete structure divert runoff into the wetland. The plants, grasses and soils filter most pollutants and fine sediments from the water. The design also includes three underground vaults, each 20 feet deep and 8 to 10 feet in diameter. As water flows through the vaults, heavier sediments are removed, and with them hydrocarbons and metals.
The vaults have shut-off valves that would operate and protect the slough in case of a major spill of a hazardous or toxic substance. “We wanted to be sure that we could capture pollutants from an event such as a diesel fuel spill and control it so it wouldn’t end up in the slough,” McCausland says.
Before the project was authorized, the state Department of Environmental Quality (DEQ) reviewed the plans for compliance with all environmental regulations, then issued a permit that covered erosion control during construction.
Everything was ready to go until the U.S. Army Corps of Engineers, which also had jurisdiction, questioned whether Native American artifacts would be affected. The city hired a Native American expert who confirmed that artifacts most likely would not be found in the area of construction, and in fact none were found.
Collaborative effort
The design of the wetland was a collaborative effort. Planning meetings included representatives of the cities of Gresham, Fairview and Portland, Boeing, and the Columbia Slough Watershed Council, a broad-based citizen group that fosters action to protect, enhance, restore and revitalize the slough and its watershed.
Even in the design phase, the project team considered maintenance issues that would arise after construction. For example, a maintenance supervisor gave input on how his crew would need to travel onto the site to conduct maintenance. One innovative aspect of the facility was the use of pervious asphalt and pavers to build the access roads. Maintenance is critical to the wetland’s long-term stability. “At the beginning we conducted frequent inspections to make sure everything was functioning smoothly,” says McCausland. “We were looking for evidence of accumulated pollutants, and if we found any, we started to remove them. Our experienced maintenance crew knows how to look for clogging of pipes and inlet grates. You wouldn’t believe what you can find there — tires, trash, anything can end up in the storm-water system.”
Meanwhile, plans have been drawn for a 30-acre industrial park that will surround the wetland. “The wetlands were designed large enough to treat stormwater runoff from the area,” Fancher says.
Comprehensive approach
The Columbia Slough facility is only one way in which Gresham addresses stormwater runoff. The city maintains 220 miles of storm sewer pipe, some 5,700 catch basins, 1,000 drywells, 150 sedimentation manholes, 137 underground structural detention facilities with control-release manholes, 15 treatment swales, 32 neigh-borhood ponds, 15 miles of roadside drainage ditches, 581 outfalls and more.
The city’s Green Streets program helps manage runoff from city streets. Rain gardens and stormwater planters form an interconnected network of shallow, heavily planted landscapes that convey, treat, retain and absorb stormwater.
In addition, in an outreach program for streamside property owners, Americorps volunteers went door-to-door to inform people not to treat their yards with pesticides and fertilizers, which will find their way into the waterways. “The goal was to make them better stewards of the environment,” Fancher says.
Fancher and colleagues take special pride in the Columbia Slough wetland. The best things about the project, he says, were “improving water quality and removing pollutants from the system. Whether it’s water we drink or water in a stream, we don’t want to pollute it. Even people who don’t consider runoff can enjoy recreational opportunities in hiking and biking and beautiful open spaces as a respite from the city.”
