On days with heavy rain, we first hear its patter, then its drums. Rain water rolls off roofs and churns down driveways. It picks up speed and chemicals along the way and careens down city gutters in torrents. But when stormwater drops into underground pipes, pollutants of modern life disappear from both sight and mind.
Household chemicals, gas, oil, bacteria, pesticides, pet waste, and even copper dust from brake pads can foul natural areas and drinking water. You can’t point to the sources of this pollution because there are too many of them.
Roofs, roads, and sidewalks that do not allow water to soak in are the starting place of “nonpoint” pollution, the largest polluter of Oregon’s rivers, lakes, and streams. Because of these impervious surfaces, a typical city block generates more than five times the runoff of a woodland area of the same size, according to the U.S. Environmental Protection Agency. When storms hit developed areas, runoff pours across parking lots and streets, rushes untreated into waterways and can strain the capacity of water treatment plants, overflowing stormwater and sewage.
In contrast, the natural water cycle seems almost romantic compared to urban stormwater runoff. Rain falls on forests and meadows, buffered by foliage as it percolates into the soil, filtered by plants. Watershed experts have discovered new tools to slow stormwater from torrents to trickles with strategies for “low impact development” that replicate natural watershed hydrology. These tools, including rain gardens (also called bioswales), are surprisingly effective—and beautiful.
“Unfortunately, we have replaced the natural landscape with hard surfaces that can’t absorb water,” said Robert Emanuel of Oregon Sea Grant and Oregon State University Extension. “But with rain gardens, we turn stormwater management on its head. Instead of funneling excess water into storm sewers, we retain it on site and allow it to slowly filter through vegetation and soil, a process much like natural hydrology.”
A bioretention system is a sunken, landscaped area that retains stormwater long enough to allow the water to soak into the soil. Native plants accustomed to flooding and drying are placed at the base, on the slope, and at the top of the swale to help slow down and clean the water. The plants help clean water naturally because they have deep root systems that anchor soil and act as filters, according to OSU horticulturist Linda McMahan. She has developed a list of 50 plants native to Oregon that are well suited to three rain garden planting zones: moist, moderate, and dry.
Consider shrubs such as the fragrant Douglas spirea or golden current; flowering perennials such as large-leaved lupine and Oregon iris; or feathery curly sedge and tufted hair grass. Now imagine these beauties in parking lot islands and street right-of-ways. Practical rain garden retention systems can be as elegant as water features in an English garden.
According to Sam Chan of Oregon Sea Grant Extension, man-made bioswales trap most pollutants, and water coming out of a rain garden can be cleaner than when it goes in. Water pollution was once an easy-to-trace problem in the United States, when industry and sewage-treatment plants discharged their chemical waste through pipes at specific places, usually into rivers. The federal Clean Water Act of 1972, one of the first big environmental cleanup measures, was successful in regulating this “point pollution.” The Clean Water Act was updated in 1987 to address nonpoint pollution in surface and groundwater resources at the coast and inland.
Water quality has improved dramatically in the last 38 years, but nonpoint pollution continues to accumulate in stormwater runoff, according to Chan. It is more cost-efficient to treat stormwater close to where it flows, rather than piping it miles away to be treated in a multi-million-dollar water treatment facility.
One of the trade-offs that came with building our modern cities was replacement of our natural vegetative water systems with culverts and pipes, according to John Lambrinos, a landscape ecologist with the OSU horticulture department. “Bioswales and green roofs that help protect our water are inexpensive compared to the price of culverts and pipes that just keep getting bigger.” Lambrinos helped create a green roof landscape atop the Portland Building – 17 stories above Portland’s city streets – that absorbs rainwater and limits the runoff that gushes into city sewers after a storm.
Portland has had to spend billions of dollars on stormwater management to meet new clean water requirements. The “Big Pipe” project and other smaller-scale stormwater projects have made the city a leader in management of runoff, Chan said. The city is restructuring old stormwater and sewage treatment systems that for years pumped raw sewage into the Willamette River. Now, pipes as tall as a house are being installed under the city to carry sewage and storm water to a huge holding tank before treatment as wastewater.
Most small cities don’t have the resources to engineer such expensive water treatment systems. Low impact development, however, can protect wetlands, riparian areas, and forests as cities grow. After Emanuel moved to Tillamook as part of OSU Extension’s water resources faculty, he found grant money to demonstrate the ease and effectiveness of bioretention. His demonstrations prompted two north-coast city ordinances to em- brace the idea.
With Oregon Sea Grant funding, Emanuel gathered volunteers to help dig bioswales along the parking area of a park adjacent to Houquarton Slough in Tillamook. The shallow swales are nearly 200 square feet in size. In and around the swales, the volunteers planted shrubs and perennials from McMahan’s list of native plants that can thrive without chemicals in both drought and flood. When stormwater rushes into the rocks and naturally filtering plants, it slows down and loses its eroding edge. When tamed, it flows gently back into the slough. The greenery helps slow and clean the water and adds a friendly feeling to what would otherwise be asphalt and concrete.
Emanuel and his colleagues have introduced city administrators to other low-impact development innovations such as clustered new housing, fewer streets, and shorter driveways. One of the most effective actions is simply to plant and protect trees.
Another larger demonstration rain garden at Bay City receives runoff that flows from a residential area down a roadway near a creek. It took less than half a day for a backhoe to shape the bowl and place the rocks to create the bioswale, according to Dave Pace, public works superintendent for the city of Bay City.
“We had drainage problems before we had the bioswale, and it’s working better than expected. There’s good flow into the swale, the ground saturates, there’s no overflow, and the neighbors like how it looks and works,” he said.
The city plans to require such low impact development techniques in its development standards, he said. “We’re very proactive about eliminating pollution here. We want to learn to do it right.”