THE NEED FOR NATURAL STORMWATER INFRASTRUCTURE:
King, Snohomish and Pierce Counties are growing at a breakneck pace, with a combined population of nearly four million and one million more expected by 2040. The Puget Sound coastal cities where most of this growth is concentrated lie at the bottom of a hydrologic basin, dependent on healthy rivers and streams for clean water and flood control. Unfortunately, their growth is placing strains on existing built stormwater infrastructure and the natural systems that absorb toxics and reduce flooding. Polluted stormwater poses a serious environmental threat to the waters of the Sound, and a significant health threat to the growing human communities that rely on these waters to live. Much of the region’s gray infrastructure is aging and prohibitively expensive to replace, and there is a pressing need for healthy natural systems to play a larger role in stormwater management.
Booming development has reduced stormwater absorption, with new construction replacing soil, vegetation and natural depressions with impermeable surfaces that prevent infiltration (U.S. Geologic Survey Fact Sheet 076-03). The volume of stormwater entering combined sewer and stormwater systems during heavy rains often causes
overflows of raw sewage into the Sound and other sensitive water bodies. Unable to infiltrate into soils, rainwater carries toxics and chemicals directly into local streams and rivers, and eventually the Sound. Oil and gas deposits as well as chemicals like copper, zinc and lead are carried off roofs, driveways, playfields, lawns, parking lots and roads (WA State Department of Ecology, 2011 Publication No. 11-03-024) and into our water. Seventy-five percent of pollution entering the Sound is carried by stormwater runoff.
Unchecked stormwater also increases flooding and other hazards to human populations. The rapid flow of water overland and in pipes increases the amount of water flowing to streams and rivers, resulting in hazardous flooding that yields a higher peak discharge faster and more frequently, and increases the risk of fatal landslides. Since 1990, flood damage has cost Washington State taxpayers $1.4 billion (2013 WA State Hazard Mitigation Plan), and the costs are expected to rise as the population grows. Climate change threatens to further exacerbate the problem. Less snowpack, higher sea levels and heavier rainfall will result in more surface water and increased flooding, while prolonged droughts will place significant burdens on the potable water supply (WA State Department of Ecology, March 2016 Publication No. 16-03-006).
THE GREEN-DUWAMISH WATERSHED:
The 93-mile Green-Duwamish River is ground zero for addressing these challenges. The river flows from the Cascade Mountains to the Sound, traversing several cities along the way including Auburn, Kent and Tukwila. It is one of the keystone watersheds of the Puget Sound basin and a significant water resource for fast-growing urban communities and Tribes, who have fished its waters for centuries. It is the City of Tacoma’s primary source of drinking water (over 200,000 people) and the Port of Seattle’s commercial terminus point. With a population of over 350,000 people, the Green-Duwamish River valley contains some of the most racially and economically diverse zip codes in King County and the nation as a whole (EPA Urban Waters Partnership website, 2017)–including many underserved immigrant communities.
The watershed’s varied geography makes it well-suited for a wide variety of land uses including agriculture, industry, urban development, commercial warehousing, native fisheries and recreation. Unfortunately, intensive exploitation has taken a toll: the lower five miles of the river are highly polluted and were declared a Superfund Site in 2001 by the Environmental Protection Agency, with high amounts of contaminants like petroleum, PCBs, PAHs, mercury, as well as fecal coliform from sewage overflow, surface runoff and farm drainage. An Ecological Risk Assessment conducted by the WA Department of Ecology and the EPA found that river otters there were exposed to such high levels of PCBs that the chances for survival of their offspring was significantly reduced (EPA Region 10 website, 2017).
The river and related tributaries provide habitat for several species of salmon listed as threatened or endangered, including Chinook, winter steelhead and coho. Once abundant, these salmon were a dependable source of food for Tribes and settlers alike, and the watershed still serves as a major resource for subsistence fishing (WA Department of Ecology, 2016). Unfortunately, salmon face overfishing, loss of native habitat, water pollution and other stresses. The collapse of salmon populations has profound implications: commercial, environmental and, for the Tribes, cultural. In 2016, American Rivers designated the Green-Duwamish one of America’s Most Endangered Rivers.
While the risks to people and wildlife are obvious within the Green-Duwamish watershed, the health of the river also impacts the Puget Sound, affecting water quality and communities across the entire region. Intervention is therefore critical. In its 2016 water quality assessment, King County’s Department of Natural Resources and Parks developed a series of recommendations to reduce pollution in the Green-Duwamish. While much of the focus was on prevention, the assessment also called for restoring natural river flow and functions, especially through the “re-greening” of shoreline habitat–including restoring permeable surfaces that absorb runoff and installing native plants to cool water for the benefit of salmon (“Tackling Stormwater Challenges on the Green-Duwamish,” American Rivers, 2016).
OUR WORK IN THE GREEN-DUWAMISH WATERSHED:
Given its social, economic and environmental importance and the myriad stormwater challenges it faces, [Organization name withheld] has prioritized the Green-Duwamish River as a keystone watershed. It is at a critical juncture, with growing populations and climate change bound to place new strains on built stormwater infrastructure in the coming decades. This requires a watershed-scale response that targets both mainstem and tributaries with a multi-year strategy that includes natural stormwater infrastructure. In many ways, it is the perfect demonstration site for the long-term efficacy of our overall riparian restoration efforts. We are also carrying out extensive bank restoration activities on the Cedar River and Bear Creek, and are in the tentative stage of bringing this work to the Sammamish River basin.
The Green-Duwamish watershed is well served by government entities and nonprofits, but we fulfill a critical niche by stressing natural infrastructure over built gray and green infrastructure, and can draw on decades of experience working with private property owners to restore native vegetation on their properties. We emphasize coordination, and work to ensure all efforts are aligned with the work of King County, The Boeing Company, The Nature Conservancy, Puget Sound Partnership and others pursuing restoration and stewardship projects. Only by working together and bringing our unique skills to the table can we have a lasting watershed-scale impact.
SUPPORTED BY SCIENCE:
There is an extensive body of research highlighting the role functioning natural systems can play in stormwater mitigation. Healthy riparian corridors and floodplains effectively slow surface runoff and filter pollutants while replenishing groundwater stores. Natural drainage solutions, like functioning riparian zones, temporarily store and filter stormwater for use onsite and return it as clean water to the local source rather than sending it downstream as a source of pollution. The best available science indicates that intact riparian forest cover is the most effective system for intercepting and absorbing stormwater. As soils are exposed, stripped or compacted during the course of development, there is measureable loss of water storage (Forest Cover, Impervious-Area, And The Mitigation of Stormwater Impacts, Booth et al., Journal of the American Water Resources Association, Vol. 38, No. 3, 2002).
Trees and shrubs, particularly plant communities native to the Pacific Northwest, have the ability to grow in a variety of conditions with little required maintenance. Unlike the fibrous roots of common riparian weeds, native trees and shrubs form stabilizing roots and create organic soils that can hold and store water. Mature trees intercept most rainwater before it reaches the ground, absorbing it through bark, branches and leaves. Conifers have been shown to absorb more water than broadleaf (deciduous) species (Surface Water Storage Capacity of Twenty Tree Species in Davis California, Xiao et al., Journal of Environmental Quality January 11, 2016), and are generally prioritized over planting other species. However, planting a diverse selection of native plants is the best approach to achieve survival success in dynamic riverine environments.
ADDRESSING COMMUNITY NEED:
This project addresses community need by reducing the burden on existing built infrastructure through the restoration of natural, permeable surfaces that slow runoff and absorb pollutants in our towns and cities. Natural green stormwater infrastructure is a low-cost, flexible solution that provides multiple benefits, including stormwater retention and absorption, improved water and air quality, increased biodiversity, more robust salmon populations, reduced heat island effect and outdoor educational opportunities.
Moreover, with increased risk of flooding due to climate change, having natural infrastructure may reduce risk to residents, property and municipal assets within the watershed. A 2009 study, “Costs and Benefits of Storm-Water Management: Case Study of the Puget Sound Region” concluded that the most cost effective stormwater strategy is “implementing preventative stormwater measures in the planning stages of development projects, such as preserving natural drainage areas, and restricting development in flood-prone areas” (Journal of Urban Planning and Development, December 2009).