Impervious surfaces have gained a lot of attention over the last decade. They are everywhere in our urban environment; from roads, to roofs, to parking lots. The high concentration of impervious surfaces in our cities accelerates the volume and speed of the rainwater causing flooding, erosion, pollution, and combined sewer overflows (CSOs). This hydrograph illustrates the difference in stormwater flows between pre-settlement and urbanized conditions, with a high speed and high volume peak.
Portland and Seattle are ahead of the curve in promoting low-impact development (LID) practices that slow, disperse and absorb rainwater where it falls to the ground. Seattle now refers to these practices as green stormwater infrastructure (GSI). Portland uses the term “green streets” to describe many of the alternative measures that address stormwater in the right-of-way beyond traditional gray infrastructure (pipes and sewers). Many of these strategies are now being marketed to developers (see National Resource Defense Council graphic below).
The Seattle Green Factor is a framework that has helped Seattle address stormwater runoff, since its introduction in 2006 by the Department of Planning and Development. This municipal code was the first of its kind in the U.S. and was based on Berlin’s Biotope Area Factor. It includes strategies such as green roofs, green walls, permeable paving, cisterns, bioretention cells and native plantings.
The Green Factor uses a scoresheet to calculate a score for new construction and each project must meet a minimum score based on zoning. It is a critical strategy for addressing urban stormwater that creates flexibility for developers to meet the code while contributing value and delight to urban built environment.
In 2009 both the Department of Planning and Seattle Public Utilities issued a “Director’s Rule” that clarifies requirements for implementing green stormwater infrastructure (GSI) to the maximum extent feasible (MEF) at the same time expanding the scope and updating the Green Factor code.
More recently, Portland continues to expand its commitment to reducing runoff from impervious surfaces, as steep Environmental Protection Agency (EPA) fines for every CSO (as much as $16,000 per daily violation) loom. In the fall 2013 the city established a progressive “stormwater fee.” It is designed to be more equitable because the fee is based on runoff quantity instead of being based on water usage. A similar fee is also being implemented in Maryland.
This type of framework could address existing properties impervious surface and work in concert with the Green Factor that addresses new construction projects. Together these types of measures create incentives for property owners to address runoff in whatever way that they see fit.
With the private and public sector working together on this issue, Seattle might have a chance a reaching the ambitious goal that former mayor Mike McGinn set in the spring of 2013. He issued an Executive Order setting the goal for Seattle to manage 700 million gallons of stormwater by 2025 using green stormwater infrastructure (GSI). By expanding the use of strategic frameworks, Seattle and Portland will continue to be leaders in protecting their waterways.