On a rainy winter evening, three raindrops fall over Portland.
Raindrop A falls into Forest Park, passing through the canopy and the undergrowth and landing softly in the soil. ‘A’ makes its way across the forest floor, travelling over terrain, around fallen trees and through beds of needles until it enters a seasonal creek. Life speeds up here and before long A arrives at the Willamette River, the Columbia River and the Pacific Ocean.
At the same time, raindrop ‘B’ lands in Portland’s downtown, seven miles upstream. B splashes on the pavement and rolls across the ground into a catch basin grate. B enters the combined sewer and mixes with other raindrops as well as sewage from nearby buildings. Continuing downhill to the Ankeny Pump Station under the west end of the Burnside Bridge, B is pumped across the Willamette River and travels through another 20 miles of sewer before arriving at the Columbia Boulevard Wastewater Treatment Plant in North Portland. The plant, one of two in Portland, will clean B through a series of physical and chemical processes and eventually discharge it to the Columbia River, about 10 miles upstream from where A entered.
Though they ended up in the same place, there are many differences between the routes A and B followed. First, the forest ecosystem avoids concentrating flows and encourages local disposal of stormwater while the urban streetscape is designed to concentrate flows and deliver runoff from a huge area to a single place. In both cases this is by design. The forest evolved over millions of years to value water as a resource. The city sewer system was designed to avert the health, safety and convenience issues that come with standing and improperly treated water.
By slowing and spreading flows, the forest protects the quality of its water. Slower moving water does not pose the same risk of erosion that fast moving, concentrated flows do. If water does become polluted with sediment or excess nutrients, the sediment will likely settle out and the nutrients will be used by native vegetation. Conversely, nearly every step of the urban process increases water pollution. Tail pipe exhaust is trapped on pavement surfaces and washed into the sewer. Stormwater in combined sewers mixes with sewage making black water, a blend that is both difficult and expensive to clean.
There is no single way to reconcile the gap between natural and urbanized systems and still maintain the level of stormwater infrastructure needed to support a city. There is, though, a growing toolbox of stormwater facility designs that has given engineers the means to begin to address the issue.
Imagine now raindrop C, falling on a new greenway trail along the Willamette. The project design team prioritized establishing a healthy canopy so C lands in a tree and may not ever reach the ground. If it does, C might land on pervious pavement and soak into the soil. Though the trail supports a high level of traffic, it is bicycle traffic, so if C leaves the pavement it will not carry pollutants with it. Once on the shoulder, C might run through a short swale, allowing sediment to be filtered away before it enters a rain garden where it will sit until it either soaks into the ground or evaporates. Much like raindrop A, each step along C’s path keeps flow dispersed, encourages local disposal and protects water quality.
C’s path illustrates just a few of the tools in the stormwater toolbox, a collection that has been evolving for several decades now and will continue to do so. As it does, it is important to remember that, like the forest, the process starts small. Creating a more natural urban environment takes time, and significant improvements are only possible with continued commitment and dedication from everyone involved.
This post is the first in a series about green stormwater infrastructure and low impact development. Check back over the coming weeks for more on design considerations and some examples from KPFF’s project experience.