top of page

Green Stormwater Infrastructure

Environmental resilience depends upon several factors, one of which is stormwater. Capitol Hill is home to more than 37,000 people, about 5% of the City of Seattle population. Seattle’s population growth has caused the built environment to harden, and tree canopies, native soils, and vegetation have declined. As our neighborhood expands in infrastructure and grows in population, Green Stormwater Infrastructure (GSI) has become an essential part of strengthening our urban ecosystem – a critical element of public life for the Capitol Hill community.

The Capitol Hill EcoDistrict has centered part of its Public Life planning towards building environmental resilience through GSI projects. Explore some of the EcoDistrict's previous and upcoming GSI projects below. 

The impact of stormwater

The challenge: Our city is expanding, and most of those expansions have impervious surfaces. As our infrastructure grows, our natural spaces have begun to decrease dramatically. Instead of rain absorbing naturally into vegetation, it now moves quickly through streetscapes, parking lots, and sidewalks. As the water slides down the streets, it collects trash, dirt, oil, metals, fertilizers, bacteria, animal waste, and other impurities.


In some cases, this water directly reaches our waterways. However, in most cases, the water will eventually flow into stormwater pipes that direct it to a water treatment facility. The challenge is during major storms when our drains reach capacity, and the water diverts from pipes that lead to treatment facilities into pipes that lead directly into the Sound. 

Combined sewer overflows: Capitol Hill is one of many neighborhoods in the Puget Sound area that operates on a combined sewer system. This means that in some neighborhoods, the pipes that collect sewage are the same pipes that collect stormwater. During major storms, these pipes cannot hold the sewage and increased stormwater simultaneously, causing the excess wastewater to outpour into pipes that lead into Puget Sound. The diagram below shows an example of the flow of combined sewer outfall (CSO) pipes before and during a major rainstorm. 


Dry weather conditions in the combined sewer system 

Wet weather conditions in the combined sewer system 

Source: NYC Environmental Protection

With increased rainfall due to climate change, Seattle is seeing an increase in large-scale storm incidents that flood stormwater pipes to send extra flow into CSOs. Without discharge limits, CSOs add large quantities of raw sewage into lakes, rivers, and Puget Sound. Contaminants in CSOs can include pathogens, oxygen-consuming pollutants, solids, nutrients, and toxins that are unsafe for fish, other species of animals, and humans. In King County, you can track where CSO incidents occur. Use this tool to see if there are CSO overflows in your area.

The solution: Increasing GSI resources around Seattle is one of the first steps toward preserving our water quality, shorelines, and aquatic systems. GSI mimics natural systems like forests and wetlands, offering a natural filtration system for our delicate ecosystem.

GSI strategies

We can slow, detain, and clean stormwater before it enters the sewer system and local waterways using the GSI methods below. Below, learn about GSI methods that help reduce the amount of pollution that reaches our waterways.

CISTERNS (20).jpg

Permeable pavement 

A hardscape intended to break the flow of surface run-off, is designed to allow for percolation. Permeable concrete or asphalt and pavers with extra-wide gaps between them are all examples of permeable paving. The foundation for these types of paving is specially prepared to create a stable base while providing voids for water to collect and pass through on its way to native soils. These surfaces require regular cleaning or vacuuming to maintain their porosity.

GSI strategies in Capitol Hill:
Swale on Yale

Floodwater management in Capitol Hill is especially important because of the water's ability to run downhill and into surrounding neighborhoods. In 2012, a unique project came to the neighborhood, which utilized a series of bioswales to treat runoff from Capitol Hill in the Cascade neighborhood on Yale Ave. The effort was to help improve water quality in Lake Union and, ultimately, the Puget Sound. It was the first stormwater project of its kind that was developed through a public-private partnership between Vulcan Real Estate and the City of Seattle. The "Swale on Yale" currently receives and treats 435 acres of stormwater runoff from Capitol Hill. This project is a glowing example of how GSI strategies can be used to significantly impact local stormwater management. Learn more about the Swale on Yale here.


How can you be an advocate for GSI?

Start by learning about stormwater in your neighborhood! Use the Environmental Protection Agencies National Stormwater Calculator to estimate the annual amount of rainwater and the frequency of runoff in your area, then learn about the best GSI tools for your area. If you are interested in stormwater flooding in your neighborhood, check out King County's interactive floodwater map to learn about flooding events in your area. 


Special thanks to our community partner:

Seattle Established Logo.png


NEA logo.png
bottom of page