Seattle Builds Subway-Sized Tunnel — for Stormwater

The $700 million ‘stormwater subway’ is designed to handle overflows during storms, which contain toxic runoff from roadways and vehicles.

2 minute read

May 13, 2025, 11:40 AM PDT

By Diana Ionescu @aworkoffiction


Pump station with blue pipes coming out of concrete wall in Seattle, Washington.

The South Park Pump Station in Seattle, Washington. | Joe Mabel, CC BY-SA 4.0, via Wikimedia Commons / Wikimedia Commons

A new, $700 million Seattle tunnel with a diameter large enough to fit a single track train is actually designed solely for wastewater, writes Joe Cortright in City Commentary.

As Cortright explains, “The reason for this tunnel—as in many cities in the US—is to deal with periodic sewer overflows when it rains. Seattle has a system of combined storm and sanitary sewers–so stormwater mixes with domestic wastewater, which is a problem during heavy rains.  The system is sized to avoid overflows when its just handling domestic sewage, but the water flows during the rainy season exceed the capacity of the system.”

Cortright notes that half of stormwater comes from parking lots and roadways, “and much of the toxicity of stormwater is from cars and trucks (oil, tire debris, brake dust and precipitated air pollution)” — so transportation is directly responsible for added costs for sewer and stormwater infrastructure, but drivers do not contribute directly to costs. “Instead, the cost of sewage subways gets build to urban households, many of whom don’t even drive.”

According to Cortright, the city’s approach to water pollution is “both inefficient and inequitable: inefficient because we’re not sending any incentives to people to drive less, and inequitable because we’re not asking the people causing much or most of the pollution to pay for the solution.” In this way, “Not asking road users to pay for the cost of the subway built to handle their toxic runoff, and instead loading it on to the bills of households or businesses, makes driving artificially cheap, and city-living artificially expensive.”

Monday, May 12, 2025 in City Observatory

portrait of professional woman

I love the variety of courses, many practical, and all richly illustrated. They have inspired many ideas that I've applied in practice, and in my own teaching. Mary G., Urban Planner

I love the variety of courses, many practical, and all richly illustrated. They have inspired many ideas that I've applied in practice, and in my own teaching.

Mary G., Urban Planner

Cover CM Credits, Earn Certificates, Push Your Career Forward

Logo for Planetizen Federal Action Tracker with black and white image of U.S. Capitol with water ripple overlay.

Planetizen Federal Action Tracker

A weekly monitor of how Trump’s orders and actions are impacting planners and planning in America.

June 25, 2025 - Diana Ionescu

Person wearing mask walking through temporary outdoor dining setup lined with bistro lights at dusk in New York City.

Restaurant Patios Were a Pandemic Win — Why Were They so Hard to Keep?

Social distancing requirements and changes in travel patterns prompted cities to pilot new uses for street and sidewalk space. Then it got complicated.

June 19, 2025 - Diana Ionescu

Map of Western U.S. indicating public lands that would be for sale under a Senate plan in yellow and green.

Map: Where Senate Republicans Want to Sell Your Public Lands

For public land advocates, the Senate Republicans’ proposal to sell millions of acres of public land in the West is “the biggest fight of their careers.”

June 19, 2025 - Outdoor Life

Aerial view of town of Wailuku in Maui, Hawaii with mountains in background against cloudy sunset sky.

Maui's Vacation Rental Debate Turns Ugly

Verbal attacks, misinformation campaigns and fistfights plague a high-stakes debate to convert thousands of vacation rentals into long-term housing.

4 hours ago - Honolulu Civil Beat

White and purple sign for Slow Street in San Francisco, California with people crossing crosswalk.

San Francisco Suspends Traffic Calming Amidst Record Deaths

Citing “a challenging fiscal landscape,” the city will cease the program on the heels of 42 traffic deaths, including 24 pedestrians.

5 hours ago - KQED

Tents inhabited by unhoused people lined up on sidewalk in Los Angeles, California in front of industrial building.

California Homeless Arrests, Citations Spike After Ruling

An investigation reveals that anti-homeless actions increased up to 500% after Grants Pass v. Johnson — even in cities claiming no policy change.

6 hours ago - Times of San Diego

Urban Design for Planners 1: Software Tools

This six-course series explores essential urban design concepts using open source software and equips planners with the tools they need to participate fully in the urban design process.

Planning for Universal Design

Learn the tools for implementing Universal Design in planning regulations.