When complete, the new tank will collect stormwater from the street and slowly release it back into the sewer system. (Seattle DOT photo)

On May 1, construction crews working for the Seattle Department of Transportation began building a new stormwater storage tank under the street on 10th Ave. in Capitol Hill.

The tank will provide a significant environmental benefit by reducing flooding and helping contain stormwater runoff so it can be treated before it enters local waterways and Puget Sound.

This project is part of the Madison RapidRide G Line overall work. The project has already built two similar stormwater storage tanks nearby under E Pine St. (between 15th and 16th avenues) and underneath E Madison St (between 12th and 13th avenues). When taking on a major rebuild of a street to improve transit connectivity, crews also want to make sure utilities like underground water lines and storage tanks are improved. This is a coordinated One Seattle approach to best serve the public, alongside partners at Seattle Public Utilities and other city departments.

When complete, the new tank will collect stormwater from the street and slowly release it back into the sewer system. In this area, there is a combined sewer for both stormwater and sanitary waste. Sewer water is cleaned at treatment plants before it flows back into local waterways and Puget Sound.

During heavy rainfall, these combined sewers can sometimes overflow. This can lead to flooding and the release of sewer water into the natural water systems without being fully treated. Stormwater storage tanks like the new one under 10th Avenue help hold stormwater and slowly release it back into the combined sewer system. The tanks help prevent overloading the treatment plants' capacity and allow the sewer water to be fully treated before it goes back into local waterways.

When complete, the city of Seattle expects the new storage tank to be able to hold up to nearly 57,000 gal. of water, when needed.

Work Schedule

• Crews will start at the intersection of 10th Avenue and E Madison St. and work their way north to just past the E Union St intersection;
• Crews will dig a trench in the street and install piping to build the stormwater tank in sections. Once sections of the tank are installed, crews fill the trench with dirt and keep building north;
• Once the tank is fully installed, crews will need to test it. They will then fill the tank with water and monitor how well it holds;
• After the tank passes the monitoring test, crews will begin to repave the street;
• Crews will remove the remaining portions of the street around the hole. Then will grade the whole street and repave it.

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