Ship Canal project adds 112 million litres of underground wastewater storage

Seattle has completed the $255 million Ship Canal Water Quality Project, bringing into service a major underground storage system designed to reduce the amount of untreated sewage and polluted stormwater entering the Lake Washington Ship Canal, Salmon Bay and Lake Union.

The system addresses a long-standing problem in several of Seattle's older neighbourhoods, including Ballard, Fremont, Wallingford and Queen Anne, where stormwater and wastewater share the same pipes. During heavy rainfall, these combined networks can exceed their hydraulic capacity, forcing excess flows to discharge directly into nearby waterways.

At the centre of the project is a 4.3 km long storage tunnel with an internal diameter of approximately 5.7 m. The tunnel can temporarily hold more than 112 million litres of combined wastewater and stormwater during major rain events. Once conditions improve and capacity becomes available, the stored flow is released gradually and conveyed for treatment rather than being discharged untreated into the environment.

The completed system is expected to prevent up to 284 million litres of polluted water from entering Seattle's waterways each year. Beyond reducing combined sewer overflows, the additional storage provides the network with greater flexibility during intense storms, helping the city manage short-duration peaks without immediately overwhelming downstream treatment infrastructure.

Construction of the main tunnel was carried out using the 6 m diameter earth pressure balance TBM MudHoney. The machine completed its drive in 2023, breaking through into a flooded reception shaft in Wallingford. The wider project then continued with connections, conveyance infrastructure and the systems required to integrate the storage tunnel into Seattle's existing wastewater network.

Two smaller conveyance tunnels also form part of the scheme. With internal diameters of approximately 2.4 m and 2.1 m, they were excavated using remotely controlled TBMs and connect local drainage infrastructure with the main storage tunnel.

The project demonstrates why large urban wastewater schemes increasingly rely on underground storage rather than simply expanding surface infrastructure. Constructing beneath developed neighbourhoods allows substantial hydraulic capacity to be added while limiting the amount of land required at street level. It also creates a buffer within the network, storing polluted flows temporarily until treatment facilities can receive them.

With the full system now completed, the Ship Canal tunnel becomes a major piece of Seattle's wet-weather infrastructure. Its purpose is straightforward but significant: capture the flows that would otherwise enter the city's waterways during storms, hold them underground and release them later under controlled conditions for treatment.