The Henderson Street CSO tunnel, located in the Rainer Beach district of Seattle, is one of several tunnels recently constructed by King County for stormwater mitigation. Other tunnels include the Mercer Street Tunnel in downtown Seattle, and the South Interceptor Tunnel in the City of Renton. Its primary function is to store water runoff during a storm. Stormwater runoff when combined with standard sewage output can overload the regional sewage treatment system. The 4 million gallon storage capacity of the tunnel allows for metered release of stormwater into the sewage treatment system after the storm is over.
This 3000 foot tunnel consists of 778 rings with dimensions of 14'-8" i.d. x 9" thick x 4'-0" wide. Each ring consists of 6 segments. Segments and rings are gasketed and bolted together for the final connection, which provides a watertight seal.