DC’s Clean Rivers Project is building 4 long tunnels to act as a buffer for the old combined sewer system. Some of the tunnels are complete and already functioning. Previously large storms would cause the sewers to overflow into the Potomac and Anacostia rivers. The EPA wound up suing DC and the Clean Rivers Project is the result. The new tunnels have already had a huge effect.

https://www.dcwater.com/cleanrivers