In some areas of the City of Seattle, sewage and stormwater runoff are collected in the same pipes, known as combined sewers. During heavy storm events, sometimes the flow in these pipes exceeds the sewer system capacity. 

When this occurs, the system overflows at an outfall structure designed for this purpose. This is called a combined sewer overflow (CSO). There are currently 84 outfalls in the City of Seattle where CSOs can occur.

To comply with State and Federal requirements, SPU must limit the number of CSOs at each outfall around the city of Seattle to an average of no more than one per year based on a 20-year moving average.

Portage Bay's basin 138 currently exceeds this standard at almost twice the required limit. The goal of this project is to reduce the number of CSOs in order to protect the environment and public health.

Following this work, SPU will continue to monitor performance and identify if additional action is needed to limit CSOs to less than 1 per year on average.