Seattle Public Utilities

CSO Reduction Projects

Lake Union


As part of its CSO Reduction Plan (pdf), the city intends to add storage capacity, build Green Stormwater Infrastructure, optimize our existing infrastructure, and develop a long term control plan.

Lake Washington is the number-one priority for CSO reduction, based on the volume of overflows and the potential for improved stormwater quality. In particular, three basin areas – Windermere, Genesee and Henderson – require quick attention to cut the volume of overflows. Together, the three basin areas account for the majority of uncontrolled discharges into the lake, totaling an average of 24 million gallons annually. The most effective way to cut that volume is to construct additional storage facilities that can hold excess stormwater and sewage, preventing overflows.

Additional storage facilities in these basins enable the system to hold more stormwater and sewage until they can be transferred to treatment plants. This approach includes building large underground tanks to control millions of gallons of stormwater and sewage that would otherwise get discharged into local waterways. The stored water is typically held in the tank for 24 hours or less. Smaller storage tanks built underground alongside the existing pipeline (known as in-line storage) also provide opportunities to store smaller volumes.

Green solutions offer a variety of options to manage stormwater before it enters the drainage system. Those options include disconnecting roof drains, adding cisterns, or redirecting stormwater into rain gardens where it is allowed to slowly seep into the soil.

In some cases, retrofits provide a relatively cost-effective way to control smaller volumes. For example, by adjusting the height of the gates that control stormwater once it’s in the system we can increase capacity and reduce overflows. Extra attention to maintenance and monitoring also provides opportunities for small, but significant, improvements to the current system.

Sometimes, repair or replacement of the outfalls themselves is needed before CSO reduction work can be undertaken. At this time, such replacements are planned for Salmon Bay in Ballard and Leschi on Lake Washington.

Planning for the long term

The Long-term Control Plan will indicate which approaches will be used in which drainage basins so that the most appropriate solutions are designed and built.