12th Ave NW Basin Drainage

What & Why

The 12th Ave NW Basin Drainage Improvement Project will improve the drainage system in parts of west Broadview with severe flooding issues. This project is the next phase of improvements in the Broadview Program. Visit our program webpage to learn more about the Broadview Program.

We plan to reduce storm-related flooding by building new storm system improvements where there are none, to slow runoff and improve flows. The project may use a variety of tools in City of Seattle right-of-way along neighborhood streets to address flooding issues including:

  • Installing natural drainage systems in feasible locations
  • Converting ditch-and-culvert flow to a piped system
  • Installing large, underground pipes to hold water and prevent flooding during storms
  • Locating improvements to address priority flooding and allow for future expansion

This project will be designed to help flooding problems throughout the entire drainage basin. There are several known locations in the drainage basin that flood during small rain storms. Because this is a basin-wide project, the location of some of the work may not be directly located at the worst problem areas; instead the work may help reduce stormwater volumes from upstream of the worst problem locations or may be installed downstream to help stormwater flow away from the worst problem locations.

What's Happening Now?

We held neighborhood walking tours on Saturday, Nov. 3, to receive input on our preliminary natural drainage system design concepts and the potential feasible locations for NDS . If you were unable to attend a walking tour, please visit our Project Documents tab to review the design concepts and related materials. If you have questions or comments, please send an email to spu_broadviewprojects@seattle.gov.

Previously, our project team conducted surveys and tested soils in 12th Ave NW Basin to identify feasible locations for natural drainage installations. Initially we studied 24 blocks near the top of the drainage basin where capturing surface stormwater flows would be most effective. In early June, we went door-to-door to talk to residents about the possibility of siting natural drainage systems (NDS) in the area.

Natural drainage systems require well-draining, granular soils (comprised mostly of sand or a mix of sand and gravel). We found that the most favorable soils for NDS are east of 3rd Avenue NW between NW 125th and NW 130th streets. Other blocks in the original study area had significant fine-grained soils, which have low infiltration rates and are not considered suitable for NDS. We are no longer considering these blocks for NDS installations.

Next steps

  • We will continue to study the soils and technical feasibility along the blocks in the smaller study area in addition to the location of other utilities, existing infrastructure and the feedback from the community.
  • We will present refined NDS design and locations to the community in spring 2019. 
  • We are also continuing to design new underground stormwater pipes. These new underground stormwater pipes help capture stormwater and reduce flooding.

What are Natural Drainage Systems (NDS)?

NDS are engineered facilities that mimic nature by slowing or reducing stormwater runoff close to its source (streets, rooftops and parking lots). NDS helps our city act more like a forest by absorbing, filtering and cleaning stormwater runoff before it harms our waterways. NDS installed in our public spaces—such as planting strips next to the street—help absorb and clean stormwater runoff.

Learn more about NDS and why they are being installed in the Broadview neighborhood here.


The 12th Ave NW Basin Drainage Improvement Project is part of the first phase of improvements in the Broadview Program. Read about how SPU selected the 12th Ave NW Basin project, check out the Broadview Program background. 

Program Background

For more information about how SPU selected the 12th Ave NW Basin project, check out the Broadview Program background.

Project goals and benefits

The goals of the Broadview program are to:

  • Reduce sanitary sewer overflows into homes, streets, and creeks
  • Reduce the frequency and quantity of stormwater flooding to areas most impacted, especially building structures.

The 12th Avenue NW Basin Drainage project will focus on reducing storm-related flooding by slowing runoff and improving flows, seeking to maximize benefits with available budget.

Following the outcomes of our program’s options analysis process that included community input, we are now focused on making improvements along neighborhood roads, in City-owned right-of-way. As we move through design, we will work to identify where proposed improvements may require property owners to clear any encroachments beyond their property boundaries that currently extend into the City’s right-of-way (e.g. fencing, landscaping, or parking). Without doing so, we won’t be able to meet our goals.