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?

Last November, we held a drop-in session in your community to present natural drainage system concepts that we were considering and the feasible blocks where they could be located. We received a lot of thoughtful feedback at that event and would like to thank everyone for their input!

Since November, we've been doing several things to narrow down the blocks we're considering for natural drainage systems, such as:

  • Listening to community preferences and concerns regarding natural drainage system locations.
  • Conducting hydraulic modeling to figure out the amount of natural drainage systems we need to meet our project goals.
  • Looking into where we can design and construct natural drainage systems cost-efficiently.
  • Evaluating the impacts to existing underground utilities (such as gas lines or water mains) to ensure we protect existing infrastructure.

Based on the above considerations, we're narrowing down the potential blocks for natural drainage systems to:

  • The southside NW 127th St between 1st Ave NW and 3rd Ave NW
  • The westside of 1st Ave NW between NW 127 St and NW 130th St near the intersections
  • The northside NW 130th St between 1st Ave NW and 3rd Ave NW

Next steps

Now that we have identified the preferred project blocks, we are beginning design of the natural drainage systems and expect to have the locations finalized by early 2020. Once finalized, we will communicate these locations on our website and via mailers to homes nearby. We'll also continue refining the design of the drainage pipes farther south in the basin.

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 Natural Drainage Systems and why they are being installed in the Broadview neighborhood.


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.

This project is partially funded by the King County Flood Control District.

King County Flood Control District logo 

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.