12th Ave NW Basin Drainage Improvements Project

Photo of the broadview neighborhood view
The City of Seattle will construct drainage improvements in west Broadview.

Project Description

The 12th Ave NW Basin Drainage Improvement Project will improve the drainage system in parts of west Broadview with severe flooding issues.


Natural drainage systems will be located between 1st and 3rd avenues NW on NW 127th and NW 130th streets. Visit the drainage improvements map

Drainage conveyance pipes and detention pipes will be located mostly along 10th Ave NW and 11th Ave NW between NW 120th and NW 125th streets and along NW 125th St to 6th Ave NW.

What's Happening Now?

Our design team is finalizing the natural drainage systems design for 2nd and 3rd Avenues NW on NW 127th St and between 1st and 3rd Avenues NW on NW 130th St. Construction of the natural drainage systems has been pushed out to 2024 as a result of a new partnership with the Seattle Department of Transportation (SDOT) to incorporate walkways in the design. See the drainage improvements map for approximate locations of the new walkways and natural drainage systems. 

The underground detention pipe and conveyance project elements have reached 90% design. This portion of the project is located mostly along 10th Ave NW and 11th Ave NW between NW 120th and NW 125th streets and along NW 125th St to 6th Ave NW. Construction of the pipe and conveyance elements is expected to start in summer 2022. Once we have a contractor on board, we will have more details about anticipated construction sequence, staging areas, impacts, and schedule.

Community Benefits

This project addresses flooding issues in parts of Broadview and water quality in Mohlendorph Creek by improving the drainage system. 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

Anticipated Construction Impacts

The project will build two main types of new storm system improvements: natural drainage systems and a combination of conveyance pipes and large underground storage pipes (grey infrastructure) to hold water and help protect against flooding during storms. Construction impacts may include:

  • Construction-related noise, dust, vibrations
  • Restricted street parking near work zones
  • Equipment being stored in the right of way
  • Temporary access restrictions
  • Limited access to some driveways during daytime working hours

Pedestrian access and access for essential services (such as emergency vehicles, mail delivery and garbage pickup) will always be maintained.

Community Engagement

Nov. 7, 2019 open house materials

Nov 3, 2018 walking tour materials


Through 2021

  • Design

Mid 2022

  • Construction (pipe and conveyance elements)

2024 (pending design approval)

  • Construction (walkways and natural drainage systems)


The Broadview neighborhood has a history of observed sewer and drainage problems. During heavy rains, water can overwhelm Broadview’s sewer system and cause sewage backups into homes, which is a public health issue. Stormwater runoff can also flow from streets onto private property and cause damage. Broadview’s high groundwater table, poor draining soils, steep bluff to the west, lack of formal drainage system, and aging sewer pipes all contribute to the complexity of these issues.

From 2011-2016, SPU evaluated potential options to reduce stormwater flooding in Broadview. After extensive evaluation and talking with the community, SPU selected the 12th Ave NW Basin Drainage Improvements project. This project is currently in the design phase.

SPU has already completed two early action pilot projects in the Broadview neighborhood and two small projects:

  • Flood Grouting Pilot Project -- In 2011, crews sealed mainlines and side sewers using flood grouting to reduce flows of stormwater into the sewer system.
  • Backflow Preventer Pilot Project -- In 2011, SPU initiated a project to provide expedited protection to homes that have experienced multiple backups. Since installation, there has been a reduction in sanitary sewer overflows and claims.
  • 11th Ave NW flooding reduction -- In 2017, SPU completed a flooding reduction/sidewalk improvement project along the 12700 block of 11th Ave NW to reduce stormwater flooding from the street onto private properties during storm events.
  • NW 120th outfall replacement and drainage improvement -- In 2017, SPU installed a new outfall and improved stormwater drainage around 9th Ave NW and NW 120th St.

Budget & Funding

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

King County Flood Control District logo