12th Ave NW Basin Drainage Improvements (Broadview Natural Drainage Systems) Project
The City of Seattle is constructing drainage improvements in Broadview.
Project description
As part of the drainage improvements in 12th Ave NW Basin, SPU will be constructing natural drainage systems in Broadview, known as the Broadview NDS project.
Location
For the Broadview NDS project, natural drainage systems will be located between 1st and 3rd Ave NW on NW 127th and NW 130th streets.
Construction of drainage conveyance pipes and detention pipes has been completed along 10th Ave NW and 11th Ave NW between NW 120th and NW 125th streets, and NW 125th St to 6th Ave NW in 2023.
What's happening now?
Construction will begin around December 2025 to early January 2026.
See the NDS Drainage Improvements Map 2023 for approximate locations and renderings of the new walkways and natural drainage systems.
Community benefits
This project addresses flooding issues in parts of Broadview and water quality in Mohlendorph Creek by building new storm system improvements to slow runoff and improve flows. The project uses a variety of tools in the right-of-way 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
Anticipated construction impacts
- Construction-related noise, dust, vibrations
- Restricted street parking and partial lane closures near work zones
- Equipment stored in the right of way
- Temporary access restrictions to some driveways during working hours
- Pedestrian access and access for essential services (such as emergency vehicles, mail delivery and garbage pickup) will always be maintained.
2022-2023
-
Construction of pipe and conveyance elements
2023-2025
- Design of walkways and natural drainage systems
2025-2026
-
Construction of 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 community engagement, SPU selected the 12th Ave NW Basin Drainage Improvements project.
Between 2011-2017, SPU completed two early action pilot projects 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.
Natural drainage systems are living systems, and their appearance will change over time. The grasses, shrubs, and trees in the natural drainage system will grow and change as the garden matures. It may take up to three years for plants and shrubs in the natural drainage system to reach full maturity, and possibly longer for trees. The examples below show what a natural drainage system may look like over time.
Maintenance and Care
SPU is responsible for all maintenance needs of the natural drainage systems, including watering, weeding, and general upkeep. SPU will also prune trees and shrubs within the natural drainage system as needed. You will not be asked to perform any maintenance of the natural drainage systems. It is important for residents to stay out of the natural drainage system to maintain the function of the systems. Plants will mature over time and the mix of plants will likely change, but this won’t affect the system’s function.
As is typical for Seattle, property owners are responsible for maintaining portions of the planting strip, roadway shoulder, and grass-lined drainage ditches that are not part of the natural drainage system.
This project is partially funded by the King County Flood Control District.
