Holden Natural Drainage Systems Project
Example of a natural drainage system project a few years after installation
Project description
Improving creek water quality, increasing landscaping diversity, and providing roadway and pedestrian safety improvements
More than 12 million pounds of pollution are carried into our water bodies through stormwater runoff each year. Stormwater that falls on the roofs, streets, and parking lots in your neighborhood can pick up harmful pollutants before entering Longfellow Creek, which flows into Puget Sound. The pollutants can harm fish, wildlife, and our ecosystems. The Holden Natural Drainage Systems (NDS) Project is building natural drainage systems, also known as roadside rain gardens or bioretention, on SW Holden St between 16th Ave SW and 17th Ave SW to capture and treat stormwater runoff before it reaches Longfellow Creek and Puget Sound.
NDS are made up of a series of cells, or shallow depressions, along a street block designed to capture and slow stormwater and filter pollutants before they can reach our waterways. NDS cells are located in the space between the street edge and property line and are filled with a special soil and deep-rooted plants that temporarily hold and filter polluted stormwater from streets. The project will include roadway and pedestrian safety improvements such as curb bulbs and ADA ramps, in addition to new landscape plantings.
Location
The Holden NDS Project area is along SW Holden St west of 16th Ave SW and half a block west of 17th Ave SW. The map shows the project area.
What's happening now?
The project team completed the final design in March 2025. We expect to award the construction contract in late 2025 and begin construction in 2026. SPU will provide advanced notice before construction starts.
To learn more about the Holden Natural Drainage Systems Project, please view the Final Design Narrated Presentation.
Minimizing Construction Impacts
We understand that SW Holden St is a busy street for residents, businesses, and commuters. We will work to minimize construction impacts to the extent possible and will keep you informed before and during construction. Our project team will provide more details on construction timing and impacts before construction starts. The contractor is responsible for obtaining construction permits which include traffic control planning.
Construction is anticipated to start in autumn 2026. It is expected to last up to 8 months. During construction, you can expect the following:
- Work hours: Construction will typically take place Monday through Friday from 9 AM to 3 PM but may also occur on evenings or weekends.
- Road and sidewalk closures: Roads and sidewalks may be partially or fully closed, with detours posted. Anticipate lane closures on SW Holden St; westbound lanes will remain open. Pedestrian detours will be provided.
- Parking restrictions: Parking may be restricted near construction zones.
- Construction impacts: Construction can produce noise, dust, and vibrations.
- Access to homes and businesses: Access to homes and business will be maintained. If a contractor needs to close your driveway temporarily, they will coordinate with you.
- Bus stop relocation: The bus stop at SW Holden St and 16th Ave SW will be temporarily relocated.
- Communication: The project team will share email updates, phone calls, and flyers to communicate construction impacts.
Community benefits
The Holden NDS Project will offer multiple benefits to local neighborhoods and ecosystems, including:
- Improved pedestrian safety
- Improved roadway elements
- Increased landscaping diversity
- Improved stormwater/drainage water quality
- Healthier creek ecosystems
- Planning/Design 2022-2025
- Construction 2026
- Completion 2027
Natural drainage systems are living systems, and their appearance will change over time. The grasses, shrubs, and trees installed 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 maintenance of the natural drainage systems, which includes general upkeep of the plantings, ensuring that the system is functioning properly, removing sediment, and replacing or removing plantings and trees in the natural drainage system, if needed. You will not be asked to perform any maintenance related to the natural drainage system. Maintenance of the planting strip that is not part of SPU’s natural drainage system remains the responsibility of the adjacent property owner.
Natural drainage systems (NDS) help slow stormwater and filter out pollutants before they reach our creeks and waterways. This protects aquatic life and keeps our water healthier for people too.
SPU is planning to build NDS in the Longfellow Creek watershed to improve water quality in the creek and Puget Sound. These systems were once call “roadside rain gardens” but are now called natural drainage systems (NDS).
If you would like learn more about how stormwater affects our ecosystems, we recommend watching this Nature Conservancy video featuring Longfellow Creek..
Below are key updates the project team made between mid-design and final design.
- Dumar Substation update- In September 2024, the City of Seattle's Office of Housing announced that Homestead Community Land Trust was selected as the partner to develop the former Dumar Substation at SW Holden St and 16th Ave SW for affordable housing. Both projects will continue to coordinate with each other.
- Emergency route prioritization- As part of our ongoing coordination with Fire Station 11, the Seattle Fire Department (SFD) requested the removal of the existing speed cushion on SW Holden St (an emergency route) between 16th Ave SW and 17th Ave SW. The removal of the speed cushion will improve SFD the ability to respond to emergencies. This project will also provide new pavement marking by the fire station to allow for emergency vehicle route improvement
- Addressing localized flooding- The project team was made aware of a localized flooding issue at the intersection of SW Holden St and 17th Ave SW during the project’s mid-design phase outreach. As a result, the project will install new drainage infrastructures and new underground water quality treatment facility at the north side of the intersection. The new infrastructure will address both the flooding issue and provide additional water quality treatment for Longfellow Creek.
- Tree removal and replacement– The project team will need to remove two trees that conflict with where the underground water quality treatment facility will be installed. Due to site constraints, the project team will plant one tree back in the project area. SPU will provide funding to SDOT for planting five additional trees as mitigation for removing the two trees.
Site Selection Process
SPU selects potential project locations by working with other city departments (for example, when undertaking planned street or pedestrian improvements), by looking for areas where this project may address water quality and/or stormwater flow issues, and by collecting information from the community.
SPU selects sites within the Longfellow Creek watershed based on a variety of factors:
- How much stormwater can be managed and cleaned
- Ability to address flooding problems
- Location of nearby adverse conditions (steep slopes, high groundwater, contaminated sites)
- Potential impacts to existing mature trees
- Existing soil conditions
- Location and condition of existing utilities
- Community feedback
- Width of public right-of-way
- Presence of driveways
- Existing parking congestion and availability of off-street parking
- Cost/benefit evaluation balancing project construction, operation, and maintenance costs and community benefits
The Holden NDS Project site was selected because untreated stormwater from the roadways in this area ultimately reach Longfellow Creek. The Holden NDS Project site was also a good candidate as the cells installed for water quality will serve as permanent curb bulbs, replacing the temporary painted curb bulbs installed by the Seattle Department of Transportation (SDOT) at 17th Ave SW and SW Holden Street.
Potential Impacts
This project will replace some grass areas that are next to the existing sidewalk with new grass in some locations. In other locations, some of the grass areas will be converted to NDS.
Within the NDS planting areas, we are held to pre-approved plant species for specific water quality treatment function and ease of maintenance. The selection of plants for the non-NDS zone have a little more flexibility, depending on suitability, availability, equity, and maintenance.
NDS may require some driveways to be restored to the City’s standard width (typically 10’ depending on the property configuration) within the public right-of-way. SPU will work with each impacted resident to the extent allowed by City Policies and practices.
Some gas lines, side sewers, electrical lines, communication lines, and water utilities will need to be adjusted during construction. We will work with each resident who may be impacted by utility relocation.
Some mailboxes may need to be relocated due to NDS improvements.
What to Expect When the Project Is Complete
During storms, each NDS will temporarily hold up to 12 inches of water and then drain within 24 hours after the rain ends. When there are back-to-back storms or larger storms, the water level will rise and fall, which is a sign that the project is functioning properly.
The natural drainage systems are designed to completely drain within 24 hours of the storm passing. If there is more water than the project can hold at one time, you will see any excess stormwater flow out of the system and into the nearest pipe, ditch, or storm drain.
Seattle Public Utilities' Operation Response Center maintains a 24/7 hotline for people to call and report drainage issues. If the completed project near your home isn’t draining properly or requires maintenance, please call (206) 386-1800.
This project is designed to carry filtered stormwater directly to the drainage systems, not toward yards or basements. If needed, the natural drainage systems will have an underground liner for extra protection.
This project will not attract mosquitoes. Mosquitoes need stagnant water (found in bird baths, old tires, dog water bowls, etc.) and natural drainage systems are designed to constantly drain and keep water moving.
Natural drainage systems are designed with safety in mind. They will typically drain within 24 hours following a storm and, in most cases, drain even faster.