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Completed Spring 2017

King County has completed construction of an underground storage tank in the right-of-way in N.W. Blue Ridge Drive and Triton Drive N.W. This facility can store up to 380,000 gallons of untreated water when the North Beach Pump Station reaches maximum capacity.

If you have concerns about the facility's operations

Should you notice issues you believe to be related to the facility's operation (noise or exhaust concerns, lights left on at site, safety or security concerns, open gate, etc.), please contact King County WTD at 206-263-3801 as soon as possible at any time. This number is answered 24-hours a day.

Call 911 in case of emergency.

Project overview

The underground storage tank in the right-of-way in Northwest Blue Ridge Drive and Triton Drive Northwest stores peak flows when the North Beach Pump Station reaches maximum capacity. After storms have passed, an underground pump transfers stored flows to the pump station for conveyance to King County’s Carkeek Wet Weather Treatment Facility for transfer to West Point Treatment Plant or onsite treatment. The project includes an underground diversion structure (that conveys peak flows to the facility), and a new odor control and electrical facility on King County property.

Project overview map  , January 2014

Landscape site plan. Source: project newsletter  , February 2015

operations phase

24-hour emergency and odor reporting:

Contact West Point Treatment Plant at  206-263-3801.

Project location


Back to the capital projects overview map .

Learn more about the project

Why did King County need to do this project?

In 2008, King County reported that the North Beach combined sewer overflow facility had 10 overflows per year on average that discharge a total of 2.2 million gallons into Puget Sound off North Beach. At North Beach, King County is working to meet current regulations set by the Washington Department of Ecology require no more than one untreated discharge per year on a long-term average.

Understanding the sewer system in North Beach

The North Beach Pump Station has provided wastewater service for the area since 1962 by collecting flows from the City of Seattle’s local sewer system. These flows are conveyed to the Carkeek Wet Weather Facility where they are either pumped to the county’s West Point Treatment Plant or, in large storm events, treated on site. The North Beach Pump Station is built on the footprint of what was once a City of Seattle treatment plant. The walls and foundation are from the original treatment plant.

Learn more: download Understanding the sewer system in North Beach  , Winter 2014

Work in the street (Triton Drive Northwest & Northwest Blue Ridge Drive)

Project overview map  , January 2014

Underground storage tank

King County installed a storage tank approximately 230 feet long underneath the intersection of Northwest Blue Ridge Drive and Triton Drive Northwest. This facility will store peak flows during large storms when the North Beach Pump Station reaches maximum capacity, reducing discharges of untreated stormwater and wastewater to Puget Sound near a recreational beach. Pumps located on King County’s property will return stored flows to the conveyance system when capacity is restored and a flushing station installed at one end will clean the pipelines after a storm event.

The storage tank alignment was chosen because it:

  • Reduces overall construction footprint
  • Provides access to a facility that is out of the roadway, improving crew safety, and reducing traffic impacts
  • Allows for a simple, lower-maintenance flushing system that uses less water
  • Reduces interference with underground utilities


To build the new underground storage tank in the street right-of-way, King County’s contractor closed the intersection beginning in April 2014. The road was reopened on August 31, 2015. The new intersection is now designed as a three-way stop, or “T-intersection” to improve safety for drivers, pedestrians, and cyclists traveling through the area. 

Key steps for construction

  1. Install utility and sewer bypass systems. The contractor installed the gravity sewer bypass system so wastewater can continue to get to the North Beach Pump Station during construction. Other utilities like electricity, gas, cable, and water were temporarily moved to accommodate construction of the new tank.
  2. Construct the CSO storage tank. The contractor has installed the new CSO storage tank. They first excavated a large area in the street to build the tank. Crews poured concrete to form the bottom, walls, and top of the tank. Soil and gravel ("backfill") were placed over the tank in preparation for road repaving.
  3. Construct the new aboveground mechanical and electrical building. The contractor constructed the new aboveground mechanical and electrical building, located on the North Beach Pump Station while building the CSO tank.
  4. Restoration: The contractor has restored utilities (that were temporarily moved during construction) and the roadway (including reconfiguring the intersection to a safer “T” intersection), and installed landscaping.

Work at the King County site

Odor control and electrical building 

King County constructed a building approximately 55 feet long, 12 feet wide, and 15 feet tall on King County property southwest of the North Beach Pump Station. The above-ground portion of the building contains the electrical and mechanical rooms.

A new second driveway will be added on King County property to provide safer access for operations and maintenance.


Construction of the new odor control and electrical building involved excavation, shoring, and building construction. Additional site work included drain pump installation, relocating the existing fueling tank, and relocating underground utilities.

Site restoration

King County restored its property affected by the project and worked with the community on design elements (see meeting calendar).

Construction photos

rsz_20576049138_131ffb7f92_zProject site

View more photos on our Flickr site  .

Under normal operations, the combined sewer system sends flows directly to the North Beach Pump Station for conveyance to the Carkeek Park Wet Weather Facility where it is either treated or sent on to West Point for treatment. The CSO diversion structure and pipeline storage are bypassed.

During large storm events, flows up to three million gallons per day (MGD) continue to the North Beach Pump Station but excess flows travel over a weir in the underground diversion structure to the underground storage pipe.

Once the storm is passed and water levels return to normal, the stored flows are sent back through the North Beach Pump Station and on to the Carkeek Park Wet Weather Facility for conveyance to the West Point Treatment Plan or on site treatment, depending on capacity.

Once the CSO storage facility is in service, people will see one or two crew members on site about once a week for inspection and maintenance. After a storage event, more crew members may be on site to empty and flush the facility. Every three to five years, one to two days of major maintenance will be required, resulting in partial street closure. Advance notice will be provided for maintenance activities that cause traffic disruptions.

Community meetings and events

January 11, 2014
10 a.m.–2:00 p.m.
North Beach Elementary School
9018 24th Avenue Northwest, Seattle

Preconstruction open house. Drop in anytime between 10:00 a.m. and 2:00 p.m. Join us to meet Stellar J representatives and King County’s construction management team and learn about construction activities. Kids can have fun in the “kids construction zone” and become a “drain ranger” learning how to protect the sewers, streams and Puget Sound at North Beach.

Handouts/presentations and additional information

September 29, 2012
10 a.m.–1:00 p.m.
North Beach Pump Station

Community event. You will want to attend the event if you’re interested in:

  • An update on the landscape and architecture design and how community input contributed to the design
  • New information about the storage facility design
  • More detail on the construction of the new CSO control facility
  • Tours of the underground pump station (first come, first served for these very popular tours!)
  • Information about the evaluation of the North Beach outfalls and force main

Handouts/presentations and additional information

June 11, 2012
7–8 p.m.
Blue Ridge Clubhouse,
10040 15th Avenue Northwest, Seattle 

Community meeting. King County's North Beach Combined Sewer Overflow (CSO) Control Project team invites you to a meeting hosted by the Blue Ridge Board. All interested community members are invited to attend.

At this meeting, you can:

  • Review architectural and landscape concepts for the restored North Beach Pump Station site
  • Learn about elements being considered in the street improvement process, including lighting, road reconfiguration, and green stormwater infrastructure
  • Hear about options for construction staging areas
  • Ask questions and provide feedback about the architectural and landscape concepts

Handouts/presentations and additional information

May 12, 2012
10 a.m.–12:30 p.m
Carkeek Park Environmental Learning Center
950 NW Carkeek Park Road, Seattle

Design workshop. King County invites the North Beach community to attend a design workshop on May 12. The purpose of the workshop is to update the community and project stakeholders on the North Beach CSO Control Project and to provide them with an opportunity to give input on landscaping and architectural design details. Information gained from the community will help inform the site look and feel.

Handouts/presentations and additional information

January 25, 2012
6:30 –8 p.m.
North Beach Elementary School cafeteria
9018 24th Avenue Northwest, Seattle

Community meeting. King County’s project team invites you to hear an update on design of the new CSO control facility at North Beach: 

  • Results from recent field investigations
  • Location and configuration of the odor control/electrical facility and underground storage pipe
  • How community input informed preliminary design
  • Next steps and opportunities to participate

Handouts/presentations and additional information

Saturday, August 6, 2011
11 a.m.–2 p.m.
King County North Beach Pump Station
9921 Triton Drive Northwest, Seattle

Community meeting. Meet the North Beach CSO Control Project design team and take a pump station tour.

Handouts/presentations and additional information


This Facility Plan outlines improvements to the King County conveyance system serving the North Beach Basin that are necessary to control CSOs in compliance with RCW 90.48.480 and WAC 173-245-020 (22).

This plan is submitted in compliance with the first of three compliance schedule dates noted in Section S18 of the West Point Treatment Plant National Pollutant Discharge Elimination System permit (refer to page 53 of permit).

Final facility plan, September 2011

Draft facility plan, December 2010