Skip to main content

The Third Avenue entrance to the King County Courthouse is temporarily closed. Please use the Fourth Avenue entrance.

King County logo

Completed Spring 2017

Public involvement | Library

Project update

March 2, 2017

Sound muffling blankets installed

Noisy facility equipment was covered with materials to reduce sound from them in February 2017.

Drainage upgrades COMPLETE!

King County’s contractor installed drainage improvements at the corner of N.W. Neptune Place and N.W. 100th Street, and at the corner of N.W. Neptune Place and N.W. Blue Ridge Drive in mid-December.

Drainage improvements

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 description

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.

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 does 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

Project area

King County has built an underground storage tank in the right-of-way in Northwest Blue Ridge Drive and Triton Drive Northwest. This facility will store peak flows when the North Beach Pump Station reaches maximum capacity. After storms have passed, an underground pump will transfer 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 will convey peak flows to the facility), and a new odor control and electrical facility on King County property.

Project elements

Project overview map  , January 2014

Landscape site plan. Source: project newsletter  , February 2015

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.