Skip to main content



Magnolia Wet Weather Storage Facility at Smith Cove

Construction updates

See project home page for construction updates.

Configuration of the storage facility Storage facility configuration. Source: presentation board  at January 30, 2014 open house.


24-hour emergency and odor reporting:

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

Pipe installation – Pipe Bursting

King County’s contractor installed another pipeline in the existing pipe corridor using a method called pipe bursting. This method allows installation of a new pipe without losing design capacity in the existing pipe.

First, the contractor assembled a long pipe by fusing together shorter sections of pipe. A large drill rig located in Smith Cove Park pulled about 3,000 feet of pipe through the existing pipeline from the entry pit on 32nd Avenue West. To accomplish the pipe pull, a reamer on the front of the assembly cleared out dirt in the pipe. A cutter head and expander then followed to push the pipe outward to make room for the pipeline being pulled behind. The new pipe is made of polyethylene (HDPE), which works well for this method.

Pipe Bursting Graphic

The Magnolia project contractor used a reamer to clear dirt from the pipe, which was followed by a cutter head, an expander and about 3,000 feet of fused pipeline.

Pipeline construction – Horizontal Directional Drilling (HDD)

King County’s contractor used a method called Horizontal Directional Drilling (HDD) to install the sewer pipeline under Magnolia Bluff between 32nd Avenue West and 23rd Avenue West. HDD is a common trenchless technology used for utility pipe installations throughout the United States. HDD limits impacts to the surface, with excavation needed only at the entry and exit points. 

The HDD process is described in this fact sheet and video below.

Construction on 32nd Avenue West will also include new underground structures and controls that will transfer diverted flows from existing sewer lines through the new gravity pipeline to the CSO storage facility in Smith Cove.

In Spring 2015, crews are building underground structures and controls to divert excess flows from existing sewer lines After construction, people will see an above ground equipment vault on the east side of 32nd Avenue West. Operations staff will access the vault from a paved pullout.

32nd Avenue West Diversion StructureNew structures on 32nd Avenue West

The finished CSO control project includes underground structures and controls on 32nd Avenue West to send excess flows to the storage tank. Once these are completed, the contractor will restore work areas.

A paved pullout on the east side of 32nd Avenue West will allow crews access to a small aboveground vault.

Plans for vegetation address conditions in each area being restored. The County’s restoration plan, approved by the City of Seattle, increases plant diversity and replaces invasive weeds with native plants in the work area. Evergreen trees will take up rain water during the winter, and native shrubs will help control erosion and provide habitat for local wildlife. The restoration plan includes some trees left as wildlife shelter and erosion control.

Fact Sheet: learn more about restoration on 32nd Avenue West , September 2014

King County is building an underground storage tank on a site the County acquired from the Port of Seattle. The tank will store excess wastewater and stormwater during heavy rains that can fill the sewer system. When completed, King County’s site will include an underground storage tank, an above-ground equipment building, driveway access and on-site stormwater management features.

Stellar J Corporation, King County’s contractor for the storage tank construction, completed an important milestone of the project in November 2014. Stellar J’s engineers designed a complex shoring system to hold back soils while they excavated the storage tank area. Once the excavation was complete, crews installed over 500,000 pounds of steel reinforcement to form a base slab. In December 2014, the contractor carried out a mass concrete pour to form the base slab. Over the course of 16 hours, 500 concrete trucks arrived to pump just under 5,000 cubic yards of concrete to finish the base slab.

Between December 2014 and March 2015, Stellar J’s crews formed and poured the walls and the lid of the tank. Next, they will begin the above ground building. Work on this site is expected to be complete in Fall 2015.   

Source: February 2015 project newsletter

King County Wastewater Treatment Division works hard to protect the environment and be a good neighbor to communities where our facilities are located. One way we meet our goals is by establishing an environmentally friendly landscape and architecture design developed with community input.

The South Magnolia combined sewer overflow (CSO) facility site was designed to meet code requirements for on-site stormwater management and to address community interest in creating wildlife habitat in a former waterfront industrial area. King County’s project design team incorporated predominantly native plantings in bioswales and vegetative buffers to not only capture and filter stormwater runoff from the site, but also to provide habitat for local wildlife. Download the plant palette fact sheet (May 2013) to learn more.

Learn more about the project design by downloading the Spring 2013 newsletter and viewing past public meeting materials.

King County’s project architect emphasized the County’s mission to protect water quality with a distinctive scupper design feature that will create a cascade visible to passersby on 23rd Avenue West. People will see the cascade captured in a rain garden beneath the scupper.

Configuring the storage facility to accommodate future uses

King County’s project team kept in mind future uses throughout the design process. As a first step, the team optimized the facility configuration to reduce visibility to surrounding area users, and lowered the overall site footprint from the size originally proposed to project stakeholders and Washington Department of Ecology.

Configuring the storage facility to accommodate future usesDownload the facility siting and design fact sheet (May 2013) to learn more.
Site design features

A - Street trees and restored pathway according to permit conditions
B - Rain garden to capture roof runoff
C - Grassland swales to soften appearance, divided by gabion walls
D - Bioswales to retain and filter driveway runoff
E - Lift slabs to access flushing equipment
F - Trees to filter view from surrounding area
G - Successional planting of deciduous trees and coniferous trees  

Building design features

A - Scupper feature visible to passersby
B - Vegetation to prevent splashing and soften building appearance
C - Inverted roof to direct water to raingarden for infiltration
D - Tile pattern to soften building appearance
E - Glass trim to provide visual interest and let light in

Plant palette

King County’s project design team incorporated predominantly native plantings in bioswales and vegetative buffers to not only capture and filter stormwater runoff from the site, but also to provide habitat for local wildlife.

Landscaping plants