South Park Bridge - How we got here

The history of the South Park Bridge replacement project

More than 23 years of planning, engineering analysis, and community engagement have gone into the South Park Bridge replacement project. It’s a project that has touched the lives of many people in the South Park neighborhood. And it’s their input that helped shape a new bridge design that looks forward to the future while respecting the rich history of the past.

Initial planning stages

When the existing South Park Bridge was finished in 1931 it was only designed to be in service for 75 years. As early as 1987, King County started studying whether it made more sense to rehabilitate the existing bridge or replace it. One of the first steps in this process involved selecting a new alignment for a replacement bridge. After a thoughtful analysis, which involved gathering input from the community and partner agencies, the County decided to align the new bridge west of the existing one.

In 1997, the County kicked off the current planning effort to replace the bridge. Then in 1999, the County began looking at nine preliminary alternatives, which included various fixed-level and moveable-span bridges, as well as a tunnel option. These alternatives were evaluated based on seven criteria: regional mobility, local access, waterway navigation, community impacts, aquatic habitat protection, construction impacts, and cost. Three of these bridge alternatives were ultimately rejected due to their lengthy operating times and effects on traffic. The tunnel option was rejected because it would have required numerous property takes, severely disturbed river bottom sediments, and impacted fish listed as endangered species.

In 2001, the County formed a Project Advisory Committee (PAC) and Community Advisory Group (CAG) to provide feedback on the bridge replacement process. The PAC consisted of agencies such as the Cities of Tukwila and Seattle, which have jurisdiction over the project area. The CAG was made up of South Park community members and stakeholders. The CAG, in particular, provided community input on the environmental review and preferred alternative for the new bridge design. They met over the course of seven years to help scope environmental issues, develop conceptual designs for the five alternatives, and select a preferred alternative.

The proposed alternatives

In 2002, the County narrowed the options down to the following five design alternatives: No action – This option considered closing the old bridge, removing it, and not building a replacement. Federal and State environmental regulations required this alternative to be studied. Rehabilitation – This option required replacing many of the existing bridge elements to restore structural integrity The bridge was built in 1930 to the construction standards of the time, and was designed to be in service for 75 years. It was not built to current seismic, electrical or construction safety standards. As a result, the bridge had significant deterioration of its structural elements. It wasn’t possible to reconstruct the bridge without destroying its historical integrity since many new parts would have been required such as bridge piers, steel trusses, bridge railings, control towers, and light poles, among others.

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Mid-level fixed-span – This alternative would have provided 65 feet of vertical clearance from the mean high water level to the bottom of bridge deck. This clearance wasn’t enough to meet the U.S. Coast Guard’s requirement of 100 feet of clearance necessary, since this section of Duwamish Waterway is a navigation channel.

High-level fixed-span – At 100 feet above the mean high water level, this alternative would have met the Coast Guard clearance requirements for the Duwamish. However, the design would have required a south approach extending to South Trenton Street, displacing numerous businesses and homes, and requiring drastic changes to the local street network.

The Bascule Bridge – This alternative involved the construction of a dual-leaf moveable bridge similar to the existing bridge. It received the most support from the community and was the design the County selected, because it met the needs of the community as determined by all the stakeholders and agency partners.

Environmental review and preliminary engineering

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This area of the Duwamish Waterway is designated as a Superfund site and contains high levels of sediment contamination. Any work in this waterway requires an extensive environmental review process, which is referred to as an Environmental Impact Statement. As part of that process a number of studies were completed such as a hydrographic report, hazardous materials assessment, cultural and historic resources technical report, and underwater inspection report. (To see a full list of reports, please go to the South Park Bridge Replacement Project - Documents page). In September 2005, the County completed the Draft Environmental Impact Statement and selected the bascule bridge as the preliminary preferred alternative. In February 2006, the bascule bridge was formally declared as the preferred alternative after receiving approval from the Federal Highway Administration. In June 2006 the County began preparing preliminary (30%) engineering design plans for the new bascule bridge. One of the most important pieces of the preliminary design phase involved compliance with new seismic codes. The new bridge was designed to be operational after a 108-year earthquake and remain standing after a 975-year event. In December 2009 the County prepared the Final Environmental Impact Statement. Approval of the project was granted in February 2010. (To read the Record of Decision, please go to the South Park Bridge Replacement Project – Documents page). It’s important to point out that the Record of Decision only applies to the selected bridge design. Any significant changes to the design would require a lengthy and expensive process.

Design process

While the environmental review was being done, the County moved forward with finalizing the design for the bascule bridge. In July 2008, a value engineering study was conducted that found a number of design improvements and cost saving methods. The most dramatic design enhancement involved revising the foundation type from drilled shafts to sunken caissons, which cost significantly less and require a smaller footprint in the riverbed.

In September 2008, the County hosted an eco-charrette – an interactive brainstorming session used to generate sustainability goals – to explore ways green building and energy-saving techniques could be integrated into the project. One such technique the County selected was using an environmentally-friendly rain garden in lieu of a concrete drainage vault in the south approach.

During the intermediate and final design phases of the project, the landscape architect and artist worked collaboratively to develop a plan for weaving art elements into the bridge design. Existing steel rail panels and gears became new bridge pedestrian rails; existing rockers became gateway features; and weathered red bricks became meandering pathways through the rain garden.

In January 2009, more technical analysis was completed to help with the design of the pier foundations. Geotechnical borings were done to get information on the soil condition and river sediment samples were collected to determine the appropriate way to dispose of contaminated materials during construction.

By November 2009 the County had completed the final (95%) engineering design, which was advertised for industry review. The goal of the review was to solicit input on the proposed construction sequence, the constructability of the proposed in-water work within permitted fish windows (August 1 through February 15 each year), and the short timelines for procurement of materials. The completed plans, specifications, and estimate package were ready for advertisement in February 2010.

Seeking funding

With the planning and design phases well underway, the County started to seek out funding partners to build the new bridge. In 2006, the County succeeded in getting the South Park Bridge replacement included in the proposed Regional Transportation Investment District (RTID) funding package. Unfortunately, the voters didn’t pass the bond measure. In the meantime, the bridge continued to deteriorate, and the County continued to spend up to $440,000 a year on maintenance to keep it operating until its closure on June 30, 2010.

In September 2009, the County applied for a Transportation Investment Generating Economic Recovery (TIGER) grant. Although the County’s proposal made the final round of consideration, it ultimately wasn’t awarded a grant.

Undeterred, the County continued seeking funding partners and succeeded in getting $100M from various federal, state and local jurisdictions during the summer of 2010. With this overwhelming show of regional support, the County applied for a TIGER II grant in August of 2010.

TIGER II grant awarded!

The TIGER II awards were announced on Oct. 15, 2010, and King County was awarded a $34 million TIGER II grant to complete financing for a new South Park Bridge. Bridge construction is expected to be completed in 2013.

Information on the South Park Bridge website is available to people with disabilities in alternate formats upon request by calling 206-263-6482 or 711 for the TTY relay service.

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