SR 520 Bridge Replacement and HOV Program - Environmental stewardship

WSDOT is committed to building and maintaining a sustainable, integrated multimodal transportation system – one that supports healthy communities and economic vitality while protecting the environment. We plan and design our projects with the goal of limiting their environmental effects or avoiding them altogether. During construction, we require work practices to preserve the region’s natural resources and promote public health and safety. When project effects are unavoidable, we collaborate on initiatives to mitigate them. 

For the SR 520 Program, we are mitigating our work’s effects by restoring wetlands, improving creeks and ponds, building and enhancing trails, rehabilitating shorelines, and restoring fish-migration corridors. Moreover, we are treating the highway’s runoff for the first time, helping to reduce greenhouse gas emissions, and supporting more public park and green space. We’ve already completed most of these mitigation projects, while a few final projects will be completed nearer to the conclusion of the SR 520 Program.

Building an environmentally smart highway

One way we support the environment is by making good, up-front decisions in the planning and design phases of our highway program. Smart planning ensures that we reduce our environmental effects and, where possible, improve our natural surroundings. Examples include: 

Low-impact structural designs that reduce steel and concrete requirements. The West Approach Bridge North design reduced by 40% the number of in-water columns and the amount of concrete needed to build them. On the floating bridge project, our design strategies reduced rebar use by 260 tons, steel use by 1,600 tons, and concrete use by 12,500 cubic yards.

Traffic crossing the new SR 520 floating bridge on Lake Washington Dedicated transit/HOV lanes and median transit stops between I-5 and I-405, which will allow the highway to carry up to 17% more people during peak traffic (PDF 12.5MB) and 5% to 10% more vehicles.

A cross-lake bicycle and pedestrian path that now provides better connections to bus and light-rail stops, local bike paths, and, as part of the Montlake Project, will connect to the new Montlake Lid and transit hub. 
A projected 10% reduction in greenhouse gas emissions (PDF 1.8MB) within the corridor when compared to a no-build option thanks to improved traffic movement and increased transit ridership.

New stormwater management systems along the corridor, including on the new floating bridge, to capture highway runoff and improve water quality in local streams and Lake Washington.
A floating bridge design that could be retrofitted for light rail if the region chooses that option in the future.
 A constructed bioswale along SR 520 on the Eastside
Noise-reduction measures, including quieter concrete pavement, noise-absorbing materials at lid portals, taller traffic barriers, and noise walls on the Eastside.
Highway lids that reconnect neighborhoods, provide better transit connections, and increase community green space.

Promoting sustainability

WSDOT strives to reflect sustainability by designing and building an environmentally responsible, multimodal transportation system that we can efficiently operate and maintain for decades to come. Examples include:

  • Requiring our SR 520 contractors to develop and implement ecologically sustainable construction practices, such as reducing, reusing and recycling construction materials. On the floating bridge project, for example, the contractor sold all of the old bridge’s concrete pontoons for use in other regions as wharfs, piers or other marine structures.
  • Reclaiming or using existing industrial/brownfield sites for construction of SR 520 bridge components or corridor infrastructure, such as our new stormwater retention facilities. 

Read our 2014 SR 520 Sustainability Report (PDF 5.7MB) to learn more.

Employing smart construction practices

During SR 520 construction, we follow work practices that are designed to minimize the environmental effects of our work. A few examples include: 

  • Using specialized bubble curtains in Lake Washington to reduce underwater construction noise from pile driving, which can harm fish.
  • Employing truck-wash stations in construction areas to reduce dust and keep streets and highways cleaner as our construction vehicles come and go. 
  • Erecting silt and turbidity curtains around construction areas to halt erosion, prevent runoff and contain dredged sediments.
  • Using vegetable-based hydraulic fluids in construction equipment to minimize environmental damage if a spill occurs.

Enhancing parks

Within the densely populated SR 520 corridor, we work with communities to mitigate the environmental effects of our construction program, in part by making significant improvements to local parks. A wetlands within the Washington Park Arboretum in SeattleSome of our parks enhancements include:

  • Improvements to the Washington Park Arboretum, including:
    • A new multiuse trail.
    • Restoration work to Arboretum Creek and the Waterfront Trail.
    • an enhanced SR 520 pedestrian undercrossing on Foster Island.
    • Removal of SR 520 on- and off-ramps in the Arboretum and the never-completed R.H. Thomson Expressway ”Ramps to Nowhere,” which will reduce traffic through the Arboretum and create a more open and natural park area.
    • In the future, a new north entry into the Arboretum with various trail and park enhancements.
  • Funding the development of the Fritz Hedges Waterway Park, a new 4-acre public park along Portage Bay near the University of Washington. The Seattle park opened to the public in fall 2020.
  • New viewpoints on the planned Roanoke lid over SR 520, between 10th Avenue East and Delmar Drive East.
  • Corridorwide connections to local parks and shared-use trails from SR 520’s new cross-lake bicycle and pedestrian path.

Enhancing natural habitats

A construction crew installing a large fish culvert under SR 520 on the EastsideThe SR 520 corridor not only includes dense urban and suburban areas, but rich and diverse natural areas as well. We have made substantial enhancements in many of these locations to offset the environmental effects of our reconstruction effort, including:

  • Bear Creek: Improved about 16 acres of important stream and riparian habitat for salmon by stabilizing creek bank, adding stream gravel and wood, and planting native vegetation. Completed in 2015.
  • Cedar River Elliott Bridge Reach: Restored about 5 acres of wetlands, side channels, floodplain and river habitat for migrating fish, including Chinook salmon. Completed in 2016.
  • Evans Creek: Restored about 32 acres of wetlands on land previously farmed at the confluence of Bear Creek and Evans Creek, near Redmond. Completed in 2013.
  • Grass Creek: Restored wetlands and shoreline habitat in the 68-acre Grass Creek site in Grays Harbor County by removing a portion of earthen dike and re-establishing tidal channels. (SR 520 pontoons were constructed in Aberdeen, Grays Harbor County.) Completed in 2011.
  • King County Mitigation Reserves Program: This program allows participants to pool financial resources for larger restoration projects with greater environmental benefits. WSDOT contributions mitigate for some of the SR 520 program’s impacts to wetlands. Financial commitments were made in 2014, 2017 and 2018.
  • South Lake Washington: Improved about 4 acres of lake-shore salmon habitat by planting native vegetation, removing decades-old industrial structures, and enhancing near-shore habitat. Completed in 2015.
  • SR 520 East Approach: Improved about 1 acre of sockeye-spawning habitat beneath the east end of the new floating bridge by adding lakebed gravel, removing bulkheads and rubble, and restoring/replanting the shoreline. Completed in 2017.
  • Union Bay Natural Area: Improves existing wetlands and buffers, and creates new wetlands – about 22 acres combined – within this University of Washington natural area northeast of Husky Stadium. Completed in 2018.
  • WSDOT Peninsula: Enhances wetlands and wetland buffers, controls invasive plant species and adds habitat features within this 6-acre WSDOT-owned property next to the Washington Park Arboretum. Mitigation planned following SR 520 reconstruction.
  • Yarrow Creek: Restored wetlands and rehabilitated the Eastside stream to improve fish and wildlife habitat, and added eight large, fish-friendly culverts within SR 520’s Eastside corridor to improve fish migration. Completed in 2014.

Enhancing historical and cultural resources

WSDOT has worked with the community to assess and protect historic and cultural resources both along the SR 520 construction corridor and at other locations where we built components for the new floating bridge.

A barge and work trestle along a new section of SR 520 bridge on Lake WashingtonOur efforts include:

  • Use of temporary work bridges and barges to minimize our construction footprint in sensitive areas as we construct replacement bridges and other permanent highway structures.
  • Consultation with stakeholders to ensure that the project’s structural and landscape designs are compatible with the historic character of neighborhoods in the corridor.
  • Involvement of an outside expert in designing new bridges within historically sensitive areas.
  • Ongoing consultation with Native American tribes, local governments, resource agencies and others to identify potential cultural resources, understand program activities, and develop appropriate mitigation steps where needed.
  • Surveys and inventories of historic homes, and preparation of nominations for listing the Montlake Historic District and Olmsted-designed parks and boulevards in the National Register of Historic Places.

Where to learn more