A top-down approach to construction allowed the team to build two new bridges on I-5 with minimal impacts to interstate traffic.
The existing 5-foot by 5-foot, 425-foot-long, concrete box culvert at I-5 was not fish passable, so the project replaced it with two interstate bridges that provided a minimum hydraulic clear span of 44 feet and an arch culvert at an adjacent city street (right).
The restored Padden Creek reach included 1,000 linear feet of stream habitat features like LWM, riparian vegetation, and carefully designed channels.
Juvenile salmon taking shelter under a piece of LWM in Padden Creek.
Habitat included more than 100 pieces of large woody material.
The team combined two sampling techniques, McNeil and Wolman sampling, to better size proposed streambed material, resulting in smaller spawning gravel that mimics the natural environment and will provide high-quality habitat for all life stages of fish development.
Across Washington State, stream crossings like the small culvert at SR 11 frequently block fish passage to upstream habitat, and the Washington State Department of Transportation is working to replace them.
To reduce traffic impacts during construction of the Padden Creek crossing at SR 11, Granite built the entire culvert in just 52 hours during a single weekend road closure.
Padden Creek Fish Passage Design-Build
Geotechnical and environmental services for complex stream crossings at I-5 and SR 11
The Washington State Department of Transportation (WSDOT) faced a unique challenge; build a sustainable fish habitat beneath an active interstate and near a busy intersection—and do it with minimal traffic disruptions.
To comply with a federal injunction requiring corrected fish barriers, WSDOT had to replace Padden Creek crossings at I-5 and SR 11 near Bellingham, Washington. A KPFF/Granite design-build team, including GeoEngineers, used innovative design and accelerated construction techniques to build 1,000 feet of new stream channel with more than 100 habitat features, two fish-friendly culverts and two bridges that restored fish passage under SR 11 and I-5 and provide access to more than five miles of upstream rearing and spawning habitat.
Approach
Complex Geotechnical Conditions: The I-5 crossing site was known to feature variable bedrock near the new culvert’s proposed foundations, but specific subsurface data was limited. So, the geotechnical team conducted additional borings and supplemented them with a microtremor array surface wave (MASW) geophysical investigation along the length of the soldier pile walls, reducing the risk of unanticipated conditions and construction delays.
Top-Down Construction: The team used a combination of creative top-down construction sequencing and innovative engineering to decrease the total number of traffic impact days on I-5 from 400 to just 225—and built the entire southbound bridge in just 37 days. They overcame complex geotechnical conditions, construction obstructions, and unexpected changes to regulatory policy to successfully replace the old crossing with two full-span bridges for north and southbound I-5 and a 53-foot-wide buried arch at an adjacent street.
FEMA Permitting: Because the new I-5 stream crossing design created a “rise condition” in the FEMA-regulated floodplain, it needed to go through the complex CLMOR/LMOR process. The project team and WSDOT worked closely with the Washington State Department of Ecology, Washington Department of Fish and Wildlife, U.S. Army Corps of Engineers, and City of Bellingham to agree on a solution that wouldn’t trigger the CLMOR/LMOR requirement. By working together, they avoided the process by redesigning the crossing so that the “rise condition” stayed on WSDOT rather than city property.
Hybrid Gravel Sampling: Streambed gravel is a critical component of fish spawning habitat, and the stream and environmental teams used an innovative approach to get this feature right for Padden Creek. The team collected six McNeil sediment samples to supplement more traditional Wolman pebble counts—a first for design-build stream crossing projects.
McNeil samples gave the team a much better understanding of the natural streambed material in Padden Creek by collecting material from as deep as nine inches below grade. The team combined McNeil and Wolman sampling data to better size proposed streambed material, resulting in smaller spawning gravel that mimics the natural environment and will provide high-quality habitat for all life stages of fish development. WSDOT has since disseminated this unique methodology to all staff and consultants working on its injunction culvert correction designs.
Habitat Enhancement: The project team added habitat-enhancing features that will evolve with the stream system over time. At the SR 11 crossing, the team shifted the stream alignment from the conceptual design location to create an entirely new off-channel habitat for fish. Stream engineers also placed large woody material (LWM) like fallen trees and root wads throughout the project reach according to a complex matrix, providing essential habitat for young fish. Within two weeks of finishing construction at the SR 11 site, salmon, steelhead and other fish were observed swimming and spawning throughout the restored Padden Creek reach, demonstrating the immediate success of the stream crossing and environmental features.
Results
The project team helped WSDOT fulfill its requirements under the 2013 federal injunction by replacing the two fish barrier culverts beneath I-5 and SR 11 with four fish-passable structures, and they did it with minimal traffic disruptions. Within two weeks of finishing construction at the SR 11 site, salmon, steelhead and other fish were observed throughout the restored Padden Creek reach, swimming, staging, and spawning both upstream of the newly installed culvert and downstream in the constructed spawning habitat, indicating long-term success of the stream crossing and environmental features.
The project’s success depended on close collaboration and trust between teaming partners, WSDOT, and regulatory agencies—and WDFW is already using it as an ideal example of a fish-passage project that delivered effective habitat enhancement on time and on budget.
Awards
Silver Best-in-State Award, ACEC Washington
Community Impact
The Padden Creek crossing at SR 11 is in a congested area, next to a busy intersection, a senior living facility, and several businesses. The team hosted a virtual Q/A session during the pandemic to listen to the community’s questions and concerns, and then modified construction plans to meet specific needs.
Original plans called for a meandering 0.25-mile pedestrian detour through an adjacent park, but that changed after listening to community concerns. Granite’s new closure detour plan included full-time access to local businesses and residents, vehicular access to the adjacent retirement community, direct pedestrian access through the work zone and alternative routing for Whatcom Transit Authority. By listening to community and stakeholder concerns and making real design changes in response, the team showed how transportation engineering can be effective, collaborative, and respectful.
