Dungeness River Pedestrian Bridge Enhances Natural Habitat, Benefits Jamestown S’Klallam Tribe
It isn’t often that our engineering profession delivers infrastructure projects that actually enhance and create salmon habitat by allowing the restoration of natural processes. This was the case for the 750-foot long river-worthy and salmon-friendly Dungeness River Pedestrian Bridge constructed in 2015 for the Jamestown S’Klallam Tribe. Now, six years later, the bridge and habitat restoration have proven to be a real benefit to the river, salmon habitat, and the 1000+ pedestrians and bikers who cross the bridge daily. The increased bridge traffic has also brought new opportunities to the Jamestown S’Klallam Tribe and plans for a bridge extension and new nature center are underway.
2015 Flooding Destroys Existing Trestle
In February 2015, during a large flood event, the Dungeness River avulsed laterally approximately 100 feet and destroyed two bents of the existing bridge trestle. The trestle carried the popular Olympic Discovery Trail near Sequim, WA and the closing of the damaged bridge sent ripples through the community. The owner, the Jamestown S’Klallam Tribe, immediately began applying for grant funding to restore the crossing, and after months of effort, secured several grants that were primarily focused on salmon recovery.
Otak was selected and began design work in May 2015. Preliminary meetings with the Tribe and the stakeholder group allowed an expedited alternatives evaluation and selection process, and the final design was completed by the end of July 2015.
New Design Reinforces Stability
The new bridge design included four main spans of 185-feet each, which were prefabricated steel trusses with a concrete deck and a 30-foot wide section in the center to create an overlook area. The bridge piers had a buried pile cap with driven piles down to bedrock and a single concrete column supporting a hammerhead cross beam.
A robust pier design ensures stability after deep scouring below the pile caps, including lateral water pressures on debris buildup. To make the bridge river worthy, the piers had to withstand river avulsions anywhere along the span. Because of the stoutness of the pier’s limited ductility for seismic performance, the superstructure was supported on seismic isolation bearings on top of the pier caps. Restrainers were incorporated into the design to limit maximum seismic displacements.
Expedited Procurement, Permitting
To expedite procurement, the bridge sections were pre-ordered in July, with the main construction bid in August 2015. The site contractor was then selected and mobilized in September 2015. Environmental permitting was expedited including the US Army Corps of Engineers permit, which was applied for in June and granted three days before mobilization.
Spawning Salmon and Habitat Reformation
During the pre-construction walk-through, a pool in the Dungeness River was almost completely black—filled with more than a thousand spawning salmon. Because of the sensitivity of the river, caution was taken to minimize disturbance near and over the water. After the foundations and piers were complete and the bridge spans erected, a notice came in late November that heavy rains were predicted. The contractor immediately removed the temporary bridge over the river, and three days later a flood hit, which not only washed out the section of the river where the temporary bridge was but shifted the thalweg another 80 feet to the west—completely exposing one of the buried piers. Not to worry though, the bridge foundations were deep and structurally robust enough to withstand the river shifting and meandering. And, by doing so, new habitats are constantly being formed.
The complete removal of the existing trestle and replacement with longer spanning structures allows the Dungeness River to naturally migrate and thereby significantly increases the potential for fish-bearing habitat. As such, the bridge has been deemed salmon-friendly. The trestle replacement not only required a high level of engineering skill, but also engineering that had an appreciation for the natural sciences, with equal focus being on salmon recovery as well as the restoration of the popular trail linkage.
The grand opening of the bridge occurred on December 30, 2015, only eight months from the start of the design. Randy Johnson, Habitat Program Manager, Jamestown S’Klallam Tribe expressed the tribe’s appreciation for the new bridge the following November, stating “In contrast to the old bridge, the new Otak-designed bridge is environmentally friendly and river worthy. It has already been tested by several floods and has performed with flying colors. Trail users are enthralled with the bridge.”
Fast forward six years to the present and the tribe, working together with Otak, is beginning work on an extension for the bridge, as well as the partial removal of an existing levee to make the flood plan even wider. The bridge extension structure will essentially replace the portion of the levee being removed. In total, this will allow the river to run more freely and will further enhance ongoing river restoration. Designs are being finalized for the new bridge extension, which is on track to be completed in the summer of 2022.
Reconstruction of the nature center on the east side of the river began earlier this year. The new extension will provide improved access for visitors to the new Dungeness River Nature Center from the bridge, as well as access to a trail to a natural amphitheater and a bi-pass for pedestrians and commuters from Port Angeles who traverse the bridge and Olympic Trail daily.