Tryon Creek Sanitary Sewer Trunk Upgrade Stream Bank Stabilization


HENDERSON design-build professionals, as part of BergerABAM’s project team to the City of Portland, are providing stream assessment, scour evaluation, hydraulic assessment and analysis, construction access feasibility, and value-engineered cost estimations for a 2,500 feet long segment of a 30-inch diameter steel-reinforced concrete sewer pipe located along and within the lower segment of the Tryon Creek located in the Tryon Creek State Park Natural Area in Portland, Oregon. It is the City’s intent to assess and evaluate the structural condition of this sanitary main line and to develop feasible options for potential upgrades while benefiting habitat.

From the City of Portland’s Tryon Creek Wastewater Treatment Plant near the Willamette River and upstream into State Park lands, the sanitary main line is located either within the Tryon Creek stream channel or in elevated pipeline segments supported by approximately 50-year old steel-reinforced concrete piers. Several piers are embedded into the hillside and upon the floor of the Tryon Creek stream channel. Access and constructability are key issues to be evaluated throughout design, as this sanitary line traverses heavily-canopied riparian forest, is developed through residential housing, and crosses major public transportation infrastructure. Recent field inspections of the sewer and stream channel identified sewer infrastructure issues along several pipeline segments throughout this reach.  Issues located within the stream channel include seeping sewer pipe joints or manholes and channel scour near pier footings.

For the lower Tryon component of this project, HENDERSON environmental design-build professionals provided bank stabilization design to protect the sanitary sewer line from collapse and/or other failure. The existing sewer line is located at the top of what was a very steep and rapidly eroding stream bank approximately 20 feet high. HENDERSON worked with regulators to design a very robust, yet permittable, bank stabilization treatment for approximately 80 feet of bank using natural materials that included boulders, native soil, biodegradable fabric, and native vegetation. HENDERSON was able to help secure permits by including fish habitat enhancements to the opposite streambank, mitigating for the use of a substantial amount of rock below the ordinary high water line. The project was constructed in 2014 with construction oversight provided by HENDERSON.

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