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Seattle University students present 30% bridge design to restore fish passage at Gribble Creek
Summary
A Seattle University civil-engineering capstone team presented a 30% design recommending a roughly 40-foot timber girder bridge to replace a failing culvert at Gribble Creek, which would restore about 1.36 miles of upstream habitat and meet WDFW and bridge-design guidance; material costs were estimated at $30,000–$35,000 (not a full project cost).
At the May 26 Skagit County Board of Commissioners meeting, Parks and Facilities and a Seattle University civil-engineering capstone team presented a 30% design to remove a failing culvert on Gribble Creek and install a bridge to restore fish passage and upstream habitat.
Ken Hansen of Parks and Facilities introduced the project and named project sponsors from Skagit Fisheries Enhancement Group and Public Works. A student presenter described the site as a partial barrier listed in the Washington Department of Fish and Wildlife (WDFW) inventory and said removing the culvert would restore approximately 1.36 miles of upstream habitat for cutthroat trout, Chinook, chum, steelhead, coho and pink salmon.
The team evaluated options under WDFW water-crossing guidance, USGS flood analyses, WSDOT and AASHTO bridge guidance and concluded that a bridge best preserves floodplain connectivity and allows lateral channel movement. Using hydrologic calculations, they reported a 2-year flow near 28 cubic feet per second and a 100-year flow near 93.5 cfs, yielding a modeled 100-year flood height of about 2.73 feet and a floodplain width near 38 feet. To provide margin for climate-change considerations, the design targets a roughly 40-foot clear span and a flat pedestrian-grade bridge.
Design details presented at 30% include a proposed timber-girder option (selected in the team’s evaluation matrix), a target bridge width of about 5 feet with 4.5-foot railings, glulam beams sized in the design (approximately 12.25 inches by 18 inches) and 3-by-12 wooden decking. The team noted there is not yet a geotechnical report; they assumed a scour line 3 feet below surface and foundation embedment to about 5 feet below grade.
On costs, the presenters said material-only estimates run about $30,000–$35,000; they cautioned this is not a full construction estimate and does not include access, site clearing, contractor mobilization or geotechnical work. Commissioner Browning asked about GluLam durability and salmon velocities; the team responded that the timber specification resists moisture and insect damage and that in extreme flood velocities fish could use the floodplain where flows are slower.
Commissioners thanked the students and Parks staff for the thorough work; Hansen said a copy of the report will be left behind and the 30% design provides a head start for bidding and budgeting future work.
