Goodman Creek Culvert Replacement
A 10-foot diameter metal plate culvert that carries Goodman Creek under the Mountain Loop Highway 9 miles south of Darrington, Washington failed. Due to abrasion and rust the pipe floor ruptured allowing flow to pass through the bottom of the pipe and erode the fill surrounding the outlet, leaving the outlet perched 6 feet above the stream bed.
WSE was retained as a specialized sub-consultant to evaluate the potential hydraulic and geomorphic risks posed by the stream and to use this information to aid in the design of a replacement crossing. WSE constructed a 2D HEC-RAS model of the stream to evaluate hydraulics for both existing conditions and proposed alternatives, evaluated wood and sediment transport dynamics, debris flow potential, and channel migration history. WSE provided recommendations for freeboard, channel design, and scour and erosion countermeasures considering the potential for debris flows and high wood transport through the crossing. WSE also provided geomorphic information to inform habitat assessment and ensure that the new crossing would allow Goodman Creek to develop high quality habitat and maintain fish passage.
WSE Role/Services:
- Constructed a 2D HEC-RAS model to evaluate site hydraulics.
- Evaluated wood, sediment and debris flow dynamics.
- Examined lateral channel migration potential.
- Estimated local scour potential at the bridge abutments.
- Recommended crossing and channel dimensions as well as scour countermeasures.
Outcome:
Culvert and bridge alternatives were considered and a bridge was selected as the preferred option. WSE’s hydraulic and geomorphic assessments were then updated and used to design the bridge. Final design is complete. Construction is anticipated to begin in 2026.
