Project Overview
A flood event in 2012 shifted Willow Creek’s main channel and washed out the north abutment of Shirley Towne Bridge, cutting off a secondary access route that would have allowed residents to evacuate when a 2019 ice jam created flooding downstream. PND evaluated the existing bridge, designed a 50-foot bridge span extension to a new northern abutment, and added approximately 100 feet of bank protection and erosion control around the new abutment.
The original bridge consisted of two spans on either side of two large corrugated metal pipe culverts. By 2020, a vegetated island had formed in front of the culvert openings, eliminating low-flow channel connectivity and causing the majority of discharge to flow along the right bank. PND used gage data and field measurements to estimate flow distribution through multiple channels and openings and designed a new bridge span, abutment, and bank erosion protection.
Our Role
- Civil Engineering: designed new bridge approaches and revetment
- Structural Engineering: created a 50-foot bridge extension, new abutment, and bridge superstructure
- Hydrology & Hydraulics: analyzed flow distribution and hydraulic conditions to support bridge and erosion protection design
Restoring a Bridge After Channel Migration
PND evaluated Shirley Towne Bridge after Willow Creek shifted from its original channel and undermined the northern approach. The project involved topographic and bathymetric survey, geotechnical investigation, hydrologic and hydraulic analysis of a high-energy stream, and civil and structural design of a new bridge span and abutment accommodating the creek’s redistributed flow. PND also developed significant bank erosion protection design, secured environmental permits for work within an anadromous stream, and provided bid- and construction-phase engineering services.
PND designed a 50-foot span with a superstructure consisting of four 36-inch plate girders. The new deck matches the existing deck and consists of glulam and timber decking, runners, and rails. The substructure incorporates the existing northernmost pile bent and a new northern abutment consisting of three pipe piles. PND also designed a riprap-armored embankment and regraded, paved, signed, and equipped the approach with guardrails. The improvements were completed on time and on budget, restoring a second crossing over Willow Creek while addressing the site’s hydraulic challenges.
Feet
length of new bridge span
Feet
bank protection and erosion control added around the new abutment
