Franz Creek Bridge Replacement

What’s Happening?

Sonoma County plans to replace the one-lane bridge over Franz Creek on Franz Valley School Road.
Location: Franz Valley School Rd (Lat/Lon: 38.583731, −122.671856)
The new bridge will be a single-span (no piers in the creek), cast-in-place reinforced concrete box girder with:

• Two 10-ft lanes
• One 5-ft shoulder and one 1-ft shoulder

The bridge will be built upstream (south) of the current bridge on a new alignment. The existing bridge will stay open during construction and then be removed after the new bridge opens.

How We Got Here

Regular inspections show the current bridge is near the end of its service life.

• Erosion (scour) from the creek is undermining the foundations.
• Caltrans’ October 2022 inspection gave a sufficiency rating of 34.6/100 (bridges under 50 are candidates for replacement).
• A hydraulics report (Aug 28, 2013) found the bridge scour-critical, meaning higher risk during major storms.

The County reviewed options and determined that rehabilitation isn’t feasible due to the foundation erosion, construction complexity, cost, and in-stream environmental impacts.

Why It Matters

Replacing the bridge will:

• Improve safety and reliability in storms and earthquakes.
• Maintain emergency response and everyday access for residents, cyclists, and pedestrians.
• Protect the creek by removing in-stream piers and allowing water to flow freely.
• Reflect the rural setting through design choices guided by community input.

What’s Next

The project is in the environmental review (NEPA/CEQA) phase with studies underway. Targeted milestones include:

• NEPA clearance: Fall 2025 (planned)
• CEQA clearance: Fall 2025
• Permitting & right-of-way: 2026–2027
• Construction: Target 2028, planned to occur in a single construction season (subject to change)

Design and Alignment

• New alignment mimics today’s roadway while meeting modern safety standards (lane/shoulder width, sight distance, barrier rail).
• Design speed reduced to 25 mph (special Caltrans approval) to match the area’s curves and calm traffic (average current approach speed ~39 mph).
• Bridge length ~80 ft; width ~30 ft (two 10-ft lanes + 5-ft shoulder + 1-ft shoulder). The wider shoulder supports safe stopping sight distance on the curve.
• No permanent lighting is proposed.
• Retaining walls are avoided; approaches use graded embankments (with a short wingwall on the west).
• Tree removal is minimized; required replanting will occur per permits.
• Aesthetics: natural colors/textures and architectural features will be refined with community input.

Timeline (high-level)

• 2018–2021: Alternatives developed; three alignments presented to the community (red/blue/green).
• 2021: Community input led to the refined “green” alignment; public exhibit on concrete colors/textures (Sept 13–19, Alexander Valley Hall).
• 2023–2025: Environmental studies and documentation; NEPA/CEQA processing.
• 2026–2027: Permits and right-of-way acquisition.
• 2028: Construction (target), with the old bridge kept open during work and removed after the new bridge opens.