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Option 1 retains the existing stone bridge and would reinforce the bridge by removing a portion of the dirt that currently fills the arch. The dirt would be replaced with a concrete interior bridge. The stone bridge rails would be moved outward to enable pedestrian pathways and to improve sight distances for drivers. The existing sewer line would remain in place and block views from Rocky Nook Park to the bridge.

 

Why reinforce the bridge? 

The existing bridge doesn’t meet current earthquake standards designed to a 7.2 magnitude earthquake. During a 7.2 magnitude earthquake, the stone walls are predicted to slide outward and rotate over. This would cause the soil interior to spill out and collapse the stone arch. 

 

In Option 1, the new concrete interior bridge could withstand the earthquake and serve as an evacuation route for the neighborhood. The stones and arch are still predicted to fail.

 

For reference, the 1925 earthquake was 3.5 times less severe (6.8 magnitude). 

According to FEMA models, the existing bridge’s 24’ arched opening is NOT enough to contain a 20-year storm event. In this event, water and debris would back up against the arch and onto Mission Canyon Road - flooding the roadway.

During a 100-year storm event, FEMA models predict that Mission Canyon Road would be under 4’ of water.

  • Arch & stone walls remain

 

  • Interior bridge for earthquake evacuations

 

  • Improved pedestrian & bike pathways

 

  • Improved driver sight-lines

  • Road flooded in large storms

 

  • Stone walls moved 

 

  • Stones bridge collapses in earthquake

 

  • Shadowing from widened bridge deck

 

  • Sewer line remains, blocking views

 

Option 2 creates a two-arch bridge to handle water flow in large storms. The bridge would be widened to accommodate pedestrian and bike pathways and improve driver visibility. The structure would be reconstructed to meet earthquake safety standards. The sewer line could be incorporated to run through the bridge, so that views from Rocky Nook Park are unobstructed to the bridge and creek. A two-arch bridge could be constructed out of concrete (interior) and existing and new stones (exterior).

A two-arch bridge can accommodate 100-year storm levels for water and debris. However, in a 100-year storm the water levels would rise, requiring new FEMA flood mapping.

  • Mission Canyon Road usable in earthquakes & large storms

 

  • Arches mirror existing bridge arch

  • Improved pedestrian & bike pathways

 

  • Improved driver sight-lines

 

  • Sewer line can be incorporated into the bridge

  • 1 arch becomes 2 arches

 

  • $$$ (only partially covered by state funding)

 

A 50’ arch bridge can accommodate 100-year storm levels for water and debris.

  • Mission Canyon Road usable in earthquakes  & large storms

 

  • 100-year+ lifespan

 

  • Least expensive option

 

  • Improved pedestrian & bike pathways

 

  • Improved driver sight-lines

 

  • Sewer line can be incorporated into the bridge

  • Existing bridge replaced with wider arch bridge

  • Could jeopardize listing on National Register of Historic Places & City Landmark status

Option 3 creates a wider, 50’ arch bridge reusing existing stones and the capstone. The arch would be constructed to mimic the existing arch, but have a wider opening for water flow and large storms. It would meet today’s earthquake and safety standards. The sewer line could be incorporated to run through the bridge, so that views from Rocky Nook Park are unobstructed to the bridge and creek.

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Each of the bridge options can be matched with any of the roadway options. Please click here to view the roadway options.

Please share your thoughts with the Mission Canyon Bridge Studies team via the link below.