We don’t need to replace the bridge

07/12/2017

What happens after the “big one” hits Point Reyes Station? The Green Bridge project engineers have misled us into focusing on the potential of the historic crossing’s failure in a large earthquake. Yet the chance of a magnitude 6.7 quake within 10 miles of town is about 0.001 in 30 years, so thinking about the big one is not a productive approach. My research has found that a retrofit of the bridge will suffice, as the bridge will not essential after an earthquake. Here’s why. 

If a magnitude 6.7 or higher earthquake hits the North San Andreas fault somewhere in West Marin, some of our bridges will fail. How will emergency responders get to the coast? Shaking will lessen to the east, with just moderate effects on the 101 corridor. There will be landslides on Highway 1, where there are several bridges between Point Reyes Station and Tomales. The coast highway is also underlain by liquefiable soils, especially north of Point Reyes. After a large quake, help will have to come from Novato and Petaluma via the Point Reyes-Petaluma Road, and we should be thinking about the integrity of all the bridges along the way. Though there are sure to be widespread landslides on many county roads, that road is more likely to stay open, as its first bridge, what we call the painted bridge, is farther from the fault line. And farther east, all routes should stay open. 

The odds of a moderate quake, with a magnitude of around 5.7, near Point Reyes Station are about 20 percent over 30 years. Most bridges will survive a moderate quake. Highway 1, both north and south, are bad escape routes due to many bridges and liquefiable soils. Sir Francis Drake will likely be closed at Levee Road due to soil liquefaction there, even in a moderate event, but we may be able to get to Inverness via Bear Valley Road. Sir Francis Drake from Levee Road to Olema runs over one or two small bridges, and then over several others to get to Fairfax and beyond. All of these bridges are short spans and may survive a moderate quake. So we should be able to get to Novato and Petaluma, as well as to San Rafael. If the Green Bridge were to fail, the alternate route to San Rafael via Platform Bridge Road would likely be open. It is also likely that the Lucas Valley Road route would stay open.  

Short spans can be replaced with temporary one-lane bridges in a few days. Caltrans has emergency bridge kits that can span up to 150 feet (longer than all of the bridges that West Marin residents would need) and be installed in a day or two, after moving the pieces to the site. Numerous companies rent and sell temporary bridges for cars, trucks and even railroads. So it appears that all of our local bridges could be rapidly replaced with temporary versions after a quake. This solution seems adequate for such an improbable threat to our historic crossing.  

The potential failure of the Green Bridge is just one problem in post-earthquake response planning, as a link in just one of three routes into Point Reyes Station. Since it’s not an indispensable piece of our emergency infrastructure, a retrofit of the bridge seems adequate.  

 

Bob Johnston, a retired professor, lives in Inverness.