The researchers said that studying how and why bridges have collapsed in the past identifies the limitation of the current risk assessment approach and demonstrates the value of new perspectives on climate change impact.

The United States is considering a US$1 trillion budget proposal to update infrastructure, including its crumbling bridges. An obstacle to spending the money wisely is that the current means of assessing bridges may underestimate their vulnerability, according to the new study published in the Journal of Infrastructure Systems. Historical analysis of hydraulic bridge collapses in the Continental United States has been written by researchers from Stanford University and Virginia Polytechnic Institute & State University.

US risk assessments generally assume that bridges may collapse when a 100-year flood occurs. This assumption underestimates risk, the paper’s authors find, because it fails to capture the full range of stream flow conditions that can cause bridge collapse.

In their analysis, the researchers considered the full variability of floods that could cause collapse, as opposed to the 100-year approach taken previously. As a result, their findings identified a greater sensitivity to changes in the underlying frequency of flooding. This result appears to support the idea that analyses considering a range of flood scenarios, as opposed to a single 100-year threshold, could be more robust and accurate.

Of 35 bridge collapses analysed, floods caused 13, scour caused 16, a hurricane caused one and other influences (such as waterborne or hydraulic debris) caused five.  The research estimated that 23 collapsed during a water flow of lesser intensity than a 100-year flood level. The authors note that a primary reason for these lower flow collapses is the fact that most of those collapsed bridges were built before modern design standards.

“To balance funding between the backlog and climate adaptation, bridge managers will need robust data on collapse risk,” said lead author Madeleine Flint, an assistant professor of civil and environmental engineering at Virginia Polytechnic Institute and State University. “Our study is a step in that direction.”

The study said that a case in point is a bridge that collapsed in March in Big Sur, California, isolating communities and costing local businesses millions of dollars. Although California’s recent unprecedented rains were likely to damage infrastructure, standard risk assessments made it hard to identify which bridges were most vulnerable.

“This winter in California has highlighted the vulnerabilities of our nation’s infrastructure,” said Noah Diffenbaugh, a professor of earth system science at Stanford Univiersity. “Updating our infrastructure will require both making up for deferred maintenance, and preparing for the increasing risk of extreme events that comes along with global warming.”

Big Sur’s damaged Pfeiffer Canyon Bridge – out until at least September – is a harbinger of things to come, said the study. As climate and land-use change drive more frequent and intense flooding, collapses among the nation’s more than 500,000 water-spanning bridges will likely increase, the authors said.