San Francisco, April 30 (IANS) Analysis considering a range of flood scenarios, as opposed to a single 100-year threshold, could be more robust and accurate about vulnerability of bridges, according to a study.

Current risk assessments in the US assume that bridges may collapse when a 100-year flood, a streamflow with one per cent probability of being exceeded in any given year, or 63 per cent over the course of a century occurs, Xinhua news agency reported on Saturday.

This underestimates risk, according to authors of a paper published in the Journal of Infrastructure Systems, because it fails to capture the full range of stream flow conditions that can cause bridge collapse.

The researchers in their analysis considered the full variability of floods that could cause collapse, as opposed to the 100-year approach, thus identifying a greater sensitivity to changes in the underlying frequency of flooding.

Of the 35 bridges analysed, 23 were estimated to have 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.

Because most US bridges, along with most infrastructure, pre-date the modern design standards, the results highlight a more general risk that extreme climate events pose to US infrastructure.

Last month, the American Society for Civil Engineering gave US bridges a C+, estimating that $123 billion is needed to clear the maintenance backlog, and that an additional $140-250 billion over the 21st century to address the increasing risks posed by climate change.

"To balance funding between the backlog and climate adaptation, bridge managers will need robust data on collapse risk," said led author Madeleine M. 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 comes when the current US administration, led by President Donald Trump, is considering a $1 trillion budget proposal to update infrastructure, including bridges.

"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," said Noah Diffenbaugh, a professor of Earth system science at Stanford University.