Bolstering bridges against climate change
Bridges are vital structures across the world’s cities, providing fast transport links and connecting regions separated by bodies of water. But with climate change set to result in rising sea levels across the globe, what is the likely impact on cities’ bridges, and what can we do about it?
Scour is a primary source of bridge failure, created when fast-moving water erodes sediments such as sand and gravel from around a bridge’s foundation, creating scour holes that compromise the integrity of the structure. Climate change is predicted to increase this problem across the world, with rising sea levels and more frequent storm surges.
A pair of researchers — David Yang and Dan M Frangopol — from Lehigh University in Pennsylvania, USA, have investigated the possible impacts of climate change on bridge safety in a paper recently published in the American Society of Civil Engineers’ Journal of Bridge Engineering.
“We know climate change will increase the frequency and intensity of natural hazards like hurricanes, heat waves, wildfires and extreme rains,” said Yang, a postdoctoral research associate in civil and environmental engineering.
“For this paper, we’re looking at increased temperature as well as increased precipitation and their impact on bridge safety. The challenge here was that we didn’t know how to quantify those impacts to predict scour risk,” he said.
In their research, Yang and Frangopol, a professor of civil engineering and the Fazlur R. Khan Endowed Chair of Structural Engineering and Architecture, had to fill the gap between the climate data and the structural safety quantification. They achieved this by using hydrologic modelling to convert climate simulation data to flow discharge data in the Lehigh River — a 175 km tributary of the Delaware River that runs through the city of Bethlehem, Pennsylvania.
“We took a holistic approach,” explained Yang. “It started with a global climate model that was downscaled to regional hydrology, then we used structural engineering to get the failure probability of a structure in a future flooding event. From that we could assess: does this structure failure pose certain risks to a community? So our model included these four steps of climatology, hydrology, structural engineering and risk assessment.”
Yang said that the research is the first to combine all four steps to quantitatively look at the effect of climate change on bridges.
In developing their model, Yang and Frangopol considered different climate futures and global climate models provided by the Intergovernmental Panel on Climate Change. To estimate the foundation depth of older bridges spanning the Lehigh River, they developed a method to back-calculate the depth based on condition ratings from the National Bridge Inventory. The researchers also took a regional and life-cycle approach to their analysis.
Frangopol is well known for his pioneering work in life-cycle engineering, which uses computational analysis to determine the long-term value and risk associated with infrastructure investments. In 2019, he was awarded the George W. Housner Structural Control and Monitoring Medal in recognition of his groundbreaking work and leadership in the field.
Yang explained that taking a regional and life-cycle approach was a novelty for the study.
“Bridges have a lot of microenvironments, and if you only look at one bridge it’s really hard to capture the trend and get the increased risk from climate change,” he said. “So we broadened this analytical horizon both spatially and temporally to capture long-term trends.”
Yang and Frangopol reached eight conclusions using their model. The most surprising was the extent to which the frequency of flooding may change.
“We realised that a 20-year flood may now become a 13-year flood at the end of the century, so that frequency nearly doubled,” Yang said. “This is why climate change may induce an increased risk to infrastructure.”
A key conclusion from the study surrounds the question of mitigation. Specifically, what engineering measures should be deployed to reduce risk, and for which bridges?
“The reality is that budgets are limited,” Frangopol said. “So it’s important to be able to determine, what is the priority here? You need to know the location of the bridge.
“For some communities, the failure of a bridge could be disastrous. For others, a bridge may not be as critical. This model helps you make that kind of decisions because risk is not only based on safety but also on the consequences of failure. You might have two bridges at the same probability of failure, but the consequences of that failure could be very different.”
Focusing on bridges along the Lehigh River was an obvious choice given the researchers’ location, but Yang and Frangopol are eager to share their model with all communities looking to assess their infrastructure.
“We were inspired to do this research in part because historically Bethlehem was hit by multiple floods since 1902, and they had a significant impact on the community, so flooding is a significant hazard throughout the Lehigh River watershed,” Yang explained.
“We wanted to devise something that the community can use to become adaptive to future climate change,” Frangopol added.
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