More than 200 state highway bridges in California are slated for repairs, reinforcement or replacement because they’re prone to a type of erosion known as ‘bridge scour.’

Bridge scour — the removal of sediment from around bridge abutments or piers — is the most common cause of bridge failures and is the primary danger facing California’s highway bridges. Bridge scour is caused when swiftly moving water scoops out scour holes, compromising the integrity of a structure.

Caltrans engineer Kevin Flora recently inspected a 53-year-old 405 Freeway bridge that spans a stretch of the San Gabriel River, just north of the Orange County line. The bridge carries an average of 282,000 vehicles a day — Flora found scour holes up to 10 feet deep and 30 feet wide around the bridge’s foundations.

Flora’s inspection of the 65-year-old Whitewater bridge on the 10 Freeway, five miles west of Palm Springs, raised similar concerns. “This bridge is a big concern for us,” he said. “The riverbed has been scoured down 15 feet to the bridge’s footing, in a river that has a habit of abruptly shifting course during storms. If it shifts again, it could undermine the footing,” he said. There are plans to reinforce the foundation with deep piling, but in the meantime, the bridge has been fortified with massive boulders and outfitted with tilt meters to monitor its movement.

These deteriorating bridges are among 230 state highway bridges slated for repairs, reinforcement or replacement, officials said, because they’re prone to scouring. In Southern California, bridges targeted for replacement include the Trancas Creek Bridge, built in 1927, on Pacific Coast Highway in Malibu.

“A bridge fails every 10 days in the United States, and it’s usually due to scour that undermined their foundations,” said Jean-Louis Briaud, a professor of civil engineering at Texas A&M University.

“Bridges built before 1990 are the ones that fall down,” he said. “The good news is that since the 1990s, the number of scour-related failures in California and across the nation has been going down because of regulatory requirements prompted by the 1987 collapse of a New York state thruway bridge, which killed 10 people.” California has also seen deaths resulting from bridge collapse — 1995, seven people died when the I-5 bridge over Arroyo Pasajero failed.

Under legislation signed by Gov. Jerry Brown, California motorists will start paying higher gas and diesel taxes in November to provide funding for, among other state transportation projects, reducing the backlog of road and bridge repairs.

Source: Los Angeles Times