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Thomas Hawk/Flickr

Scouting for evidence of erosion, the primary danger facing California’s highway bridges, is a life’s work for Kevin Flora.

On a recent weekday morning, the state Department of Transportation engineer gunned the outboard motor of an inflatable skiff and scooted over murky water on a mission to inspect a 53-year-old 405 Freeway bridge that spans a stretch of the San Gabriel River — a spot loaded with trash and teeming with green sea turtles wide as manhole covers.

Using GPS and sonar equipment, it didn’t take Flora long to find what he was looking for: holes up to 10 feet deep and 30 feet wide in the riverbed and around the foundations of the bridge that carries an average of 282,000 vehicles a day just north of the Orange County line.

“The problem here,” he said, raising his voice to be heard above the din of freeway traffic, “is that this bridge is just downstream from the mouth of a paved flood-control channel that funnels turbulent stormwater into an earthen-bottom section of the river.”

It’s among 230 state highway bridges slated for repairs, reinforcement or replacement, officials said, because they’re prone to scouring — degradation caused by swiftly moving water.

“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.”

Under legislation signed by Gov. Jerry Brown in April, 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.

In Southern California, bridges targeted for replacement include the Trancas Creek Bridge, built in 1927, on Pacific Coast Highway in Malibu.

On Friday, Flora’s inspection of the 65-year-old Whitewater bridge on the 10 Freeway — about five miles west of Palm Springs — followed a circuitous route along rocky paths, braided channels surging with snowmelt and concrete galleries covered with graffiti.

“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. “So, we plan to reinforce the foundation with deep piling.”

In the meantime, the bridge has been fortified with massive boulders and outfitted with highly sensitive “tilt meters” to monitor its movement.