Texas is one of the best states in the country for maintaining its outsize share of bridges, but rain and flooding in the wake of Hurricane Harvey have begun toppling them and undermining roads around Houston.
The Woodforest Boulevard bridge across Greens Bayou collapsed, according to the Harris County Office of Homeland Security and Emergency Management.
Two portions of Garrett Street also collapsed, Sheriff Ed Gonzalez said. And a chunk of State Highway 316 washed away south of Houston, according to the state Department of Transportation.
The problem: Moving floodwaters gnaw away at the foundation of bridges and roadways in a process engineers call scouring. As the foundation erodes, columns supporting bridges collapse or roads simply wash away.
“If a bridge isn’t on piles, if it’s just sitting on soil or rock, the fast-moving water can eat away at supporting foundation and actually cause it to tip over,” said Andy Herrmann, a past president of the American Society of Civil Engineers and a 40-year consulting engineer in New York City.
In the same way, scrubbing away the gravel foundation of a road undermines the concrete or asphalt pavement.
“Once you wash away that foundation, you can get scour holes, sinking of the roadway itself,” Herrmann said.
More destruction is likely as flooding is expected to continue for days. Harvey is projected to be the most damaging hurricane in history, dropping more than 50 inches of rain on Houston. Early estimates tally $160 billion in damage.
Even as the storm moves northeast from Texas to Louisiana, emergency officials warned that flooding is expected to linger in Houston.
“Catastrophic flooding is likely to persist days after the rain stops,” said Elaine Duke, acting secretary of Homeland Security.
Texas has a larger-than-average share of bridges: 53,488 out of the country’s 614,387. But the state also takes above-average care of them, according to a review by the American Society of Civil Engineers: Only 900 or 1.7% are rated structurally deficient by federal inspectors. For comparison, 9% of bridges nationwide are rated deficient.
“Texas has been taking care of their bridges,” Herrmann said.
A catastrophic example of what rain can do to a bridge came April 5, 1987, when an Interstate 90 bridge that was part of the New York State Thruway collapsed into Schoharie Creek during flooding from spring storms.
Ten people died as four cars and a tractor-trailer plunged 80 feet into the swollen creek. The National Transportation Safety Board ruled that the thruway authority failed to maintain adequate support around the bridge piers, which allowed severe erosion around the column footings.
The same threat follows in the wake of Harvey.
Bridges are often supported by concrete footings, which typically sit on soil or rock beneath the river. The footings serve as the foundation for columns, which shoulder the beams that hold the roadway above.
“But that fast-moving water can start eating down to the footing,” Herrmann said. “Once it gets underneath the footing, that’s when you have problems.”
Another threat to bridges is simply getting forced off their foundations. If water rises taller than the bridge, the deluge and debris such as trees could dam the flow and weigh against the concrete deck, Herrmann said.
“You’ve got that water pressure pushing against the side of the bridge,” he said. “You could push the bridge off its bearings.”
The same threat faces submerged roads if flowing water erodes the gravel or compact dirt beneath the pavement.
“If it can eat away at the gravel, you can have sinkholes or actually washing the roadways apart,” Herrmann said.