It’s a hole, but not a sinkhole

components, and caverns that are due to engineering failures, aging infrastructure or simply not building enough capacity to handle the kind of runoff experienced in San Mateo County this month. They also note it’s a distinction without a difference for anyone stuck in traffic.

“Even scientists can’t always agree whether we want to call them sinkholes,” said Randy Orndorf, research geologist for the USGS in Reston, Va. “I think about 20 years ago when I started doing research and we tried to say these are infrastructure failures and people still wanted to call them sinkholes.”

For the most part, sinkholes are limited to regions of karst terrain, which underpin about 25 percent of the United States land mass. Sinkholes are most common in these areas, where the underlying soil simply dissolves in water. Sinkholes are most common in Florida, Alabama, Missouri,

Kentucky, Texas, Tennessee and Pennsylvania, according to the USGS. No one knows how many sinkholes develop in a given year because

most likely occur in remote areas.

“The only ones we hear about are the ones that impact human lives,” Orndorf said.

Referring to the USGS National Geologic Map Database, Orndorf determined the soil under the affected portion of Highway 92 is known as alluvial fan deposits — essentially stream runoff consisting of gravel, sand, silt and other material. “It’s unconsolidated,” he said.

Orndorf said most road collapses are likely due to aging infrastructure, including water mains and culverts that in many cases are decades old.

“We are seeing a lot more of that due to aging infrastructure, particularly on the East Coast,” he said.

He said climate change plays a role, too. Greater rainfall totals over shorter duration stresses roads and other infrastructure beyond their intended capacity. And runoff takes the soil from underneath pavement and pipes.

“There is a reason why runoff looks muddy,” he said.