Is it a sinkhole? Or something else?

The difference between sinkholes and flooding-related erosion, why flooding causes the ground to collapse and what you can do to protect your property.

(Rick Egan | The Salt Lake Tribune) Flooding led to collapsed pavement on Orchard Ridge Lane in Kaysville, on Wednesday, April 12, 2023.

Last week as Utahns ditched their winter coats and temperatures pushed into the low 80s, creeks and rivers around the state surged with the first floods of spring.

In Orchard Ridge, a subdivision under construction in Kaysville, water gushed down the street and half the road crumbled, creating a roughly 10-foot-deep pit.

Brooke and Derek Schulthies live across from the newly formed pit. Standing by the rushing water coursing down their subdivision last Wednesday, they recalled the sounds of the previous night: Running water and the crash of tumbling construction dumpsters.

“The freakiest part was when you’re trying to sleep, and you’d hear the rumbles outside, and your entire house would shake — and then you go outside and another driveway fell off,” Derek Schulthies told the Tribune.

(Rick Egan | The Salt Lake Tribune) Flooding on Orchard Ridge Lane in Kaysville, on Wednesday, April 12, 2023.

While early reports described the chasm as a “sinkhole,” multiple sources told The Tribune the term was technically incorrect. “That damage is clearly caused by erosion of the roadside by surface flow in a construction area,” responded William P. Johnson, professor in the University of Utah’s department of Geology & Geophysics, in an email. “It’s not a sinkhole, which is eroded by groundwater from below.”

While the asphalt-swallowing void in Kaysville wasn’t technically a sinkhole, The Tribune set out to determine what exactly might qualify as one, and whether Utah might see more non-sinkholes created during future flood events, or even as a result of super-saturated ground from winter’s record-setting snows.

What is a sinkhole?

True sinkholes happen in areas where the underlying rock (like limestone, salt or gypsum) dissolves. When these soluble rock types come in contact with groundwater, caves can form underneath the land surface, explained Randall Orndorff, a research geologist specializing in sinkholes with the United States Geological Survey. That rock layer typically dissolves over tens of thousands of years, forming caves underground that can be difficult to detect.

Whether a sinkhole collapses dramatically has to do with the type of soil on top. In some cases, the soil acts like an hourglass and “the sand just slowly makes this bowl shape as it goes into the bottom,” Orndorff said.

Clay soils are more cohesive and will hold up while the bedrock underneath dissipates — but with certain conditions that precarious bridge of land can come crashing down.

Drought can cause the clay soils to collapse, Orndorff explained. Conversely, heavy rains and flooding can put extra weight on the ground, causing the top layer to fall into the cave that formed beneath it. “That’s going to add a lot of weight, and it just can’t hold itself anymore,” he said.

Orndorff worked on a report examining places in the United States that have soluble rock, known as “karst,” underneath. “Just about every single state has some kind of dissolvable rock,” he said.

According to a post from the Utah Geological Survey, sinkholes have formed in southern Utah, at one point swallowing La Verkin Creek for a week in 1996.

However, roadways around Salt Lake City aren’t susceptible to natural sinkholes. “Those soils typically aren’t found here in this portion of Utah because we have a very granite rock type soil in and around our areas,” said Mark Stephens, the city engineer for Salt Lake City. “What was reported as ‘sinkholes’ in Kaysville was really a washout caused by erosion from surface flooding waters,” Stephens said.

What to call a non-sinkhole hole

“I like to call these things infrastructure collapse,” said Orndorff.

Water mains, culverts, and the various pipes humans put in the ground do, indeed, create holes. If a pipe breaks, the soil will collapse into the void. “It works the same way [as a natural sinkhole],” Orndorff said, “but it’s very different causes.”

In May 2022, a water main break in Salt Lake City’s Glendale neighborhood triggered a “car-eating crater” the Tribune reported. That was also not technically a sinkhole.

(Francisco Kjolseth | The Salt Lake Tribune) Crews work to repair a water main break that created a large sink hole that swallowed a parked car near 765 W. 1300 South in Salt Lake City on Tuesday, May 3, 2022.

Just this past January, flooding in California resulted in a 50-feet-wide and 30-feet-deep void that consumed two cars, the New York Times reported. Numerous voids popped up in the state. “They were actually infrastructure collapses,” Orndorff said, not natural sinkholes that created the California chasms.

Humans can cause sinkholes when they pump out groundwater and strip an area of water-soaking substances. When it rains in a natural environment with trees and soil, water soaks into the ground more or less evenly. Once that area is covered in pavement (what geologists like Orndorff calls “impervious surfaces”) “you’re concentrating all of that water in one area and that will start an erosion that will also include eroding the soil underneath.”

More craters could be coming

But while Florida-style sinkholes may not be opening up around the Wasatch Front, flooding and runoff could result in more erosion.

“The damage is going to happen if we have to divert excess flows over streets to keep them away from structures,” said Laura Briefer, director of Salt Lake City Department of Public Utilities. When Wasatch Hollow, along the banks of Emigration Creek, started flooding last week, utilities personnel diverted flows down streets away from homes.

That did erode the soil underneath a small portion of sidewalk on 1500 East, creating a miniature arch.

(Laura Briefer, Salt Lake City Department of Public Utilities) The soil underneath a strip of sidewalk on 1500 E. eroded after the neighborhood flooded last week.

“If we have to run water over asphalt and concrete, there’s a potential that we would erode underneath the infrastructure,” Briefer explained.

To prevent washouts, city engineer Stephens said proper erosion controls measures are needed.

Home and business owners can do their part by watching where water flows on a normal rainy day and then making sure that they have”grass or rock or something that can keep the soil from washing away underneath,” Stephens said. And if they don’t have that “they just need to be vigilant and have sandbags ready to be able to divert water away from those places that they feel could wash away.”

Water will always find the weakest point, Stephens explained.

“Water is a very particular thing, and it is a very powerful thing when it comes to where it wants to go, how much of it there is, and what it can move.”