Heavy rainfall, floods reminders of sinkhole dangers in southwest Missouri

SPRINGFIELD, Mo. -- They are grassy dips that give the countryside the rustic look of rolling hills, but sinkholes can quickly turn into not-so-romantic ponds, immersing highways and flooding homes.

This is what happened in Greene County in the wake of heavy rains in late March, with water levels rising higher than what officials saw in 1993.

The rapid rainfalls might be blamed, county storm-water engineer Kevin Barnes said.

National Weather Service meteorologist Gene Hatch said the precipitation in 1993 was "over a much longer period."

County officials say the flooding is proof that structures should be built away from known sinkholes.

Since the storms, they have been monitoring and photographing flooded sinkholes, trying to detect them and learn about their workings and chronicling damages.

In the Logan-Rogersville area, a portion of westbound U.S. 60 was inundated, It was reopened to through traffic only after being closed almost a week.

Near farm roads 189 and 172, county employees have been monitoring the water level at a sinkhole. The flood water submerged a roughly 400-foot stretch of Farm Road 172.

Danny Tavares, a county water-quality specialist, estimated the water level peaked March 24, rising as much as 20 feet.

A feature of karst terrains, sinkholes are formed when the surface sinks after rainwater, for thousands of years, percolates down through the earth, dissolving enough rock to form an underground network of water channels.

A sinkhole also can form suddenly with a collapse.

That happened to Nixa resident Norm Scrivener in 2006 when the ground gave in on his property, swallowing his garage and a 2001 Chevy Cavalier.

Peter Price, environmental geology section chief at the Missouri Department of Natural Resources, said a new sinkhole near U.S. 60 in the Norwood area was reported after the heavy rains.

In Greene County, there have been numerous reports of flooded sinkholes.

Not all sinkholes are created equal -- some drain well, while others seem to have a standing-water problem.

That's because some sinkholes have "open throats," allowing water to drain quickly to the underground channels, while the others have a "closed throat" plugged with soil, Price said.

"Some will drain overnight, and some will hold water for many days," he said.

Saturation from heavy rainfalls can push up the water level in a sinkhole as water percolates for several days after the rainfall, Price said.

Barnes said it is hard to explain the workings of sinkholes without knowing the mechanism of drainage networks underneath the surface. He also was hesitant to estimate how long it would take for the flooded sinkholes in Greene County to drain.

But several things are certain: Sinkholes are directly connected to groundwater, and contaminants from sinkholes can show up in groundwater and sometimes drinking water.

And structures built near a sinkhole are threatened with flooding or collapse.

To protect the safety of its residents, home values and to safeguard groundwater, Greene County in 1991 adopted sinkhole regulations prohibiting construction in sinkholes.

The regulations also define a sinkhole: "any depression in the surface of the ground, with or without collapse of adjacent rock, that provides a means through which surface water can come into contact with subsurface water."

After recent rainfalls, "it's obvious where they are -- they are full of water," said County Administrator Tim Smith.

"During dry times, people say it is not a sinkhole, and now we can show them the pictures, 'Hmm, how about this?"'

The Missouri Department of Transportation also avoids sinkholes when building highways and, if a road must be built close to a sinkhole, it mitigates environmental impact, spokesman Bob Edwards said.

"In this part of Missouri, there's an underground honeycomb of lakes and springs all over," he said. "If we have to build close to a sinkhole, we take steps that drainage continues to occur naturally without undermining the roads."

Built decades ago, the westbound lanes of U.S. 60 that were flooded probably were not engineered, Edwards said.

Because the water came from private property and the lanes rarely get submerged, it is unlikely that MoDOT will address the flooding on U.S. 60, Edwards said.