Geologists say Kentucky's natural underground drainage system is overwhelmed following the recent massive rainfall - something that will likely cause an increased number of sinkholes across the region.
According to the Kentucky Geological Survey, more than half of Kentucky lies above a karst landscape. Karst areas are made up of sinkholes, underground streams, caves, and springs that normally serve as a natural stormwater sewer. Excess water drains into sinkholes, then into underground cave passageways, and eventually into a stream which then flows to a larger river.
With typical rainfall, those systems run smoothly, diverting stormwater through a network of underground caverns. However, with a historic amount of rainfall in a short period of time, those natural systems become overwhelmed.
"One of the most recognizable hazards with flooding in karst areas is sinkhole flooding. So, we have these thousands of sink points within the city. Every one of those developed over time in a natural setting taking a certain amount of water every year and having the natural ability to overflow when it got topped in a natural system," said Nick Lawhon, Bowling Green's city geologist.
Lawhon reported that as of Tuesday afternoon, the city of Bowling Green has responded to at least 18 new sinkholes that have appeared in the city following the storm system that dropped more than ten inches of rain.
He says that with the clay topsoil layer of normally stable sinkholes taking on excess water, that number of exposed sinkholes will likely increase.
"When clay soil gets wet, it's heavier and it's more plastic, it bends more easily, it breaks more easily. So, obviously rainfall brings both of those things, it gets heavier and it gets weaker. If you have one that's been sitting stable for a while, and then you have, let's say, I don't know, some crazy example like ten and a half inches of rain in four days, it will cause some of those to fall out," Lawhon explained.
Of those 18 reported sinkholes, the city of Bowling Green will likely be responsible for repairing 11. The rest are on county, private, or commercial properties.
Lawhon said that depending on city locations across the underground karst system, some flooded areas should expect waters to continue to rise, despite a lack of continued rainfall.
"If you look north of the river, those areas are way up above the river, they're 90 to 200 feet above the river, and the groundwater system they're trying to tap into, those cave systems that run horizontally, are 150 feet down in a lot of places. So, that's a lot of room for that water to fill up," the geologist explained.
He said that capacity means those areas will likely not see excessive flooding.
"Now, on the other side of town, down to the northwest, so Russellville Road area, Nashville road area, that area is very shallow groundwater, so the cave systems that move that water are often 15-20 feet or sometimes less below the surface. So, you don't have a lot of vertical room for water to store and the passageways are much smaller," Lawhon said.
With that decreased capacity, Lawhon said the water near that section of town, including Lost River Cave, will continue to rise, likely for several days, before the underground drainage system can operate normally.
"For instance, there's a large basin as you're going out Russellville Road on your left, that there's a stream that runs through. A spring rises there and flows across the bottom of that valley and goes into the ground. That stream is normally about three inches deep, currently it's about 12 and a half feet deep," Lawhon said. "So, we expect that there'll be some flooding downstream of that that hasn't happened yet."
He explained that while geologists can make predictions on how drainage will operate, a natural system often presents unexpected results.
"A karst aquifer is a lot like a set of pipes, the problem is that there's no record on where the pipes are, a lot of them are broken, a lot of them have weird twists and turns in them, and we just don't have any record of that," he said. "We'll have a lot of data out of this one and hopefully our map will get just a little bit better this time around."
Lawhon said that anybody in Bowling Green that encounters a new sinkhole can report it to Bowling Green Public Works at (270) 393-3628.
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