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Line of sinkholes could be sign of more to come

TAMPA — Homes in the sinkhole-plagued community of Seffner could be sitting on a fracture line linking them to about 20 other sinkholes, including the 20-foot-deep pit that opened under a home in February and killed Jeffrey Bush.

And as Hillsborough County enters what one geologist labels "sinkhole weather," the potential for sinkholes to form will only increase.

Since Bush's death brought national attention to sinkholes here, they seem to be sprouting all over Hillsborough, including several in the past week.

"You'll get areas that just seem to get active," said Sandy Nettles, a private geologist in Palm Harbor. "It could be any number of things that actually stimulated it, but usually once they start rolling into an area, you get more action."

In Plant City, Tom Manus was told to leave his home on North Country Hills Court after a sinkhole was discovered under his porch Saturday.

On Sunday, a Bob Evans restaurant in Seffner was closed after employees found cracks on the ceiling, floor and walls. Geological tests are ongoing, but that type of damage is associated with sinkholes.

Later that day, a Tampa family on Jean Street was asked to evacuate after a sinkhole developed in the front yard. The home is east of Hesperides Street and a half mile north of Hillsborough Avenue.

On Wednesday, a 30-foot-deep possible sinkhole opened under 138th Avenue at Bruce B Downs Boulevard below a 2010 Nissan Versa. The car drove safely off the road, but its tires were damaged.

Though sinkholes are as much a part of Florida as hurricanes and pythons, their frequency and media coverage in recent months have many wondering whether this string is unusual and where they're coming from.

Experts say the recent bout of heavy rains, including from Tropical Storm Andrea, could lead to a summer of sinkholes.

Susanna Martinez Tarokh, a spokeswoman for the Southwest Florida Water Management District, said there were 150 confirmed sinkholes last year in Hernando County during and immediately after Tropical Storm Debby.

"This is sinkhole weather," said Anthony Randazzo, a retired geology professor at the University of Florida and president of Geohazards Inc. "It's very much to be expected that there would be numerous sinkholes opening up following a tropical storm, especially after a period of drought. This type of weather pattern is very conducive to the triggering of sinkholes."

Sinkhole formation starts with water. Rocks such as the limestone that dominates the area begin to dissolve when they're exposed to acidic water, such as rain. For eons, acidic water has flowed through the rocks in Florida, dissolving limestone and creating underground voids. When the ground above those voids collapses, a sinkhole is formed.

All sorts of events can create similar collapses: low water levels, broken sewers and septic tanks, poorly compacted soil after construction, even buried trash and other debris. Many of these cases are not labeled sinkholes, but the effects are similar.

Though it's hard to say, the sequence of occurrences in east Hillsborough could be linked to one fracture system, Nettles said.

"We've mapped the sinkholes we work on in Seffner and plotted the locations of the two that opened up previously, and they're all on a straight line," said Nettles, who counts about 20 sinkholes on his map. "Sometimes, something in one of those fracture systems breaks lose, and it creates a whole series of movements."

Fracture lines have high potential for sinkholes, Nettles said. Intersecting and parallel fracture lines are at an even higher risk.

"When you get a system that moves, it moves until it starts to stabilize," Nettles said. "You probably had one collapse, and it just reverberates down the line and gets everything else loose."

Randazzo and Clint Kromhout, a professional geologist with Florida Geological Survey, don't agree with the idea that a few collapsed sinkholes increase the likelihood of others along the same fracture. But areas sitting on a fracture in the limestone are more likely to be exposed to the groundwater that dissolves it, Randazzo said.

"(A fracture line) can be a very common place where you would form a whole series of sinkholes," Randazzo said. "The fact that they would line up is particularly indicative of that kind of phenomena."

There's no way to determine what an average number of sinkholes for the state or a particular region would look like, Kromhout said. Many go unreported or unnoticed. And while one region of the state might be more at risk than another based on geology and rainfall, it's nearly impossible to tell whether a series of occurrences is abnormal.

"Every day in this state, there's likely a sinkhole that occurs that, A, no one sees or, B, no one thinks to say anything about," Kromhout said.

Caitlin Johnston can be reached at cjohnston@tampabay.com or (813) 661-2443.

Line of sinkholes could be sign of more to come 06/13/13 [Last modified: Friday, June 14, 2013 12:30am]

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