Sinkholes not common in Manatee County

jdeleon@bradenton.comMarch 7, 2013 

MANATEE -- The sinkhole in Hillsborough County that claimed a man's life last week has left many Floridians, including Manatee residents, pondering the likelihood of sinkholes in their own backyard.

While Florida has more sinkholes than any other state in the country, they are rarely associated with fatalities.

In Manatee County, the threat of sinkholes is much lower than in neighboring Hillsborough County.

"In all of Manatee County, sinkholes are a very rare occurrence," Manatee County Emergency Management Chief Don Hermey said.

The Southwest Florida Water Management District has recorded only four sinkholes in Manatee County, Hermey said.

The exact number of sinkholes that occur each year is unclear since all reports are not confirmed by the Florida Geological Survey, the agency under the Department of Environmental Protection that records sinkholes.

The Florida Geological Survey does keep records of reports of subsidence, a natural caving in of an area of land. However all cases of subsidence are not sinkholes.

Harley Means, assistant state geologist with the Florida Geological Survey, said there are sinkholes in Florida because of the geology of the ground under Florida, which consists of limestone.

"Limestone, because of its chemical makeup, is capable of being dissolved by a weak acid, in this case rainwater," Means said.

When rainwater penetrates the soil, it trickles down in the limestone, slowly dissolving it. Eventually cavities are formed and sometimes the land above gives way to the void.

Manatee's geology, however, makes the area less prone to this natural phenomenon.

"They do develop more frequently north of Tampa Bay where the limestone base is closest to the land surface and the supporting land and clay layers are thin," Hermey said.

But Manatee emergency officials still include sinkholes in their training and planning processes.

"You never know when and where they are going to occur," Hermey said.

Warning signs that could indicate a sinkhole are cracks in a home's foundation, cracks in driveways, cracked tiles in the flooring, cracks in the ceiling and offsets of windows or doors.

Means warns that subsidence can occur for other reasons, as well.

Homes built on top of certain kinds of clay can shrink or swell, Means said. Also susceptible are homes built on top of former wetlands or with a broken water main underneath.

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