published: Saturday, March 09, 2013

Don't fall into a sinkhole panic

By CHRISTOPHER TUFFLEY

christopher.tuffley@newssun.com

SEBRING -- Ever since the man-eating sinkhole in Seffner opened up a week ago, Highlands County's director of Emergency Operations Center, Scott Canady, has received a stream of calls from panicked homeowners reporting sinkholes on their property. He got two calls Wednesday morning alone.

"The news coverage (of the Seffner sinkhole) has got people on edge," Canady said. So far, he added, he's been able to say, "No. You really don't have a sink hole," at every site he's inspected. He said he has only been to one site in the county "where you'd say, 'Oh yeah, that's a sinkhole.'"

There is a good reason Canady has seen only a single example. Highlands County's geology is stable. Located on the Lake Wales Ridge, the layers of sand, clay and limestone are thick.

This is important because Florida is built on a foundation of limestone, a rock created out of the remains of once living creatures. It erodes easily, because the rock is porous. Water seeps into the rock's natural cracks -- forming channels, eroding it from the inside.

Over millennia, caverns are created in the limestone, underground rivers flow through the rock, and aquifers form.

When a cavern roof is eroded to the point its ceiling is just below the surface crust, the weight of the crust -- and everything on it -- can break though the weakened sedimentary rock, creating a deep hole.

The same dynamic comes into play when water is withdrawn from the aquifer -- either through drought or human consumption -- voids are created and the brittle limestone, suddenly without support, caves in under the weight of the surface.

The sinkhole Canady witnessed occurred in Venus in August of 2012.

"Ben Henley and I stood and watched it grow," Canady said. The sand poured into the circular hole that grew wider.

"It's like the sand in an hourglass," Canady said, "an upside down hourglass. It's trying to fill the underground hole."

According to the Florida geological survey conducted in 2004, there are very few sinkholes here in Highlands County - most of them to the west and very, very small. There have only been three of any size.

A map of sinkholes in Florida shows the great majority of them in the central west coast counties -- like Hernando, Hillsborough, Pasco and Pinellas counties. The northern part of the state and the panhandle have them in number too, as does Polk County. South Florida has virtually none.