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Under weight of heavy rains, experts say ground in Tampa Bay could be more susceptible to sinkholes

TAMPA — The onslaught of rain that drenched the Tampa Bay area in the past month unleashed widespread flooding that left the ground soppy and saturated.

Geologists say that under all that water weight, another problem could be brewing out of sight: sinkholes.

They stopped just short of saying bay area residents should be alarmed, but emphasized the importance of watching for warning signs like dips in the yard or new structure cracks in buildings and sidewalks.

"Any time there is a significant change in the weather pattern in Florida, it may or may not play a factor in the increase in sinkhole formation," said Jonathan Arthur, director of the Department of Environmental Protection's Florida Geological Survey.

Big rains over an extended period of time weigh down the soil and sand that make up the earth's surface, increasing the likelihood that it will collapse into holes in the bedrock below and form depressions or sinkholes.

"When you get 10 or 20 inches of rain in such a short time period, it's kind of a trigger feature," said Jerry Black, a geologist with the Gainesville-based company Geohazards.

Just last week, engineers attributed this phenomenon to the re-opening of a deadly sinkhole in Seffner that swallowed a man as he slept in his house in February 2013. Jeffrey Bush's body was never recovered.

On Wednesday, the same hole cracked open again, measuring 17 feet wide and 20 feet deep. Engineers said the gravel they originally used to fill in the gaping hole settled after more than 2 feet of rain soaked the area since late July.

Black said less than 2 percent of sinkholes are as devastating or tragic as the one in Seffner.

What Floridians should be more concerned about, he said, are the slow-forming depressions that grow over time if not detected and resolved early.

"Not all depressions and not all low areas are sinkholes," he said. "But since we're in Florida, that's what people think of first."

The difference is nuanced, but important, both geologists say.

Sinkholes are naturally occurring formations brought on by a gradual weakening of the limestone that lines much of Florida. Over time, acidic water eats away at the rock, creating openings that eventually give way to the soil and sand above it. A golf-ball sized hole would take thousands of years to form, Black said.

The limestone beneath the bay area is between 5 million and 35 million years old.

"It's had a long time to dissolve and for these large voids to form," Black said. "The system is already set up."

What exacerbates the risk in places like Tampa is that the limestone is much closer to the earth's surface — as close as 5 feet in some areas. Hillsborough County, as well as areas of north Pinellas and western Hernando and Pasco counties, have soil made up of sand, which is far more likely to fall through those limestone voids than a sturdier material like clay.

Clearwater, Largo and St. Petersburg have 40 feet of clay above their limestone bedrock.

Other human-caused factors can lead to depressions in the earth, such as rotting debris, pipe collapses or infrastructure failures during construction, Arthur said.

To help Florida residents visualize areas of the state that are more susceptible to sinkholes, Arthur's department has partnered with the state's division of emergency management to develop a "sinkhole likelihood" map.

A Federal Emergency Management Agency grant is funding the project, which has been under way for two years and is expected to publish next summer.

The map depicts the nature of the soil, the depth of the limestone and the depth of the water table.

In the meantime, Arthur advises Floridians to keep an eye out for the most obvious signs, like cracks in driveways, misfitting door jambs or changes in nearby vegetation.

"Sometimes, you can notice that the grass is getting dry or the bushes look thirsty," he said.

The most obvious signs of a sudden sinkhole are seemingly unprompted popping sounds inside a building or the breaking of glass windows.

His advice in those times: Get out, fast.

Contact Katie Mettler at kmettler@tampabay.com or (813) 226-3446. Follow @kemettler.

EDITOR'S NOTE: This story has been updated to reflect the following correction. Jonathan Arthur is the director of the Florida Geological Survey, which is an arm of the state's Department of Environmental Protection.

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