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Despite heavy rain, sinkhole odds low

When it comes to sinkholes, the recent heavy downpours could be bad news. But not necessarily.

Storms like the ones that dumped 7 to 9 inches of rain on Pinellas late last month can trigger the sudden appearance of holes in the earth. But the rain can also prevent sinkholes, experts say, by filling the aquifer.

Tony Gilboy, a geologist for the Southwest Florida Water Management District in Brooksville, said that when underground water levels are low, which usually happens November through May, and sudden rains hit in summer months, sinkholes are likely to develop.

But Pinellas shouldn't have to worry this year, he said. Pinellas, along with other counties, has high water levels in the aquifer.

"In Citrus County, I would be worried, but Pinellas is in a different situation geographically," Gilboy said.

Sinkholes are nothing new to Pinellas, though.

Doug Smith, a professor of geological sciences at the University of Florida in Gainesville, said the area from northern Clearwater through Port Richey has a lot of sinkhole activity.

Smith said Dunedin, Palm Harbor and New Port Richey are of particular concern because of population density. Not that population density causes sinkholes, he said, but when houses are more densely located, a sinkhole that develops is more likely to affect someone's home.

Leonard Marsland of Palm Harbor knows about this. He walked out of his mobile home in December to find a hole 10 feet wide and more than 6 feet deep in his back yard.

"I thought, "Who is digging a hole?' then I realized no one would leave a hole, then that's when I realized, uh-oh, that's a sinkhole," Marsland said.

Sinkholes form when rainwater dissolves the layer of limestone that lies under the surface earth in Florida.

Add heavy rains and "the ground becomes so saturated or weighted with water, it can collapse in those cavities," Gilboy said.

A rash of sinkholes struck Pinellas' northern areas in the past year.

In Tarpon Springs in November, a 25-foot-wide and 30-foot-deep hole opened under the rear corner of a condominium.

A 5-foot sinkhole opened 2 feet from a home on Eniswood Parkway in Palm Harbor last summer.

And in May, a sinkhole opened up on Ridge Road, slowing traffic and causing authorities to close two northbound lanes at 10th Avenue SW in Largo.

Despite there being no injuries from recent sinkhole incidents, Gary Vickers, director of emergency management for the county, cautions residents to notify authorities at the slightest hint of a depression.

"At any sign, call 911," Vickers said. "If you see one, it's virtually impossible to tell how severe the situation is. . . . We can't tell how much danger a person is in."

Vickers said one or several homes may need to be evacuated if a sinkhole should occur, which is one reason it is crucial to alert officials.

"If we don't get out and cordon the area off, it could have all kinds of severe consequences."

To protect yourself, Vickers said, aggressively look out for warning signs like the level of a lake decreasing. If it's high one day and low the next, the water went somewhere.

"If it's a depressed area, it could be indicative of a sinkhole forming," he said.

Also, when a sinkhole begins to form the ground starts to shift, which misaligns a structure. Walls will start to shift, so significant cracking misaligned windows or doors should be noted.

For more information on sinkholes, go to /sinkhole/sinkpage.htm.

_ Information from Times files was used in this report. Angie Green can be reached at 445-4224 or

What to do if a sinkhole develops:

1. Call 911.

2. Keep children away.

3. If lake and water levels or groundwater quality are affected, report it to the Southwest Florida Water Management District at 800-423-1476.

4. If your home is threatened, contact your homeowners insurance company.

_ Source: Southwest Florida Water Management District and Pinellas County Emergency Management

How sinkholes form

A combination of drought conditions, heavy rains and excessive pumping has led to the recent proliferation of sinkholes.

1. Water percolating through the ground dissolves carbon dioxide gas from air and soil creating a weak acid that breaks down limestone and forms cavities.

2. Aggressive pumping from the aquifer lowers the water table, which reduces fluid pressure and accelerates water filtration from the surface.

3. This sudden imbalance stresses the subsurface, causing the cavity to grow.

4. Heavy rain or vibration from construction sites eventually cause the surface to collapse, creating the sinkhole.

Sinkhole warning signs

_ Fresh exposure on fence posts, foundations and trees that result when the ground sinks.

_ Slumping, sagging or slanting fence posts, trees or other objects.

_ Doors and windows that fail to close properly.

_ Small ponds of rainfall forming where water has not collected before.

_ Wilting vegetations. This happens because the moisture that normally supports vegetation is draining into the sinkhole that is developing below the surface.

_ Muddy water in nearby wells.

_ Cracks in walls, floors, pavement and in the ground.

Two types of sinkholes

Solution sinkholes occur where limestone is exposed or is covered only by thin layers of soil. As the underlayer of limestone erodes, the soil begins sinking gradually creating a bowl-shaped depression, which often becomes a pond or marsh.

Collapse sinkholes occur suddenly. When an underground cavern cannot support the surface above, it collapse and a hold opens without any warning, resulting in possible loss of life and property.

Sources: Southwest Florida Management District and Knight-Ridder Tribune

Palm Harbor firefighter Brent Breuer, left, and Lt. Milton Smith measure the sinkhole outside George and Bonnie Quirk's living room window in August to see if it had grown overnight. A rash of sinkholes struck Pinellas' northern areas last year.