During May 1981, in Winter Park, the largest sinkhole ever recorded consumed a three-bedroom home, an Olympic-sized swimming pool, a Porsche repair shop, along with five Porches, a pick-up truck and parts of a road. The sinkhole eventually grew to about 350 feet in diameter and had a depth of about 90 feet. The massive hole was eventually stabilized with dirt and concrete, and is now filled with water and resembles a lake.
Harley Means, the assistant state geologist for the Geological Investigations Section of the Florida Department of Environmental Protection, Florida Geological Survey (FGS), has investigated many sinkholes over the course of his career, including a notable one called "Porter Sink."
Porter Sink is a sinkhole located in the bed of Lake Jackson, a 4,000-acre lake located in Leon County. "The entire lake has drained through this sinkhole numerous times," said Means, who has actually climbed down into the sink and mapped the cave passages underneath the lake. "We measured the passages both for length and direction, drilled cores nearby to investigate which geologic units were involved in the sinkhole activity, shot video and took numerous photos, and mapped the elevations of the lake bed when it was dry," said Means, whose job includes documenting sinkhole events and providing information to the public, local officials and state agencies about how to best manage them.
Many of the 30,000 lakes in Florida have formed by sinkhole activity. According to the Census of Agriculture (2007-2010), there are 922,095,840 acres of farmland, upon which some those lakes are located.
Certain areas of Florida are more vulnerable to sinkholes than others. "The more vulnerable parts are where limestone or dolostone are in close proximity to the surface," said Means. Those areas are located in the Big Bend coastline from central Wakulla County down to Pasco and parts of Pinellas and Hillsborough counties, as well as in an area that centers around Jackson County, and includes parts of surrounding counties.
Means explained that the reason for the vulnerability is because limestone and dolostone are susceptible to being dissolved away over geologic time. As this happens, void spaces are created.
"Periodically the sediment overlying one of the void spaces can collapse into the void causing a sinkhole to form," said Means, who said that the topography that results from this activity is called karst. Means explained that the underlying geology, or karst terrain, is what makes the difference in vulnerability.
Although there is no way to predict exactly when and where a sinkhole is likely to form, there are some techniques that can be used to determine where they are more likely to occur. "These techniques include ground-penetrating radar, and resistivity and drilling, which can be helpful in trying to characterize the subsurface geology," said Means. However, these measures will not determine if or when a sinkhole will form even if void spaces are detected.
The first alert to a sinkhole development often occurs with the naked eye. "Typically, the subsidence, or the land surface, becomes depressed in an area. Concentric cracks at the land surface can develop and eventually a hole may appear. If this happens under a structure (like a home), cracks may occur in tiled areas, walls and ceilings. Also, windows and doors may no longer close properly," said Means, who explained that just because there are signs that subsidence is occurring, not all of these types of incidents are necessarily related to sinkhole activity.
Sinkholes are natural phenomena, and in the case of Lake Jackson, the sinkhole was healthy for the lake. "Lake Jackson has drained many times in the geologic past and the ecology of the lake is adapted to this periodic draining event," said Means, who added that reoccurrences are typically associated with drought cycles.
No matter what we have heard in the headlines about sinkholes being partial to the Florida landscape, they also occur in other parts of the United States and throughout the world.
It is impossible to know how often sinkholes happen and how many have occurred, as some are small in size (10-20 feet in diameter) and many occur in heavily vegetated, remote areas. "We can use aerial imagery and other types of remote sensing techniques to estimate the number of sinkholes but the exact number is not known," said Means.
For more information about sinkholes, visit: http://www.dep.state.fl.us/geology/feedback/faq.htm#1