Subsidence Incident Reports
What is a Subsidence Incident?
Sinkholes are closed depressions in areas underlain by soluble rock such as limestone, dolostone, and in some states gypsum and salt. Sinkholes form when surface sediments subside into underground voids created by the dissolving action of groundwater in the underlying bedrock.
Other subterranean events can cause holes, depressions, or subsidence of the land surface that may mimic sinkhole activity. These include subsurface expansive clay or organic layers that compress as water is removed; decomposing stumps or organic debris; collapsed or broken sewer and drain pipes or broken septic tanks; improperly compacted soil after excavation work; or decomposing or compressed buried trash and other debris.
Commonly, a reported depression is not verified by a licensed professional geologist to be a true sinkhole, and the cause of subsidence is not known. Such an event is called a subsidence incident. The Florida Geological Survey maintains and provides a downloadable database of reported subsidence incidents statewide. While this data may include some true sinkholes, the majority of the incidents have not been field-checked and the cause of subsidence is not verified.
Origins of the Subsidence Incident Reports
The database was started by the Florida Geological Survey (FGS) for scientific research purposes only. We are unsure what year (the FGS is over 110 years old). In the early 1980s, the database was moved to the newly formed and legislatively mandated Florida Sinkhole Research Institute (FSRI); however, in the early 1990s FSRI was eliminated and the database came back to the FGS. There was never a legal requirement that sinkhole occurrences or sinkhole insurance claims be entered into the database. The data collected originated from citizens reporting sinkholes, state and local governments, and the Florida Department of Transportation. All incidents were reported voluntarily.
Currently, a majority of the records come from the State Watch Office, which is the clearinghouse for emergency response calls involving human-made and natural disasters. The State Watch Office has a special reporting form that county, city and state dispatchers fill out (should they choose to) if a call comes in regarding a possible sinkhole occurrence.
The second source of records comes from members of the public who either fill out and submit the FGS subsidence incident report form (via mail, email, or fax) or call the FGS. The third source is via emergency situations where a swarm of sinkholes occurs and the FGS is called in by emergency officials to help survey the sinkhole hazard (examples include Tropical Storm Debby and the January 2010 frost/freeze event in the Plant City area). There are some records in the database that are associated with sinkhole insurance claims; however, those represent a small fraction of the total reports.