Sinkholes are common features in parts of Florida, and the Florida Geological Survey maintains an online database of sinkhole incident reports (SIRs) that was started in 1965. The incident reports are accepted “as-is” without verification; sinkhole location, length, width, and depth are included in SIRs. A desktop assessment of sinkhole activity in northern Florida was developed on the basis of SIRs that were available in GIS (shape file) format from the Florida Geological Survey website and an understanding that sinkhole activity needed to be normalized to length for use in pipeline risk assessments.
The rate of sinkhole development in northern Florida was quantified by extracting sinkhole locations within 10 miles of a 230-mile-long hypothetical alignment of a pipeline and lateral. Over 500 sinkholes were located within the approximately 4,700-square mile polygon. Sinkhole trends aligned to highways indicate more complete reporting; therefore, 33 road segments comprising a combined length of about 944 miles within the polygon were used for statistical analysis. The SIR database was accepted as an accurate portrayal of sinkholes during its 47-year existence. Sinkhole activity was portrayed as annual frequency for sinkhole width or length ranging from 1 to 500 feet and normalized to 1 square mile and 1 lineal mile. A sinkhole 2 feet or larger in width occurs on average 8 times per year somewhere within 10 miles of the hypothetical alignment; whereas, a similar sinkhole occurs on average 4 times per year along a length of 944 miles. A 2-foot or larger sinkhole occurs on average about every 600 years within any 1 square mile of the 4,700-square-mile area and about every 200 years along any 1 mile of alignment length. On a per-lineal-mile basis, the expected 1,000-year sinkhole would be at least 9 feet wide; whereas the 1,000-year sinkhole would be at least 22 feet wide. Performance of specific pipelines under loading conditions associated with sinkholes of various widths can be assessed and used in a pipeline risk assessment.