Karst / Sinkhole

Background and Extent

Types of Sinkholes

Solution Sinkholes are formed by the weathering by dissolution of exposed soluble bedrock (limestone, dolomite, marble, and rock salt) at the land surface. Surface water collects in the natural depressions and slowly dissolves a sinkhole.

Collapse Sinkholes form when the surface materials suddenly sink into a subsurface cavity or cave. Cavities form slowly over time as groundwater moves along fractures in soluble bedrock which enlarges them through dissolution. Collapses may occur when the cavity gets sufficiently large and the “roof” becomes too thin to support the weight of any overlying rock or sediment causing the cavity to collapse; or if groundwater levels are lowered causing the overlying sediment to first erode and then collapse into the dewatered cavity.

Subsidence Sinkholes – Similar to solution sinkholes, except the soluble bedrock is covered by a thin layer of soil and/or sediment. Surface water infiltration dissolves cavities where the bedrock is most intensely fractured resulting in the overlying sediment to gradually move downward into the expanding cavity. Sinkhole collapse is the hazard most commonly associated with karst. This hazard occurs in the soil that lies on top of soluble bedrock.

Why Do Sinkholes Collapse?

The Kentucky Geological Survey (KGS) identifies two ways in which sinkholes collapse:

1. The roof of a cave becomes too thin to support the weight of the material above it and collapses.
   
2. Limestone bedrock develops a fracture that is enlarged by water dissolving the limestone. As the limestone erodes, the soil above it slowly falls into the developing sinkhole. This type of cover-collapse is typically very slow and occurs over a long period of time. Only in rare instances will limestone bedrock dissolution cause a rapid collapse.

Location

Karst-related hazards happen quickly and without warning. However, scientists have mapped a large portion of the BRADD region to determine where these hazards are most likely to occur. A snapshot of this mapping is located above. to view a fully interactive map and explore sinkholes by jurisdiction, click on the link below.

Past Events

In February of 2014, the BRADD Region received national attention after the floor of the Corvette Museum in Bowling Green collapsed into a 30-foot deep sinkhole. At the time of the collapse, the damage to the facility and the eight corvettes that succumbed to the sinkhole was estimated at $3.2 million.

Other notable past events include a significant sinkhole collapse in 2002 on Dishman Lane in Bowling Green. Extensive repairs to the road cost $1 million.

More recently, after three days of extensive rainfall and flooding in 2021, a number of sinkholes occurred across farmland in Hart County. On one Hammonsville farm, a new 8-foot by 8-foot  sinkhole appeared near an existing sinkhole. Additionally, the existing sinkhole grew as a result of rainfall accumulation. New and exacerbated sinkholes on farmland pose a risk to livestock; a calf had to be rescued from the aforementioned sinkhole after falling in.

Sources:

Detroit Free Press, Hart County News-Herald 2021, Hart County News-Herald 2002

Probability

The dashboard linked below shows karst potential in the BRADD Region. Sinkholes are a quintessential feature of karst terrain, and the presence of karst terrain significantly increases a community’s risk to sinkholes. Moderate sinkhole or subsidence potential is defined as karst terrain where sinkholes have not presented themselves. Major sinkhole or subsidence potential is defined as karst terrain where 1-9 sinkholes per square kilometer have been recorded in the past.  As shown below, moderate and major karst potential is present in all BRADD counties. This, in part, is due to the many cave systems prominent throughout the region.

Overall Probability

Overall, the BRADD Region ranks the probability of sinkhole occurrence as highly likely because of the extensive number of sinkholes identified in the region and moderate to high karst potential in each county. This was determined using the classification definitions outlined by FEMA (click here for more information).

Vulnerability & Impact

Hazard Vulnerability Summary Analysis

Overall, the BRADD Region and its counties experience moderate vulnerability to sinkholes with some past events having caused serious property damage.

While this analysis applies to all BRADD Counties, it should be recognized that some counties, especially Hart, Edmonson, Simpson, and Warren, are more likely to experience sinkholes due to karst terrain.  To view a more detailed analysis of each county’s vulnerability, click the links below.