KENTUCKY KARST
opment at the surface, were also col- lected for Warren County. Sinkholes and cave entrance locations were ground- truthed using GPS. Field measurements were conducted to collect overburden measurements at cave entrance loca- tions and were extrapolated to repre- sent the overburden of the entire cave system. Known cave entrances were field-checked and collapse type charac- terized, inventoried and georeferenced. Overburden and epikarst, the upper and most permeable boundary of a karst sys- tem, were measured at each bedrock col- lapse using standard geologic mapping methods. The ratio of cave passage width to overburden thickness was calculated to assess the relative stability of cave passages known as cover ratios (Table 1). The cover ratio for the Dishman Lane col- lapse was taken from Crawford (2002).
Digital and field data were compiled within a GIS to assess geological and hydrological controls on sinkhole loca- tions and development (Figure 8). Extent of sinkhole drainage, cave locations, overburden and major and moderate karst development zones were ana- lyzed to identify sinkhole risk potential (Figure 9). Based on a series of derivative maps produced, including an overburden isopach map showing the overburden thickness above known cave passages and existing bedrock collapses, a bedrock collapse geohazard map was made in order to identify areas at risk for bedrock collapses (Figure 10).
Discussion
All the largest sinkholes in Bowling Green, Kentucky area are associated with conduits that currently drain the surface topography of the city or were paleo-drainage conduits that are cur- rently not active. The active conduits are passages associated with the Lost River Cave System. The Corvette Museum collapse occurred within an inactive drainage conduit not associated with the Lost River Cave System.
GIS analysis of the previous bedrock collapses show that construction activi- ties and the redirection of flood waters into the epikarst create an imposed load over an underlying cave passage, caus- ing the weakened cave roof to collapse. Collapses could have been avoided had more detailed information about the subsurface cave features been available.
The maps produced during this study not only elucidate the subsurface of the City of Bowling Green, but also provide tools with which to assess the stability of the landscape and the potential risk of sinkhole development. The bedrock collapse geohazard maps are a proactive
means for informing future infrastruc- ture development in Bowling Green, Kentucky. They have the potential to help with risk assessment in sinkhole- prone areas. The maps would also serve as a good resource for public education and outreach.
The structural integrity of different parts of each cave system will evolve differently. While one site may exhibit a structurally sound roof with relatively intact rock, another site in the same cave or same passage may have areas exhibiting breakdown or stoping, break- down of a cave roof in a series of steps or layers. Such conditions are common in a karst environment and can be mapped to reduce the difficulty they present for developers during construc- tion. However, in order to avoid the damage caused by bedrock collapse sink- holes, careful site investigation, which includes the use of tools such as the maps created in this project, is required for every construction within a terrain of cavernous karst.
Kentucky annually incurs $20 million in damages due to karst subsidence and sinkhole collapse (Weary 2015). There are approximately 22,735 households in Bowling Green, Kentucky, leaving thousands of citizens at risk for sinkhole damages. The potential for sinkhole collapse increases as the rate and scale of development in this city increases. Although bedrock collapse is considered to be less frequent than cover collapse, its mitigation is more costly.
Correspondingly, land values in sink- hole-prone areas can suffer, impacting landowners, as well as counties and municipalities that are dependent on property tax revenue (Weary 2015). Many citizens are not aware of the hid- den costs, in the form of higher taxes and an increased cost of living, that are imposed by the need for the remediation of sinkholes. Sinkholes also serve as drainage points for surface water to reach the subsurface. This creates sig- nificant issues in groundwater quality and often leaves structures vulnerable to flood damage (Currens 2002).
Conclusion
While some Kentucky cities have recognized a need for planning when building on karst, there are no build- ing regulations relating to sinkholes in Bowling Green or Warren County. Many planning regulations currently in place have no provision for sinkholes, but certain ordinances that do provide guidelines for sinkholes are limited to the protection of water quality. This is the typical practice for managing
construction in karst terrain across the country (Fleury 2009). While no formal practices for building on karst exist, both the Kentucky Geological Survey and the National Cave and Karst Research Institute offer guidelines for government agencies to work from or adopt. The Bowling Green Public Works Department claims that sinkholes do not pose a threat to the Warren County area, and that best management practices are currently being used in the city. However, recent sinkhole events, and the rapid increase in sinkhole develop- ment due to imposed loads from con- struction seem to suggest otherwise.
Maps that provide an in-depth look at the subsurface environment would aid in the avoidance of poor construc- tion planning and enhance preparation and remediation tactics where imposed loads currently exist. Education about the Bowling Green area subsurface will allow for better management practices to be implemented, including, but not limited to, the passing of ordinances and guidelines for building on karst. Continued efforts to map the subsurface and karst environments in Bowling Green will make available more data to map larger expanses of the city and allow for better risk assessment.
References
Crawford, Nicholas C., 2002, Dishman Lane Sinkhole Collapse Investigation. Bowling Green: Center for Cave and Karst Studies.
Currens, James, 2002. Kentucky is Karst Country. Information Circular 4, Series XII, Lexington: Kentucky Geological Survey.
Fleury, Spencer, 2009. Land Use Policy and Practice on Karst Terrains. New York City: Springer.
Polk, Jason S., Leslie A. North, Ric Federico, Brian Ham, Dan Nedvidek, Kegan McClanahan, Pat Kambesis, and Michael J. Marasa. 2015. “Cars and Karst: Investigating the National Corvette Museum Sinkhole.” 14th Sinkhole Conference NCKRI Symposium 5. Rochester, MN. 477 - 481.
Waltham, Tony, Fred Bell, and Martin Culshaw. 2005. Sinkholes and Subsidence: Karst and Cavernous Rocks in Engineering and Construction. Chichester, UK: Springer-Praxis.
Waltham, Tony. 2007. “Natural and anthro- pogenic rock collapse over open caves.” Geological Society London Special Publications. 13-21.
Weary, David J. 2015. “The cost of karst subsidence and sinkhole collapse in the United States compared with other natu- ral hazards.” 14th Sinkhole Conference, NCKRI Symposium 5. Reston, VA: U.S. Geological Survey. 433-446.
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