Trigg County, Kentucky

Geologic Hazards

The most prominent geologic hazard for Trigg County is the karst development on the terrain underlain by thick limestone, unit 5 on the map. Sinkholes shown in red on the map are the surface expression of solution cavities such as caves and flow channels. Karst can be particularly hazardous if not treated properly during urban development. Significant damage can occur if sinkholes open beneath a structure, and flooding can worsen if subsurface drainage through sinkholes and caves are plugged or impaired. Groundwater supplies may be polluted if waste is improperly dumped into sinkholes, which ultimately affects surface water.

Never use sinkholes as dumps. All waste, but especially pesticides, paints, household chemicals, automobile batteries, and used motor oil, should be taken to an appropriate recycling center or landfill.

Make sure runoff from parking lots, streets, and other urban areas is routed through a detention basin and sediment trap to filter it before it flows into a sinkhole.

Make sure your home septic system is working properly and that it's not discharging sewage into a crevice or sinkhole.

Keep cattle and other livestock out of sinkholes and sinking streams. There are other methods of providing water to livestock.

See to it that sinkholes near or in crop fields are bordered with trees, shrubs, or grass buffer strips. This will filter runoff flowing into sinkholes and also keep tilled areas away from sinkholes.

Construct waste-holding lagoons in karst areas carefully, to prevent the bottom of the lagoon from collapsing, which would result in a catastrophic emptying of waste into the groundwater.

If required, develop a groundwater protection plan (410KAR5:037) or an agricultural water-quality plan (KRS224.71) for your land use.

(From Currens, 2001)

Limestone terrain can be subject to subsidence hazards, which usually can be overcome by prior planning and site evaluation. "A" shows construction above an open cavern, which later collapses. This is one of the most difficult situations to detect, and the possibility of this situation beneath a structure warrants insurance protection for homes built on karst terrain. In "B," a heavy structure presumed to lie above solid bedrock actually is partially supported on soft, residual clay soils that subside gradually, resulting in damage to the structure. This occurs where inadequate site evaluation can be traced to lack of geophysical studies and inadequate core sampling. "C" and "D" show the close relationship between hydrology and subsidence hazards in limestone terrain. In "C," the house is situated on porous fill (light shading) at a site where surface and groundwater drainage move supporting soil (darker shading) into voids in limestone (blocks) below. The natural process is then accelerated by infiltration through fill around the home. "D" shows a karst site where normal rainfall is absorbed by subsurface conduits, but water from infrequent heavy storms cannot be carried away quickly enough to prevent flooding of low-lying areas. Adapted from AIPG (1993).

Other Geologic Hazards

None of the faults in Trigg County are considered to be active; however, the proximity of active seismic zones, such as the New Madrid, Wabash, or East Tennessee, calls for precautions to be taken for earthquake damage mitigation.

Flooding may be a problem in Trigg County, especially, along major streams. Urban development often exacerbates flooding, and therefore potential flooding should always be considered in urban development plans. Areas of steep walled drainage, such as that formed in terrain underlain by limestone, are conducive to flash flooding, especially in developed areas. Areas underlain by alluvium are generally subject to flooding. Also, alluvium contains abundant clay minerals, which may result in significant shrink/swell conditions.

Steep slopes are present, especially along streams, in areas underlain by limestone in the western part of Trigg County. Steep slopes can develop soil creep and landslides if not properly treated during development. Proper engineering techniques should be followed when developing on hillsides, and care should be taken not to affect property above and below a development site on a hillside.

Earthquake Hazard Information

Peak ground acceleration at the top of rock that will probably occur in the next 500 years in Kentucky

Although we do not know when and where the next major earthquake will occur, we do know that an earthquake will cause damage. Severity depends on many factors, such as earthquake magnitude, distance from the epicenter, and local geology. Information on earthquake effects is obtained by monitoring earthquakes and performing research. Such information is vital for earthquake hazard mitigation and risk reduction.

The most important information for seismic-hazard mitigation and risk reduction is ground-motion hazard. One way of predicting ground-motion hazard is by determining the peak ground acceleration (PGA) that may occur in a particular timeframe. The map above shows the PGA at the top of bedrock that will likely occur within the next 500 years in Kentucky (Street and others, 1996). It shows, as expected, that PGA would be greatest in far western Kentucky near the New Madrid Seismic Zone. Ground-motion hazard maps for the central United States and other areas are available from the U.S. Geological Survey. These maps are used to set general policies on mitigating damage. For example, maps produced by the USGS in 1996 were used to determine seismic design in building codes. For additional information about earthquake hazards, visit the Kentucky Geological Survey Web site at www.uky.edu/KGS/geologichazards/geologichazards.html.

Development

The population of Trigg County grew 21.6 percent between 1990 and 2000, according to the U.S. Census Bureau. Because of this growth, home construction is a growing land use through out the county. Photo by Glynn Beck, Kentucky Geological Survey.

Two developed industrial parks are in Trigg County, and one park is under construction. Eleven businesses are located on 230 acres. Photo by Glynn Beck, Kentucky Geological Survey.

Water Resources

Lake Barkley was formed when the Cumberland River was impounded in the early 1960’s. Over 18,500 acres of the lake are located in Trigg County. Lake Barkley is an excellent resource for boating, fishing, and other activities. Photo by Glynn Beck, Kentucky Geological Survey.

Land Between The Lakes (LBL) is an inland peninsula, which was formed after the Cumberland and Tennessee Rivers were impounded to create Kentucky Lake and Lake Barkley. In 1963 the area was deemed a National Recreation Area. Approximately 68,800 acres of the 170,000 acres comprising LBL are located in Trigg County. Photo by Glynn Beck, Kentucky Geological Survey.

Groundwater

In the eastern two-thirds of Trigg County, more than three-quarters of the drilled wells in the uplands are adequate for a domestic supply. Yields as high as 50 gallons per minute have been reported from wells penetrating large solution channels. In the low-lying areas of the Little River and its major tributaries, most wells are inadequate for domestic use unless the well intercepts a major solution opening in the limestone; in that case, the yield could be very large. Wells in the uplands of the Land Between the Lakes in the western third of the county generally do not yield enough water for domestic use. In the lowlands adjacent to Kentucky and Barkley Lakes, however, three-fourths of the wells yield enough for a domestic supply and can sometimes produce more than 5 gallons per minute.

Springs with flows ranging from a few gallons per minute to 3,000 gallons per minute are found in the county. Minimum flow generally occurs in early fall, and maximum flows in late winter. For more information on groundwater in the county, see Carey and Stickney (2001).

Agriculture

Agriculture is a major land use in Trigg County. In 2002, 445 acres of burley tobacco (light green above) and 475 acres of dark fired tobacco (dark green above) were grown. Also, out of the 269,658 land acres available in Trigg County, 36,700 acres (14 percent) where planted in corn and soybeans, according to 2002 statistics from the Kentucky Agricultural Statistics Service. Photo by Glynn Beck, Kentucky Geological Survey.