SECTION 4.3.11: RISK ASSESSMENT – SUBSIDENCE/SINKHOLES Bedford County Hazard Mitigation Plan 4.3.11-1 October 2017 4.3.11 Subsidence and Sinkholes This section provides a profile and vulnerability assessment for the subsidence/sinkhole hazard for Bedford County. Subsidence/sinkholes may be natural or related to underground mining activities. The predominant cause of subsidence and sinkholes in Bedford County is its underlying carbonite bedrock composition, which can include limestone and dolomite. Although underground mining is not considered the primary cause of sinkholes or subsidence in the county, subsidence/sinkholes may still occur in the future because of mining activity. Thus, information will be presented to highlight this hazard cause and its potential impacts. Although underground mining is not considered a geologic hazard, it will be treated as such in this document, due to its relation with the potential for subsidence events. Land subsidence can be defined as the sudden sinking or gradual downward settling of the earth’s surface with little or no horizontal motion, owing to the subsurface movement of earth materials (U.S. Geological Survey [USGS] 2007). Subsidence often occurs through the loss of subsurface support due to mining or in karst terrain, which may result from a number of natural and human-caused occurrences. Karst is a distinctive topography, in which the landscape is largely shaped by the dissolving action of water on carbonate bedrock (usually limestone, dolomite, or marble). Karst features are defined as pockets of limestone or dolomite bedrock located within more stable geological formations that could cause subsidence or sinkholes. The density of karst features ranges from 0 to 600 features per square mile, with wide variations in size. Fewer karst features have been mapped in existing urban areas; however, this is likely a result of development activities that disguise, cover, or fill existing features rather than an absence of the features themselves (Pennsylvania Emergency Management Agency [PEMA] 2013). Sinkholes are a natural and common geologic feature in areas with underlying limestone, carbonate rock, salt beds, or other rocks that are soluble in water. Over periods of time measured in thousands of years, the carbonate bedrock can be dissolved through acidic rainwater moving through fractures or cracks in the bedrock. This creates larger openings in the rock through which water and overlying soil materials travel. Over time, the deposited soils compromise the strength of the bedrock, until it is unable to support the land surface above, causing a collapse or sinkhole. In this example the sinkhole occurs naturally; however, in other cases, the root causes of a sinkhole are anthropogenic, especially those that involve changes to the water balance of an area including over-withdrawal of groundwater, diverting surface water from a large area and concentrating it in a single point, artificially creating ponds of surface water, and drilling new water wells. These actions can also serve to accelerate the natural processes of bedrock degradation, which can directly impact sinkhole creation. Both natural and manmade sinkholes can occur without warning. Specific signs that a sinkhole is forming include slumping or falling fence posts, trees, or foundations; sudden formation of small ponds; wilting vegetation; discolored well water; and/or structural cracks in walls and floors. Sinkholes can form into steep- walled holes or into bowl- or cone-shaped depressions. When sinkholes occur in developed areas, they can cause severe property damage, injury, and loss of life; disruption of utilities; and damage to roadways. In urban and suburban areas, sinkholes can destroy highways and buildings. Two common causes of subsidence in Pennsylvania are (1) dissolution of carbonate rock, such as limestone or dolomite; and (2) mining activity. Water passing through naturally occurring fractures and bedding planes dissolves bedrock, leaving voids below the surface. Eventually, overburden on top of the voids collapses, leaving surface depressions resulting in karst topography. Characteristic features associated with karst topography include sinkholes, linear depressions, and caves. Often, subsurface solution of limestone will not result in the immediate formation of karst features. Collapse sometimes occurs only after a large amount of activity, or when a heavy burden is placed on the overlying material (PEMA 2013). The following sections discuss the location and extent, range of magnitude, previous occurrence, future occurrence, and vulnerability assessment associated with the subsidence/sinkhole hazard for Bedford County.
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4.3.11 Subsidence and Sinkholes H… · formations that could cause subsidence or sinkholes. The density of karst features ranges from 0 to 600 features per square mile, with wide
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Bedford County Hazard Mitigation Plan 4.3.11-1 October 2017
4.3.11 Subsidence and Sinkholes
This section provides a profile and vulnerability assessment for the subsidence/sinkhole hazard for Bedford
County. Subsidence/sinkholes may be natural or related to underground mining activities. The predominant
cause of subsidence and sinkholes in Bedford County is its underlying carbonite bedrock composition, which
can include limestone and dolomite. Although underground mining is not considered the primary cause of
sinkholes or subsidence in the county, subsidence/sinkholes may still occur in the future because of mining
activity. Thus, information will be presented to highlight this hazard cause and its potential impacts. Although
underground mining is not considered a geologic hazard, it will be treated as such in this document, due to its
relation with the potential for subsidence events.
Land subsidence can be defined as the sudden sinking or gradual downward settling of the earth’s surface with
little or no horizontal motion, owing to the subsurface movement of earth materials (U.S. Geological Survey
[USGS] 2007). Subsidence often occurs through the loss of subsurface support due to mining or in karst terrain,
which may result from a number of natural and human-caused occurrences. Karst is a distinctive topography,
in which the landscape is largely shaped by the dissolving action of water on carbonate bedrock (usually
limestone, dolomite, or marble).
Karst features are defined as pockets of limestone or dolomite bedrock located within more stable geological
formations that could cause subsidence or sinkholes. The density of karst features ranges from 0 to 600 features
per square mile, with wide variations in size. Fewer karst features have been mapped in existing urban areas;
however, this is likely a result of development activities that disguise, cover, or fill existing features rather than
an absence of the features themselves (Pennsylvania Emergency Management Agency [PEMA] 2013).
Sinkholes are a natural and common geologic feature in areas with underlying limestone, carbonate rock, salt
beds, or other rocks that are soluble in water. Over periods of time measured in thousands of years, the carbonate
bedrock can be dissolved through acidic rainwater moving through fractures or cracks in the bedrock. This
creates larger openings in the rock through which water and overlying soil materials travel. Over time, the
deposited soils compromise the strength of the bedrock, until it is unable to support the land surface above,
causing a collapse or sinkhole. In this example the sinkhole occurs naturally; however, in other cases, the root
causes of a sinkhole are anthropogenic, especially those that involve changes to the water balance of an area
including over-withdrawal of groundwater, diverting surface water from a large area and concentrating it in a
single point, artificially creating ponds of surface water, and drilling new water wells. These actions can also
serve to accelerate the natural processes of bedrock degradation, which can directly impact sinkhole creation.
Both natural and manmade sinkholes can occur without warning. Specific signs that a sinkhole is forming
include slumping or falling fence posts, trees, or foundations; sudden formation of small ponds; wilting
vegetation; discolored well water; and/or structural cracks in walls and floors. Sinkholes can form into steep-
walled holes or into bowl- or cone-shaped depressions. When sinkholes occur in developed areas, they can cause
severe property damage, injury, and loss of life; disruption of utilities; and damage to roadways. In urban and
suburban areas, sinkholes can destroy highways and buildings.
Two common causes of subsidence in Pennsylvania are (1) dissolution of carbonate rock, such as limestone or
dolomite; and (2) mining activity. Water passing through naturally occurring fractures and bedding planes
dissolves bedrock, leaving voids below the surface. Eventually, overburden on top of the voids collapses, leaving
surface depressions resulting in karst topography. Characteristic features associated with karst topography
include sinkholes, linear depressions, and caves. Often, subsurface solution of limestone will not result in the
immediate formation of karst features. Collapse sometimes occurs only after a large amount of activity, or when
a heavy burden is placed on the overlying material (PEMA 2013).
The following sections discuss the location and extent, range of magnitude, previous occurrence, future occurrence, and vulnerability assessment associated with the subsidence/sinkhole hazard for Bedford County.