Subsidence and Sinkholes

Size: px

Start display at page:

Download "Subsidence and Sinkholes"

Neal Lewis Wilson
5 years ago
Views:

1 Subsidence and Sinkholes This section provides a profile and vulnerability assessment for the subsidence and sinkhole hazard for the Armstrong County Hazard Mitigation Plan (HMP). Two common causes of subsidence in Pennsylvania that impact Armstrong County are (1) dissolution of carbonate rock, such as limestone or dolomite; and (2) mining activity. In the first case, 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 structures 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. Areas of the County underlain by carbonate rock may be more susceptible to sinkholes as they are common where this type of rock is below the surface. As the rock dissolves, spaces and caverns develop underground (U.S. Geological Survey [USGS] 2018). Human activity can also result in subsidence or sinkhole events. Leaking water pipes or structures that convey storm-water runoff may also result in areas of subsidence as the water dissolves substantial amounts of rock over time. Poorly-managed stormwater has particularly been an exacerbating factor in subsidence events in Cumberland County, Lebanon County, and Palmyra. In some cases, construction, land grading, or earthmoving activities that cause changes in stormwater flow can trigger sinkhole events (Pennsylvania Emergency Management Agency [PEMA] 2018). Subsidence or sinkhole events may also occur in the presence of mining activity, even in areas where bedrock is not necessarily conducive to their formation. Mining activity is concentrated in the southwestern region of the state, as well as Schuylkill, Northumberland, and Carbon Counties. Because subsurface (i.e., underground) extraction of materials such as oil, gas, coal, metal ores (i.e., copper, iron, and zinc), clay, shale, limestone, or water may result in slow-moving or abrupt shifts in the ground surface, these areas have a higher potential to be impacted by sinkholes and subsidence (PEMA 2018). Sinkholes often develop where the cover above a mine is thin. Piggott and Eynon (1978) indicated that sinkhole development normally occurs where the interval to the ground surface is less than 3 to 5 times the thickness of the extracted seam and the maximum interval is up to ten times the thickness of the extracted seam. In western Pennsylvania, most sinkholes develop where the soil and rock above a mine are less than 50 feet thick. A study of subsidence in the Pittsburgh area revealed that the majority of sinkholes, which constituted about 95 percent of all reported subsidence incidents, occurred on sites located less than 60 feet above mine level (PEMA 2018). The following sections discuss the location and extent, range of magnitude, previous occurrence, future occurrence, and vulnerability assessment associated with the subsidence and sinkhole hazard for Armstrong County Location and Extent Approximately 2.5 percent of Armstrong County (3.6 square miles) is underlain by carbonate bedrock. Figure shows the distribution of carbonate rock areas in Armstrong County. The following municipalities have identified near-surface limestone: Kiskiminetas Township South Bend Township Armstrong County Hazard Mitigation Plan

2 Figure Armstrong County Carbonate Bedrock Geology Source: Pennsylvania Bureau of Topographic and Geologic Survey 2001 Armstrong County Hazard Mitigation Plan

3 Figure shows the approximate location of abandoned mines and land hazards created by past coal mining; information is based on a subset of data contained in the Office of Surface Mining Reclamation and Enforcement (OSMRE) Abandoned Mine Land Inventory. In addition, detailed maps of abandoned mines are available for 649 mines in Armstrong County through the National Mine Map Repository (NMMR), maintained by OSMRE. The NMMR contains over 183,000 maps from the 1790s to the present day, providing information for both surface and underground mines throughout the United States (OSMRE 2018). Armstrong County Hazard Mitigation Plan

4 Figure Abandoned Mines in Armstrong County Source: Pennsylvania Department of Environmental Protection (PADEP) 2014 Note: Red areas indicate abandoned mines that have been identified as subsidence areas. Armstrong County Hazard Mitigation Plan

5 Range of Magnitude No two subsidence areas or sinkholes are exactly alike. Variations in size and shape, time period under which they occur (i.e., gradually or abruptly), and their proximity to development ultimately determines the magnitude of damage incurred. Events could result in minor elevation changes or deep, gaping holes in the ground surface. Subsidence and sinkhole events can cause severe damage in urban environments, although gradual events can be addressed before significant damage occurs. Primarily, problems related to subsidence include the disruption of utility services and damages to private and public property including buildings, roads, and underground infrastructure. If long-term subsidence or sinkhole formation is not recognized and mitigation measures are not implemented, fractures or complete collapse of building foundations and roadways may result (PEMA 2018). Damage from mine subsidence can impact structures, surface water and groundwater, and wells and springs (PADEP 1999) Past Occurrence According to the USGS, Pennsylvania is one of the top seven states most likely to receive damage from sinkholes. The other states include Florida, Texas, Alabama, Missouri, Kentucky, and Tennessee. Neither the Pennsylvania Department of Conservation and Natural Resources (PA DCNR) (PA DCNR 2018) nor the 2018 Pennsylvania State HMP (PEMA 2018) show any sinkholes in Armstrong County Future Occurrence Although sinkhole occurrence will continue to be a possibility in Armstrong County, the probability of a sinkhole or subsidence event is difficult to predict because of the low number of previous events. Areas to monitor for future sinkhole and subsidence events based on their geologic bedrock are listed above in Section The identified hazards of concern for Armstrong County were ranked for relative risk in Section 4.4 of the HMP. The probability of occurrence, or likelihood of the event, is one parameter used for ranking hazards. Based on historical records and reference to the Pennsylvania State Hazard Mitigation Plan, the probability of occurrence for subsidence and sinkhole events in the County is considered possible. Section 4.4 includes further information on PEMA s risk factor methodology and the risk factors used to determine each hazard s risk rank Vulnerability Assessment To understand risk, a community must evaluate the assets that are exposed or vulnerable in the identified hazard area. This section discusses the potential impact of the subsidence and sinkhole hazard on Armstrong County in the following subsections: Overview of vulnerability Data and methodology used for the evaluation Impact on (1) life, health, and safety; (2) general building stock; (3) critical facilities; (4) economy; (5) the environment; and (5) future growth and development Effects of climate change on vulnerability Overview of Vulnerability Table summarizes the municipalities that are potentially vulnerable to sinkholes and subsidence events based on the presence of limestone bedrock and/or abandoned mines. Armstrong County Hazard Mitigation Plan

6 Table Municipalities Vulnerable to Sinkholes and Subsidence Events Municipality Carbonate Rock Abandoned Mine Apollo Borough Applewold Borough Atwood Borough Abandoned Mine noted as Subsidence Area Bethel Township Boggs Township Bradys Bend Township Burrell Township Cadogan Township Cowanshannock Township Dayton Borough East Franklin Township Elderton Borough Ford City Borough Ford Cliff Borough Freeport Borough Gilpin Township Hovey Township Kiskiminetas Township Kittanning Borough Kittanning Township Leechburg Borough Madison Township Mahoning Township Manor Township Manorville Borough North Apollo Borough North Buffalo Township Parker City Parks Township Perry Township Pine Township Plumcreek Township Rayburn Township Redbank Township Rural Valley Borough South Bend Township South Bethlehem Borough South Buffalo Township Sugarcreek Township Valley Township Armstrong County Hazard Mitigation Plan

7 Abandoned Mine noted as Municipality Carbonate Rock Abandoned Mine Subsidence Area Washington Township Wayne Township West Franklin Township West Kittanning Borough Worthington Borough Source: Pennsylvania Bureau of Topographic and Geologic Survey 2001; PADEP 2014 Data and Methodology Unlike the flood, wind, and earthquake hazards, no standard loss estimation models or methodologies exist for the subsidence and sinkhole hazard. To estimate the County s vulnerability, the portion of the region underlain by limestone bedrock is considered exposed to natural subsidence and sink holes. To determine the assets that are exposed to this hazard, available and appropriate bedrock geology spatial data generated by the Pennsylvania Bureau of Topographic and Geologic Survey were overlaid upon the assets. The limitations of this analysis are recognized and are only used to provide a general estimate. Over time, additional data will be collected to allow better analysis for this hazard. Available information reviewed and a preliminary assessment are provided in the sections below. Impact on Life, Health, and Safety To estimate the population exposed to the hazard, the approximate hazard area (limestone bedrock) was overlaid upon the 2010 U.S. Census population data. The Census blocks with their center (centroid) within the boundary were used to calculate the estimated population exposed to this hazard. Please note U.S. Census blocks do not align with the limestone bedrock polygon in the spatial data, and these estimates are for planning purposes only. Only two municipalities have populations exposed to the limestone; 30 people in Kiskiminetas Township (less than 1 percent of the total population) and 22 people in South Bend Township (approximately 1.9 percent of the total population) are exposed to the hazard area. Impact on General Building Stock As noted above, no standard loss estimation models exist for the subsidence and sinkhole hazard. In general, the built environment located on limestone is exposed to this hazard. In an attempt to estimate the general building stock potentially vulnerable to this hazard, the associated building replacement values (buildings and contents) were determined for the identified Census blocks within the approximate hazard area. The Countyprovided spatial layer for building structures was also used to determine the number of structures located within the hazard area. In Kiskiminetas Township, approximately six buildings with an associated replacement cost value of $3.3 million are exposed to the hazard area (less than 1 percent of the total building stock). In South Bend Township, approximately seven buildings with an associated replacement cost value of $2.9 million are exposed to the hazard area (approximately 1.3 percent of the total building count and approximately 2.5 percent of the total replacement cost value). Impact on Critical Facilities There are no critical facilities exposed to the subsidence and sinkhole hazard in Armstrong County. Impact on the Economy Subsidence and sinkholes can severely impact roads and infrastructure. As noted earlier, limestone formations underlie approximately 2.5 percent of the County. However, there are no major roadways in the County located Armstrong County Hazard Mitigation Plan

8 above limestone bedrock. It is not possible to estimate potential future economic losses caused by subsidence and sinkhole events at this time. Impact on the Environment Sinkholes can have negative effects on local groundwater. Groundwater in limestone and other similar carbonate rock formations can be easily polluted, because water moves readily from the earth s surface down through solution cavities and fractures, thus undergoing very little filtration. Sinkholes have the potential to cause damage to chemical infrastructure such as pipelines and facilities that store or transport hazardous materials. The result from a breach of one of these systems may result in a hazardous materials release and damage the environment. Contaminants such as sewage, fertilizers, herbicides, pesticides, or industrial products are of concern. Vegetation is usually damaged during abrupt subsidence events. However, regrowth takes place over time (PEMA 2013). Future Growth and Development Areas targeted for potential future growth and development in the next 5 to 10 years have been identified across the County at the municipal level and are described in Section 4.4 of this Plan. New development occurring within the identified hazard areas may be exposed to risks associated with the subsidence and sinkhole hazard. Effect of Climate Change on Vulnerability Climate change factors such as an extended growing season, higher temperatures, and the possibility of more intense, less frequent summer rainfall may lead to changes in water resource availability. Sinkholes are caused by 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 will cause sinkholes. These actions can also serve to accelerate the natural processes of bedrock degradation, which can have a direct impact on sinkhole creation. The climate of Pennsylvania is already changing and will continue to change over the course of this century. Since 1900, temperatures in the northeastern United States have increased an average of 1.5 F. The majority of this warming has occurred since In terms of winter temperatures, the northeastern United States has seen an increase in the average temperature by 4 F since 1970 (Northeast Climate Impacts Assessment [NECIA] 2007). The projection in the increase of average temperatures may lead to an increase in the frequency of droughts. Sinkhole activity intensifies in some karst areas and increases during periods of drought. With an increase in drought periods, the number of sinkholes can increase (Linares et al. 2016). The potential effects of climate change on Armstrong County s vulnerability to subsidence and sinkhole events will need to be considered as a greater understanding of regional climate change impacts develop. Armstrong County Hazard Mitigation Plan

The Richter Scale. Micro Less than 2.0 Microearthquakes, not felt. About 8,000/day

The Richter Scale. Micro Less than 2.0 Microearthquakes, not felt. About 8,000/day Geologic Hazards General s are very rare in Pennsylvania and have caused very little damage and no reported injuries or casualties. Since the Commonwealth is not on an active fault, the earthquakes that