4.3.9 Subsidence/Sinkhole
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SECTION 4.3.9: RISK ASSESSMENT – SUBSIDENCE/SINKHOLE 4.3.9 Subsidence/Sinkhole This section provides a profile and vulnerability assessment for the subsidence/sinkhole hazard. According to the U.S. Geological Survey (USGS), “ground failure” is the term used to describe zones of ground cracking, fissuring, and localized horizontal and vertical permanent ground displacement that may be caused by surface rupture along faults; secondary movement on shallow faults; shaking-induced compaction of natural deposits in sedimentary basins and river valleys; liquefaction of loose, sandy sediment (USGS, 2005); landslides; and land subsidence and sinkholes. For the purpose of this HMP, the ground failure hazard to which the Lehigh Valley is vulnerable includes, but is not limited to, land subsidence or sinkholes, which are further defined as follows: 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 (USGS, 2007). Subsidence often occurs through the loss of subsurface support 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). Sinkholes, the type of subsidence most frequently seen in the Lehigh Valley, 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 rain water moving in fractures or cracks in the bedrock. This creates larger openings in the rock through which water and overlying soil materials will travel. Over time, the deposited soils compromise the strength of the bedrock, until it is unable to support the land surface above, and a collapse or sinkhole occurs. In this example the sinkhole occurs naturally, but 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 have a direct impact on sinkhole creation. Both natural and man-made sinkholes can occur without warning. 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, are all specific signs that a sinkhole is forming. They can form into steep-walled holes to 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. 4.3.9.1 Location and Extent Forty-seven of the 62 municipalities in Lehigh and Northampton counties (or about 76% of municipalities) are underlain entirely or in part by carbonate bedrock. The carbonate rock formations have developed karst landforms, resulting in significant land subsidence problems. These limestone and dolomite formations underlie the heart of the Lehigh Valley’s urban core, and soils produced from the weathering of carbonate bedrock also provide the area’s most fertile farmland. The bedrock itself serves as a source of raw material for the cement industry. The Saucon Valley of Lehigh County is one of the most common sinkhole locations throughout Pennsylvania. DMA 2000 Hazard Mitigation Plan Update – Lehigh Valley, Pennsylvania 4.3.9-1 March 2013 SECTION 4.3.9: RISK ASSESSMENT – SUBSIDENCE/SINKHOLE Figure 4.3.9-1 illustrates the geology across the Lehigh Valley. Figure 4.3.9-2 illustrates the areas of Pennsylvania subject to natural subsidence due to the presence of limestone bedrock. Locations of known subsidence and sinkhole events as well as cave locations are also shown. More specifically, Figure 4.3.9-3 shows the distribution of limestone in the Lehigh Valley and the areas vulnerable to subsidence. Figure 4.3.9-4 shows the areas of the region that are underlain by carbonate bedrock that are characterized by closed depressions, sinkholes and caves (karst features). DMA 2000 Hazard Mitigation Plan Update – Lehigh Valley, Pennsylvania 4.3.9-2 March 2013 SECTION 4.3.9: RISK ASSESSMENT – SUBSIDENCE/SINKHOLE Figure 4.3.9-1. Lehigh Valley Geology Source: LVPC, 2011; ESRI, 2009 DMA 2000 Hazard Mitigation Plan Update – Lehigh Valley, Pennsylvania 4.3.9-3 March 2013 SECTION 4.3.9: RISK ASSESSMENT – SUBSIDENCE/SINKHOLE Figure 4.3.9-2. Areas of Pennsylvania Subject to Natural Subsidence Due to the Presence of Limestone Bedrock Source: 2010 PA HMP (highlight added) DMA 2000 Hazard Mitigation Plan Update – Lehigh Valley, Pennsylvania 4.3.9-4 March 2013 SECTION 4.3.9: RISK ASSESSMENT – SUBSIDENCE/SINKHOLE Figure 4.3.9-3. Lehigh Valley Limestone Geology Source: LVPC, 2011; ESRI, 2009 DMA 2000 Hazard Mitigation Plan Update – Lehigh Valley, Pennsylvania 4.3.9-5 March 2013 SECTION 4.3.9: RISK ASSESSMENT – SUBSIDENCE/SINKHOLE Figure 4.3.9-4. Karst Features in the Lehigh Valley Source: PASDA, 2007 (Bureau of Topographic and Geologic Survey, Department of Conservation and Natural Resources) DMA 2000 Hazard Mitigation Plan Update – Lehigh Valley, Pennsylvania 4.3.9-6 March 2013 SECTION 4.3.9: RISK ASSESSMENT – SUBSIDENCE/SINKHOLE The following municipalities have identified near-surface limestone, and are therefore vulnerable to sinkholes: Lehigh County Northampton County Alburtis Borough Allen Township City of Allentown Bath Borough City of Bethlehem City of Bethlehem Catasauqua Borough Bethlehem Township Coplay Borough Bushkill Township Emmaus Borough East Allen Township Fountain Hill Borough City of Easton Hanover Township Forks Township Lower Macungie Township Freemansburg Borough Lower Milford Township Glendon Borough Macungie Borough Hanover Township North Whitehall Township Hellertown Borough Salisbury Township Lower Mt. Bethel Township South Whitehall Township Lower Nazareth Township Upper Macungie Township Lower Saucon Township Upper Milford Township Moore Township Upper Saucon Township Nazareth Borough Weisenberg Township Northampton Borough Whitehall Township North Catasauqua Borough Palmer Township Plainfield Township Portland Borough Stockertown Borough Tatamy Borough Upper Mt. Bethel Township Upper Nazareth Township West Easton Borough Williams Township Wilson Borough DMA 2000 Hazard Mitigation Plan Update – Lehigh Valley, Pennsylvania 4.3.9-7 March 2013 SECTION 4.3.9: RISK ASSESSMENT – SUBSIDENCE/SINKHOLE In summary, in Lehigh County, 19 of the 25 municipalities and approximately 80,399 acres are within the carbonate area. In Northampton County, 29 of 38 municipalities with approximately 87,516 acres are within the carbonate area for a total of 167,915 acres (262.4 square miles) in the Lehigh Valley. In Lehigh County, only five (5) of the 19 municipalities have less than 50% of their total acres in carbonate areas. These include Lower Milford Township, North Whitehall Township, Salisbury Township, Upper Milford Township and Weisenberg Township. Both Lower Milford Township and Weisenberg Township have less than 5% of their total acres in a carbonate area and, therefore, have a much lower hazard risk than the other municipalities. In Northampton County, only eight (8) of the 29 municipalities have less than 50% of their total acres underlain by carbonate bedrock. These include Allen Township, Bushkill Township, Lower Mt. Bethel Township, Lower Saucon Township, Moore Township, Plainfield Township, Upper Mt. Bethel Township, Williams Township, and the Borough of Hellertown. Of these, Bushkill Township, Moore Township, Plainfield Township and Upper Mt. Bethel Township have less than 5% of their total acres in a carbonate area and also have a much lower hazard risk than the other municipalities. For purposes of this plan, we have assumed that the higher the percentage of carbonate bedrock in a municipality, the higher the risk for sinkhole formation. While fewer karst features have been mapped in existing urban areas, human activity can often be the cause of a subsidence or sinkhole event. Furthermore, the lack of karst features exhibited in maps of urban areas is likely a result of development activities that disguise, cover, or fill existing features rather than an absence of the features themselves (PADCNR, 2003). 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. In some cases, construction, land grading or earthmoving activities that cause changes in stormwater flow can trigger sinkhole events. Subsidence or sinkhole events may occur in the presence of mining activity, especially in areas where the cover of a mine is thin, even in areas where bedrock is not necessarily conducive to their formation. Piggott and Eynon (1978) indicated that sinkhole development normally occurs where the interval to the ground surface is less than three to five times the thickness of the extracted seam and the maximum interval is up to ten times the thickness of the extracted seam. Sub-surface (i.e. underground) extraction