Dam Failure Hazard Profile

Transcription

1 Appendix F Dam Failure Hazard Profile F-1

2 THIS PAGE INTENTIONALLY LEFT BLANK F-2

3 Dam Failure Location and Extent Dam failures most often occur during or after a massive rainfall, flooding, or spring thaws, sometimes with little to no warning. Depending on the size of the water body where the dam is constructed, water contributions may come from distant upstream locations. As shown in Figure , Cumberland County has 33 dams regulated by the PA Department of Environmental Protection (DEP). DEP s dataset does not distinguish type of ownership, but permit holders throughout Pennsylvania include DEP, PA Department of Conservation and Natural Resources, PA Game Commission, PA Fish and Boat Commission, PA Department of Public Welfare, PA Turnpike Commission, PA Department of Corrections, PA Historical and Museum Commission, local municipalities, local water authorities, private companies, and private citizens. Of the 33 dams in Cumberland County regulated by the DEP, classifications include one B-1, two C-1, one C-2, one B-3, two C-3, and 26 C-4 dams. Dam classification definitions are provided in Section Details are provided below for all but the C-4 dams. Opossum Lake Dam: This B-1 dam is located on a Commonwealth Fire Commission Lake in Lower Frankford Township, a sparsely populated community. This 59-acre lake was drained in late 2008 after routine inspections found cracks and leaks in the spillway. The PA Fish and Boat Commission (PFBC) rebuilt the spillway from May 2010 to February 2012 with local and state funds, and the lake was reopened on March 30, 2013 (PFBC, 2013). An Emergency Action Plan has been developed for this dam. Laurel Lake Dam: This C-1 dam owned by the Department of Conservation and Natural Resources is located in Pine Grove Furnace State Park in Cooke Township, a mountainous, seasonally populated area. An Emergency Action Plan and inundation maps have been developed for this dam. Oyler Lake Dam: This C-1 dam is located in Dickinson Township. An Emergency Action Plan and inundation maps have been developed for this dam. Wolfe Farm Dam: This C-2, privately-owned, agricultural dam is located in a sparsely populated fruit-production area of South Middleton Township. An Emergency Action Plan has been developed for this dam. Toland Quarry Dam: The B-3 Toland Quarry Dam was located in Dickinson Township and owned by Hempt Brothers. At the time of production of the 2014 HMP, the Toland Quarry impoundment was being filled. Children s Lake Dam: This C-3 dam is owned by the Pennsylvania Fish and Boat Commission and is located in South Middleton Township. There is no Emergency Action Plan currently on file as of the time of the 2014 HMP update. Carlisle Raw Water Intake Dam: This C-3 dam is located in North Middleton Township. An Emergency Action Plan has been developed for this dam. F-3

4 Figure : Locations of DEP-Regulated Dams in Cumberland County (2014). F-4

5 In addition to the dams listed above, there are two dams located outside of Cumberland County which have the potential to affect municipalities within the County. Roxbury Dam is located on the Conodoguinet Creek in Letterkenny Township in Franklin County and has an Emergency Action Plan in place. Raystown Lake Dam is owned by the USACE and is located in Huntington County upstream of the confluence of the Juniata River and the Susquehanna River. The USACE has a federal plan for this dam, but there has been minimal outreach to downstream communities within Cumberland County. Both of these dams would impact riverside communities in Cumberland County in the event of a dam break. While not available for all dams, downstream inundation maps can be obtained from the Pennsylvania Department of Environmental Protection Range of Magnitude Dam failures can pose a serious threat to communities located downstream from major dams. The impact of a dam failure is dependent on the volume of water impounded by the dam and the amount and distance of population or assets located downstream. Catastrophic failures are characterized by the sudden, rapid, and uncontrolled release of impounded water or any other fluid or semi-fluid from a dammed impoundment or water body. The DEP defines a high hazard dam as any dam so located as to endanger populated areas downstream by its failure [Def. added May 16, 1985, P.L.32, No. 15]. High hazard dams receive two inspections each year once by a professional engineer on behalf of the owner and once by a DEP inspector (PADEP, 2008). Dam failures may or may not leave enough time for evacuation of people and property, depending on their abruptness. Seepages in earth dams usually develop gradually, and, if the embankment damage is detected early, downstream residents have at least a few hours or days to evacuate. Failures of concrete or masonry dams tend to occur suddenly, sending a wall of water and debris down the valley at more than 100 miles per hour. Survival would be a matter of having the good fortune not to be in the flood path at the time of the break. Dam failures due to overtopping of a dam normally give sufficient lead time for evacuation. Dam failures may also be intentional, as their potential to cause serious destruction may make them a potential terrorism target. The environmental impacts of dam failures can be devastating. Depending on the size of the event and number or type of structures located in the inundation area, water contamination from hazardous material facilities could occur. Water velocities could result in total destruction of trees and other vegetation. Severe erosion both during and after the failure event are probable. Additionally, if the dam s purpose is water supply, downstream communities will lose access to potable water. Finally, all of the environmental impacts expected during a flood are possible during a dam failure (see Section ). The worst case scenario for dam failure in Cumberland County would occur at the dam with the highest hazard potential. The Opossum Lake Dam in Lower Frankford Township is classified as a B-1 dam, which indicates the potential for substantial loss of life and excessive economic loss (see Table and ). The 2012 reconstruction of the spillway at this dam is described in Section F-5

6 Past Occurrence There have been two significant dam failures in Pennsylvania. The worst dam failure to occur in the U.S. took place in Johnstown, PA in 1889 which claimed 2,209 lives. Another dam failure took place in Austin, PA (Potter County) in 1911 which claimed 78 lives. No significant dam failures have occurred in Cumberland County. According to PEMA, minor dam failures occur annually, but the impact of these events is minimal Future Occurrence Provided that adequate engineering and maintenance measures are in place, high hazard dam failures have a moderate risk of occurring in Cumberland County, according to the Risk Factor criteria (see Section 4.4.2). The DEP inventories and generally regulates all dams that meet one of the following criteria (PADEP, 2008): Impound water from a drainage area of greater than 100 acres; Have a maximum water depth greater than 15 feet; Have a maximum storage capacity of 50 acre-feet or greater. The construction, operation, maintenance, modification, and abandonment of dams regulated by the DEP is reviewed and monitored by the Department s Program of Dam Safety. Dams are evaluated based on categories such as slope stability, undermining seepage, and spillway adequacy. The presence of structural integrity and inspection programs significantly reduces the potential for major dam failure events to occur. Minor dam failures are more common since low hazard structures are minimally regulated, but the impact of these events is minimal. Dam Emergency Action Plans drafted in accordance with the Federal Guidelines for Dam Safety identify the risk related information including the inundation area and the time lapse between dam failure and flooding reaching specific destinations downstream. These plans are also reviewed and approved by PEMA. The County noted during the 2014 update that all Emergency Action Plans that were due for revision or update in Cumberland County were completed during this reporting period, and Silver Spring Township noted that they have developed a system to notify residents along the creek about dam failure and rising waters. Dams regulated by federal agencies are subject to the dam safety offices of the regulating agency. The FERC Office of Energy Projects Dam Safety and Inspections division conducts construction, operation, exemption, special, pre-license, and environmental and public use inspections of energy production dams to minimize risk associated with FERC dams. USACE dams are inspected and maintained by the district in which the dam is located. Overall, the probability of future dam failures can be considered unlikely according to the Risk Factor Methodology (See Table 4.4-1) Vulnerability Assessment The Pennsylvania Code classifies dams based on impoundment storage, dam height, loss of life, and economic loss (Table and ). Vulnerability is defined by identifying the location of dams having high hazard potential, as defined by The Pennsylvania Code ( F-6

7 Classification of dams and reservoirs). Specifically, category 1 dams were identified, indicating that the loss of life would be substantial or that economic loss would be excessive to extensive residential, commercial, or agricultural resources and would cause substantial public inconvenience. Notably, in 2011, the provisions for dam hazard potential classification changed; a fourth category of dam was added to capture instances where there might be damage to property but not loss of human life. Table : Dam Size Classification (The Pennsylvania Code). CLASS IMPOUNDMENT STORAGE (acre feet) DAM HEIGHT (feet) A Equal to or greater than 50,000 Equal to or greater than 100 B Less than 50,000 but greater than 1000 Less than 100 but greater than 40 C Equal to or less than 1000 Equal to or less than 40 Table : Dam Hazard Potential Classification- Extent of Development (The Pennsylvania Code, amended 2011). CATEGORY LOSS OF LIFE ECONOMIC LOSS Substantial (Numerous homes or small businesses or a large business or school) Few (A small number of homes or small businesses.) None expected (no permanent structures for human habitation or employment.) None expected (no permanent structures for human habitation or employment.) Excessive such as extensive residential, commercial, or agricultural damage, or substantial public inconvenience. Appreciable such as limited residential, commercial, or agricultural damage, or moderate public inconvenience. Significant damage to private or public property and short duration public inconvenience such as damage to storage facilities or loss of critical stream crossings. Minimal damage to private or public property and no significant public inconvenience. Flood inundation studies for dams are required as part of the development of Emergency Action Plans (EAPs). EAPs are required for category 1 and category 2 dams. According to DEP, dam permittees are responsible for having the dam breach analysis and inundation mapping done. Frequently, inundation areas are submitted as both printed and electronic images, but not as a GIS layer. The extent of downstream inundation areas vary based on dam and reservoir characteristics. The inundation map provided as Figure shows the area that is projected to be impacted by flooding due to a dam failure in Cumberland County, but inundation areas are only available for dams that are required to have an EAP. F-7

8 Figure : Inundation Areas of EAP Dams in Cumberland County (2014). F-8

9 The numbers of structures and critical facilities vulnerable to dam failures in Cumberland County are provided in Table , and the location of these structures by generalized land use type is provided in Table A total of 327 structures in Cumberland County are vulnerable to dam failures (0.3 percent), and zero of these are critical facilities. Of the 327 structures vulnerable to dam failures, 318 are residential. F-9

10 Table : Structures and Critical Facilities Vulnerable to Dam Failures for Cumberland County (Cumberland County GIS, 2014). Structures in Dam Inundation Percent of Structures in Dam Inundation Areas Total Critical Facilities in Dam Inundation Areas Percent of Critical Facilities in Dam Inundation Areas Municipality Total Structures in Municipality Areas Total Critical Facilities in Municipality Borough of Camp Hill 3, % % Borough of Carlisle 8, % % Township of Cooke % % Township of Dickinson 2, % % Township of East Pennsboro 8, % % Township of Hampden 12, % % Township of Hopewell % % Borough of Lemoyne 2, % % Township of Lower Allen 7, % % Township of Lower Frankford % % Township of Lower Mifflin % % Borough of Mechanicsburg 4, % % Township of Middlesex 2, % % Township of Monroe 2, % % Borough of Mount Holly Springs % % Borough of New Cumberland 3, % % Borough of Newburg % % Borough of Newville % % Township of North Middleton 4, % % Township of North Newton % % Township of Penn 1, % % Borough of Shippensburg 1, % % Township of Shippensburg 1, % % Borough of Shiremanstown % % Township of Silver Spring 6, % % Township of South Middleton 6, % % Township of South Newton % % Township of Southampton 2, % % Township of Upper Allen 7, % % Township of Upper Frankford % % Township of Upper Mifflin % % Township of West Pennsboro 2, % % Borough of Wormleysburg 1, % % TOTAL 100, % % F-10

11 Table : Structures Vulnerable to Dam Failure by Generalized Land Use Type (Cumberland County GIS, 2014). Agricultural Civic/ Institutional Commercial Emergency Services Total Structures in Municipality Municipality Borough of Camp Hill 3, Government Hospitals and Medical Centers Borough of Carlisle 8, Township of Cooke Township of Dickinson 2, Township of East Pennsboro 8, Township of Hampden 12, Township of Hopewell Borough of Lemoyne 2, Township of Lower Allen 7, Township of Lower Frankford Township of Lower Mifflin Borough of Mechanicsburg 4, Township of Middlesex 2, Township of Monroe 2, Borough of Mount Holly Springs Borough of New Cumberland 3, Borough of Newburg Borough of Newville Township of North Middleton 4, Township of North Newton Township of Penn 1, Borough of Shippensburg 1, Township of Shippensburg 1, Industrial Other/ Unspecified Religious Residential School Transportation Grand Total F-11

12 Table : Structures Vulnerable to Dam Failure by Generalized Land Use Type (Cumberland County GIS, 2014). Agricultural Civic/ Institutional Commercial Emergency Services Total Structures in Municipality Municipality Borough of Shiremanstown Government Hospitals and Medical Centers Township of Silver Spring 6, Township of South Middleton 6, Township of South Newton Township of Southampton 2, Township of Upper Allen 7, Township of Upper Frankford Township of Upper Mifflin Township of West Pennsboro 2, Borough of Wormleysburg 1, TOTAL 100, Industrial Other/ Unspecified Religious Residential School Transportation Grand Total F-12