4.3.17 Levee Failure - Bedford County, Pennsylvania · 2019-12-09 · SECTION 4.3.17: RISK ASSESSMENT – LEVEE FAILURE Bedford County Hazard Mitigation Plan 4.3.17-1 October 2017

SECTION 4.3.17: RISK ASSESSMENT – LEVEE FAILURE

Bedford County Hazard Mitigation Plan 4.3.17-1 October 2017

4.3.17 Levee Failure

Levees and flood walls are manmade structures designed to protect specific areas within a community from

flooding. These structures fail when flood waters exceed the height of the protective levee structure, or when

the maximum pressure exerted by the flood waters against the levee or flood wall exceeds its capability.

Levee failures, like dam failures, have the potential to place large numbers of people and great amounts of

property at risk. Unlike dams, levees are built parallel to a river or another body of water to protect the population

and structures behind it from risks to human health and property damage that could be caused by flooding events

(Federal Emergency Management Agency [FEMA] 2008). Levees do not serve a purpose beyond providing

flood protection and (less frequently) recreational space for community residents. Dams, on the other hand, can

serve to store water or generate energy, in addition to protecting areas from flooding.

Levee failures can be caused by a number of factors, and can be catastrophic. Damage to the area beyond a

failed levee could be more significant than damage caused by the uninhibited flow of flood water (FEMA 2008).

Levees are designed to provide a specific level of protection; therefore, excessive water from a flooding event

could overtop a levee if the water volume exceeds the levee specifications. Additionally, because levees can fail

if they are allowed to decay or deteriorate, regular maintenance is critical.

4.3.17.1 Location and Extent

Bedford County has two levee systems, one located in Everett Borough and one located in Hyndman Borough,

as shown below in Figures 4.3.17-1 and 4.3.17-2. The Everett Flood Protection Project was created in 1968 and

is owned and operated by Everett Borough (U.S. Army Corps of Engineers [USACE] 2016). According to the

National Levee Database (NLD), the Everett Borough levee is 0.96 mile in length (USACE NLD 2015).

The Hyndman Borough levee system consists of two levees: one on Wills Creek that is 0.34 mile long, and one

along a back channel that is 0.67 mile long. Both were created in 2008, but are not shown as providing flood

protection on the FIRM (USACE 2016).



Figure 4.3.17-1. Everett Borough Levee

Source: USACE NLD 2016

Note: Shading indicates the area protected by the levee.



Figure 4.3.17-2. Hyndman Borough Levee

Source: USACE NLD 2016

Note: Shading indicates the area protected by the levee.



4.3.17.2 Range of Magnitude

A levee failure or breach causes flooding in the developed land adjacent to the failed levee structure. The failure

of a levee or other flood protection structure could be devastating depending on the level of flooding for which

the structure is designed and the amount of land development present. Large volumes of water may be moving

at high velocities, potentially causing severe damage to buildings, infrastructure, trees, and other large objects.

The environmental impacts of a levee failure can include significant water-quality and debris-disposal issues.

Flood waters can back up sanitary sewer systems and inundate waste water treatment plants, causing raw sewage

to contaminate residential and commercial buildings and the flooded waterway. The contents of unsecured

containers of oil, fertilizers, pesticides, and other chemicals get added to flood waters. Hazardous materials may

be released and distributed widely across the floodplain. Water supply and waste water treatment facilities could

be off line for weeks. After the flood waters subside, contaminated and flood-damaged building materials and

contents must be properly disposed of. Contaminated sediment must be removed from buildings, yards, and

properties. In addition, severe erosion is likely; such erosion can negatively impact local ecosystems.

The effects of a levee failure are exacerbated when the failure occurs abruptly or with little warning and if it

results in deep, fast-moving water through highly developed areas. The worst-case scenario for a levee failure in

Bedford County would be the complete failure of the levee systems. If this occurred during a flood with a 1

percent annual chance of occurrence, the failure would lead to effects consistent with those described in Section

4.3.4 (Flood, Flash Flood, and Ice Jams).

4.3.17.3 Past Occurrence

There have been no known levee failures in Bedford County.

4.3.17.4 Future Occurrence

Similarly to dam failures, levee failures can occur at any time given certain circumstances. However, the

probability of future occurrence can be reduced through proper design, construction, and maintenance measures.

Most levees are designed to meet a specified level of flooding. While FEMA focuses on mapping levees that

will reduce the risk of a 1-percent annual chance flood, other levees may be designed to protect against smaller

or larger floods. FEMA design specifications provide information regarding the percent annual chance flood that

a levee structure is expected to withstand, assuming that the levee has been adequately constructed and

maintained. The probability of a levee failure in Bedford County cannot be determined, but based on the Risk

Factor Methodology Probability Criteria in Section 4.1, it is considered unlikely.

4.3.17.5 Vulnerability Assessment

To understand risk, a community must evaluate the assets exposed and/or vulnerable within the identified hazard

area. For the levee hazard, the area protected by the Everett Levee, as depicted on the FEMA DFIRM flood

maps, is examined. The following sections evaluate and estimate potential impact of flooding in Bedford County

presenting specifically:

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) the economy;

and (5) future growth and development

Effects of climate change on vulnerability



Overview of Vulnerability

The Hyndman Levee System is not reflected on the Digital Flood Insurance Rate Maps (DFIRM); therefore, the

areas, structures, and population vulnerable to the failure of the levees in this system cannot be determined at

this time. Failure of these levees during the 1-percent annual chance flood would result in flood waters reaching

the areas shown on the DFIRMs. The area protected by the Everett Levee System is reflected on the DFIRM

maps, and is used to estimate exposure to the dam failure hazard. The hazard area is located within Everett

Borough (refer to Figure 4.3.17-3).

Figure 4.3.17-3. Everett Borough Areas Protected by Levee



Data and Methodology

Data from the Digital Flood Insurance Rate Map (DFIRM) dated March 2012, including polygons representing

the area protected by the levee, were used to estimate exposure. These areas are located adjacent to the floodplain

and would otherwise be exposed to the flood hazard if the levee were not present.

Impact on Life, Health, and Safety

Impacts of levee failure on life, health, and safety depend on several factors including severity of the event,

protection level of the level, and whether or not adequate warning time is provided to residents. Assumedly, the

population living in or near floodplain areas and in the levee protection area could be impacted by a failure event.

To estimate the population exposed to the levee failure hazard, the FEMA DFIRM boundaries were overlaid

upon the 2010 U.S. Census population data in Geographic Information Systems (GIS) (U.S. Census 2010). The

U.S. Census blocks do not follow the boundaries of the DFIRM data. When utilizing the centroids or intersects

of the U.S. Census blocks with the levee failure hazard area, the population exposed may be grossly

overestimated or underestimated. The limitations of these analyses are recognized, and as such the results are

used only to provide a general estimate. For more information on the impact of life, health, and safety, refer to

Section 4.3.4 (Flood, Flash Flood, and Ice Jam). According to the analysis, approximately 410 people (22.4%

of total borough population) in Everett Borough are located within the levee-protected area.

Impact on General Building Stock

After consideration of the population exposed, the built environment was evaluated. Similar to the population,

the building stock data are presented by U.S. Census block. To estimate the value of building stock exposed to

the levee failure hazard, the FEMA DFIRM boundaries were overlaid upon the HAZUS-MH building stock data

in GIS. Using the default general building stock, the replacement cost values of the Census blocks with their

centroids in the area were totaled. Approximately $139 million worth of buildings and their contents are exposed

to the hazard area in Everett Borough. This represents approximately 31.7 percent of the borough’s total general

building stock replacement value inventory ($439 million). As described above, the U.S. Census blocks do not

follow DFIRM boundaries and these estimates should only be used for planning purposes.

To estimate the number of structures exposed to the FEMA DFIRM floodplain boundary, Bedford County’s

spatial layer of structures was overlaid by the 1-percent flood event boundary. In total, 266 structures, or 21.8

percent of the Everett Borough’s building stock, are located in the levee protected area.

Impact on Critical Facilities

In addition to considering general building stock at risk, the hazard risk for critical facilities, utilities, and user-

defined facilities was evaluated. There are three critical facilities in Everett Borough located within the hazard

area. The facilities include one fire station, one police station, and a wastewater pump station.

Impact on the Economy

For more information regarding the impact of levee failure and flooding on the economy, refer to Section 4.3.4

(Flood, Flash Flood, and Ice Jams).

Impact on the Environment

Similar to dam failure events, the environmental impacts of a levee failure event result in significant water quality

and debris disposal issues. Flood waters can affect sanitary sewer systems by inundating wastewater treatment

plants and causing raw sewage to flow from the sewer system and contaminate residential and commercial

properties. Oil, fertilizers, pesticides, and other chemicals are at risk of polluting the waterway and surrounding

areas if they are not located in a secure location. It could take weeks to regain adequate water supply and



wastewater treatment capabilities; contaminated and flood-damaged building material and contents would also

need to be cleaned and disposed of once the floodwater subside. Subsequent contaminated soil would also need

to be removed (PA SHMP, 2013).

Future Growth and Development

As discussed in Section 2.4, areas targeted for future growth and development have been identified across the

Bedford County. Any areas of growth could be impacted by the flood hazard if within identified hazard

areas. The county intends to discourage development in vulnerable areas or to encourage higher regulatory

standards on the local level.

Effect of Climate Change on Vulnerability

Climate is defined not simply as average temperature and precipitation but also by the type, frequency, and

intensity of weather events. Both globally and at the local scale, climate change can alter the prevalence and

severity of extremes such as flood events. While predicting changes of flood events under a changing climate

is difficult, understanding vulnerabilities to potential changes is a critical part of estimating future climate change

impacts on human health, society, and the environment (U.S. Environmental Protection Agency [EPA] 2006).

Pennsylvania’s Department of Environmental Protection (PADEP) was directed by the Climate Change Act (Act

70 of 2008) to initiate a study of potential impacts of global climate change on the Commonwealth. The June

2009 Pennsylvania Climate Impact Assessment’s main findings indicate that Pennsylvania is very likely to

undergo increased temperatures in the 21st century. An increase in variability of temperature and precipitation

may lead to increased frequency and/or severity of storm events. Summer floods and general stream flow

variability are projected to increase due to increased precipitation. Even with the anticipated increase in winter

precipitation occurring as rain rather than snow, increased winter temperatures and a reduced snowpack may

decrease rain-on-snow events and thus affect major flooding events in Pennsylvania. This conclusion, however,

remains speculative until further studies can validate it. Future improvements in modeling smaller-scale climatic

processes are expected, and will lead to improved understanding of how the changing climate will alter

temperature, precipitation, storms, and flood events in Pennsylvania (Shortle et al. 2009).

Documents

4.3.17 Levee Failure - Bedford County, Pennsylvania · 2019-12-09 · SECTION 4.3.17: RISK ASSESSMENT – LEVEE FAILURE Bedford County Hazard Mitigation Plan 4.3.17-1 October 2017