4.3.7 Landslide - Bedford County, Pennsylvania...This section provides a profile and vulnerability...

SECTION 4.3.7: RISK ASSESSMENT – LANDSLIDE

Bedford County Hazard Mitigation Plan 4.3.7-1 October 2017

4.3.7 Landslide

This section provides a profile and vulnerability assessment of the landslide hazard. A landslide is described

in the Commonwealth of Pennsylvania 2013 Standard All-Hazard Mitigation Plan (PA HMP) as downward

and outward movement of slope-forming soil, rock, and vegetation reacting to the force of gravity. Materials

can move at speeds as high as 120 miles per hour (mph) or more; slides can last a few seconds or a few

minutes, or can be gradual, slower movements over several hours or days. Several different types of landslides

include:

Rock Fall involves detachment of mass from a steep slope or cliff, and descent by free fall, bounding,

or rolling.

Rock Topple involves tilt or rotation of a mass forward as a unit.

Slide involves displacement of a mass on one or more recognizable surfaces, which may be curved or

planar.

Flow involves movement of a mass downslope with a fluid motion. A significant amount of water

may or may not be part of the mass (PEMA 2013).

Landslides may be triggered by both natural and human-caused changes in the environment, including heavy

rain, rapid snow melt, steepening of slopes through construction or erosion, earthquakes, and changes in

groundwater levels. Areas generally prone to landslide hazards include previous landslide areas, bases of steep

slopes, bases of drainage channels, developed hillsides, and areas recently burned by forest and brush fires

(Delano and Wilshusen 2001). Human activities that contribute to slope failure include alteration of the

natural slope gradient, increase of soil water content, and removal of vegetation cover.

4.3.7.1 Location and Extent

According to the 2013 PA HMP, landslides have occurred in many parts of Pennsylvania but are most

abundant and troublesome in much of the western and north-central portions of the State and adjacent states.

Rockfalls and other slope failures can occur in areas of Bedford County at locations of moderate to steep

slopes. Areas undergoing erosion, decline in vegetation cover, and earthquakes are also susceptible to

landslides. Figure 4.3.7-1 shows areas of low, moderate, and high landslide susceptibility as identified by the

U.S. Geological Survey (USGS). Most of Bedford County ranks as having high susceptibility and a moderate

number of incidents, while a swath along the western part of the County falls within the Low Incidence zone

with less than 1.5 percent of the area involved in landsliding. Figure 4.3.7-2 shows areas in Bedford County

on 25% or greater slopes, as generated by the Natural Lands Trust (2010). Almost every municipality in the

County has at least some slopes with 25% or greater steepness.



Figure 4.3.7-1. U.S. Geological Survey. Landslide Incidence and Susceptibility.

Source: PEMA 2013

Note: Highlight added.



Figure 4.3.7-2. Natural Lands Trust. Slopes Greater than 25%.

Source: Natural Lands Trust 2010



4.3.7.2 Range of Magnitude

Landslides damage transportation routes, utilities, and buildings. They can also create travel delays and other

side effects. Fortunately, deaths and injuries caused by landslides are rare in Pennsylvania, and most

landslides in the State are moderate to slow moving, damaging things rather than people. Almost all known

deaths caused by landslides have occurred when rockfalls or other slides along highways have involved

vehicles. Storm-induced debris flows are the only other type of landslide likely to cause death and injuries.

Hazards from these events will also increase as residential and recreational development increases on and near

steep mountain slopes.

The worst-case scenario for a landslide in Bedford County would be an event similar to one in Beaver County

in 1942 (PEMA 2013). In that event, 150 cubic yards of rock fell from a highway cut onto a bus. Twenty-two

people were killed and four others were injured. In Bedford County’s worst-case scenario, the landslide would

hit the Pennsylvania Turnpike, Route 30, or another busy highway. Depending on the time of day, and the

number of vehicles on the road at that time, this could trigger a severe traffic accident, resulting in multiple

fatalities. Closure of a major transportation route would affect commerce in the County, as well as the

Commonwealth, because the Pennsylvania Turnpike is a major east-west transportation corridor. This is the

worst-case scenario because it could exert the greatest impact on the County, surrounding counties, and the

Commonwealth. The most likely landslide would occur within an unpopulated area and likely would be

undetected.

4.3.7.3 Past Occurrence

Outside of impacts on important transportation routes, the history of landslides is not documented as

completely (if at all) as other hazards, primarily because landslides are not always seen, and therefore

historical landslide occurrences in Bedford County are not well known. The National Climatic Data Center

does not have any records of landslides in the County (NOAA-NCDC 2016). No deaths, serious injury, or

property damages have been reported from landslides in local records.

Pennsylvania has no history of federally declared disasters as a result of landslides. One federally declared

disaster included mudslides, in June 2006. Bedford County was not included in that declaration. PEMA also

notes only one disaster incident including mudslides, in April 2005, which did not include Bedford County.

This event was eligible for individual assistance, public assistance, and hazard mitigation.

4.3.7.4 Future Occurrence

Mismanaged, intense development within steeply sloped areas could increase frequency of landslides in

Bedford County. Often, building and road construction contributes to potential for landslides by undermining

or steepening otherwise stable soil.

Current landslide events occur within steeply sloped areas that do not feature extensive land development or

many structures. However, increased deforestation, timbering, and soil disturbances caused by development

within sloped areas significantly increases risk of landslides.

Based on available historical data, future occurrence of landslides can be considered unlikely as defined by the

Risk Factor Methodology probability criteria (refer to Section 4.4).

4.3.7.5 Vulnerability Assessment

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

The following section discusses potential impacts of the landslide hazard on Bedford County, including:

Overview of vulnerability

Data and methodology used for the evaluation

Impacts on (1) life, (2) health and safety, (3) general building stock, (4) critical facilities and

economy, and (5) future growth and development



Effect of climate change on vulnerability

Additional data and next steps.

Overview of Vulnerability

Vulnerability to ground failure hazards is a function of location, soil type, geology, type of human activity,

use, and frequency of events. Effects of landslides on people and structures can be reduced by total avoidance

of hazard areas or by restricting, prohibiting, or imposing conditions on hazard-zone activity. Local

governments can reduce effects of landslides through land use policies and regulations. Individuals can reduce

their exposure to hazards by educating themselves on the past hazard history of the site and by inquiring about

hazards to planning and engineering departments of local governments (National Atlas 2007).

Overall, most of the County is exposed to the landslide hazard, with the most vulnerable portion of the County

located within the high-susceptibility/moderate-incidence hazard area; approximately 75.5% of the County is

within this hazard area (refer to Figure 4.3.7-1 earlier in this section). East St. Clair Township, Harrison

Township, Kimmel Township, King Township, Manns Choice Borough, New Paris Borough, Pleasantville

borough, Schellsburg Borough, and St. Clairsville Borough are completely within the low incidence hazard

area and are not susceptible to the landslide hazard. Because of widespread cover of high

susceptibility/moderate incidence, areas on slopes of 25% or greater were determined at greater risk from the

landslide hazard. Further information regarding these hazard areas is described below.

Data and Methodology

Unlike the flood, wind, and earthquake hazards, no standard loss estimation models for the landslide hazard are

available. In an attempt to estimate Bedford County’s vulnerability, the Geology – Landslide Incidence and

Susceptibility geographic information system (GIS) layer from the National Atlas was used to coarsely define

the general landslide-susceptible area (“approximate hazard area”) (Figure 4.3.7-1). Limitations of this

analysis are recognized, and results of it are used only to provide a general estimate. Over time, additional

data will be acquired to allow better analysis of this hazard. Available information and a preliminary

assessment appear below.

According to Radbruch-Hall and others, the Landslide Incidence and Susceptibility GIS layer from National

Atlas:

“….was prepared by evaluating formations or groups of formations shown on the geologic

map of the United States (King and Beikman, 1974) and classifying them as having high,

medium, or low landslide incidence (number of landslides) and being of high, medium, or

low susceptibility to landsliding. Thus, those map units or parts of units with more than

15 percent of their area involved in landsliding were classified as having high incidence;

those with 1.5 to 15 percent of their area involved in landsliding, as having medium

incidence; and those with less than 1.5 percent of their area involved, as having low

incidence. This classification scheme was modified where particular lithofacies are known to

have variable landslide incidence or susceptibility. In continental glaciated areas, additional

data were used to identify surficial deposits that are susceptible to slope movement.

Susceptibility to landsliding was defined as the probable degree of response of the areal rocks

and soils to natural or artificial cutting or loading of slopes or to anomalously high

precipitation. High, medium, and low susceptibility are delimited by the same percentages

used in classifying the incidence of landsliding. For example, it was estimated that a rock or

soil unit characterized by high landslide susceptibility would respond to widespread artificial

cutting by some movement in 15 percent or more of the affected area. We did not evaluate the

effect of earthquakes on slope stability, although many catastrophic landslides have been

generated by ground shaking during earthquakes. Areas susceptible to ground failure under static conditions would probably also be susceptible to failure during earthquakes.”

(Radbruch-Hall 1982).



The Natural Lands Trust generated a GIS layer displaying areas with slopes greater than 25% within the

Central Appalachian Forest Ecoregion using the U.S. Geological Survey’s National Elevation Dataset (Figure

4.3.7-2). This layer was used to determine the County’s vulnerability to steep slopes.

Impacts on Life, Health, and Safety

As discussed above, most of the County is within the high-susceptibility/moderate-incidence hazard area;

therefore the steep slopes hazard was used to estimate the population vulnerable to the hazard. Potential

landslide events can directly and indirectly damage the County’s population via impacts on buildings. To

estimate the population located on slopes greater than 25%, hazard area boundaries were overlaid upon

2010 Census population data (U.S. Census 2010). Census blocks with their centers (centroids) within the

boundaries of steep slope hazard areas were used to calculate the estimated population considered exposed to

this hazard. Because census blocks do not align exactly with hazard areas, these estimates should be

considered for planning purposes only. Table 4.3.7-1 summarizes the population exposed by municipality

(U.S. Census 2010). Populations downslope of landslide hazard areas are particularly vulnerable to this

hazard. Identifying populations vulnerable to mass movements of geological material by reference only to

census block data is difficult. But via this approach, 4,696 people, or 9.4% of the overall population, are

within areas with steep slopes.

Table 4.3.7-1 Estimated Bedford County Population Located in the Steep Slope Hazard Area

Municipality

Total

Population

Population in the

Hazard Area

Percent Population in

Hazard Area

Bedford Borough 2,841 18 <1%

Bedford Township 5,395 621 11.5%

Bloomfield Township 1,016 3 <1%

Broad Top Township 1,687 450 26.7%

Coaldale Borough 161 0 0.0%

Colerain Township 1,195 5 <1%

Cumberland Valley Township 1,597 247 15.5%

East Providence Township 1,854 110 5.9%

East St. Clair Township 3,042 122 4.0%

Everett Borough 1,832 0 0.0%

Harrison Township 978 123 12.6%

Hopewell Borough 230 27 11.7%

Hopewell Township 2,010 240 11.9%

Hyndman Borough 910 8 <1%

Juniata Township 954 137 14.4%

Kimmel Township 1,616 123 7.6%

King Township 1,238 0 0.0%

Liberty Township 1,418 275 19.4%

Lincoln Township 425 73 17.2%

Londonderry Township 1,856 745 40.1%

Mann Township 500 95 19.0%

Manns Choice Borough 294 0 0.0%

Monroe Township 1,336 74 5.5%

Napier Township 2,198 84 3.8%



Municipality

Total

Population

Population in the

Hazard Area

Percent Population in

Hazard Area

New Paris Borough 186 0 0.0%

Pavia Township 295 31 10.5%

Pleasantville Borough 198 0 0.0%

Rainsburg Borough 133 0 0.0%

Saxton Borough 686 0 0.0%

Schellsburg Borough 338 0 0.0%

Snake Spring Township 1,639 175 10.7%

South Woodbury Township 2,155 147 6.8%

Southampton Township 976 372 38.1%

St. Clairsville Borough 78 0 0.0%

West Providence Township 3,212 186 5.8%

West Saint Clair Township 1,736 172 9.9%

Woodbury Borough 284 0 0.0%

Woodbury Township 1,263 33 2.6%

Bedford County (Total) 49,762 4,696 9.4%

Sources: U.S. Census 2010; Natural Lands Trust 2010.

Impact on General Building Stock

As discussed above, most of the County is within the high-susceptibility/moderate-incidence area, and areas of

steep slope will be referenced to estimate vulnerability of the County’s general building stock. Direct building

losses are estimated costs to repair or replace damage caused to buildings.

Similar to the population, building stock data are presented by census block. To estimate the value of building

stock exposed to steep slopes, hazard area boundaries were overlaid upon HAZUS-MH building stock data in

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

centroids in hazard areas were totaled. Approximately $444 million of buildings/contents are on steep slopes

in Bedford County. This represents approximately 5.9% of the County’s total general building stock

replacement value inventory ($7.5 billion).

To estimate the number of structures exposed to the hazard boundary, the County’s spatial layer of structures

was overlaid by the steep slopes layer. In total, 1,040 structures, or 1.8% of building stock, would be exposed

to the hazard. Building stock exposures per municipality are listed in Table 4.3.7-2.

Table 4.3.7-2 Estimated General Building Stock Within the Steep Slope Hazard Area

Municipality

Total

Number of

Buildings

Total

Replacement

Cost Value

(RCV)

Number of

Buildings

% of

Total RCV

% of

Total

Bedford Borough 1,892 $646,059,000 6 <1% $3,532,000 <1%

Bedford Township 5,482 $1,064,751,000 36 <1% $97,957,000 9.2%

Bloomfield Township 1,053 $98,910,000 2 <1% $3,526,000 3.6%

Broad Top Township 1,989 $210,095,000 124 6.2% $38,051,000 18.1%

Coaldale Borough 101 $12,009,000 2 2.0% $0 0.0%



Municipality

Total

Number of

Buildings

Total

Replacement

Cost Value

(RCV)

Number of

Buildings

% of

Total RCV

% of

Total

Colerain Township 1,879 $124,874,000 3 <1% $908,000 <1%

Cumberland Valley

Township 2,167 $186,632,000 86 4.0% $21,501,000 11.5%

East Providence Township 2,599 $278,118,000 27 1.0% $8,833,000 3.2%

East St. Clair Township 3,216 $370,063,000 23 <1% $606,000 <1%

Everett Borough 1,222 $438,564,000 0 0.0% $0 0.0%

Harrison Township 1,664 $163,407,000 51 3.1% $25,726,000 15.7%

Hopewell Borough 164 $24,173,000 16 9.8% $1,649,000 6.8%

Hopewell Township 2,146 $222,875,000 39 1.8% $27,248,000 12.2%

Hyndman Borough 778 $117,166,000 2 <1% $0 0.0%

Juniata Township 1,979 $125,361,000 36 1.8% $17,074,000 13.6%

Kimmel Township 1,852 $207,126,000 14 <1% $15,957,000 7.7%

King Township 1,354 $128,234,000 6 <1% $225,000 <1%

Liberty Township 1,764 $190,571,000 44 2.5% $20,578,000 10.8%

Lincoln Township 462 $43,153,000 23 5.0% $8,813,000 20.4%

Londonderry Township 2,507 $197,714,000 115 4.6% $27,897,000 14.1%

Mann Township 1,125 $84,599,000 35 3.1% $4,425,000 5.2%

Manns Choice Borough 269 $32,878,000 11 4.1% $0 0.0%

Monroe Township 2,558 $164,383,000 63 2.5% $12,708,000 7.7%

Napier Township 3,539 $277,952,000 55 1.6% $14,122,000 5.1%

New Paris Borough 135 $21,772,000 0 0.0% $0 0.0%

Pavia Township 559 $46,739,000 57 10.2% $22,290,000 47.7%

Pleasantville Borough 170 $22,172,000 0 0.0% $0 0.0%

Rainsburg Borough 157 $14,504,000 0 0.0% $0 0.0%

Saxton Borough 504 $168,466,000 2 <1% $0 0.0%

Schellsburg Borough 266 $41,027,000 0 0.0% $0 0.0%

Snake Spring Township 1,768 $383,646,000 29 1.6% $3,808,000 1.0%

South Woodbury Township 2,245 $245,720,000 11 <1% $208,000 <1%

Southampton Township 1,932 $133,937,000 59 3.1% $29,097,000 21.7%

St. Clairsville Borough 73 $10,568,000 0 0.0% $0 0.0%

West Providence Township 3,696 $618,794,000 28 <1% $19,007,000 3.1%

West Saint Clair Township 1,790 $179,339,000 28 1.6% $17,970,000 10.0%

Woodbury Borough 238 $31,161,000 0 0.0% $0 0.0%

Woodbury Township 1,614 $198,967,000 7 <1% $0 0.0%

Bedford County (Total) 58,908 $7,526,479,000 1,040 1.8% $443,716,000 5.9%

Sources: HAZUS-MH 3.1, Natural Lands Trust 2010, Bedford County 2016.



Critical Facilities and the Economy

As discussed above, most of the County is within the high-susceptibility/moderate-incidence landslide hazard

area. As with impacts on population and general building stock of the County, the steep slope hazard area was

referenced to estimate vulnerabilities of critical facilities within the County. One hazmat facility in Kimmel

Twp, one municipal building in Pavia Twp, and two wastewater treatment plants, one in Napier Township and

one in South Woodbury Township, are on slopes of 25% or greater.

A landslide’s impact on the economy and estimated dollar losses are difficult to measure. As stated earlier,

landslides can impose direct and indirect impacts on society. Direct costs include actual damage sustained by

buildings, property, and infrastructure. Indirect costs, such as cleanup costs, business interruption, loss of tax

revenues, reduced property values, and loss of productivity, are difficult to measure. Additionally, ground

failure threatens transportation corridors, fuel and energy conduits, and communication lines (USGS 2003).

Losses to the County’s total building inventory replacement value would affect the local tax base and

economy.

Future Growth and Development

Areas targeted for potential future growth and development within the next 5 to 10 years have been identified

across Bedford County. Refer to Section 2.4 of this HMP. New development within identified landslide

hazard areas will be exposed to these risks.

Effect of Climate Change on Vulnerability

Climate is defined not just as average temperature and precipitation but also by type, frequency, and intensity

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

extremes such as severe storms, including those that may bring intense or prolonged precipitation (U.S.

Environmental Protection Agency [EPA] 2006). An increase in rainfall intensity and duration will saturate the

soil and potentially erode the local landscape and impair slope stability, leading to an increase of landslide

events in Bedford County.

While predicting changes in these types of 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 (EPA 2006). Potential effects of climate change on the County’s

vulnerability to landslide events must be considered as understanding of regional climate change impacts

increases.

Additional Data and Next Steps

More detailed landslide susceptibility zones can be generated so that communities can more specifically

identify high hazard areas. A pilot study conducted for Schenectady County, New York, as described in the

2014 New York State Hazard Mitigation Plan, developed higher resolution images of landslide susceptibility

zones. The methodology included use of the Natural Resource Conservation Services (NRCS) Digital Soil

Survey soil units and their associated properties, including the American Association of State Highway and

Transportation Officials (AASHTO) rating, liquid limit, hydrologic group, percentage of silt and clay, erosion

potential, and slope, derived from high-resolution digital elevation models. Determining historical damages to

buildings and infrastructure incurred from landslides will also help improve loss estimates and future modeling

efforts, given a margin of uncertainty. Furthermore, research on rainfall thresholds for forecasting landslide

potential may also be an option for Bedford County.

4.3.7 Landslide - Bedford County, Pennsylvania...This section provides a profile and vulnerability...

Documents

Transcript of 4.3.7 Landslide - Bedford County, Pennsylvania...This section provides a profile and vulnerability...