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2015 - Hazard Identification



North Carolina and its inland communities are vulnerable to a wide range of natural hazards that threaten life and property.  The hazards identified by the Mecklenburg County Hazard Mitigation Planning Team for inclusion in this risk assessment are those determined to be of actual potential threat to Mecklenburg County and its incorporated jurisdictions and are consistent with the potential natural hazards identified by the State of North Carolina and the Federal Emergency Management Agency for this part of the State and this region of the country.[1]  These hazards consist of the following:


Some of these hazards can be interrelated (for example, hurricane events can cause flooding and tornado activity), and thus discussion of these hazards may overlap where necessary throughout the risk assessment.  Also, some hazards consist of hazardous elements that are not listed separately above (for example, discussion of severe thunderstorms includes lightning and hail activity; discussion of hurricanes and tropical storms includes nor’easters and coastal erosion). 


44 CFR Requirement

Part 201.6(c)(2)(i): The risk assessment shall include a description of the type, location and extent of all natural hazards that can affect the jurisdiction.  The plan shall include information on previous occurrences of hazard events and on the probability of future hazard events.


This CFR requirement is met in the Hazard Identification and Hazard Analysis sections of this risk assessment.

This section provides a general description for each of the hazards listed above, including their damage-causing characteristics, written largely from a national perspective.[2]




Flooding is the most frequent and costly of all natural hazards in the United States, and has caused more than 10,000 deaths since 1900.  Approximately 90 percent of presidentially declared disasters result from flood-related natural hazard events.  Taken as a whole, more frequent, localized flooding problems that do not meet federal disaster declaration thresholds ultimately cause the majority of damages across the United States.



Flooding remains one of Charlotte-Mecklenburg’s most frequent and problematic natural hazards – often causing severe, repetitive property damages and posing significant threats to people unable to evacuate flood hazard areas as well as drivers of vehicles entering flooded roadways.  (Photo courtesy of Robert Lahser, Charlotte Observer)

Floods are generally the result of excessive precipitation, and can be characterized as follows: general floods, in which precipitation occurs over a given river basin for a long period of time; and flash floods, which are the product of heavy localized precipitation falling in a short time period over a given location.  The severity of a flood event is determined by the following factors: a combination of stream and river basin topography and physiography, hydrology, precipitation and weather patterns, recent soil moisture conditions, and the degree of vegetative clearing in and around flood-prone areas.

General floods may last for several days or even weeks.  The primary types of general flooding include riverine, coastal and urban flooding.  Riverine flooding is a function of excessive precipitation levels and water runoff volumes within a stream or river.  Coastal flooding is typically a result of storm surge, wind-driven waves, and heavy rainfall produced by hurricanes, tropical storms, nor’easters and other large coastal storms.  Urban flooding occurs where man-made development has obstructed the natural flow of water and decreased the ability of natural groundcover to absorb and retain surface water runoff.

Most flash flooding is caused by slow-moving thunderstorms in a localized area or by heavy rains associated with hurricanes and tropical storms.  Flash flooding can also occur due to accelerated snow melt, a dam or levee failure, or from a sudden release of water held by an ice jam.  Although flash flooding occurs often along mountain streams, it is also common in urbanized areas where much of the ground is covered by impervious surfaces.  Flash flood waters can move at very high speeds and “walls” of water have been known to reach heights of 10 to 20 feet.  Flash flood waters and the accompanying debris can uproot trees, roll boulders, destroy buildings, and obliterate bridges and roads.

The periodic flooding of lands including and adjacent to rivers, streams, and shorelines, referred to as the floodplain, is a natural and inevitable occurrence that can be expected to take place based upon established recurrence intervals.  The recurrence interval of a flood is defined as the average time interval, in years, expected between a flood event of a particular magnitude and an equal or larger flood.  As the magnitude of a hypothetical flood scenario increases the recurrence interval increases.  That is, the greater the magnitude of a given event, the less likely it will occur over time.

Floodplains are delineated by the frequency of the flood that is large enough to cover them.  For example, the 10-year floodplain will be covered by a 10-year flood (should it occur) and the 100-year floodplain by the 100-year flood.  Flood frequencies such as the 100-year flood are determined by plotting a graph of the size of all known floods for an area and determining how often floods of a particular size occur.  Another way of expressing the flood frequency is the chance of occurrence (expressed as a percent) in a given year of a flood event of a given magnitude.  For example, the 100-year flood has a 1 percent chance of occurring in any given year. 



Hurricanes and tropical storms, along with nor’easters and typhoons, are classified as cyclones and are any closed circulation developing around a low-pressure center in which the winds rotate counter-clockwise in the Northern Hemisphere (or clockwise in the Southern Hemisphere) and whose diameter averages 10 to 30 miles across.  A tropical cyclone refers to any such circulation that develops over tropical waters.  Tropical cyclones act as a “safety-valve,” limiting the continued build-up of heat and energy in tropical regions by maintaining the atmospheric heat and moisture balance between the tropics and the pole-ward latitudes.  The primary damaging forces associated with these storms are high-level sustained winds, heavy precipitation, and tornadoes.  Coastal areas are particularly vulnerable to storm surge, wind-driven waves, and tidal flooding which can prove more destructive than cyclone wind.



Although Hurricane Hugo made landfall nearly 200 miles away in Charleston, South Carolina, the storm crossed Mecklenburg County as a Category 2 hurricane causing significant wind-related damages across the area in September 1989. (Photo courtesy of NOAA)

The key energy source for a tropical cyclone is the release of latent heat from the condensation of warm water.  Their formation requires a low-pressure disturbance, warm sea surface temperature, rotational force from the spinning of the earth, and the absence of wind shear in the lowest 50,000 feet of the atmosphere.  The majority of hurricanes and tropical storms form in the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico during the official Atlantic hurricane season, which encompasses the months of June through November.  The peak of the Atlantic hurricane season is in early to mid-September.  Based on a long-term average, approximately six storms reach hurricane intensity per year.

Figure 4.1 shows, for any particular location, the chance of a hurricane or tropical storm affecting the area sometime during the Atlantic hurricane season.  The figure was created by the National Oceanic and Atmospheric Administration’s (NOAA) Hurricane Research Division, using data from 1944 to 1999.  The figure shows the number of times a storm or hurricane was located within approximately 100 miles (165 kilometers) of a given spot in the Atlantic basin.


Figure 4.1: Empirical Probability of a Named Hurricane or Tropical Storm


 Source: National Oceanic and Atmospheric Administration, Hurricane Research Division

As an incipient hurricane develops, barometric pressure (measured in millibars or inches) at its center falls and winds increase.  If the atmospheric and oceanic conditions are favorable, it can intensify into a tropical depression.  When maximum sustained winds reach or exceed 39 miles per hour, the system is designated a tropical storm, given a name, and is monitored by the National Hurricane Center in Miami, Florida.  When sustained winds reach or exceed 74 miles per hour the storm is deemed a hurricane.  Hurricane intensity is further classified by the Saffir-Simpson Scale which rates hurricane intensity on a scale of 1 to 5, with 5 being the most intense.  The Saffir-Simpson Scale is shown in Table 4.1.


Table 4.1: Saffir-Simpson Scale








Greater than 980















155 +

Less than 920


Source: National Hurricane Center

The Saffir-Simpson Scale categorizes hurricane intensity linearly based upon maximum sustained winds, barometric pressure, and storm surge potential, which are combined to estimate potential damage.  Categories 3, 4, and 5 are classified as “major” hurricanes, and while hurricanes within this range comprise only 20 percent of total tropical cyclones making landfall, they account for over 70 percent of the damage in the United States.  Table 4.2 describes the damage that could be expected for each hurricane category.


Table 4.2: Hurricane Damage Classifications








No real damage to building structures.  Damage primarily to unanchored mobile homes, shrubbery, and trees.  Also, some coastal flooding and minor pier damage.




Some roofing material, door, and window damage.  Considerable damage to vegetation, mobile homes, etc.  Flooding damages piers and small craft in unprotected moorings may break their moorings.




Some structural damage to small residences and utility buildings, with a minor amount of curtainwall failures.  Mobile homes are destroyed.  Flooding near the coast destroys smaller structures, with larger structures damaged by floating debris.  Terrain may be flooded well inland.




More extensive curtainwall failures with some complete roof structure failure on small residences.  Major erosion of beach areas.  Terrain may be flooded well inland.




Complete roof failure on many residences and industrial buildings.  Some complete building failures with small utility buildings blown over or away.  Flooding causes major damage to lower floors of all structures near the shoreline.  Massive evacuation of residential areas may be required.


Sources: National Hurricane Center and the Federal Emergency Management Agency

While not directly relevant to the planning area, storm surge is another common element of hurricane activity.  A storm surge is a large dome of water often 50 to 100 miles wide and rising anywhere from four to five feet in a Category 1 hurricane up to 20 feet in a Category 5 storm.  The storm surge arrives ahead of the storm’s actual landfall and the more intense the hurricane is, the sooner the surge arrives.  Water rise can be very rapid, posing a serious threat to those who have not yet evacuated flood-prone areas.  A storm surge is a wave that has outrun its generating source and become a long period swell.  The surge is always highest in the right-front quadrant of the direction in which the hurricane is moving.  As the storm approaches shore, the greatest storm surge will be to the north of the hurricane eye.  Such a surge of high water topped by waves driven by hurricane force winds can be devastating to coastal regions, causing severebeach erosion and property damage along the immediate coast.

Storm surge heights and associated waves are dependent upon the shape of the continental shelf (narrow or wide) and the depth of the ocean bottom (bathymetry).  A narrow shelf, or one that drops steeply from the shoreline and subsequently produces deep water close to the shoreline, tends to produce a lower surge but higher and more powerful storm waves.


Hurricane Floyd brought a devastating 15 feet of storm surge that damaged or destroyed hundreds of houses along the ocean front of Long Beach on Oak Island, North Carolina in September 1999. A prime example of successful hazard mitigation, the elevated home (right) survived while the older, ground-level block foundation of the home on the left was undermined and crushed. (Photo by Dave Gatley/FEMA News Photo)

Damage during hurricanes may also result from spawned tornadoes and inland flooding associated with heavy rainfall that usually accompanies these storms.  Hurricane Floyd, for example, was at one time a Category 4 hurricane racing towards the North Carolina coast.  As far inland as Raleigh, the state capital located more than 100 miles from the coast, communities were preparing for winds exceeding 100 miles per hour.  While Floyd made landfall as a Category 2 hurricane it caused the worst inland flooding disaster in North Carolina’s history.  Rainfall amounts exceeded 20 inches in certain locales and 67 counties sustained damages.

Similar to hurricanes, nor’easters are ocean storms capable of causing substantial damage to coastal areas in the Eastern United States due to their strong winds and heavy surf.  Nor'easters are named for the winds that blow in from the northeast and drive the storm up the East Coast along the Gulf Stream, a band of warm water that lies off the Atlantic coast.  They are caused by the interaction of the jet stream with horizontal temperature gradients and generally occur during the fall and winter months when moisture and cold air are plentiful.

Nor’easters are known for dumping heavy amounts of rain and snow, producing hurricane-force winds, and creating high surf that causes severe beach erosion and coastal flooding.  There are two main components to a nor'easter: (1) a Gulf Stream low-pressure system (counter-clockwise winds) generated off the southeastern U.S. coast, gathering warm air and moisture from the Atlantic, and pulled up the East Coast by strong northeasterly winds at the leading edge of the storm; and (2) an Arctic high-pressure system (clockwise winds) which meets the low-pressure system with cold, arctic air blowing down from Canada.  When the two systems collide, the moisture and cold air produce a mix of precipitation and have the potential for creating dangerously high winds and heavy seas.  As the low-pressure system deepens, the intensity of the winds and waves increase and can cause serious damage to coastal areas as the storm moves northeast. [3]




According to the National Weather Service, more than 100,000 thunderstorms occur each year, though only about 10 percent of these storms are classified as “severe.”  Although thunderstorms generally affect a small area when they occur, they are very dangerous because of their ability to generate tornadoes, hailstorms, strong winds, flash flooding, and damaging lightning.  While thunderstorms can occur in all regions of the United States, they are most common in the central and southern states because atmospheric conditions in those regions are most ideal for generating these powerful storms.


Multiple cloud-to-ground and cloud-to-cloud lightning strokes observed during a nighttime thunderstorm. (Photo courtesy of NOAA Photo Library, NOAA Central Library; OAR/ERL/ National Severe Storms Laboratory)

Thunderstorms are caused when air masses of varying temperatures meet.  Rapidly rising warm moist air serves as the “engine” for thunderstorms.  These storms can occur singularly, in lines, or in clusters.  They can move through an area very quickly or linger for several hours.

Lightning is a discharge of electrical energy resulting from the buildup of positive and negative charges within a thunderstorm, creating a “bolt” when the buildup of charges becomes strong enough.  This flash of light usually occurs within the clouds or between the clouds and the ground.  A bolt of lightning can reach temperatures approaching 50,000 degrees Fahrenheit.  Lightning rapidly heats the sky as it flashes but the surrounding air cools following the bolt.  This rapid heating and cooling of the surrounding air causes thunder.  On average, 89 people are killed each year by lightning strikes in the United States.

The National Weather Service collected data for thunder days, number and duration of thunder events, and lightening strike density for the 30-year period from 1948 to 1977.  A series of maps was generated showing the annual average thunder event duration, the annual average number of thunder events, and the mean annual density of lightning strikes.

Figure4.2 illustrates thunderstorm hazard severity based on the annual average number of thunder events from 1948 to 1977. 


Figure 4.2: Annual Average Number of Thunder Events


Source: Federal Emergency Management Agency

Straight-line winds, which in extreme cases have the potential to cause wind gusts that exceed 100 miles per hour, are responsible for most thunderstorm wind damage.  One type of straight-line wind, the downburst, can cause damage equivalent to a strong tornado and can be extremely dangerous to aviation.  Figure 4.3 shows how the frequency and strength of extreme windstorms vary across the United States.  The map was produced by the Federal Emergency Management Agency (FEMA) and is based on 40 years of tornado history and over 100 years of hurricane history.  Zone IV, the darkest area on the map, has experienced both the greatest number of tornadoes and the strongest tornadoes.  As shown by the map key, wind speeds in Zone IV can be as high as 250 MPH. 


Figure 4.3: Wind Zones in the United States


Source: Federal Emergency Management Agency


Large hail collects on streets and grass during a severe thunderstorm. Larger stones appear to be nearly two to three inches in diameter. (NOAA Photo Library, NOAA Central Library; OAR/ERL/National Severe Storms Laboratory)

Hailstorms are another potential damaging outgrowth of severe thunderstorms.  Early in the developmental stages of a hailstorm, ice crystals form within a low-pressure front due to the rapid rising of warm air into the upper atmosphere and the subsequent cooling of the air mass.  Frozen droplets gradually accumulate on the ice crystals until, having developed sufficient weight, they fall as precipitation—as balls or irregularly shaped masses of ice greater than 0.75 in. (1.91 cm) in diameter.  The size of hailstones is a direct function of the size and severity of the storm.  High velocity updraft winds are required to keep hail in suspension in thunderclouds.  The strength of the updraft is a function of the intensity of heating at the Earth’s surface.  Higher temperature gradients relative to elevation above the surface result in increased suspension time and hailstone size.  Figure 4.4 shows the annual frequency of hailstorms in the United States.


Figure 4.4: Annual Frequency of Hailstorms in the United States


Source: Federal Emergency Management Agency



A tornado is a violent windstorm characterized by a twisting, funnel-shaped cloud extending to the ground.  Tornadoes are most often generated by thunderstorm activity (but sometimes result from hurricanes and tropical storms) when cool, dry air intersects and overrides a layer of warm, moist air forcing the warm air to rise rapidly.  The damage caused by a tornado is a result of the high wind velocity and wind-blown debris, also accompanied by lightning or large hail.  According to the National Weather Service, tornado wind speeds normally range from 40 to more than 300 miles per hour.  The most violent tornadoes have rotating winds of 250 miles per hour or more and are capable of causing extreme destruction and turning normally harmless objects into deadly missiles.


The most comprehensively observed tornado in history, this tornado south of Dimmitt, Texas developed June 2, 1995 curving northward across Texas Highway 86 where it entirely removed 300 feet of asphalt from the road tossing it more than 600 feet into an adjacent field. It also caused F4 damage at an isolated rural residence just north of the road. (NOAA Photo Library, NOAA Central Library; OAR/ERL/National Severe Storms Laboratory)

Each year, an average of over 800 tornadoes is reported nationwide, resulting in an average of 80 deaths and 1,500 injuries (NOAA, 2002).  They are more likely to occur during the spring and early summer months of March through June and can occur at any time of day, but are likely to form in the late afternoon and early evening.  Most tornadoes are a few dozen yards wide and touch down briefly, but even small short-lived tornadoes can inflict tremendous damage.  Highly destructive tornadoes may carve out a path over a mile wide and several miles long.

Waterspouts are weak tornadoes that form over warm water and are most common along the Gulf Coast and southeastern states.  Waterspouts occasionally move inland, becoming tornadoes that cause damage and injury.  However, most waterspouts dissipate over the open water causing threats only to marine and boating interests.  Typically a waterspout is weak and short-lived, and because they are so common, most go unreported unless they cause damage.

The destruction caused by tornadoes ranges from light to inconceivable depending on the intensity, size, and duration of the storm.  Typically, tornadoes cause the greatest damages to structures of light construction such as residential homes (particularly mobile homes), and tend to remain localized in impact.    Table 4.3 shows the Enhanced Fujita Scale for Tornadoes which was developed to measure tornado strength and associated damages.


Table 4.3: Enhanced Fujita Scale for Tornadoes




3 Second Gust (mph)

Description of Damages





Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted trees; damages to sign boards.





The lower limit is the beginning of hurricane wind speed; peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos pushed off the roads; attached garages might be destroyed.





Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees snapped or uprooted; light object missiles generated.





Roof and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted.





Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown and large missiles generated.





Strong frame houses lifted off foundations and carried considerable distances to disintegrate; automobile sized missiles fly through the air in excess of 100 meters; trees debarked; steel re-enforced concrete structures badly damaged.


Source: NOAA, FEMA


According to the NOAA Storm Prediction Center (SPC), the highest concentration of tornadoes in the United States has been in Oklahoma, Texas, Kansas and Florida respectively.  Although the Great Plains region of the Central United States does favor the development of the largest and most dangerous tornadoes (earning the designation of “tornado alley”), Florida experiences the greatest number of tornadoes per square mile of all U.S. states (SPC, 2002).  Figure 4.5 shows tornado activity in the United States based on the number of recorded tornadoes per 1,000 square miles.


Figure 4.5: Tornado Activity in the United States


Source: American Society of Civil Engineers

The tornadoes associated with tropical cyclones are most frequent in September and October when the incidence of tropical storm systems is greatest.  This type of tornado usually occurs around the perimeter of the storm, and most often to the right and ahead of the storm path or the storm center as it comes ashore.  These tornadoes commonly occur as part of large outbreaks and generally move in an easterly direction.



A winter storm can range from a moderate snow over a period of a few hours to blizzard conditions with blinding wind-driven snow that lasts for several days.  Some winter storms may be large enough to affect several states, while others may affect only a single community.  Many winter storms are accompanied by low temperatures and heavy and/or blowing snow, which can severely impair visibility.


A heavy layer of ice was more weight than this tree in Kansas City, Missouri could withstand during a January 2002 ice storm that swept through the region bringing down trees, power lines and telephone lines. (Photo by Heather Oliver/FEMA News Photo)

Winter storms may include snow, sleet, freezing rain, or a mix of these wintry forms of precipitation.  Sleet—raindrops that freeze into ice pellets before reaching the ground—usually bounce when hitting a surface and do not stick to objects; however, sleet can accumulate like snow and cause a hazard to motorists.  Freezing rain is rain that falls onto a surface with a temperature below freezing, forming a glaze of ice.  Even small accumulations of ice can cause a significant hazard, especially on power lines and trees.  An ice storm occurs when freezing rain falls and freezes immediately upon impact.  Communications and power can be disrupted for days, and even small accumulations of ice may cause extreme hazards to motorists and pedestrians.

A freeze is weather marked by low temperatures, especially when below the freezing point (zero degrees Celsius or thirty-two degrees Fahrenheit).  Agricultural production is seriously affected when temperatures remain below the freezing point.



An earthquake is the motion or trembling of the ground produced by sudden displacement of rock in the Earth's crust.  Earthquakes result from crustal strain, volcanism, landslides or the collapse of caverns.  Earthquakes can affect hundreds of thousands of square miles; cause damage to property measured in the tens of billions of dollars; result in loss of life and injury to hundreds of thousands of persons; and disrupt the social and economic functioning of the affected area.



Many roads, including bridges and elevated highways, were damaged by the 6.7 magnitude earthquake that impacted the Northridge, California area January 17, 1994. Approximately 114,000 structures were damaged and 72 deaths were attributed to the event. Damage costs were estimated at $25 billion. (FEMA News Photo)

Most property damage and earthquake-related deaths are caused by the failure and collapse of structures due to ground shaking.  The level of damage depends upon the amplitude and duration of the shaking, which are directly related to the earthquake size, distance from the fault, site and regional geology.  Other damaging earthquake effects include landslides, the down-slope movement of soil and rock (mountain regions and along hillsides), and liquefaction, in which ground soil loses the ability to resist shear and flows much like quick sand.  In the case of liquefaction, anything relying on the substrata for support can shift, tilt, rupture or collapse.

Most earthquakes are caused by the release of stresses accumulated as a result of the rupture of rocks along opposing fault planes in the Earth’s outer crust.  These fault planes are typically found along borders of the Earth's 10 tectonic plates.  These plate borders generally follow the outlines of the continents, with the North American plate following the continental border with the Pacific Ocean in the west, but following the mid-Atlantic trench in the east.  As earthquakes occurring in the mid-Atlantic trench usually pose little danger to humans, the greatest earthquake threat in North America is along the Pacific Coast.

The areas of greatest tectonic instability occur at the perimeters of the slowly moving plates, as these locations are subjected to the greatest strains from plates traveling in opposite directions and at different speeds.  Deformation along plate boundaries causes strain in the rock and the consequent buildup of stored energy.  When the built-up stress exceeds the rocks' strength, a rupture occurs.  The rock on both sides of the fracture is snapped, releasing the stored energy and producing seismic waves, generating an earthquake.

Earthquakes are measured in terms of their magnitude and intensity.  Magnitude is measured using the Richter Scale, an open-ended logarithmic scale that describes the energy release of an earthquake through a measure of shock wave amplitude (see Table 4.4).  Each unit increase in magnitude on the Richter Scale corresponds to a 10-fold increase in wave amplitude, or a 32-fold increase in energy.  Intensity is most commonly measured using the Modified Mercalli Intensity (MMI) Scale based on direct and indirect measurements of seismic effects.  The scale levels are typically described using roman numerals, with a I corresponding to imperceptible (instrumental) events, IV corresponding to moderate (felt by people awake), to XII for catastrophic (total destruction).  A detailed description of the Modified Mercalli Intensity Scale of earthquake intensity and its correspondence to the Richter Scale is given in Table 4.5.


Table 4.4: Richter Scale



Less than 3.5

Generally not felt, but recorded.


Often felt, but rarely causes damage.

Under 6.0

At most slight damage to well-designed buildings.  Can cause major damage to poorly constructed buildings over small regions.


Can be destructive in areas up to about 100 kilometers across where people live.


Major earthquake. Can cause serious damage over larger areas.

8 or greater

Great earthquake. Can cause serious damage in areas several hundred kilometers across.


Source: North Carolina Division of Emergency Management


Table 4.5: Modified Mercalli Intensity Scale for Earthquakes







Detected only on seismographs




Some people feel it




Felt by people resting; like a truck rumbling by




Felt by people walking



Slightly Strong

Sleepers awake; church bells ring




Trees sway; suspended objects swing, objects fall off shelves



Very Strong

Mild Alarm; walls crack; plaster falls




Moving cars uncontrollable; masonry fractures, poorly constructed buildings damaged




Some houses collapse; ground cracks; pipes break open




Ground cracks profusely; many buildings destroyed; liquefaction and landslides widespread



Very Disastrous

Most buildings and bridges collapse; roads, railways, pipes and cables destroyed; general triggering of other hazards




Total destruction; trees fall; ground rises and falls in waves


Source: North Carolina Division of Emergency Management

Figure 4.6 shows the probability that ground motion will reach a certain level during an earthquake.  The data show peak horizontal ground acceleration (the fastest measured change in speed, for a particle at ground level that is moving horizontally due to an earthquake) with a 10 percent probability of exceedance in 50 years.  The map was compiled by the U.S. Geological Survey (USGS) Geologic Hazards Team, which conducts global investigations of earthquake, geomagnetic, and landslide hazards.


Figure 4.6: Peak Acceleration with 10 Percent Probability of Exceedance in 50 Years


Source: United States Geological Survey



A landslide is the downward and outward movement of slope-forming soil, rock, and vegetation, which is driven by gravity.  Landslides may be triggered by both natural and human-caused changes in the environment, including heavy rain, rapid snow melt, steepening of slopes due to construction or erosion, earthquakes, volcanic eruptions, and changes in groundwater levels.



Landslides can damage or destroy roads, railroads, pipelines, electrical and telephone lines, mines, oil wells, buildings, canals, sewers, bridges, dams, seaports, airports, forests, parks, and farms. (Photo by Lynn Forman)

There are several types of landslides: rock falls, rock topple, slides, and flows.  Rock falls are rapid movements of bedrock, which result in bouncing or rolling.  A topple is a section or block of rock that rotates or tilts before falling to the slope below.  Slides are movements of soil or rock along a distinct surface of rupture, which separates the slide material from the more stable underlying material.  Mudflows, sometimes referred to as mudslides, mudflows, lahars or debris avalanches, are fast-moving rivers of rock, earth, and other debris saturated with water. They develop when water rapidly accumulates in the ground, such as heavy rainfall or rapid snowmelt, changing the soil into a flowing river of mud or "slurry."  Slurry can flow rapidly down slopes or through channels, and can strike with little or no warning at avalanche speeds.  Slurry can travel several miles from its source, growing in size as it picks up trees, cars, and other materials along the way.  As the flows reach flatter ground, the mudflow spreads over a broad area where it can accumulate in thick deposits.

Landslides are typically associated with periods of heavy rainfall or rapid snow melt and tend to worsen the effects of flooding that often accompanies these events.  In areas burned by forest and brush fires, a lower threshold of precipitation may initiate landslides.  Some landslides move slowly and cause damage gradually, whereas others move so rapidly that they can destroy property and take lives suddenly and unexpectedly.  Among the most destructive types of debris flows are those that accompany volcanic eruptions.  A spectacular example in the United States was a massive debris flow resulting from the 1980 eruptions of Mount St. Helens, Washington.  Areas near the bases of many volcanoes in the Cascade Mountain Range of California, Oregon and Washington are at risk from the same types of flows during future volcanic eruptions.

Areas that are generally prone to landslide hazards include previous landslide areas; the bases of steep slopes; the bases of drainage channels; and developed hillsides where leach-field septic systems are used.  Areas that are typically considered safe from landslides include areas that have not moved in the past; relatively flat-lying areas away from sudden changes in slope; and areas at the top or along ridges, set back from the tops of slopes.

In the United States, it is estimated that landslides cause up to $2 billion in damages and from 25 to 50 deaths annually.  Globally, landslides cause billions of dollars in damage and thousands of deaths and injuries each year.  Figure 4.7 delineates areas where large numbers of landslides have occurred and areas which are susceptible to landsliding in the conterminous United States.  This map layer is provided in the U.S. Geological Survey Professional Paper 1183, Landslide Overview Map of the Conterminous United States, available online at


Figure 4.7: Landslide Overview Map of the Conterminous United States


Source: United States Geological Survey



Sinkholes are a natural and common geologic feature in areas with underlying limestone and other rock types that are soluble in natural water.  Most limestone is porous, allowing the acidic water of rain to percolate through their strata, dissolving some limestone and carrying it away in solution.  Over time, this persistent erosional process can create extensive underground voids and drainage systems in much of the carbonate rocks.  Collapse of overlying sediments into the underground cavities produces sinkholes.



Collapses, such as the sudden formation of sinkholes, may destroy buildings, roads, and utilities. (Photo: Bettmann)

The three general types of sinkholes are:  subsidence, solution, and collapse.  Collapse sinkholes are most common in areas where the overburden (the sediments and water contained in the unsaturated zone, surficial aquifer system, and the confining layer above an aquifer) is thick, but the confining layer is breached or absent.  Collapse sinkholes can form with little warning and leave behind a deep, steep sided hole.  Subsidence sinkholes form gradually where the overburden is thin and only a veneer of sediments is overlying the limestone.  Solution sinkholes form where no overburden is present and the limestone is exposed at land surface.

Sinkholes occur in many shapes, from steep-walled holes to bowl or cone shaped depressions.  Sinkholes are dramatic because the land generally stays intact for a while until the underground spaces get too big.  If there is not enough support for the land above the spaces, then a sudden collapse of the land surface can occur.  Under natural conditions, sinkholes form slowly and expand gradually.  However, human activities such as dredging, constructing reservoirs, diverting surface water, and pumping groundwater can accelerate the rate of sinkhole expansions, resulting in the abrupt formation of collapse sinkholes.

Although a sinkhole can form without warning, specific signs can signal potential development:

  • Slumping or falling fenceposts, trees, or foundations;
  • Sudden formation of small ponds;
  • Wilting vegetation;
  • Discolored well water; and/or
  • Structural cracks in walls, floors.

Sinkhole formation is aggravated and accelerated by urbanization.  Development increases water usage, alters drainage pathways, overloads the ground surface, and redistributes soil.  According to FEMA, the number of human-induced sinkholes has doubled since 1930, insurance claims for damages as a result of sinkholes has increased 1,200 percent from 1987 to 1991, costing nearly $100 million.



Drought is a natural climatic condition caused by an extended period of limited rainfall beyond that which occurs naturally in a broad geographic area.  High temperatures, high winds and low humidity can worsen drought conditions, and can make areas more susceptible to wildfire.  Human demands and actions can also hasten drought-related impacts.


A USGS streamflow gaging station at the Ogeechee River near Eden, Georgia in July 2000 illustrates the drought conditions that can severely affect water supplies, agriculture, stream water quality, recreation, navigation and forest resources. (Photo courtesy of the United States Geological Survey)

Droughts are frequently classified as one of four types: meteorological, agricultural, hydrological or socio-economic.  Meteorological droughts are typically defined by the level of “dryness” when compared to an average or normal amount of precipitation over a given period of time.  Agricultural droughts relate common characteristics of drought to their specific agricultural-related impacts.  Emphasis tends to be placed on factors such as soil water deficits, water needs based on differing stages of crop development, and water reservoir levels.  Hydrological drought is directly related to the effect of precipitation shortfalls on surface and groundwater supplies.  Human factors, particularly changes in land use, can alter the hydrologic characteristics of a basin.  Socio-economic drought is the result of water shortages that limit the ability to supply water-dependent products in the marketplace.  Figure 4.8 shows the Palmer Drought Severity Index (PDSI) summary map for the United States from 1895 to 1995.  PDSI drought classifications are based on observed drought conditions and range from -0.5 (incipient dry spell) to -4.0 (extreme drought).  As can be seen, the Eastern United States has historically not seen as many significant long-term droughts as the Central and Western regions of the country. 


Figure 4.8: Palmer Drought Severity Index, 1895-1995 Percent of Time in Severe and Extreme Drought


Source: National Drought Mitigation Center



A wildfire is any fire occurring in a wildland area (i.e., grassland, forest, brush land) except for fire under prescription.[4]  Wildfires are part of the natural management of the Earth’s ecosystems, but may also be caused by natural or human factors.  Over 80 percent of forest fires are started by negligent human behavior such as smoking in wooded areas or improperly extinguishing campfires.  The second most common cause for wildfire is lightning.


On Sunday, August 6, 2000, several forest fires converged near Sula, Montana, forming a firestorm that overran 100,000 acres and destroyed 10 homes. Temperatures in the flame front were estimated at more than 800 degrees. (Photo by John McColgan/U.S. Forest Service Firefighter)

There are three classes of wildland fires: surface fire, ground fire, and crown fire.  A surface fire is the most common of these three classes and burns along the floor of a forest, moving slowly and killing or damaging trees.  A ground fire (muck fire) is usually started by lightning or human carelessness and burns on or below the forest floor.  Crown fires spread rapidly by wind and move quickly by jumping along the tops of trees.  Wildland fires are usually signaled by dense smoke that fills the area for miles around.

State and local governments can impose fire safety regulations on home sites and developments to help curb wildfire.  Land treatment measures such as fire access roads, water storage, helipads, safety zones, buffers, firebreaks, fuel breaks, and fuel management can be designed as part of an overall fire defense system to aid in fire control.  Fuel management, prescribed burning, and cooperative land management planning can also be encouraged to reduce fire hazards.

Fire probability depends on local weather conditions, outdoor activities such as camping, debris burning, and construction, and the degree of public cooperation with fire prevention measures.  Drought conditions and other natural disasters (hurricanes, tornadoes, etc.) increase the probability of wildfires by producing fuel in both urban and rural settings.  Forest damage from hurricanes and tornadoes may block interior access roads and fire breaks, pull down overhead power lines, or damage pavement and underground utilities.

Many individual homes and cabins, subdivisions, resorts, recreational areas, organizational camps, businesses, and industries are located within high fire hazard areas.  The increasing demand for outdoor recreation places more people in wildlands during holidays, weekends, and vacation periods.  Unfortunately, wildland residents and visitors are rarely educated or prepared for the inferno that can sweep through the brush and timber and destroy property in minutes.



Worldwide interest in dam and levee safety has risen significantly in recent years.  Aging infrastructure, new hydrologic information, and population growth in floodplain areas downstream from dams and near levees have resulted in an increased emphasis on safety, operation and maintenance.


Dam failure can result from natural events, human-induced events, or a combination of the two. Failures due to natural events such as hurricanes, earthquakes or landslides are significant because there is generally little or no advance warning.


There are approximately 80,000 dams in the United States today, the majority of which are privately owned.  Other owners include state and local authorities, public utilities and federal agencies.  The benefits of dams are numerous: they provide water for drinking, navigation and agricultural irrigation.  Dams also provide hydroelectric power, create lakes for fishing and recreation, and save lives by preventing or reducing floods.

Though dams have many benefits, they also can pose a risk to communities if not designed, operated and maintained properly.  In the event of a dam failure, the energy of the water stored behind even a small dam is capable of causing loss of life and great property damage if development exists downstream of the dam.  If a levee breaks, scores of properties are quickly submerged in floodwaters and residents may become trapped by this rapidly rising water.  The failure of dams and levees has the potential to place large numbers of people and great amounts of property in harm’s way.



There are many different types of space weather that can result in what is referred to as a “solar event.” Although these naturally occurring hazards are relatively new to the sphere of hazard mitigation planning, there are concerns raised by geomagnetic storms, solar radiation storms, and radio blackouts that are very real and relevant to local hazard mitigation planning teams. NOAA, DHS, and NASA are among the Federal agencies that are publishing information on solar events and providing warnings and alerts to interested parties.  

Significant geomagnetic storms—one type of solar event—happen less frequently than other natural hazards, but have the potential to cause considerable damage across the globe with a single event. In the past, geomagnetic storms have disrupted space-based assets as well as terrestrial assets such as electric power transmission networks. Extra-high-voltage transformers and transmission lines may be particularly vulnerable to geomagnetically induced currents caused by the disturbance of Earth’s geomagnetic field. The simultaneous loss of large numbers of these assets could cause a voltage collapse and lead to cascading power outages, resulting in significant economic costs to the Nation. An extreme geomagnetic storm is a low-probability, high-consequence event that could pose a systemic risk to the Nation.[5]

The three main types of solar events are geomagnetic storms (described above), solar radiation storms (defined as elevated levels of radiation that occur when the numbers of energetic particles decrease), and radio blackouts (defined as disturbances of the ionosphere caused by X-ray emissions from the Sun).[6]

The NOAA Space Weather Prediction Center has developed a set of intensity scales for each of these types of solar events as shown in Tables 4.6 through 4.8. These tables provide further explanation of the nature of each type of solar hazard and the potential effects of each. There are some notification procedures in place based on these scales to notify stakeholders of potential solar events. These are tracked by FEMA and are based on the severity of the anticipated event (i.e., G3 and above for geomagnetic storms, S3 and above for solar radiation storms, R2 and above for radio blackouts, etc.).


Table 4.6: NOAA Space Weather Scale for Geomagnetic Storms







Duration of event will influence severity of effects





Power systems: Widespread voltage control problems and protective system problems can occur, some grid systems may experience complete collapse or blackouts. Transformers may experience damage.

Spacecraft operations: May experience extensive surface charging, problems with orientation, uplink/downlink and tracking satellites.

Other systems: Pipeline currents can reach hundreds of amps, HF (high frequency) radio propagation may be impossible in many areas for one to two days, satellite navigation may be degraded for days, low-frequency radio navigation can be out for hours, and aurora has been seen as low as Florida and southern Texas (typically 40° geomagnetic lat.).

Kp = 9

4 per cycle (4 days per cycle)



Power systems: Possible widespread voltage control problems and some protective systems will mistakenly trip out key assets from the grid.

Spacecraft operations: May experience surface charging and tracking problems, corrections may be needed for orientation problems.

Other systems: Induced pipeline currents affect preventive measures, HF radio propagation sporadic, satellite navigation degraded for hours, low-frequency radio navigation disrupted, and aurora has been seen as low as Alabama and northern California (typically 45° geomagnetic lat.).

Kp = 8, including a 9-

100 per cycle (60 days per cycle)



Power systems: Voltage corrections may be required, false alarms triggered on some protection devices.

Spacecraft operations:  Surface charging may occur on satellite components, drag may increase on low-Earth-orbit satellites, and corrections may be needed for orientation problems.

Other systems: Intermittent satellite navigation and low-frequency radio navigation problems may occur, HF radio may be intermittent, and aurora has been seen as low as Illinois and Oregon (typically 50° geomagnetic lat.).

Kp = 7

200 per cycle
(130 days per cycle)



Power systems: High-latitude power systems may experience voltage alarms, long-duration storms may cause transformer damage.

Spacecraft operations:  Corrective actions to orientation may be required by ground control; possible changes in drag affect orbit predictions.

Other systems: HF radio propagation can fade at higher latitudes, and aurora has been seen as low as New York and Idaho (typically 55° geomagnetic lat.).

Kp = 6

00 per cycle
(360 days per cycle)



Power systems: Weak power grid fluctuations can occur.

Spacecraft operations: Minor impact on satellite operations possible.

Other systems:  Migratory animals are affected at this and higher levels; aurora is commonly visible at high latitudes (northern Michigan and Maine).

Kp = 5

1700 per cycle
(900 days per cycle)

Source: NOAA Space Weather Prediction Center


Table 4.7: NOAA Space Weather Scale for Solar Radiation Storms







Duration of event will influence severity of effects

Flux level of > = 10 MeV particles (ions)

Number of events when flux level was not met (number of storm days)



Biological: Unavoidable high radiation hazard to astronauts on EVA (extra-vehicular activity); passengers and crew in high-flying aircraft at high latitudes may be exposed to radiation risk.

Satellite operations: Satellites may be rendered useless, memory impacts can cause loss of control, may cause serious noise in image data, star-trackers may be unable to locate sources; permanent damage to solar panels possible.

Other systems: Complete blackout of HF (high frequency) communications possible through the polar regions, and position errors make navigation operations extremely difficult.


Fewer than 1 per cycle



Biological: Unavoidable radiation hazard to astronauts on EVA; passengers and crew in high-flying aircraft at high latitudes may be exposed to radiation risk.

Satellite operations: May experience memory device problems and noise on imaging systems; star-tracker problems may cause orientation problems, and solar panel efficiency can be degraded.

Other systems: Blackout of HF radio communications through the polar regions and increased navigation errors over several days are likely.


3 per cycle



Biological: Radiation hazard avoidance recommended for astronauts on EVA; passengers and crew in high-flying aircraft at high latitudes may be exposed to radiation risk.

Satellite operations: Single-event upsets, noise in imaging systems, and slight reduction of efficiency in solar panel are likely.

Other systems: Degraded HF radio propagation through the polar regions and navigation position errors likely.


10 per cycle



Biological:  Passengers and crew in high-flying aircraft at high latitudes may be exposed to elevated radiation risk.

Satellite operations: Infrequent single-event upsets possible.

Other systems: Small effects on HF propagation through the polar regions and navigation at polar cap locations possibly affected.


25 per cycle



Biological: None.

Satellite operations: None.

Other systems: Minor impacts on HF radio in the polar regions.


50 per cycle

Source: NOAA Space Weather Prediction Center


Table 4.8: NOAA Space Weather Scale for Radio Blackouts







Duration of event will influence severity of effects

GOES X-ray peak brightness by class and by flux

Number of events when flux level was met; (number of storm days)



HF Radio: Complete HF (high frequency) radio blackout on the entire sunlit side of the Earth lasting for a number of hours. This results in no HF radio contact with mariners and en route aviators in this sector.

Navigation: Low-frequency navigation signals used by maritime and general aviation systems experience outages on the sunlit side of the Earth for many hours, causing loss in positioning. Increased satellite navigation errors in positioning for several hours on the sunlit side of Earth, which may spread into the night side.


 (2 x 10-3

Less than 1 per cycle



HF Radio: HF radio communication blackout on most of the sunlit side of Earth for one to two hours. HF radio contact lost during this time.

Navigation: Outages of low-frequency navigation signals cause increased error in positioning for one to two hours. Minor disruptions of satellite navigation possible on the sunlit side of Earth.



8 per cycle

 (8 days per cycle)



HF Radio: Wide area blackout of HF radio communication, loss of radio contact for about an hour on sunlit side of Earth.

Navigation: Low-frequency navigation signals degraded for about an hour.



175 per cycle

 (140 days per cycle)



HF Radio: Limited blackout of HF radio communication on sunlit side, loss of radio contact for tens of minutes.

Navigation: Degradation of low-frequency navigation signals for tens of minutes.


 (5 x 10-5)

350 per cycle

 (300 days per cycle)



HF Radio: Weak or minor degradation of HF radio communication on sunlit side, occasional loss of radio contact.

Navigation: Low-frequency navigation signals degraded for brief intervals.



2000 per cycle

 (950 days per cycle)

Source: NOAA Space Weather Prediction Center

Historical Examples

To illustrate the vulnerability of various types of infrastructure to geomagnetic storms, it is worth presenting three historical examples: the October-November 2003 “Halloween” event; the Quebec Power Outage of 1989; and the Carrington Event of 1859.[7]

From late October to early November 2003, large geomagnetic storms, which peaked at a severity of-410nanoTeslas, affected the power system infrastructure, the aviation industry, and satellite communications in Europe and North America. In Sweden, a large power utility experienced transformer problems, which led to a system failure and a subsequent power outage. During the 2003 Halloween event, the international airline industry experienced communication problems on a daily basis, with significantly degraded communications at high-latitudes. The Federal Aviation Administration (FAA) could not provide GPS navigational guidance for approximately 30 hours.

On March 13, 1989, a geomagnetic storm that registered -640 nanoTeslas affected Canadian and U.S. power systems, resulting in a major power outage for nine hours for the majority of the Quebec region and for parts of the northeastern United States. Geomagnetically induced currents flowing through the power system severely damaged seven static compensators in the Hydro-Quebec grid, causing them to trip or shut down automatically before preventive measures were possible. The unavailability of new equipment to replace damaged equipment prevented power restoration to the transmission network. After nine hours, 83 percent of full power was restored, but one million customers were still without electrical power.

The most severe space weather event recorded in history is the Carrington Event of 1859, measured at -850 nanoTeslas. From August 28 to September 4, 1859, auroral displays, often called the northern or southern lights, spanned several continents and were observed around the world. According to modern experts, the auroras witnessed were actually two intense geomagnetic storms. Across the world, telegraph networks experienced disruptions and outages as a result of the currents generated by the geomagnetic storms. The economic costs associated with a catastrophic geomagnetic storm similar to that of the Carrington Event could measure in the range of several trillion dollars.


Large, violent eruptions of plasma and magnetic fields from the Sun’s corona, known as coronal mass ejections, form the origin of geomagnetic storms. Coronal mass ejections shock waves create solar energetic particles—consisting of electrons and coronal and solar wind ions—that when they approach Earth create disturbances that affect the planet’s magnetic field. It takes approximately one to three days after a coronal mass ejections launches from the Sun for a geomagnetic storm to reach Earth and to affect the planet’s geomagnetic field. Countries located in northern latitudes, such as Canada, the United States, and Scandinavia, are particularly vulnerable to geomagnetic storms. Power systems in these countries are more likely to experience significant geomagnetically induced currents because of their location in the northern latitudes, the soil type (igneous rock) surrounding electrical infrastructure, and the fact that transmission networks in these countries cover longer distances to the load center. Power systems located in the northern regions of the North American continent are also particularly vulnerable because of their proximity to the Earth’s magnetic north pole.

Figure 4.9 shows the potential widespread power outages that may occur during a severe geomagnetic storm.


Figure 4.9: Power System Disturbance and Outage Scenario


Source: National Research Council

Figure 4.10 shows the correlation between the probability of occurrence, severity, and geographic extent relative to geomagnetic storms.


Figure 4.10: Geomagnetic Storm Risk Management



Source: CENTRA Technology, Inc.


Despite the potentially serious consequences of a severe geomagnetic storm, a literature review indicates that the state of the art for assessing the security risk from geomagnetic storms is still in development. There are examples of analyses that describe threat, vulnerability, and consequence, but they are not integrated, primarily because of the weakness in the threat analysis. Without a sense of the likelihood of such events or at least a mechanism for relative comparisons, cost-benefit analyses have been unable to demonstrate the utility of investing either in hardening or in testing and maintaining operational procedures. The Federal government lacks comprehensive national-level geomagnetic storm risk management assessments and strategies, and no standing entity exists to coordinate cross-Federal government geomagnetic storm risk analysis.[8]

Despite these limitations, several states have developed legislation that seeks to address this hazard. This includes the State of Maine which recently passed “An Act To Secure the Safety of Electrical Power Transmission Lines” in the event of a geomagnetic storm (LD 131).

Other recent state-level legislation activities related to solar events include, but are not limited to, the following:

  • The Florida Senate HM 1251: Electromagnetic Pulse Threats, Cyber-Attacks, and Geomagnetic Storms
  • The Florida Senate HB 1342: Relating to Electromagnetic Pulses and Geomagnetic Storms
  • Georgia General Assembly HB 1148: Public utilities; evaluation of electromagnetic field levels and protection of the transmission and distribution systems against damage from an electromagnetic pulse or a geomagnetic storm
  • The State of New Jersey: An Act Establishing the "New Jersey Electromagnetic Infrastructure Advisory Commission" (A275)



The following primary data sources were among those used to collect the information presented in this section.


[1] The process used to arrive at this list of hazards is documented in the Planning Process section.  Refer to Table 2.2 for details.

[2] As stated, the Hazard Identification section of the risk assessment provides general descriptions from a national perspective whereas the Hazard Analysis and Vulnerability Assessment sections contain information specific to Mecklenburg County, the City of Charlotte, and the towns of Pineville, Mint Hill, Huntersville, Cornelius, Matthews and Davidson.

[3] Due to the inland nature of Mecklenburg County and its communities, nor’easters are viewed primarily as winter storm-type events, as the coastal storm characteristics and coastal impacts of nor’easters would not likely be observed within the county.  The Dolan-Davis Nor’easter Intensity Scale, which shows levels of coastal degradation based on beach and dune erosion, overwash and coastal property damage is not relevant to Mecklenburg County and therefore is not discussed here.

[4] Prescription burning, or “controlled burn,” undertaken by land management agencies is the process of igniting fires under selected conditions, in accordance with strict parameters.)

[5] Risk Management Issue Brief, May 2011, Geomagnetic Storms: An Evaluation of Risks and Risk Assessments By the Office of Risk Management and Analysis.

[6] NOAA Space Weather Prediction Center.

[7] Risk Management Issue Brief, May 2011, Geomagnetic Storms: An Evaluation of Risks and Risk Assessments By the Office of Risk Management and Analysis.

[8] Risk Management Issue Brief, May 2011, Geomagnetic Storms: An Evaluation of Risks and Risk Assessments By the Office of Risk Management and Analysis.

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