Overview of Natural Hazards

HURRICANES—TROPICAL SYSTEMS

Hurricanes and tropical systems have tracked over or have passed close to Southeastern states many times in the past, with a recent example of Hurricane Irene directly affecting Virginia occurring in August 2011, resulting in the declaration of a state of emergency, the evacuation of US Navy ships from Norfolk and Newport News, the US Army base Fort Monroe, and Langley Air Force Base [1, 2, 3].

A hurricane is an intense tropical weather system with a well-defined circulation pattern and maximum sustained winds of 74 miles per hour (mph) or more. A tropical storm is also an organized weather system with well-defined circulation, but its maximum sustained winds are lower— between 39 and 73 mph. A tropical depression is a low-level circulation system of persistent clouds and thunderstorms with maximum sustained winds of 38 mph or less. While far less powerful than hurricanes, tropical storms and tropical depressions can cause substantial damage. As a hurricane weakens and dissipates, it may revert to a tropical storm and eventually a tropical depression.

Hurricane strength is often given in categories using the Saffir-Simpson Hurricane Wind Scale, which rates hurricanes from 1 to 5 based on the intensity of the sustained winds. The table to the right shows expected wind-related damage from the different hurricane categories [4]. It is important to note that the Saffir-Simpson Scale only illustrates the “sustained winds” of a hurricane. Wind gusts can reach up to 135 mph for a Category 2 storm and 160 mph for a Category 3 storm. Hurricanes may also produce tornadoes that add to their destructive power.

During a hurricane, there is a triple threat of damage from high winds, storm surge, and flooding associated with heavy rains. Storm surge is a large dome of water, often 50 to 100 miles wide, that sweeps ashore near where a hurricane strikes land. Storm surge typically accounts for 90 percent of storm-related deaths.

A surge height of 10 feet or more can cause severe flooding far inland and cause severe damage along the coast, particularly when storm surge coincides with high tide. Keep in mind that hurricane rating does not predict storm surge! Hurricane size, depth of near-shore waters, topography, and the hurricane’s forward speed and angle to the coast all affect the surge that is produced.

Rainfall totals of 10 inches or more are common when a tropical storm or hurricane moves across a coastal location. Rainfall totals of this magnitude can result in destructive flash flooding near streams and rivers. Flooding also causes extensive property damage and agricultural losses. Torrential rains continue in upland areas long after the high winds of a hurricane diminish.

A common misconception is that a hurricane or other tropical system will only impact Virginia’s immediate coast, and therefore, inland residents do not need to prepare. However, records show that all areas of Virginia are vulnerable to potentially devastating impacts of tropical storm force winds, torrential rains and associated flooding, and potentially deadly tornadoes.

A review of Virginia’s tropical system history clearly illustrates the need for all of Virginia to prepare. In 1970, Hurricane Camille caused significant flooding across the state. 153 people died of blunt trauma sustained during mountain slides caused by flash flooding in Nelson County, which experienced at least 27 inches of rainfall during the deluge[5]. Hurricane Hazel (1954) produced wind gusts of 70 mph in Richmond; Agnes (1972) produced the worst river flooding in Virginia history, causing the James River to crest at 36 and a half feet in Richmond, washing out all but one of the bridges across the river [6]. The photograph to the left (courtesy of Martin Taylor) shows the Weston Golf Club in Richmond underwater during Hurricane Hazel.

The photograph to the right (courtesy of Virginia Institute of Marine Science) shows waters during Hurricane Isabel (2003) rendering roadways completely impassable. Gaston reached Virginia in 2004 as only a tropical storm, but still overwhelmed drainage systems with 10-12 inches of rain. It is clear that any type of tropical system can cause widespread damage and destruction throughout the state. Thus, it is very important for all Virginia residents to be fully prepared.

NORTHEASTERS—EXTRATROPICAL STORMS

While not as powerful in terms of wind speeds as hurricanes, northeasters (also called nor’easters) occur more frequently in Virginia. Because they cover a larger area and are typically slow-moving storms, northeasters usually affect a large portion of the coast and exert significant impacts on beaches, dunes, buildings, and roads over several successive tides. Northeasters are most damaging when they stall off the coast. Northeasters are a year-round threat to Virginia but occur more frequently during winter and spring months. These intense storms move along the coast with winds blowing directly from the northeast, right off the Atlantic Ocean onto the shoreline. They develop around regions of low pressure and derive their energy from the strong temperature gradients that commonly occur when cold and warm fronts collide.

Northeasters typically produce winds ranging from 30 to 40 mph, with gusts that can exceed 74 mph. These strong winds can create waves ranging from 5 to 15 feet high, depending on the storm’s duration and location relative to the shoreline. The size and strength of these waves can erode beaches and dunes and demolish buildings, boardwalks, and roads. Tidal flooding is also a serious hazard associated with northeasters. Storm-tide heights of 3 to 10 feet above normal are especially damaging when they bracket several tidal cycles. The torrential rainfall from northeasters can cause extensive flooding in both coastal and inland areas.

One of the most-damaging coastal storms to impact Virginia was a northeaster that occurred in March 1962. The March (Ash Wednesday) 1962 storm was extremely severe because it stalled off the coast for more than three days and coincided with the highest monthly astronomical tides through five successive high-tide cycles. The extreme storm surge combined with strong northeast winds and wind-driven waves broke the concrete boardwalk and sea wall at Virginia Beach. The maximum tide height recorded during the 1962 storm is the second-highest tide of record in Virginia, and the largest for a Northeaster. Of the nine highest tidal elevations at Sewell’s Point since 1928, four were caused by Northeasters [7].

Northeasters are also responsible for beach erosion and overwash, which can damage road ways near the coast. Erosion and breaching of dunes leads to severe flooding and property damage as sea-water breaks past the usual barriers, and wind gusts cause damage and power outages. The photograph to the left, courtesy of the United States Geological Survey, compares the the effects of a 2009 nor’easter on Cedar Island, VA, before and roughly two weeks after the storm occurred.

Flooding is probably the most common natural hazard in Virginia. Flooding can be caused not only by a hurricane, but also by a tropical storm, tropical depression, northeaster, or any other weather system that produces heavy rain. Flooding can build up gradually over a period of days or suddenly in a few minutes (commonly known as a flash flood). Coastal flooding can result from high tides (usually on either side of a new or full moon), storm surge, and waves generated by storms located hundreds or thousands of miles from Virginia. Flooding can be associated with living near a body of water such as an ocean, stream, river, or reservoir. To determine whether you are in a high-risk flood area, look at the Federal Emergency Management Agency’s (FEMA) Flood Insurance Rate Maps (FIRMs). These maps show what areas are susceptible to flooding and high velocity wave action (for those near coastal areas) from a 1 percent annual chance event (a.k.a. 100-year flood). Electronic copies of the maps can be downloaded from FEMA’s Map Service Center.

Flood Zones are geographic areas that FEMA has defined according to varying levels of flood risk. They are depicted on a community’s FIRM and each zone reflects the flood risk severity or type of flooding in the area. Generally, these zones can be identified as one of three risk classifications. Even if you are not in a high-risk flood zone, you may nonetheless be at risk from flooding. According to FEMA, nearly 20 percent of flood insurance claims come from low-to-moderate risk areas. Go to floodsmart.gov and type in your street address to determine a very general estimate of the flood risk for your property.

A good way to determine the risk of flooding for your house is to observe and study your property, looking for potential nearby sources of flooding or blockages of surface flow. If your property is immediately adjacent to a road or drainage ditch, try to evaluate the potential for water to accumulate in the ditch due to blocked culverts. If the crown of the nearby road is higher than your driveway or crawlspace, this may be an indication that heavy rainfall runoff could accumulate on your property, regardless of whether you are in a mapped floodplain. Even inland properties may be susceptible to flooding if there is poor localized drainage or if recent development has altered the ability of water to drain out of your area. If your property floods during small rain events, then the problem will be greater during a severe storm or hurricane. You can protect yourself by improving the local drainage, making your house resistant to floods, and purchasing flood insurance. You do not need to be in a high risk flood zone to obtain flood insurance.

For those located within a flood zone, elevating a building’s lowest floor above predicted flood elevations by a small additional height (known as “freeboard” [9]) has very little effect on the look of a home, yet it can lead to substantial reductions in damages caused by flooding as well as reductions in flood insurance cost. Consult with your local floodplain manager to determine how much freeboard (if any) is needed for your property for flood insurance savings, and click on “FEMA: Freeboard Factsheet” in the Learn More section. Even if you are not in a flood zone, you should consider purchasing flood insurance, especially as properties located outside of an official flood zone might also be at risk from flooding. The rates for properties outside declared flood zones are very affordable and are invaluable if a flood event should occur

One of nature’s most violent storms, a tornado, is characterized by a twisting, funnel-shaped cloud extending to and in contact with the ground. Tornadoes most often result from the intersection and interaction of cool dry air as it overrides warm moist air, causing the warm air to rise rapidly. These conditions are also associated with severe thunderstorm activity, so it follows that tornadoes are most often generated by thunderstorms (including those associated with tropical systems such as hurricanes). With wind speeds ranging from 40 to more than 300 mph, tornadoes can cause fatalities and devastate a neighborhood in seconds. The result is catastrophic failure of structures and facilities, as well as the potential for injury and death. Damage paths can be in excess of 1 mile wide and 50 miles long [10].

The size of a tornado is not necessarily an indication of its intensity; large tornadoes can be weak and small tornadoes can be violent. The Fujita (F) Scale is used to estimate tornado wind speeds based on damage left behind by a tornado. An Enhanced Fujita (EF) Scale, developed by a forum of nationally renowned meteorologists and wind engineers, makes improvements to and replaces the original F scale [11]. While tornadoes are most frequently reported east of the Rocky Mountains during spring and summer months, peak tornado season in Delaware is March through May. They are most likely to occur between the hours of 3 and 9 p.m., but can occur at any time. Tornadoes generally occur near the trailing edge of a thunderstorm or accompany a tropical storm or hurricane as it moves onshore. The average forward speed of a tornado is 30 mph but may vary from stationary to 70 mph. Before a tornado hits, the wind may die down and the air may become very still. A cloud of debris can mark the location of a tornado even if a funnel is not visible. It is not uncommon to see clear, sunlit skies behind a tornado. Waterspouts are tornadoes that form over water, which, though rare, can be dangerous.

The entire state of Virginia is at nearly equal risk for tornadoes. From 1950 to 2001, 376 tornadoes were documented in Virginia. On August 6, 1993, one tornado killed four and injured 238 people in Petersburg. In 2004, remnants of hurricanes Gaston, Frances, and Ivan spawned a total of 69 tornadoes over the span of three weeks [12].

SEVERE THUNDERSTORMS

Although thunderstorms typically impact a small area, they can be extremely dangerous due to their capability of generating tornadoes, hail, strong winds, flash floods, and damaging lightning. These storms can move through an area very quickly or linger for several hours, with longer duration resulting in the possibility of excessive precipitation and increased likelihood of flash floods. The National Weather Service considers a thunderstorm to be severe if it produces hail at least one inch in diameter, winds of 58 mph or stronger, or a tornado. Straight-line winds associated with severe thunderstorms can exceed 125 mph and are responsible for most thunderstorm damage. Straight-line winds, or downbursts, are much more common than tornadoes and tend to cause more damage than the typical tornado in Virginia.

DROUGHT/EXTREME HEAT

Drought conditions are the result of extended periods of limited precipitation. Human activities, high temperatures, high winds, and low humidity can exacerbate drought conditions and may also make areas more susceptible to wildfire. Periods of drought can have significant negative impacts on agriculture, water reservoir levels, surface and groundwater supplies, or any water-dependent resource or product. An extreme heat condition is commonly identified when prolonged temperatures are greater than or equal to 10 degrees above the average high temperature for a region. Periods of extreme heat in Virginia are also often accompanied by high humidity. Extreme heat can cause medical problems and pose significant risks to humans, especially the elderly, young children, and people with respiratory difficulties.

WILDFIRE

Wildfires, or any naturally occurring fire in a grassland, brush, or forested area, are especially dangerous hazards during periods of drought. The most common cause of wildfires is negligent human behavior (causing 80 percent of forest fires). Lightning strikes are the second most common cause and typically occur during summer months. Areas with large amounts of dry fuel, such as vegetation, debris, or trees, are particularly susceptible to wildfires caused by lightning strikes. Fire probability depends on local weather conditions, human activity, and implementation of community fire prevention measures.

EARTHQUAKES

Every year approximately 3 million earthquakes occur worldwide. Between 2000 and 2009, the United States experienced approximately 32,000 earthquakes; six were considered major and occurred in either Alaska or California. Earthquakes do not occur exclusively in the western United States. Seven events with magnitudes greater than 6.0 have occurred in the central and eastern sections of the United States since 1811. A magnitude 5.8 earthquake occurred with an epicenter in Virginia in August of 2011, and felt across a dozen states and several Canadian provinces, resulting in building damage between $200 million and $300 million[13, 14].

TSUNAMIS

While tsunamis are not considered to be a high-risk hazard in Virginia, it is possible that a tsunami could impact the Virginia coast. Tsunamis in the Atlantic Basin are most commonly generated by earthquakes and landslides. Primary sources of tsunami-producing earthquakes in the Atlantic are located near Puerto Rico, Portugal, and the Canary Islands. Tsunamis in the Atlantic Ocean may also be caused by underwater landslides, usually occurring near the continental shelf and slope. Since 1600, 40 tsunamis and tsunami-type waves have been documented in the eastern United States. East Coast tide gauges can detect even small tsunami waves caused by distant earthquakes. For example, Atlantic basin gauges recorded 5 to 10 inch waves generated by the December 2004 Indian Ocean tsunami.