A flash flood is a rapid and extreme flow of high water into a normally dry area or a rapid rise in the water level of a stream or creek above a predetermined flood level. Flash flooding differs from flooding in that flooding may last longer than flash flooding — floods may happen and last over days or weeks, while flash floods generally happen within six hours. Furthermore, the causes of flooding are not exclusive to rainfall. Floods can also be expected when typically dry areas are inundated due to rising water in existing waterways such as a river, stream, or drainage ditch.
"Runoff" is a term commonly associated with flooding. It is the movement of water from land downhill in the direction of the ocean due to gravity. Normally, water on the earth's surface after rainfall evaporates infiltrates the soil or runs over the surface. Surface runoff happens when water that the ground does not absorb flows over the land surface. The action of rainfall intensity exceeding the soil's evaporation rate and infiltration capacity is the cause of surface runoff. Flash floods differ from other forms of flooding, such as river floods, coastal floods, storm surges, and inland flooding.
River floods occur when water levels rise over the top of river banks due to excessive rain from tropical systems making landfall, persistent thunderstorms over the same area for extended periods, combined rainfall and snowmelt, or an ice jam. A coastal flood occurs after heavy rainfall and strong onshore winds. It is an inundation of coastal areas primarily caused by an excessively high tide. Storm surge is an abnormal rise of water generated by a storm over and above the predicted astronomical tides. Inland flooding is typically an overflow of flood waters into land areas away from the coast.
The main distinction between these types of flooding and flash floods is that the latter occurs rapidly, usually within minutes or hours of the rainfall, and usually lasts for a short time. Despite this short duration, a flash flood can be very hazardous. During flash floods, there is a rapid rise in water, high velocities, and large amounts of debris.
Most flash floods occur due to weather events such as slow-moving or stalled thunderstorms, thunderstorms repeatedly moving over the same area, and heavy rains from hurricanes and tropical storms. Some other causes of flash floods include dam or levee failure, the release of ice jams, and the collapse of debris dams, among others. Floods from levee or dam failure result from inadequate or poorly constructed dams or levees. Many private or locally built levees and dams may provide limited flood protection. Floods from dam failure are especially dangerous because they are difficult to predict, so there is hardly any time to evacuate. But it is possible.
An ice jam is an accumulation of pieces of floating ice carried with a stream's current, which constitutes an obstruction to the stream flow. A debris dam is similar, but the pieces are floating debris and not made of ice. These obstructions can develop near river bends, mouths of tributaries, points where the river slope decreases, downstream of dams, and upstream of bridges or obstructions. When the ice or floating debris is an obstacle to flowing water, it may result in upstream flooding. Conversely, there could be further flash flooding downstream if the obstruction abruptly releases.
Coastal areas with high or excessive rainfall are the most vulnerable to flash floods. However, these disasters can also happen in areas without nearby bodies of water because the primary trigger—excessive rainfall—is ubiquitous.
Flash floods are common in areas with a dry climate and rocky terrain because the lack of soil or vegetation allows torrential rains to flow overland rather than infiltrate into the ground. The same is true for urban areas compared to the suburbs or countryside. The conversion of land, from fields or woodlands to roads and parking lots, causes it to lose its ability to absorb rainfall. Urbanization increases runoff two to six times over what would occur on natural terrain. This is due to impermeable tarred or concrete surfaces in those areas.
In addition, areas damaged by wildfires are particularly susceptible to flash floods and debris flows during rainstorms because torched soils develop a layer that repels water. Therefore, the forest canopy, which will usually absorb rainfall and loose tree litter and duff on the ground, cannot do so, and the water will instead runoff. This consequence is that much less rainfall is required to produce a flash flood. Moreover, the potential for debris flows increases with the loss of plant material that holds the soil in place.
Flash floods can occur at any time throughout the year. It is possible, however, for certain regions to see more or less flooding at different times of the year. In coastal regions, flooding is more common in hurricane seasons (between June and November). Springtime and during heavy Summer rains is the period with the highest flooding risk in the Midwest.
Weather patterns are an important determinant of where and when floods occur. Frontal storms can cause floods in the Northern and Eastern parts of the United States during Winter and Spring. In the Southwest, thunderstorms can cause flash floods in smaller streams in late Summer and Fall. In the Western parts of the US, cyclones or storms that bring moisture inland from the ocean can cause floods in the Winter and early Spring.
Floods kill more people each year than tornadoes, hurricanes, or lightning. The National Weather Service (NWS) records that the national 30-year average for flood deaths is 88. This record is alarming in comparison to lightning, tornadoes, and hurricanes, which are 41, 68, and 45, respectively.
Although drowning is the major cause of fatalities in a flash flood, getting dragged through the debris-filled floodwaters and slammed into immovable objects can also result in traumatic damage to victims. These flood-related disasters may also cause deterioration of health conditions owing to waterborne diseases. Victims of flash flooding may also suffer damage to their homes, office buildings, vehicles, and other property.
The combination of the force of the water and the debris can cause heavy structural damage to homes and other buildings, making them uninhabitable. The National Centers for Environmental Information (NCEI) reports that the economic loss following flooding events has an average cost of $4.7 billion per event. The Federal Emergency Management Agency (FEMA) estimates that just one inch of floodwater can cause up to $26,000 in damage.
The agricultural sector can also be severely impacted by flash flooding. Some negative effects include: preventing root crop expansion, causing premature harvest, rotting crops, increasing brooder pneumonia (aspergillosis) in poultry, causing foot rot disease on animals, and reducing the nutritional value of livestock feed. Flash floods also cause erosion because powerful floods tear away the top layers of the soil.
Flash floods are a real threat to critical coastal infrastructures such as pumps, floodgates, and embankments. Transport networks are also vulnerable to coastal flooding. In urban areas, infrastructure such as bridges and power lines can also be severely affected by flash flooding. Debris carried by floods can also cause substantial damage to roads or block road access.
Flooding is a significant threat in North Carolina. Among several counties in the state, Tyrell County, NC, is one of the most flood-prone areas. Nearly all of the entire low-lying rural county is considered to fall within the 100-year flood plain. The wetlands are more prone to flooding than the agricultural lands. This is because of the higher possibility for surface runoffs to occur in wetlands.
Flooding is most common in the Eastern region of North Carolina due to the state's location—on the Atlantic coast. However, powerful storms and other non-weather-related events may also cause flood disasters in other cities in North Carolina. Studies by the FEMA Digital Flood Insurance Rate Maps (DFIRM) and the Geographic Information System (GIS) have marked out about 0.3 percent of the Southeastern NC Region as special flood hazard areas. Counties that fall within this region are Brunswick County, New Hanover County, Onslow County, and Pender County.
In the state's history, hurricanes are responsible for spawning the most severe flash flood disasters. An example was Hurricane Floyd, which produced unprecedented flooding across much of Eastern North Carolina in September 1999. Floyd made landfall as a Category 2 in Cape Fear, NC, on September 16. Eastern North Carolina recorded 35 deaths and was the hardest hit area in the state, with many casualties caused by inland freshwater flooding. Agricultural livestock was also seriously affected as nearly 750,000 turkeys, and 100,000 hogs were lost to flooding in Duplin County alone. Hurricane Floyd reportedly cost the state an estimated $3 to $6 billion in damage.
Another historical flash flood was caused when Hurricane Matthew made landfall in Southeastern North Carolina in October 2016. Matthew caused catastrophic flooding and killed 28 people. It was difficult for residents and local officials to adequately prepare for the storm because the models forecasting its path were unpredictable and varied. Widespread flash floods were also reported during the storm. Matthew is considered the tenth-most-destructive hurricane to affect the United States. The NOAA National Centers for Environmental Information (NCEI) estimates that wind and water damage caused by Matthew totaled approximately $10.0 billion.
Making adequate preparation and paying serious attention to flood safety tips will help to mitigate the impacts of flash floods. The first step to take when preparing for a flood is to find out the flood potential of the area to properly assess the risk. Also, make sure to identify the location of a high ground closest to the area and map out the quickest route to access the high ground.
When preparing for a flash flood, it is advisable to have an emergency plan drawn out. In addition to the emergency plan, make an emergency kit containing items needed for survival. Family members need an emergency plan to know where to go in case of evacuation and how to communicate. Ensure to make medical information for family members accessible and also add necessary medication to the emergency kit for medical emergencies.
Remember to include pets and service animals in the emergency plan, and they should also have their supply kits. Keep a supply of necessary medication and treats that can last for at least a week. If evacuations are necessary, take pets along for their safety. Remember not to allow pets to roam free during an emergency, as they can get seriously injured. Bring them indoors at the first warning of a storm. It is also advisable to identify nearby animal shelters, boarding kennels, veterinary clinics, grooming facilities, and the homes of family and friends where pets may stay in an emergency.
Furthermore, protect the home from being severely damaged by flash floods by carrying out structural repairs on vulnerable or worn-out parts of the building. Building barriers will help to prevent water from entering the building. Sealing the walls in basements with waterproofing compounds is also recommended. Residents in areas prone to flooding will find it helpful to raise the furnace, water heater, and electric panel in the home. This will help to reduce the risk of these appliances being engulfed in water during flash floods. Residents in coastal areas likely to be in the path of a hurricane, which may spawn a flash flood, may want to have storm shutters or plywood to board up windows.
Cars, trucks, and boats should be removed from probable floodways and moved to secure locations to reduce the risk of damage. Additionally, remove the patio furniture and other outdoor equipment from the yard to prevent them from being carried away during the flood disaster. Documents containing sensitive information should be kept in water-resistant bags and stored in secure locations. It may also be helpful to make copies of the documents and store the originals in off-site locations or secure password-protected digital spaces.
Flood awareness can be especially critical for campers. It may be unsafe to camp or hike near a stream or river if there are thunderstorms in the area. Always remember to check the National Weather Service forecast before leaving home. Be alert for changing weather conditions, sudden rain storms, water flowing into low areas, or the sound of rushing water while visiting the forest. Devices like a weather radio, a terrestrial radio, or a smartphone application can help visitors stay tuned in during outdoor activities.
The financial costs of floods can be very severe, especially for unprepared homeowners. Homeowners are strongly advised to get insurance coverage to protect against flooding disasters. Prospective policyholders should note that renter and homeowner's insurance policies do not always cover flood losses. So, acquire separate flood insurance to cover damage from flood-related disasters. It is also advisable to get insurance coverage for cars, trucks, and boats that may be damaged by flooding. Policies are available whether the building is in or out of a known flood-prone area. Insurance providers should be asked about getting comprehensive insurance against floods. It is also helpful for existing policyholders to review existing policies to ascertain the amount and level of coverage.
The National Flood Insurance Program (NFIP) provides insurance resources and advice to property owners, renters, and businesses to help reduce the socioeconomic impact of floods. This program also makes federally-backed flood insurance available in areas that agree to adopt and apply floodplain management rules to lower future flood damage.
In North Carolina, several forms of flash flood warnings and alerts may be issued to notify residents about predicted flash floods. The National Oceanic and Atmospheric Administration (NOAA) issues important warnings through the National Weather Service (NWS). These warnings are very important and can be decisive in ensuring flood safety. They are:
Flood watch – state and local authorities issue a flood watch when rainfall is heavy enough to cause rivers to overflow their banks, and flooding is possible.
Flood warning – state and local authorities issue a flood warning when flooding is happening or is very likely to happen in affected areas. If told to leave, do so immediately.
Flash flood watch – state and local authorities issue a flash flood watch when flash flooding in specified areas is possible, and it may become necessary to take quick action. Residents in a flood-prone area may be directed to seek higher ground immediately. If advised, evacuation should be done immediately.
Flash flood warning – state and local authorities issue a flash flood warning when flash flooding is happening or is likely to happen along certain areas. In such a situation, residents of such an area are expected to act quickly. Get to a safe place immediately. Move to higher ground or climb to safety before flood waters cut off access to safety.
State and local governments and the National Weather Service can send emergency messages via television, radio, weather radios, social media, and alerts on cellular phones. Individuals must make sure they have a way to monitor severe weather conditions and receive emergency alerts at all times. NOAA Weather Radio, local radio, or television are common sources for the latest weather and river forecasts.
NOAA Weather Radio All Hazards (NWR) is a nationwide network of radio stations broadcasting continuous weather information directly from the nearest National Weather Service office. The NWR broadcasts official Weather Service warnings, watches, forecasts, and other hazard information 24 hours a day, seven days a week. Among the 750 Weather Radio transmitters found across the United States, over 27 of those transmitters, which are located within North Carolina and three neighboring states, provide necessary weather updates to the state. NOAA weather radios are sold at most local electronics stores.
The Emergency Alert System (EAS) is a national public warning system that broadcasts emergency messages via radio and TV. It is implemented, maintained, and operated at the federal level by the Federal Emergency Management Agency (FEMA) in partnership with the Federal Communications Commission and NOAA. Participation by radio and TV stations is voluntary, but organizations that join the EAS must agree to the rules in the North Carolina Emergency Alert System Plan.
Wireless Emergency Alerts (WEA) are short text-like alert messages sent by government-approved staff during an emergency to give people a free and fast way to get key details. It also sends details about AMBER alerts or other problems in your area. Residents who wish to access these options do not need to get an app or subscribe to the service. However, you'll need to turn on emergency alerts in the notifications settings on the mobile device.
Flash floods represent different forecast and detection challenges because they are not always caused by meteorological phenomena. Therefore, many variables come into play when predicting when a flash flood will occur. Although forecasters can usually tell in advance when conditions are right for flash floods, there is often little lead time for an actual warning. Thus, it is important to immediately follow all issued warnings and alerts and prepare to carry out the flash flood emergency plan.
Flood risk is based on many factors, including rainfall, landscape, flood-control measures, river flow, tidal-surge data, flood history, and changes due to new construction and development. Flash flood risks can be determined through several sources of information, including rainfall records, historical information on past flash flood occurrences, and even regional records of flash flooding in nearby areas.
The North Carolina Flood Risk Information System provides a Digital Flood Insurance Rate Map (DFIRM) that enables business leaders and residents to more accurately predict flood hazards and prepare for flood risks. The system requires individuals to fill in their address, city, county, or zip code. The North Carolina Floodplain Mapping Program (NCFMP) performs flood hazard mapping studies that help inform, protect, and preserve the lives and properties of the citizens in North Carolina. More information on flood risks is on the state's website, which provides details on flood risks at specific addresses, including flood hazards, structural and content impacts, potential insurance rates, mitigation opportunities, and the location of the nearest flood warning sites.
When caught in a flash flood, the paramount objective is to remain safe. To achieve this, individuals must follow all safety guidelines and be alert. The water levels and the rate at which water flows can quickly change during a flash flood. It is important to avoid flood water and evacuate as soon as safety officials give the order. Seek higher ground immediately.
Electrocution is a likely hazard during a flash flood. Do not go into a basement or room if water covers the electrical outlets or cords are submerged. Immediately get out of rooms where there are sparks or buzzing, crackling, snapping, or popping noises.
As little as six inches of fast-moving water can knock people off their feet. When trapped by moving water, try to climb to the highest possible point and call emergency services if possible. Never attempt to cross a flowing stream of water or a body of water that has an undetermined depth. Water may be deeper than it appears and can hide hazards such as sharp objects, washed-out road surfaces, electrical wires, and chemicals.
A vehicle caught in swiftly moving water can be swept away in seconds. The CDC reports that the most common flash flood fatalities are vehicle-related. Do not drive into flooded roadways or around a barricade. Turn around instead if you can. If a vehicle gets stuck, abandon the car and get to higher ground quickly. It takes 12 inches of rushing water to carry away most cars, and two feet of rushing water can carry away SUVs and trucks.
For campers and hitchhikers, it may be nearly impossible to call for help because cell phones and digital data services may be limited in remote areas. So, immediately identify and move to high points away from the low-lying flooded areas. Also, call for help if possible. Do not attempt to wade into the water as it may be difficult to ascertain the water depths and the force of the current, especially when a flash flood happens at night. Maps may make provision for evacuation routes and should be followed for safety.
Residents returning home after a flood should make sure to go back only when local authorities say it is safe. Also, pay attention to news reports and local officials on whether the community's water supply is safe to drink.
Electrical appliances should be turned off and should only be turned back on by a trained technician to avoid the risk of electric shock. Look out for downed power lines, gas leaks, and structural damage. Report service outages and downed power lines directly to the service company. Take basic steps to keep the home safe. Generators should be used outdoors and never indoors or in the garage because of the risk of carbon monoxide poisoning.
Remember to wear protective clothing when cleaning up after a flood. Children should not be allowed to assist in cleaning up unsupervised. It may be expedient to have help or support cleaning up to avoid getting overwhelmed by the workload.
Thorough cleaning and drying after flooding are important to prevent mold growth. Exposure to molds may cause a variety of health issues to vulnerable persons. Exposure can lead to symptoms such as stuffy nose, wheezing, and red or itchy eyes or skin. People with allergies to molds or with asthma may have more intense reactions.
Remember to avoid standing water as it may be electrically charged from underground or downed power lines. Always assume downed power lines are active and keep away. Moreover, it is recommended to stay away from floodwaters because water may be dirty with oil, gasoline, or untreated sewage.
Sewer systems may have been damaged by flood water and may consequently cause serious health problems. It is advisable to fix broken septic tanks, cesspools, pits, and leaching systems as soon as possible. When seeking to employ the services of cleanup or repair contractors, ensure they are trained and have valid licenses.
Persons insured against flood disasters may contact their homeowners' insurance providers to file a claim. It is advisable to take photographs and document the extent of damage before cleaning up. Also, make an inventory of damaged or lost items and file a claim as soon as possible.
Additionally, certain government programs offer resources and advice to insurance policyholders. The NFIP offers a wide range of resources to policyholders, agents, and other services to help them navigate the flood insurance process before, during, and after the disaster, including publications, videos, graphics, and online tools.