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This past summer, while wildfires ravaged the drought-stricken West, states along the East Coast experienced devastating floods. In a one-week period during August, North Carolina, Tennessee, New Jersey, and New York all had major floods, each one separate from the other.
In a typical year in the United States, floods kill more people than lightning, tornadoes, or hurricanes. In 2020, 59 people died in floods, which was actually below the 10-year average of 94 deaths per year. In addition to fatalities, major floods, which now occur somewhere in the U.S. every year, also cause billions of dollars in damage. In Canada, floods are listed as the “most frequent natural hazard,” proving they’re nothing to take lightly.
Types of Freshwater Floods
Simply put, a flood is said to occur when a significant amount of water covers an area that’s usually dry. There are different types of floods, varying by location and cause. In this article, I’m going to focus on freshwater flooding, which can occur anywhere and is usually the result of excessive rainfall or melting snowpacks. Coastal flooding – which can occur along lakes as well as oceans – is area-specific and typically caused by strong onshore winds during major storms.
Freshwater floods can be broken down into two major categories according to time frame: floods and flash floods. Flash floods occur quickly – officially “within six hours of the causative event,” according to the National Weather Service, but they often occur much faster, sometimes in less than an hour. They tend to be localized and usually subside within hours. Floods, on the other hand, are inundations by water that take longer than six hours to develop – sometimes days. They cover more area than flash floods do, and they subside more slowly, often taking at least a day.
Flash floods are usually produced by thunderstorms, which can drop tremendous amounts of rain in short periods of time. During a flash flood, rain can fall at rates of 2 to 8 inches per hour, which exceeds the time it takes water to safely run off or sink into the ground. The worst flash floods occur when storms are slow-moving, or when multiple storms form and move over the same area repeatedly. The majority of flash-flooding events occur in spring and summer, when warm temperatures promote thunderstorm development.
For example, in the early morning of August 21, 2021, a thunderstorm hit Hickman County, Tennessee, near the Piney River. Throughout the morning, additional thunderstorms continued to pelt the area with heavy rain. At one point, a weather observer measured 4 inches of rain in one hour. When the rain finally stopped, the observer had measured an amazing 17.26 total inches of rainfall. As a result of the heavy, rapid rainfall, the Piney River rose quickly and crested at 31.8 feet. Unfortunately, 22 people lost their lives in those Tennessee floods.
Flash flooding due to thunderstorms occurs more frequently in the eastern part of the United States during spring and continues into summer. In the Southwest, summer monsoons occur when moisture-laden air overspreads the normally dry region, sometimes extending northward into the Rockies. Daily heating and orographic effects promote thunderstorm development. Even normally “dry washes” can quickly fill with water.
If the conditions are right for flash floods, avoid any place where water normally flows, such as streams and small rivers, even if the water level is low or the stream bed is dry. Heavy rains can fill these areas with water extremely quickly, especially in hilly or mountainous terrain, where rainwater gets channeled into valleys. One such event occurred on July 31, 1976, when hundreds of people were camping along the scenic and seemingly benign Big Thompson River in Colorado.
It didn’t rain where they were, but in the upper reaches of the canyon, a nearly stationary thunderstorm dropped an estimated 12 inches of rain. As a result, a 20-foot-high wall of water roared down the canyon, destroying everything in its path and killing 143 people.
Urban areas aren’t immune to flash floods either. Paved roads and sidewalks don’t allow water to soak into the soil, so it instead runs off and can accumulate to dangerous depths. Underpasses and other low spots are particularly susceptible to flooding.
Flash floods account for most flood fatalities. Over half of all flood-related drownings occur when people become trapped in their cars. In 2020, 63 percent of all flood fatalities in the U.S. occurred in vehicles. Most of these fatalities happened when people drove into a flooded area. It only takes about 1 foot of fast-moving water to carry away most cars, and 2 feet of water for heavier SUVs and trucks. When a vehicle gets caught in fast-moving water, it can then be swept into deeper water and quickly be submerged.
In recent years, the NWS has promoted a motto to help prevent vehicle-related drownings: “Turn around, don’t drown.” In locations where road flooding is frequent, yellow road signs carrying this message are permanently displayed. In addition, temporary pink incident signs bearing the same warning may be used when and where necessary.
To avoid vehicle-related flood accidents, never drive into a flooded area. The water will often be deeper than it appears, and you won’t know if the road beneath the water has washed away. Don’t drive around signs or barricades closing a road for flooding. If your car stalls in floodwaters, get out immediately and head for higher ground.
Floods are more widespread and slower-developing than flash floods. They can occur anywhere, but river floods are most common. River floods are defined by the NWS as when a “river overflows its natural banks, causing or threatening damage.” River flooding occurs when an excessive amount of rain falls over a broad area. River floods can also happen in spring when melting snow occurs in conjunction with significant rainfall. Major winter storms can cause flooding problems, particularly in the western United States and Canada. Moisture-laden air coming off the Pacific Ocean that lifts over the mountainous terrain can produce heavy rainfall at lower elevations. If warmer air causes significant snowmelt in the mountains, it increases flooding potential. The combination of large spring storms and melting snow often produces river flooding east of the Rockies in both the United States and Canada.
In my article on hurricanes (“Into the Eye of the Storm,” May/June 2019), I talked about the massive amounts of rain these tropical systems can produce, even the weaker storms. With this much rain, both flash flooding and river flooding are possible, and the threat can extend inland many miles from the coast and last for days. Late summer and fall are peak times for hurricanes.
One of Canada’s worst natural disasters was caused by flooding from a hurricane in Ontario. In 1954, Hurricane Hazel dropped more than 11 inches of rain in 48 hours in an area that seldom gets heavy, prolonged rains. Toronto was hit particularly hard, and 81 people died. Adjusted for inflation, the property damage was in the billions of dollars.
In late August 2017, Hurricane Harvey dropped 60 inches of rain in the Houston, Texas, area, resulting in flooding that took multiple lives and caused over $100 billion in property damage.
Dam or levee failure, and Ice Jams
Dam or levee failure (a “levee” being an earthen dam) can produce either flash floods or river floods. Dam and levee failures have produced some of the worst and deadliest floods in history. Such an event can send a massive wall of water downstream of the failure. These are often the result of heavy rainfall that overloads the structure. Ice jams – dams made of ice that can block river flow – are another cause of floods.
On May 31, 1889, the South Fork Dam on the Little Conemaugh River in Pennsylvania broke after days of heavy rain. The torrent of water that broke loose roared down the river channel to Johnstown, some 14 miles downstream. The Johnstown Flood killed more than 2,200 people.
Mud and Debris Slides
Besides flooding, mud and debris slides can also result from heavy rainfall if the topographic conditions are right. The force of this material moving quickly downhill can do tremendous damage to structures and people. Areas that have been laid barren by wildfires are extremely prone to this scenario.
With some exceptions, such as those mentioned above, major river flooding typically results in fewer deaths than flash floods, because people have more time to seek safety. They often result in catastrophic property and land damage, however. Cities and towns along a river can suffer major infrastructure and building damage. Crops can be destroyed, and agricultural lands can be left unusable for periods of time. Financial losses can be in the millions, or even billions, of dollars. According to the NWS, there have been 35 flood events in the U.S. that have resulted in damages totaling at least $1 billion since 1980. In March of 2019, a major flood event involving the Missouri River and its tributaries resulted in nearly $11 billion of damage in the Upper Midwest.
Keep Up with Weather Conditions
To stay safe in the event of a flood, keep up to date about weather conditions for your area, whether you’re at home or away. Are you in or near a flood plain, where flooding can be occasionally expected? Would possible escape routes still be passable during a flood? Always keep track of the weather. Even if you don’t live in a flood plain, keep in mind that a changing climate is causing floods in areas that have previously been unaffected.
The NWS is a reliable source for information on flooding potential, and offers location-specific data. Critical information will also be broadcast on the National Oceanic and Atmospheric Administration (NOAA) Weather Radio and distributed to local media.
For advance notice about flooding, both the NWS and the NOAA will issue “flood watches” or “flash flood watches.” This means conditions favorable for flooding may develop, but it’s not certain or imminent. Normally issued hours or more before flooding could occur, a watch gives people time to take any necessary precautions and to be alert for future developments. A “flood warning” or “flash flood warning” means flooding is imminent or even currently occurring. Warnings can be very specific to location, even particular sections of a town or city. As events develop, your local NWS office will typically issue “flood statements” or “flash flood statements” that further describe the current status of the flooding.
The NWS might also use the term “urban flooding” when urban areas may be subjected to flooding in streets, underpasses, low-lying areas, or storm drains. According to the NWS, this type of flooding is “mainly an inconvenience, not generally life-threatening.” Sometimes, such an advisory would include “small stream flooding,” indicating that small rural or urban streams may fill or overflow, resulting in some home or road damage. For river flooding, the NWS will also give specific flooding information for points along a river when flooding amounts may vary. The expected flooding will also be characterized by severity, ranging from “minor” to “record flooding.”
In Canada, the provinces issue “provincial flood watches” when needed, as well as “watershed condition statements” pertaining to flood potential. Local authorities will also issue “watershed condition statements” and “flood watches” for their areas, as well as “flood warnings” when flooding is imminent or already occurring.
Prepare Your Property For A Flood
If a flood event hits your area, here are a few things you can do ahead of time to better protect your family, property, and animals. Most of this preparation needs to be done well before a flood, so plan early. Visit the American Red Cross “Flood Safety” for more information about preparing your family for a flood.
For your family and pets:
- Assemble a flood emergency kit with enough food and clean water to last at least a week.
- Develop an evacuation plan, and make sure each family member knows what to do.
- Some emergency shelters only allow service animals, so make a plan for what you’ll do with pets in the event of an evacuation.
For your property:
- Proactive land management is your best defense against flood damage on your property. Techniques vary, but can include ditches, natural barriers, and drainage canals. Keep culverts and watercourses free of debris to help avoid water backup.
- In addition to permanent water barriers, sandbags and other temporary barriers can be placed around your property if you have time to do so safely.
- Make sure the land around your buildings is properly graded to encourage water runoff.
For your home and other buildings:
- Install a sump pump with a battery backup.
- Keep gutters and downspouts clean.
- Keep important documents in a dry, elevated location, preferably in a watertight safe.
- If you have time to do so safely, move valuable items to upper levels or higher surfaces.
- Turn off all utilities if floodwater enters your home, and especially if you evacuate.
For your animals:
- Make sure all animals are marked with some type of identification in case they get lost during a flood.
- Ensure there’s a way to restrict access to parts of your property that are likely to become more dangerous in a flood, such as low-lying buildings and fields. If an evacuation seems likely, move confined animals to higher ground. Unconfined animals are typically able to care for themselves as long as they have access to higher ground.
- Make sure animals have access to clean water and food. Make a plan for how you’ll feed and water animals in the event of long-term flood conditions.
- If flooding is a regular concern, consider installing “flood mounds” around your property for animals to escape high water.
Flood Safety Resources
For more information on how to prepare for a flood, visit the National Weather Service’s “Flood Safety Tips and Resources” page at Flood Safety Tips. In Canada, visit the Government of Canada’s “Get Prepared” page.
For localized weather, find a nearby National Weather Service. Additionally, tune in to local radio stations to stay up to date on current flood conditions.
For more than 30 years, Ed Brotak taught thousands of college students about weather, and he’s helped many of them pursue careers in meteorology. He lives in Asheville, North Carolina, with his wife, Liz.
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