Guide to Flash Floods: What to Do Before, During and After

A flash flood usually occurs within six hours, but sometimes as little as three. It can happen due to heavy rain, a levee or dam breaking or mudslides.

There are a few factors that determine how severe a flash flood can be:

• Where the rain falls and its frequency.

• Force of the rain.

• Soil condition (if the ground is very dry or wet).

• Soil type (rocky or clay earth doesn’t soak up the rain as quickly as other soil types).

• What the land is being used for (a forest, rural, construction site, etc.).

What damage can a flood cause?

Because flash floods move swiftly, they can carry even large items, causing significant damage to your home and property.

What kind of damage can a flood do?

Structural damage includes cracks along the foundation or ceiling or the floor buckling. In extreme cases, the building could shift or entirely collapse.

Property damage can include floodwaters uprooting trees and shrubbery, damage to your septic tank or well, or cracks or potholes in your driveway.

Damage to your electrical system can cause power outages or exposed wires. If you leave appliances plugged into the grid, they can short out and be permanently damaged.

Debris or sediment may be left after the floodwater drains, especially if your home is downhill. Floodwater can potentially damage the soil or surrounding area, leaving behind rocks or branches.

Blackwater can contaminate your home or belongings, depositing disease-causing germs or leading to other potential health hazards.

Mold can start growing in a day or two from exposure to water or even from a simple leak inside your home. Mold can be difficult to remove and it can affect the health of infants, children and anyone else with allergies or lung issues.

Where is flash flooding most common?

Flash flooding can happen anywhere, from city streets to your backyard. If you live in a flood zone area or you receive a Flash Flood Watch, you should always exercise extreme caution.

Top 5 states most at risk for flooding

• 50.56% of Louisiana is in a flood zone area, encompassing a little less than 25,000 square miles.

• Florida also has a high flood risk zone, since 40.08% of the state is within the flood state hazard area.

• 23.14% of Mississippi is in the flood risk zone. 

• Arkansas has 22.57% of its lands in a flood hazard area.

• 19.27% of New Jersey is in a flood hazard area, making it the fifth most at-risk state.

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Do you live in a flood zone? Check FEMA’s Flood Map to find out.

Flash floods can happen in urban areas.

Especially where there are a lot of buildings, highways, parking lots or other manufactured structures. If there isn't enough soil around to absorb the rain, runoff can happen along buildings and concrete, sometimes even creating small rivers of water. Urbanizing areas increase runoff two to six times more than in natural settings [2].

Rural or forested areas are also prone to flooding.

• Mountains, sheer valleys or steep hills can create rapid runoff or quickly rising waters.

• Canyon streams or rivers can swell during heavy rainfalls, leading to flash flooding.

• Soil that has experienced recent rainfall and is already saturated can overflow.

• Overly rocky areas can prevent water from being absorbed into the ground.

• Water runoff can occur where a recent controlled or uncontrolled burn took place.

• Low river or other water crossings can become dangerously waterlogged during heavy rain and lead to flash flooding.

Be careful when camping near streams or rivers.

In less than an hour, a thunderstorm can turn that 6-inch creek into a 10-foot raging river. And even if you don't immediately see an increase in the water level, that may be because it's happening upstream from your current encampment [3].

Low-lying areas are prone to both floods and flash floods in residential and rural settings. 

Areas at the bottoms of hills or mountains can become incredibly dangerous, especially if the site has rocky soil. 

You should also be cautious when going under underpasses, basements or underground parking garages, which can trap you in quickly-rising water.

When rainwater collects inside storm drains or ditches, it can carry debris.

Debris may get clogged inside drains during heavy rainfall, leading to an increased risk of flooding.

If a dam or levee fails.

This can lead to devastating amounts of water being released into the surrounding area in a short period.

Snowmelts or ice jams can cause sudden increases in the water level.

A snowmelt flood happens when the snow melts due to fluctuating temperatures. Because snowmelt slowly releases water over time, some water continues to be stored until it completely melts away. If snowmelt combines with rain, flash floods can occur.

An ice jam can happen on a river or lake when melting ice is obstructed, and it can't correctly drain or flow away. As the snow melts or a sudden rainstorm occurs, it may increase the riverbed and break up pieces of ice that get stuck against bridges or other natural obstructions. The water behind the ice accumulates until the ice suddenly breaks or the water overflows, leading to flash flooding.

Why is contamination such a risk with floodwater?

When water spreads through roads, sewers and yards, it can pick up sewage and other contaminants that spread as the waters rise. Sometimes, this type of water is called brown or blackwater, or Category 3 Water.

You should never drink this water, as it can house E. coli, Salmonella, or other harmful bacteria.

What to do before a flash flood?

If you know that a flash flood is imminent, it’s crucial to monitor the situation through local weather stations, radio, or the NOAA website.

If a Flash Flood Warning is issued, you should take it seriously and stay calm. If it evolves into a Flood Warning, you should make your way to higher ground. 

What types of flash flood warnings are there?

There are three central flash flood warnings: Flash Flood Watch, a Flood Advisory and a Flash Flood Warning.

A Flash Flood Watch means that the current weather conditions are favorable for a flash flood, but it may not necessarily happen. Keep an eye on changing weather patterns and be prepared. 

A Flood Advisory is issued if a minor flood is expected, but it may not reach flash flood levels. If you're living in a low-lying area or driving during this time, you should take extreme caution and continue to monitor the situation. You should never drive on a flooded road.

A Flash Flood Warning means that a flash flood is going to happen or is already happening. This is a potentially life-threatening situation, so you should move to higher ground, especially if you're near a low-lying area.

Before a flash flood happens, there are a few things you can do to help prepare yourself and your home:

1. Practice an emergency escape plan and ensure that family members know what to do during a flood and where to go if your home becomes inaccessible.

2. Prepare an emergency kit with food, water, extra batteries, NOAA weather radio and other crucial first aid items.

3. Store important documents in a fireproof and waterproof safe or container. These could include immunization records and other medical records, deeds, tax returns, social security cards, passports and wills; any document you don't want to get lost in a flood should go in this container.

How can you help protect your property from flood damage?

You should know your area's risk for flood and be aware of any rivers or streams on your property. If you're in a high-risk flood area, you may be required to have flood insurance.

If there is an active stream or even a dry streambed on your property, it’s good to remove any large debris that might impede water flow and lead to excess flooding towards your home. 

• If you have any outdoor furniture that might be carried away from a flood, like your grill, trash cans, patio furniture or decorations, you should bring them inside. 

• Add flood detectors or sensors to your basement, garage or other areas that may experience flooding. Flood detectors can send you instant alerts if water is leaking.

• You should place appliances or other important items on raised surfaces like tables, beds, or inside cabinets.

• Seal your basement with waterproofing materials to prevent leaks and cracks from developing during a flood.

• If you live in a flood-prone area, you may want to construct a barrier or levee to divert water away from your home. You should consult with your local laws and regulations regarding any type of construction like this.

• Plug up showers or drains with approved flood stoppers to help prevent sewage leaks or water backup.

Resources for flash flooding:

• The National Weather Service constantly updates its “Watches, Warning and Advisories” page.

• USGS Surface-Water Data for the Nation can be found here and here.

• The Red Cross “Find Open Shelters” page can help you stay safe during an emergency

What to pack in an emergency flood kit?

Make sure you have enough supplies for at least three days, but a week or two is better. Keep an eye on expiration dates on canned goods and throw out any that appear dented, swollen, rusted or corroded. You should check your food supplies every six months and replace them if they're expired.

Bottled water doesn’t have a shelf life if stored properly. Though the FDA doesn’t require an expiration date on bottled water, the CDC recommends replacing this water every six months.

• You should have at least one gallon of water for drinking and sanitation for every day and every person or pet. 

• Consider packing non-perishable foods that don't need refrigeration or water and require minimal cooking. This could include canned tuna, chicken or soup, dried fruits and nuts, peanut butter, powdered milk or juice or dehydrated camping meals.

Pro-tip: Don’t forget to add nonperishable pet food to your emergency kit for your furry friend, too!

• Pack a manual can opener, disposable utensils, plastic wrap, paper towels, toilet paper, portable camping stove, extra fuel canisters and matches. Exercise caution when using a portable camping stove and never use a stove with natural gas, propane, gas, charcoal or kerosene indoors.

• An NOAA weather radio with extra batteries.

• Hand-crank flashlight or flashlight with extra batteries. 

• A medical kit that includes emergency blankets, hand sanitizer and extra medications.

• Whistle, siren or noisemaker if you need to alert someone of your whereabouts.

What to do during a flood?

If you’re experiencing a Flood Warning, make sure your phone is charged, so you can keep an eye on weather conditions as they change. Keep your NOAA Weather Radio charged and ready, too.

Do

• Stay calm and assess your surroundings.

• If flooding is immediate, move to a shelter or higher ground.

• If you have to evacuate, you should turn off your home’s electrical supply and switch off the gas.

• Take your emergency flood kit and other important documents as you evacuate.

• Avoid walking or driving near sewage lines, gutters, drains and anything else meant to channel water.

• Cover food once you’re done eating it to reduce contaminants from the water getting into your food.

• Add a floor drain plug or standpipe to your basement to help prevent sewage backups.

Don’t

• Drive onto flooded roads or through standing water.

• Avoid walking through floodwater, especially deep or dark water.

• Drink flood water or eat anything that may have encountered contaminated water.

• Go near any electrical poles, fallen power lines, trees or debris.

• Use generators or camping stoves indoors, since this can increase your risk of carbon monoxide poisoning.

• Allow small children to play in any floodwater or blackwater.

• Use the toilet if the sewer line or pipers are damaged.

• Come home until officials allow it.

What to do after a flash flood?

Be on the lookout for any warnings from the Health Department so you know when it's safe to drink tap water again or go back home. If your property and home are severely damaged, you may want to consult with flood specialists before moving back home.  

Besides drinking water, you should also avoid using it to wash dishes or clothes or even brush your teeth. If there's a Boil-Water Advisory, use only bottled water or treat your water accordingly. 

If you believe any food or water has become contaminated or come into contact with floodwater, you should throw it out!

What is a Boil-Water Advisory?

If the local authorities have announced a Boil-Water advisory, you can use bottled water or otherwise boil or treat the water. 

If you’re boiling water, make sure that it achieves a rolling boil for at least a minute, then drink or store the water once it’s cool.

ADT can help protect your home against water leaks with flood sensors.

Flood detectors can sense early detection of leaks in your home, whether it's from damaged pipes or an overflowing washing machine. ADT can help you keep an eye on those conditions in your home with automatic updates sent straight to your phone.

Contact ADT today for more home safety solutions.