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Types of Floods
Flash Floods: Flash floods usually develop shortly after a nearby heavy rain. The rainfall creates a rapid rise of water which accumulates in low-lying areas like floodplains. This often catches populations downstream by surprise, causing severe damage and loss of life.
Flash flooding may also occur due to ice jams, overflowing levees, or dam failures (described below). This is especially common in the western United States where normally dry areas near steep terrain might fill with rushing water.
River Flooding: River flooding, like flash floods, can be caused by heavy rainfall, dam failures, rapid snowmelt, and ice jams. Unlike flash floods, some river flooding may cause water levels to rise slowly but steadily. Either way, the result threatens structures and populations along its course.
Storm Surges: Tropical (or even non-tropical) storm systems can bring heavy winds; storm surge flooding, however, is responsible for more damage. Storm surge is the rise in water generated by the storm above normal tide levels. When the storm approaches the coast, high winds cause large waves that can inundate structures, damage foundations, and cause significant loss of life.
Burn Scars: The Western U.S. has, in recent years, experienced significant wildfire activity. After a fire, the bare ground can become so hardened that water can’t be absorbed into the ground. This is known as a “burn scar”. Burn scars are less able to absorb moisture, leading heavy rains to accumulate water wherever gravity takes it.
Ice Jams: Northern areas of the continental U.S. and Alaska may have flooding as a result of ice jams. When waterways are blocked by an obstruction, such as ice and debris, water is held back. This causes flooding upstream. When the obstruction is finally removed, flash flooding occurs downstream. Many ice jams occur at bends in a river.
Snowmelt: Snowmelt flooding is common in mountainous northern U.S. states. Until temperatures rise above freezing, snow is just stored water. When it gets warmer, the snowmelt acts as if it were rain and flooding can occur.
Barrier Failures: When a dam or levee breaks, it can be due to excessive rainfall, erosion, landslides, earthquakes, and many other natural causes. Some dams fail as a result of man-made issues, such as negligence, improper maintenance, and even sabotage. As a result, water levels can overflow the barrier or seep through defects in the structure. Failures of infrastructure, such as the sudden collapse of a dam, may cause catastrophic flooding: The 1889 Johnstown, Pennsylvania event took 2,200 lives.
Major flooding may also be caused by the effects of an earthquake or even a volcanic eruption. If these events occur offshore, they may result in tidal waves (“tsunamis”) which have both short- and long-term effects. Short-term issues which cost lives include the inability of emergency medical services to access flooded areas. Penetration of saltwater causes widespread failure of freshwater delivery systems which cause crop failures and may make farmland infertile, sometimes for years. When agriculture and certain industries fail, an economic collapse in the affected area may be the result. Civil unrest often follows.
The nightmare scenario above is not purely hypothetical: Disasters related to flooding have sometimes caused millions of casualties. In 1931, major flooding of rivers in China resulted both directly and indirectly in three million deaths. If you take out war, famine, and pandemics, flooding is, potentially, the deadliest of human tragedies.
FLOOD SAFETY
Most people have heard of hurricane or tornado watches and warnings, but the U.S. weather services also tries to warn the populace of flooding. Just like storm alerts, a “flash flood watch” means that flash flooding is possible in the near future; a “flash flood warning” means that flooding is imminent or already occurring in the area.
To make it safely through a flood, consider the following recommendations:
Pay attention to flood alerts: If you live in a low-lying area, especially near a dam or river, you should heed warnings when they are given and be prepared to evacuate quickly. Timing is important: Make the decision to leave for higher ground before flooding occurs and roads are blocked. Rising flood waters could easily trap people in their home, and you don’t want to have to perch on your roof waiting for help. Having a NOAA weather radio will keep you up to date on the latest advisories.
Be careful walking Through flood Waters: Drowning is the most common cause of death during a flood, especially a flash flood. Rapidly moving water can knock you off your feet even if it’s less than a foot deep. The calmest flood waters are often murky and hide debris that can cause injuries if you walk through them.
Wounds exposed to contaminated flood water are more likely to become seriously infected. Wash wounds immediately with clean drinkable water, cover with dry sterile dressings, and change dressings twice daily. Keep dressings dry and be sure to clean with drinkable water during each dressing change until fully healed. See the section on wound care for more information.
Don’t drive through flooded areas: Roads and bridges could easily be washed out if you wait too long to leave the area. In a flood, many people drown in their cars as they stall out in moving water. Average-sized vehicles can be carried away by water just two foot deep. In one recent event, 130 people required rescue from partially submerged cars. Try to plan a “high road” route to safety before a flood occurs.
Beware of downed power lines: Electrical current is easily conducted through water, so beware of downed power lines. You don’t have to physically touch the downed line to be electrocuted, only step in the highly conductive water nearby. There are numerous instances of electrocutions occurring as a result of rescuers jumping into the water to try to save victims of a shock.
Never touch someone who has been electrocuted without first shutting off the power source. if you can’t shut off the power, you will, somehow, have to move the victim. Use a dry nonmetal object, such as a wooden broom handle or rope. Be aware that, although dry wood may not be conductive of electricity, wet wood conducts it very well and can kill you.
Don’t drink the water: Flood water is not clean water. It is contaminated by debris and water treatment plants may have been compromised by the disaster. As a result, water-borne illness such as cholera may run rampant unless the water is disinfected. Have a reliable way to purify water and a good supply of clean water stored away. It is, indeed, a medical supply item. 8 drops of 6% household bleach will sterilize a gallon of water (1/3 teaspoon for 4 gallons), but twice the amount will be needed for cloudy water. Remember that a filter might also be needed to eliminate debris. Wait 30 minutes after adding bleach before drinking. This and other water disinfection methods are discussed in the sanitation section of this book.
Have supplies handy: Flood waters may not recede quickly. Besides water, have non-perishable food, bottled water, heat and light sources, batteries, tools, extra clothing, a medical kit, a cell phone, and a NOAA weather radio among your supplies. Don’t forget extra prescription medications.
Turn off the power: If you have reason to believe that water will get into your home, turn off the electricity before the flooding occurs. If the water reaches the level of the electric outlets, you could easily get electrocuted. Some warning signs might be sparks or strange sounds like crackling, popping, or buzzing.
Beware of intruders and pests: Animals that have been flooded out of their homes may seek shelter in yours. Mosquitoes and the diseases they carry will be on the rise as flood waters increase potential breeding grounds. Use insect repellent, mosquito netting or stay inside air-conditioned buildings if possible. Snakes, raccoons, and other refugees may decide your residence is now their territory. Beware of human intruders as well; they may be interested to see what valuables you left behind.
Watch your step: After a flood, watch where you step when you enter your home; there will, likely, be debris everywhere. The floors may also be covered in mud, causing a slip-and-fall hazard.
Check for gas leaks: Don’t use candles, lanterns, stoves, or lighters unless you are sure that the gas has been turned off and the area is well-ventilated.
Avoid exhaust fumes: Only use generators, camping stoves, or charcoal grills outside. Their fumes can be deadly.
Clean out saturated items completely: If cans of food got wet in the flood, their surfaces may be covered with mud or otherwise contaminated. Thoroughly wash food containers, utensils, and personal items before using. Don’t use appliances or motors that have gotten wet unless they have completely dried. You might have to take some apart to clean the debris out.
Mold is going to be an issue after the water recedes. Remove all materials susceptible to mold growth as soon as possible. For things that can’t be removed, clean with a 10% bleach solution.
Use waterproof containers for Important items: Waterproof containers can protect food, personal items, important documents, and more.
Joe Alton MD
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