Flood

What a flood is and the forms it takes

A flood is an overflow of water onto land that is normally dry. The National Weather Service tracks several types, and each one behaves differently and threatens different households.

Flash floods are the deadliest. They form within minutes to hours of heavy rain, dam failure, or a levee breach, and the NWS reports they cause more flood deaths than any other type because they give so little warning. River floods build more slowly, as rain and snowmelt swell a river until it spills its banks over days. Coastal floods come from storm surge, the wall of ocean water a hurricane or strong storm pushes ashore, and NOAA identifies surge as the leading cause of hurricane deaths. Urban floods are the quiet one. They happen anywhere pavement and drains cannot absorb heavy rain fast enough, which means a home can flood without a river or coastline anywhere near it.

Water finds the path of least resistance. It seeps through foundation cracks, backs up through drains and toilets, pours through doors and windows, and rises through the ground into basements and crawl spaces. Once it is inside, it soaks into everything porous and stays.

The damage runs deep for how shallow the water can be. FEMA reports that just one inch of floodwater inside a home can cause about $25,000 in damage. Water wicks up drywall, saturates insulation, warps flooring, and ruins furniture, appliances, and electrical systems. It also destabilizes the structure itself, weakening walls and floors that looked sound before the flood. The house holds the water long after the flood recedes, which is where the health problems begin.

Floodwater is not clean rain. It is a moving mix of whatever it crossed on the way to your home, and much of it is hazardous. The CDC warns that floodwater can contain sewage, and overwhelmed treatment systems and backed-up sewer lines routinely spill human waste into the water that enters homes and streets.

The rest of the mix is no better. Floodwater picks up gasoline, oil, pesticides, and household and industrial chemicals from garages, yards, and businesses it passes through. It carries bacteria, viruses, and parasites, including the organisms that cause diarrheal illness and, in rare cases, more serious infections. It hides physical hazards too, from sharp debris and broken glass to downed power lines that can electrify standing water. The CDC advises treating all floodwater as contaminated, because there is no reliable way to tell by looking what a given flood is carrying.

What flooding does to health, during and after

The immediate danger is drowning, and most of it happens in vehicles. The NWS reports that just 12 inches of moving water can carry away a small car, and 6 inches can knock an adult off their feet. Electrocution from downed lines, injuries from debris, and carbon monoxide poisoning from generators run too close to the home add to the toll during and right after a flood.

The slower harm comes from what the water leaves behind. Contact with contaminated floodwater can cause skin infections, and swallowing it can bring stomach and intestinal illness, the CDC reports. Mold is the exposure that outlasts everything else. The EPA reports that mold begins to grow on wet surfaces within 24 to 48 hours, and it triggers coughing, wheezing, and asthma attacks for weeks after a flood. The mental toll is documented too. The Substance Abuse and Mental Health Services Administration reports that flood survivors commonly experience anxiety, depression, and post-traumatic stress, and the strain of losing a home or facing a long recovery can last well past the physical repairs.

Who is most at risk

Flooding does not threaten every household equally. Older adults face the greatest danger, because they are less able to evacuate quickly and more likely to depend on power for medical equipment that fails in a flood. People with disabilities or chronic illness carry similar risk when floodwater cuts off medication, mobility, or powered devices.

Location and resources widen the gap. Roughly 13 million Americans live in a FEMA-designated high-risk flood zone, and many more face risk that official maps understate, according to research from the First Street Foundation. Low-income families often live in older or lower-lying housing, have fewer resources to evacuate or repair, and recover more slowly. Renters are frequently uninsured, since standard renter and homeowner policies do not cover flood damage. Children face added danger from contaminated water and from the mold and displacement that follow. People who have never flooded before are vulnerable in their own way, because more than 40% of National Flood Insurance Program claims come from outside high-risk zones, FEMA reports.

How flooding connects to other exposures

A flood rarely causes a single problem. It sets off a chain of them. Water is the link that ties flooding to nearly every other environmental exposure in the home, starting with mold. The moisture a flood leaves in walls, floors, and insulation feeds mold growth that the EPA calls one of the most common and lasting hazards a family faces after a flood.

The reach extends to the water you drink and the air you breathe. Floods damage water systems and knock out treatment plants, and boil-water advisories routinely follow major flooding when pipes break or lose pressure. Private wells are especially vulnerable, since floodwater can seep in and contaminate them, and the CDC advises testing a flooded well before drinking from it. Air quality suffers as mold, generator exhaust, and cleanup dust build up indoors. Floodwater can also drive contamination into soil and leave behind sediment laced with the chemicals and sewage it carried. One flood can turn a healthy home into a source of mold, unsafe water, and poor air at the same time.

What a household can do

Prepare before the water rises, because the most important steps take time to put in place. FEMA recommends knowing your flood risk first, which you can check by address through the FEMA Flood Map Service Center. Consider flood insurance, since standard policies do not cover flooding and a National Flood Insurance Program policy typically takes 30 days to take effect. Build an emergency kit with water, food, and medications for at least three days, sign up for local alerts, and keep insurance papers and identification in a waterproof container.

Move fast when a warning comes. If officials tell you to evacuate, go, and never drive or walk through floodwater, which can hide debris, live wires, and depths that are impossible to judge. Move valuables and important documents to a higher floor, and shut off electricity at the breaker if water threatens to enter the home. After the flood, treat all floodwater as contaminated. Wear rubber boots, gloves, and an N95 mask during cleanup, throw out porous items that soaked in floodwater, and dry out wet materials within 24 to 48 hours to stop mold before it starts. Do not drink tap water until officials lift any boil-water advisory, and test a private well before using it. Run generators outdoors only, at least 20 feet from the home, and watch for the hot skin and confusion of heat or the headache and dizziness of carbon monoxide while you work.