Why floods so often kill people in cars

The 13 North Carolina deaths attributed so far to Hurricane Matthew follow a familiar pattern: drivers’ fatal challenges of water-covered roads and bridges.

Flooding killed an average of 84 Americans a year between 2006 and 2015, the National Oceanic and Atmospheric Administration says. About half of them – the Weather Channel says nearly two-thirds between 1995 and 2010 – died in their vehicles.

Most made the worst possible decision: driving into water of unknown depth.

In Wilson, east of Raleigh, floodwater swept away the car of a 63-year-old woman driving home from work Saturday night. By the time rescuers found her, alive, she had clung to a tree for three hours.

Other drivers did not survive. Of the 13 confirmed deaths in North Carolina, state emergency-response officials said, nine or 10 resulted from drivers trying to navigate flooded roads.

They included a 19-year-old woman whose car was swept away Saturday night in flood waters on Interstate 95 in Johnson County. Her body was found early Sunday.

“As we have learned from past experience, the most deaths occur after the storm has passed,” state emergency management director Mike Sprayberry said in a statement Monday. “Citizens can reduce their risk of injury or harm by avoiding driving through flooded roads and knowing what action to take should flooding occur.”

Six inches of fast-moving floodwater can knock over an adult, NOAA says. Twelve inches of rushing water can carry away a small car.

Physics explains why the weight of a 3,000-pound car is no match for a foot of moving water. Cars float for the same reasons aircraft carriers do.

Moving water exerts pressure that grows as the water deepens, the agency explains. Physics says that the buoyancy of an object is equal to the weight of the water displaced by that object.

As floodwater rises up the side of a car, the displaced water can soon outweigh the vehicle, making it float. Mud and sediment on the road reduces friction force, making the surface slippery.

Two feet of water is enough to carry away most vehicles, NOAA says.

This story was originally published October 10, 2016 at 4:44 PM with the headline "Why floods so often kill people in cars."