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Floodwater: Road Crossing Hazards

Updated September 12, 2019
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Alvaro Garcia

SDSU Extension Agriculture and Natural Resources Program Director & Professor

If you are among those who take precautions, quit golfing, and look for shelter during a thunderstorm, think about the following statistic. According to the National Weather Service, twice as many people die each year from flash floods in the U.S. compared to lightning! Flash floods pose an inherent danger because of the force of the running water. A water current of just six inches deep can throw you off balance with relative ease. It can also carry debris and other inert objects that can make wading through even more hazardous. According to the National Weather Service statistics however, the majority of the casualties (64 percent) happen in cars, compared with outdoor recreation (22.5 percent) or homes/buildings (9 percent).

We all have places to go—be sure to get there safely!

During flooding, and when driving in the countryside we oftentimes encounter a creek or stream running on top of the road. Things that should immediately come to the driver’s mind are: water depth, strength, and if the road is firm and safe to cross. It is sometimes difficult to assess the risk of crossing, and a typical mistake is to misjudge the water's depth and the soundness of the underlying surface.

The Federal Emergency Management Agency (FEMA) shares these tips on water depth:

  1. Six inches of water reach the bottom of a conventional car.
  2. At one foot deep, the conventional passenger car will start to float. Bear in mind that, according to the U.S. Geological Survey, one foot of water carries a force of 500 pounds. Thus, there is a great likelihood that the water current will sweep the car away. The water can sweep you away as well should you decide to step out of the vehicle.
  3. At two feet, the running water will sweep a vehicle down current almost right away.

First thing’s first.

A car being towed a flooded, washed out gravel road by a national guard truck. Photo by Senior Master Sgt. David H. Lipp, U.S. Department of Defense
Photo by Senior Master Sgt. David H. Lipp, U.S. Department of Defense

If we are crossing and the running water starts to sweep the vehicle, the first thing to consider is to avoid ending up trapped inside the vehicle. Before calling for help, unbuckle yours and the other passenger’s seatbelts, unlock the doors, and open the windows. This will help you be ready in case the car ends up getting dragged into deeper water. Do not attempt to contact emergency help before increasing your chances of exiting the car. Every second counts at this point! If you fall in the current, you are going to get soaked in frigid water, so it is important to understand what the risks are if it happens. The body's cold shock response happens first and usually between 2 to 3 minutes following immersion. The time it takes for it to occur varies between individuals, and it is usually associated with body mass. Heavy-built subjects will tolerate cold water longer compared to their skinnier counterparts. A state of panic with gasping and hyperventilation characterizes this stage. The first thing a person needs to do is to remain as calm as possible, in an attempt to get his or her respiration rate under control. People that are afraid of water are predisposed to enter this stage sooner, as panicking compounds the problem.

Be aware that a course of water running over the road can turn into a very dangerous, even life-threatening situation if you attempt to cross it with your vehicle. There is likely no place one needs to get to that urgently as to risk life. The best advice? “When in doubt back out!”