Every year in the United States flash flooding causes some $3 billion dollars in damages and kills between 100-150 people. And while the damage is not likely to be avoidable most of the deaths are. Because many people do not understand the realities of flash flooding hazards they don’t take proper precautions, and find themselves in danger that could have been avoided.
A definition: A flash flood is a flood that occurs because of heavy rainfall over saturated ground, where the ground can no longer absorb any more water. This can be because of an unusually damp turn of the weather, a microcell opening over a local area, or rain over frozen ground or permafrost, or heavy rains in an urban or developed area where the prevalence of concrete and asphalt prevents the water from going into the ground. This kind of flooding is different from a river flood, where a river is receiving for an extended period of time more water than it is able to pass on downstream. River floods generally take time to build up, and allow for preventative measures, or at least limiting measures, can be taken. A flash flood on the other hand often occurs rapidly, with little warning, and is usually in a narrow locale.
One of the most dangerous myths about flash floods is that they only happen when it’s raining. This is not true: There only needs to be rain coming down further upstream for flash flooding to flow down a dry arroyo. This sort of flash flooding occurs most often in the southwest, but it is possible most anywhere there is a path for water to come down from a higher elevation.
Another flash flooding myth is that if an area has had a ‘100 year flood’ within the past several years, the area is guaranteed to be safe from another such flood for several years. The ‘100 year flood’ is only a statistical tool used to judge flood potential for a given area – not any kind of statistical guarantee. In some areas of the country ‘100 year floods’ have occurred three years in a row. A better way to look at the term ‘100 year flood’ is to realize that each year there is a 1% chance that there may be flooding of that severity in the area.
Also related to the ‘100 year flood’ concept: Just because an area nearby had had it’s ‘100 year flood’ last year doesn’t mean that your area is less likely to have a ‘100 year flood’ this year. Remember, it’s an average concept, and only an estimate at that.
Finally, the most deadly flash flood myths are those relating to driving a car through a flooded area. Some people believe that if they know what the pavement hidden under the flood waters should look like, they can know how much water they would be attempting to drive through. This is dangerously false, there is no guarantee that the road way still exists under the water, or that the roadbed hasn’t been undercut by the flood waters, just waiting for a little additional weight to make it collapse. IF YOU CANNOT SEE THE BOTTOM OF THE FLOODED SECTION, DO NOT ATTEMPT TO CROSS IT!
In general, for every foot of flood water it will lower the effective weight of your vehicle by 1500 pounds, so two feet of water make it unlikely for any car to be able to keep on the road surface. Another rule of thumb to consider: a foot of water moving a mere 8 miles per hour exerts 264 pounds per square foot force on anything in its path. Never try to walk or drive through moving flood waters more than 4-6 inches deep, and never try driving through standing water more than 12 inches deep.
