Assessing Your Flood Risk

Compare Risks

Your chances of being flooded are much greater than some other risks you face daily. If you live in a 100-year floodplain, there is more than a 1 in 4 chance that you will be flooded during your 30-year mortgage. And during a 30-year mortgage period you are 27 times more likely to experience a flood than having a fire.

Event % chance of happening during the next year
25-year flood 4 chances in 100
Involved in a
car accident
3 chances in 100
Some form of cancer 3 chances in 100
Victim of larceny 2 chances in 100
50-year flood 2 chances in 100
Victim of burglary 1 chance in 100
Injured in a car accident 1 chance in 100
100-year flood 1 chance in 100
Victim of auto theft 1 chance in 300
Victim of
aggravated assault
1 chance in 500
Victim of robbery 1 chance in 1,000
Residential fire 4 chances in 10,000
Killed in car accident 3 chances in 10,000

Floods and Your Family brochure,
U.S. Army Corps of Engineers

Understanding the "100-year floodplain"

The first thing you need to know about the concept of a 100-year floodplain is that it is based on a statistical probability needed by the insurance industry as a standard upon which to base insurance policies. Both the federal government and the private sector assist the insurance industry in gathering the scientific measurements that exist to generate a "*best guess" of stream flow peaks over a time. All this information goes into a formula/statistical model that generates elevations on tracts of land throughout a watershed that have what is termed a "one-in-one hundred chance (1 percent) of occurrence of flooding in any given year, or a "return period" of once every 100 years."

100-year floodplains are not arbitrary but they are:
1. Limited to the "best information at the time"
2. Not a determination of where and how frequently actual flood damage will occur.
3. Subject to change

With urbanization, better calculations and the lessons from recent flood events, many communities (like Boulder) are realizing they have more households in harms way from floods.

*We say "best guess" for several reasons, chiefly that streamflow data has only been collected for a maximum period of 150 years (much less in many areas) which is a small sampling in the context of regional weather patterns and actual flood events.

Source: The Flood Safety Project

  • Properties can also be at higher or lower risk within a floodplain depending where in the floodplain they are located. For example, one house in the 25-year floodplain may flood 2 feet deep during a storm, but their neighbor deeper in the floodplain may flood 6 feet deep.
  • If you are a typical homeowner, living in a single-story $100,000 home, without a basement, you can expect to suffer the following damages to your house and contents:
    1 foot of water = $14,000
    3 feet of water = $27,000
    Also add the cost of cleanup of mud and residue.

    Source: Floods and Your Family brochure, U.S. Army Corps of Engineers.

  • Myths:
    • It has never flooded here, so it never will.
    • There was a flood problem, but it has been fixed.
    • If flooding were a problem, someone would have told us.
    • It's only water. It's no big deal to be flooded.
    • The government will bail me out.
    • My homeowner's policy will cover any flood damage.
    • We just had a "100-year flood", so my family will be safe from future floods for the rest of their lives.
  • Facts:
    • We can't predict when floods will occur, but we can usually tell where they will occur.
    • Just because it hasn't flooded in the past doesn't mean it won't in the future.
    • Just because you had a flood does not mean it won't happen again soon.
    • Floods are caused by weather conditions and are unpredictable. If the conditions are right, floods will occur again.
    • It is almost impossible to "fix" a flood problem.
    • Real estate agents usually don't know whether flooding has ever occurred on the property.
    • Government assistance after a flood is usually limited to loans which have to be repaid. Who needs a second mortgage?
    • Figuring out how to cope with a flood is your responsibility.

    Source: Floods and Your Family brochure, U.S. Army Corps of Engineers.