Skip to main content

100-Year Storm#

Rainfall Standards: The 100-Year Storm#

Floods will happen.

The City's floodplain regulations and capital improvement projects are designed to mitigate flooding.

A 100-year storm is the amount of rainfall measured at a certain location, during a specified length of time, that has a 1% chance in any given year of being equaled or exceeded.

It is normal in Colorado to experience many storms that locally equal or exceed rainfall defined as the 100-year storm. It is likely that between 100 and 300 "100-year storms" occur somewhere in Colorado in a typical year. This graphic compares 100-year storms to storms of other sizes.

What are the Odds?#

100-Year Flood
A 100-year storm (rainfall) does not always create a 100-year flood (runoff). Many variables exist that determine how much water runs off and where it goes.

In mapping the 100-year floodplain, officials assume that the 100-year storm occurs uniformly over the entire basin. Therefore, if a 100-year rainfall occurs in only an isolated location, it may not result in a 100-year flood.

Similar to the 100-year storm, the definition of the 100-year flood is the specific streamflow that has a 1% chance in any given year of being equaled or exceeded. That streamflow amount is determined from:

  • Long-term streamflow records, such as records for the Poudre River, or
  • Modeling of the runoff from the 100-year storm

Flood Risk
A property in the 100-year floodplain has a 1% chance in any given year of being flooded. Over a 30-year mortgage, that percentage jumps to 26%. As a comparison, only a 5% chance exists that the house will catch fire during that same 30-year period. This is why flood insurance is recommended. Some properties have an even higher risk of flooding since they are in areas where smaller, more frequent storms cause damage.

Event Odds of Occurring:

Structure in the 100-year floodplain being flooded in any given year

1 in 100

Matching 1 number plus the Powerball in the Powerball lottery

1 in 124

Structure in the 500-year floodplain being flooded in any given year

1 in 500

Annual chance of being killed in a car accident if you drive 10,000 miles a year

1 in 4,000

Being struck by lightning

1 in 600,000

Winning the Powerball lottery jackpot (matching 5 numbers plus the Powerball)

1 in 120,526,770

Did You Know?

Information about business-related programs and rebates regularly goes out in the Keep Current e-newsletter.

Setting your refrigerator to recommended 38-40 degrees Fahrenheit and freezer to 0 degrees Fahrenheit helps save energy.

Only pee, poo and toilet paper should be flushed. Everything else should be thrown in the trash.