What is a 1-100 year flood?

In flood hazard mapping, some areas are indicated as being at risk for a 1-100 year flood. Flood mapping is used to illustrate risk in certain areas to allow municipalities and communities to plan for the possibility of flooding.

Frequently the meaning of 1-100 year flood is misinterpreted as meaning a flood will occur in the highlighted area only once every 100 years. 1-100 year is actually a statistical designation meaning that in any given year there is a 1% chance a flood risk area will flood. It is similar to rolling a 100 sided dice each year where the number “1” means the area will flood. Further, some areas have experienced more than one 1-100 year flood in the same year.

 

Recurrence interval in years

Probability of occurrence in any given year

Percent chance occurrence in any given year

The same as rolling a 1 on a ____ sided dice.

250

1-250

0.4%

250

100

1-100

1%

100

50

1-50

2%

50

40

1-40

2.5%

40

10

1-10

10%

10

6

1-6

16.7%

6

 

Multiple 1-100 Year Floods In Less Than 100 Years

Every year there is a risk that the flood hazard area may experience a flood. These areas might not be affected by floods for decades, while some decades may see multiple floods or even more than one flood in a year. So why do multiple 1-100 year floods happen in short timespan? Sometimes the unlikely happens and the 100 sided dice rolls a 1 twice in a row. Other times it’s due to human influence on the watershed, or an initial assessment that did not have enough data.

 

Unlikely is Possible; and Increased Time, Increased Risk

The amount of time spent in an area identified as being at risk for flooding increases the likelihood of being impacted by a flood. The following table illustrates the likelihood of an identified area being affected by at least one flood over various periods of time.

Years

Likelihood of a flood

   
 

1-50 (2%)

1-100 (1%)

1-250 (0.4%)

10

18.3%

9.6%

3.9%

30

45.5%

26.0%

11.3%

50

63.6%

39.5%

18.2%

70

75.7%

50.5%

24.5%

100

86.7%

63.4%

33.0%

130

92.8%

72.9%

40.6%

150

95.2%

77.9%

45.2%

250

99.4%

91.9%

63.3%

 

Changes to the Watershed

Historical records of river activity are often used to calculate the 1-100 year; however, changes to the river and riparian areas over time may influence present day river activity. A river that has had one or more dams installed, several kilometers of wetlands and riparian areas removed, or rapid urbanization will behave differently. It is possible that there is now a systematic error in the calculation vs modern day events. Figure 1 demonstrates the new designation given as more data is collected and re-evaluated by scientists.

 What is a 1-100 year flood?

Increased urban development in the Mercer Creek Basin increased the streamflow and therefore changed the data to be considered.[1]

 

New Information and Additional Data

If the historical data being used happens to represent a period where there was lower or higher number of floods than normal, then by random chance the initial calculation is incorrect.  

 What is a 1-100 year flood?

New data over time changed the expected magnitude of a 1-100 year flood in the Chehalis River. [2]

 

[1] Dinicola, Karen. (2014). The "1OO-Year Flood". U.S. Geological Survey. Retrieved fromhttp://pubs.usgs.gov/fs/FS-229-96/

[2] Ibid.