In flood-frequency terminology, what annual exceedance probability defines a 100-year flood magnitude?
Correct Answer
C) 1% chance
A 100-year event means a magnitude with 1% annual probability of being equaled or exceeded under the model. It is not a prediction of one flood each century, nor does it prevent events in consecutive years. Parcel-specific risk depends on location and other evidence.
Why This Is the Correct Answer
Option C is correct because a 100-year flood is the flood having a 1 percent chance of being equaled or exceeded in any given year. The probability is 1 divided by 100 years. Such floods can occur in consecutive years, since each year carries its own 1 percent chance.
Why the Other Options Are Wrong
Option A: 10% chance
Option A, a 10% chance, corresponds to a 10-year flood (1 divided by 10), not a 100-year flood.
Option B: 5% chance
Option B, a 5% chance, corresponds to a 20-year flood (1 divided by 20), not a 100-year flood.
Option D: 0.1% chance
Option D, a 0.1% chance, corresponds to a 1,000-year flood (1 divided by 1,000), not a 100-year flood.
One Over the Years
Divide 1 by the return period: 1 over 100 years equals a 1 percent annual chance.
How to use: Use this division to convert any return period into an annual probability.
Exam Tip
Convert the return period into a percentage by dividing 1 by the number of years. Reject answers that describe a 100-year flood as a once-a-century event.
Common Mistakes to Avoid
- -Believing a 100-year flood occurs only once per century
- -Assuming a recent flood lowers next year's chance
- -Confusing the 10-year and 100-year probabilities
Concept Deep Dive
Analysis
This question tests the meaning of the term 100-year flood. It refers to a flood magnitude with a 1 percent chance of being equaled or exceeded in any given year. It does not mean the flood happens once per century.
Background Knowledge
Flood-frequency terms express the annual probability that a flood of a given size will be equaled or exceeded. The 1 percent annual chance flood is the base flood commonly used to identify special flood hazard areas. Actual risk at a parcel depends on its location, elevation and other evidence.
Real-World Application
A buyer tells the appraiser that the area flooded last year, so another 100-year flood is unlikely for decades. The appraiser notes the subject's flood zone designation and understands that each year carries the same 1 percent annual chance.
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