6g if a 5 Year Flood Occurs This Year – What Is the Chance That One Will Happen Again Next Year
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Flood frequency analysis uses historical records of peak flows to produce guidance about the expected behavior of future flooding. Two primary applications of flood frequency analyses are:
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In this section you will learn to:
- Define the meaning of return period (i.e., the 100-year flood).
- Explain the exceedance probability and its relationship to return period.
- Understand the two primary applications of flood frequency analyses.
Topics in this section include:
Flood Frequency Analysis in the Flood Prediction Process
Basic Terminology
Time Distribution of Floods
Flood Frequency Misconceptions
Estimate of Flood Magnitude
Estimate of Return Period
Review Questions
- Choose a section above by clicking it, or scroll down to continue the module.
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Role in Flood Prediction Process
Flood frequency analysis provides information about potential floods based on either current or forecast conditions. It also uses information from past floods to update statistical representations of possible future flood events.
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Basic Terminology
Many people have heard terms like the "100-year flood" or the "500-year flood" used in the news media and within professional organizations like the National Weather Service and emergency management agencies. Although many are familiar with these terms, not everyone really knows what they mean.
Let's look at the term "return period," also referred to as the "recurrence interval." The return period is the time period over which it is likely that a particular magnitude flood will occur. Thus, a 100-year flood is defined as a flood that can occur on average once every 100 years. In this example, 100 years is considered the return period. However, floods do not occur in exact cyclic events. That is, they don't occur at nicely spaced 100-year intervals.
Another way to think of the 100-year flood is of a flood that has a one one-hundredth, or 1 percent, chance of occurring in any given year. This is called the exceedance probability or the probability of occurrence, and is simply 1 divided by the return period.
A 100-year flood is less frequent but larger in magnitude than a 25-year flood.
The 25-year flood occurs on average once every 25 years and has an exceedance probability of 1 over 25, or 4 percent, in any given year. That's a 4 times greater chance than the 100-year flood, which only has a 1 percent exceedance probability in any given year.
The 100-year flood is smaller in magnitude and 5 times more frequent than the rare 500-year flood, which has a 1 over 500, or 0.2 percent chance of occurring in any given year.
Additional Resources:
The "100-Year Flood"
U.S. GEOLOGICAL SURVEY Fact Sheet 229-96
by Karen Dinicola
http://pubs.usgs.gov/fs/FS-229-96/
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Time Distribution of Floods
The 2-year flood occurs on average 25 times over a 50-year period. To meet this definition, the time distribution of the 2-year flood could be evenly spaced, that is, every two years. The 2-year flood definition could also be met if the flood occurs every year for the first 25 years in a row, and then not at all for another 25 years. However, either of these scenarios is very unlikely. The most likely time distribution of the 2-year flood would be unevenly distributed over the 50-year period. The 2-year event may have several consecutive years of occurrences, or there may be multi-year periods with no occurrences.
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Flood Frequency Misconceptions
Just as the 25-year flood would occur on average once every 25 years, the 25-year rainstorm would also occur on average once every 25 years. A common misconception is that a 25-year rainstorm will always produce a 25-year flood.
While this may be a good first guess, the conditions on the ground greatly affect the runoff generated from any particular storm.
Click the image above to view animation.
For example, if the soil is saturated, it is possible that a 25-year rainfall would become runoff so efficiently that it could result in a runoff event larger than the 25-year flood. Likewise, dry ground conditions could mitigate the impact of a large rainstorm and produce a smaller flood.
Another common misconception is that a 100-year flood occurs only once every 100 years or will occur only once during a 100-year time span. In truth, there is an equal (and equally unlikely) chance for a flood of this magnitude to occur in any year or even multiple times in a single year.
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Estimate of Flood Magnitude
There are two basic applications for flood frequency analysis. The first is to estimate the potential flood magnitude that can occur in a given time interval.
For example, we may want to know the peak flow expected over a 100-year period at a certain location. Using a fictitious example, an engineer may ask, "What is the magnitude of a 100-year flood at the Main Street Bridge over Spring Creek?" The information from flood frequency analysis provides an answer such as: the 100-year flood has an estimated magnitude of 30,000 cubic feet per second, or 840 cubic meters per second.
This information can then used by a hydrologist or by a civil engineer to determine the size of a bridge clearance. It's possible that the opening under the bridge may need to be large enough to accommodate large, infrequent flows.
Here we see an example of flood debris from a flood that was obviously greater than the flood for which the bridge was designed.
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Estimate of Return Period
The second primary goal of flood frequency analysis is to estimate the return period for a certain magnitude flood.
To continue with our example, let's assume that we know flooding will occur at the newly proposed Plaza Mall if Spring Creek reaches a peak discharge of 20,000 cubic feet per second (560 cubic meters per second).
We may ask, "What is the return period, or recurrence interval, of 20,000 cubic feet per second (560 cubic meters per second)?" Flood frequency analysis can then tell us that such a flow has a 50-year return period. In other words, 20,000 cubic feet per second (560 cubic meters per second) is a 50-year flood. This information can be used by people such as urban planners and civil engineers to determine the best locations and construction practices for new development.
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Review Questions:
1. Flood frequency analysis is an important element in _____.
(Choose the best answer.)
a) designing transportation infrastructure such as bridges and culverts
b) estimating how often a certain magnitude flood may occur
c) estimating the flood magnitude of a particular probability
d) all of the above
2. The 100-year flood is _____.
(Choose all that apply.)
a) less frequent than the 500-year flood
b) lower magnitude than the 500-year flood
c) one that occurs once every 100 years
d) one with a 1 percent chance of occurring in any given year
3. A 100-year rainstorm will always produce a 100-year flood.
(Choose the best answer.)
a) True
b) False
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Review Question Feedback
1. Flood frequency analysis is an important element in _____.
(Choose the best answer.)
a) designing transportation infrastructure such as bridges and culverts
b) estimating how often a certain magnitude flood may occur
c) estimating the flood magnitude of a particular probability
d) all of the above
The correct answer is d).
2. The 100-year flood is _____.
(Choose all that apply.)
a) less frequent than the 500-year flood
b) lower magnitude than the 500-year flood
c) one that occurs once every 100 years
d) one with a 1 percent chance of occurring in any given year
The correct answers are b) and d).
3. A 100-year rainstorm will always produce a 100-year flood.
(Choose the best answer.)
a) True
b) False
The correct answer is b).
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End of Section One: Introduction to Flood Frequency Analysis
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Source: http://stream1.cmatc.cn/pub/comet/HydrologyFlooding/flood/comet/hydro/basic/FloodFrequency/print_version/01-introduction.htm
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