(Solution) 100-Year Flood-It's All About Haven't We Already Had One This Century? Chance What Is A Flood? How Accurate Are Estimates Of The 1-percent A On The | Snapessays.com


(Solution) 100-Year Flood-It's All About Haven't we already had one this century? Chance What is a Flood? How accurate are estimates of the 1-percent A On the


can you please do this lab for me? instruction all in theworksheetGeneral Information Product 106

 

April 2010

 

100-Year Flood–It’s All About

 

Chance

 

U.S. Department of the Interior

 

U.S. Geological Survey

 

In the 1960’s, the United States government 

 

decided to use the 1-percent annual 

 

exceedance probability (AEP) flood as 

 

the basis for the National Flood Insurance 

 

Program. The 1-percent AEP flood was 

 

thought to be a fair balance between 

 

protecting the public and overly stringent 

 

regulation.  Because the 1-percent AEP flood 

 

has a 1 in 100 chance of being equaled or 

 

exceeded in any 1 year, and it has an average 

 

recurrence interval of 100 years, it often is referred to as the “100-year flood”.  

 

Scientists and engineers frequently use statistical probability (chance) to put a context to 

 

floods and their occurrence. If the probability of a particular flood magnitude being equaled 

 

or exceeded is known, then risk can be assessed. To determine these probabilities all the 

 

annual peak streamflow values measured at a streamgage are examined.  A streamgage is 

 

a location on a river where the height of the water and the quantity of flow (streamflow) 

 

are recorded. The U.S. Geological Survey (USGS) operates more than 7,500 streamgages 

 

nationwide (see map) that allow for assessment of the probability of floods. Examining all the 

 

annual peak streamflow values that occurred at a streamgage with time allows us to estimate 

 

the AEP for various flood magnitudes. For example, we can say there is a 1 in 100 chance 

 

that next year’s flood will equal or 

 

exceed the 1-percent AEP flood.  

 

More recently, people talk about 

 

larger floods, such as the “500-

 

year flood,” as tolerance for risk is 

 

reduced and increased protection 

 

from flooding is desired. The 

 

“500-year flood” corresponds 

 

to an AEP of 0.2 percent, which 

 

means a flood of that size or 

 

greater has a 0.2-percent chance 

 

(or 1 in 500 chance) of occurring 

 

in a given year.

 

               

 

A flood is any relatively high streamflow 

 

overtopping the natural or artificial banks in 

 

any reach of a stream. Floods occur for many 

 

reasons, such as long-lasting rainfall over a broad 

 

area, locally intense thunderstorm-generated 

 

rainfall, or rapid melting of a large snow pack 

 

with or without accompanying rainfall.  Because 

 

floods result from many different circumstances, 

 

not all floods are equal in magnitude, duration, or 

 

effect.   Placing floods in context allows society to address such issues as the risk to life and 

 

property, and to study and understand the environmental benefits of floods. Trying to place 

 

contextual framework around floods is where such terms as “100-year flood” came into 

 

being.

 

What is a Flood?

 

So what is a 100-year ±ood and how is it determined?

 

How accurate are estimates of the 1-percent A

 

Exceedance Probability (AEP) Flood (also know

 

100-year ±ood)?

 

Speaking of chance.

 

..

 

The “100-year 

 

flood” is an 

 

estimate of the 

 

long-term average 

 

recurrence interval, 

 

which does not 

 

mean that we really 

 

have 100 years 

 

between each flood 

 

of greater or equal 

 

magnitude. Floods 

 

happen irregularly.

 

Consider the 

 

following: if we 

 

had 1,000 years of 

 

streamflow data, 

 

we would expect to see about 10 floods of equal or greater magnitude than the “100-

 

year flood.” These floods would not occur at 100-year intervals. In one part of the 

 

1,000-year record it could be 15 or fewer years between “100-year floods,” whereas 

 

in other parts, it could be 150 or more years between “100-year floods.” 

 

The graph above shows how irregularly floods have occurred during the past 98 

 

years on the Embarras River near Ste. Marie, IL. The magnitude of the 10-year flood 

 

has been determined through statistical analysis to be approximately 31,100 cubic 

 

feet per second (ft

 

3

 

/s). You can see from the graph that the actual interval between 

 

floods greater than this magnitude ranged from 4 to 28 years, but the average of 

 

these intervals is about 10 years.

 

Admittedly, use of such terms as the “100-year flood” can confuse or unintentionally 

 

mislead those unfamiliar with flood science. Because of the potential confusion, the 

 

U.S. Geological Survey, 

 

along with other agencies, 

 

is encouraging the use of 

 

the annual exceedance 

 

probability (AEP) 

 

terminology instead of 

 

the recurrence interval 

 

terminology. For example, 

 

one would discuss the 

 

“1-percent AEP flood” as 

 

opposed to the “100-year 

 

flood.” 

 

Current streamfow conditions For the United States

 

are available on the World Wide Web:

 

waterwatch.usgs.gov

 

The accuracy of the 1-percent AEP flood varies depending on the amount of data available, 

 

the accuracy of those data, land-use changes in the river drainage area, climate cycles, 

 

and how well the data fits the statistical probability distribution. As a demonstration of 

 

the uncertainty in the estimates of flood probability, the flood probability relation for the 

 

Big Piney River near Big Piney, MO, is plotted in the figure below as the solid black line. 

 

Above and below that solid black line are two dashed lines that represent the 90-percent 

 

confidence intervals of this relation. These confidence intervals simply mean that we are 

 

90-percent confident that the true flood magnitude for a particular AEP lies between the 

 

confidence limit lines; or, there is a 10-percent chance that the true value lies somewhere 

 

outside the confidence interval lines. The 1-percent AEP flood (“100-year flood”) for the 

 

Big Piney River at this location has an estimated magnitude of 44,300 cubic feet per second 

 

(ft

 

3

 

/s). We know that 44,300 ft

 

3

 

/s is an estimate, but by looking closer at the graph, we 

 

can say that we are 90-percent confident that the true value of the 1-percent AEP flood is 

 

between 36,600 ft

 

3

 

/s and 56,400 ft

 

3

 

/s.

 

Most policy makers and water managers often are more concerned with the height of 

 

the water in the river (river levels) than the streamflow quantity. The uncertainty for the 

 

streamflow quantity of the 1-percent AEP flood for the Big Piney River can be translated 

 

into an uncertainty of the river level.  A streamflow of 36,600 ft

 

3

 

/s corresponds to a river 

 

level of 20.6 ft, whereas a streamflow of 56,400 ft

 

3

 

/s corresponds to a river level of 22.85 ft. 

 

Stated another way, the flood probability analysis reveals that we are 90-percent sure that 

 

the river elevation will be between 20.6 and 22.85 on the Big Piney River at Big Piney for 

 

the 1-percent AEP flood.

 

  

 

Solid line through data indicates

 

±tted Frequency curve; dashed lines

 

indicate 90-percent con±dence lim-

 

its For the Big Piney River near Big

 

Piney, MO.

 

The 1-percent AEP flood has a 1-percent chance of occurring in any given year; however, 

 

during the span of a 30-year mortgage, a home in the 1-percent AEP (100-year) floodplain 

 

has a 26-percent chance of being flooded at least once during those 30 years! The value of 

 

26 percent is based on probability theory that accounts for each of the 30 years having a 

 

1-percent chance of flooding.

 

On the river near me, we have had two 100-year ±oods

 

in 15 years…I really am confused about this 100-year

 

±ood stu².

 

Haven’t we already had one this century?

 

New Information and Additional Data

 

Installation of Flood Controls

 

Urban Development

 

The designation of the

 

“100-year ±ood” was

 

changed for my river

 

recently—Why?

 

Robert R. Holmes, Jr. and

 

Karen Dinicola

 

Author information

 

For more information on this poster contact:

 

O²ce oF SurFace Water

 

415 National Center

 

Reston, Virginia 20192

 

703-648-5301

 

ANNUAL PEAK DISCHARGE

 

CUBIC FEET PER SECOND

 

56,400 cubic feet per second

 

44,300 cubic feet per second

 

36,600 cubic feet per second

 

Upper band oF 90-percent

 

con±dence interval

 

Lower band oF 90-percent

 

con±dence interval

 

 100,000

 

 10,000

 

Annual Peak Streamflow

 

Cubic Feet Per Second

 

 1,000

 

99.5

 

98

 

95

 

90

 

80

 

70

 

50

 

30

 

20

 

10

 

5

 

1

 

.5 .2

 

        Annual Exceedance Probability, Percent

 

Cedar River at Cedar Rapids, IA

 

Annual Peak

 

The estimate oF the magnitude oF the 1-percent AEP (100-year) food using only the ±rst 20 years oF data

 

(1903-1922) is quite di³erent From the estimate using all available data (1903-2008).

 

This is an example oF

 

the value oF long-term streamfow data.

 

Collecting more data and updating the 1-percent AEP (100-year) estimate provides better inFormation For

 

agencies charged with managing food-prone areas and protecting liFe and property.

 

1-percent AEP

 

(100-year) flood

 

1-percent AEP (100-year) flood

 

1903 to 1922 data

 

1903 to 2008 data

 

Green River near Auburn, WA

 

Annual Peak

 

´lood control dams on the Green River in Washington State have

 

reduced the magnitude oF foods.

 

1-percent AEP (100-year) flood

 

1-percent AEP (100-year) flood

 

1937 to 1961 data

 

1962 to 2007 data

 

Boneyard Creek at Urbana, IL

 

Annual Peak

 

Urban development in Champaign-Urbana, IL has increased the magnitude oF

 

fooding oF the Boneyard Creek.

 

1-percent AEP (100-year) flood

 

1-percent AEP (100-year) flood

 

1948 to 1963

 

data

 

1964 to 2007 data

 

Incidence oF the 10-year food For the Embarras River at Ste. Marie, IL (03345500). The variability in time

 

between “10-year foods” ranges From 4 to as many as 28 years between foods.

 

17 yrs

 

11 yrs

 

5 yrs7 yrs7

 

yrs4 yrs7 yrs

 

28 yrs

 

“10-year ±ood”

 

31,140 CFS

 

6 yrs6

 

yrs

 

Annual Peak

 

17 yrs

 

11 yrs

 

5 yrs

 

7 yrs

 

7 yrs

 

4 yrs

 

7 yrs

 

6 yrs

 

6 yrs

 

28 yrs

 

10-year flood

 

31,100 cubic feet per second

 

yrs = years

 


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