FLOOD PROBABILITY- POTENTIAL WORKSHEET QUESTIONS
Recurrence Interval in Years
100
50
25
10
5
1
Probability of Occurrence in
Any Given Year
1 in 100
1 in 50
1 in 25
1 in 10
1 in 5
1 in 2
Percent Chance of Occurrence
In Any Given Year
1
2
4
10
20
50
http://ga.water.usgs.gov/edu/100yearflood.html
1. In the table, rank the discharges in order from (1) being the largest discharge to (35) being the
smallest discharge. Fill these numbers in the rank column.
2. After you have filled in the rank column, calculate the recurrence interval using the Weibull
equation RI=(n+1)/m, where n is the number of years data was collected (35 years in this case).
Fill in the Recurrence Interval column with these values.
3. Use the graph on the next page to plot your data. Discharge values plotted on the vertical axis
(y-axis) and Recurrence Interval values on the horizontal axis (x-axis). The x-axis is a logarithmic
scale, so try to best estimate where the point will fall.
4. Once all points have been plotted, draw a best-fit line through the data points. Be sure to draw a
line that represents all data points accurately.
5. Calculate the probability that the river will flood (i.e., overtop its banks) in any given year. Add
these values to the Probability column. Probability is simply the inverse of the recurrence
interval: % probability = (1 / RI) x 100 %
6. What is the recurrence interval for floods with a peak flood discharge of 10,300cfs?
7. Calculate the probability that a flood with a peak flood discharge of 10,300cfs will occur in a
given year.
Year
1976
1977
1978
1979
1980
1981
1982
1983
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
Peak Flood
Discharge (cfs)
1030
3540
3360
2590
632
1050
6520
1940
1810
4100
809
1660
1040
10200
1180
1220
6760
864
1910
4350
2830
1160
2900
1950
1860
1700
632
987
500
515
2380
2840
3510
2210
FLOOD DATA TABLE
Rank (m)
Recurrence
Interval
Probability of
Occurrence