Flood Information- University of Iowa Intro to Environmental Science Lab
Discharge
The volume of water passing through a channel cross section in a given interval of time is a measure of
the stream’s discharge. The common unit for recording discharge is cubic feet per second (ft.3 /s or cfs).
Discharge is determined using the following equation, which is also known as Darcy’s law:
Discharge=cross-sectional area of the channel x flow velocity
Q=A x V
Flood
A flood is defined as the temporary overflow of a river onto adjacent lands not normally covered by
water. The stage (or water level) of a river or stream is measured and recorded at a gaging station. Stage is
recorded in feet or meters above a known base height.
A flood recurrence level (or RI) is the average interval between floods of a particular size. The Weibull
method calculates the recurrence interval by using the average time between two floods of equal or
greater magnitude. This method uses the formula:
RI= (n+1)/m
RI equals the recurrence interval in years, n is the number of years of record, and m is the magnitude (or
rank) of the annual flood discharges from the largest (ranked 1) to the smallest on record.
We can better understand the frequency of flood events by creating a flood-frequency curve. A stream’s
peak flood discharge values (in cfs) are plotted on the y-axis against the recurrence interval on the x-axis.
Next, a best-fit line is drawn through the data points. The flood-frequency curve provides an estimate for
how often a flood of a particular magnitude will occur. For example, if Creek X has a discharge of 1,000
cfs every 2 years, then in 10 years there should be 5 occurrences of a 1,0000 cfs discharge.
Probability
The probability that a flood will occur during any given year is the reciprocal of the recurrence interval
multiplied by 100. The formula to use is:
P= (1/RI) x 100%
P is the percent probability of a certain-sized flood occurring in any given year, and RI is the recurrence
interval.
Example: What is the percent probability of a 50-year flood occurring in a given year?
P= (1/50) x 100= 0.02 x 100= 2%
Frequency Analysis
A statistical technique (called frequency analysis) is used to estimate the probability of the occurrence of
a given event. In this case, the event is a flood of a given size.
Because it’s a statistical calculation, you can’t calculate a Recurrence Interval for one flood. You need a
set of data that’s been collected over several years.
Ten or more years of data are required to perform a frequency analysis for the determination of recurrence
intervals. More confidence can be placed in the results of a frequency analysis based on, for example, 30
years of record than on an analysis based on 10 years of record.
A recurrence interval of 100 means that there is a 1 in 100 chance that a flood of this size will occur in
any given year.
Notice that the bigger the recurrence interval (i.e., the bigger the flood), the lower the
probability of it occurring.