Geology Reference
In-Depth Information
PART A. FLOOD FREQUENCY
AND MAJOR FLOODS
Flood Discharge
The
discharge
of a stream is the volume of flow that
passes a specific location in a given period of time. Dis-
charge rates are usually expressed in cubic feet/second
(cfs) or cubic meters /second (m
3
/s). If the area of the
wetted channel cross section (the measured width of
the channel at a site multiplied by the depth of the
water) and the velocity of the stream are known,
then the discharge can be determined by the following
formula:
very large discharges have been recorded. Estimates of
100-year floods are better if they are based on 100 or
more years of record. Where there are few years of
flood data, such as in Table 10.1, reliable estimates of
100-year floods may be difficult.
The annual probability of exceedence, P, is the
reciprocal of T. Written as a formula
P = 1/T
This is the probability or chance that in a single
year the annual maximum flood will equal or exceed a
given discharge. A flood having a recurrence interval
of 10 years is one that has a 10 percent chance of recur-
ring in any year; a 100-year flood has a 1 percent
chance of recurring in any year.
The 100-year flood, as defined statistically, is a
legal definition of areas that are likely to be flooded. If,
in the United States, someone chooses to purchase a
home in the 100-year floodplain, they must obtain
flood insurance.
Q = A
X
V
where Q is the discharge, in cfs or m
3
/s, A is the
cross-sectional area of the stream in square feet or
square meters, and V is the velocity in ft/s or m/s.
Flood Frequency
Where flood records are available, computations of
flood frequency are based on peak annual floods (the
maximum discharge for the year at a specific station).
Flood frequency is expressed as a recurrence interval
(or return period), which is the average time interval
(in years) between the occurrence of two similar floods,
with the same water levels. The recurrence interval
(T, in years) for a flood of a given discharge is deter-
mined by this formula:
SOMEWHERE IN THE UNITED STATES, YEAR
2000 PLUS OR MINUS. Nature takes its inex-
orable toll. Thousand-year flood causes untold
damage and staggering loss of life. Engineers
and meteorologists believe that present storm
and flood resulted from •"
1
conditions [that] ''
1
occur once in a millennium. Reservoirs, levees,
and other control works which have proved
effective for a century, and are still effective up
to their design capacity, are unable to cope with
enormous volumes of water. This catastrophe
brings home the lesson that protection from
floods is only a relative matter, and that eventu-
ally nature demands its toll from those who
occupy flood plains. (Hoyt and Langbein, 1955)
T = (n + l)/m
where n equals the number of years of record,
and m is the rank or order of the annual flood
discharges from the greatest (1), to the smallest
for the number of years of record.
QUESTIONS (10, PART A)
To understand better the impact and frequency
of floods and to assist in flood prediction, one method
is to plot flood data on a flood frequency graph with
the discharge plotted on the vertical axis and recur-
rence interval on the horizontal axis. A straight
"best-fit" line is drawn to join the points for each
year. The average number of years that will elapse
until a given magnitude flood occurs again can be
estimated from this line. In other words, based on
the line there is a certain probability that a flood of a
given magnitude can be expected to occur x times
within a fixed time interval. For example, if the
recurrence interval for an annual maximum dis-
charge of 500 cfs is found to be 4 years, then we
would expect to see a discharge of 500 cfs or greater
five times during a 20-year period (20 years divided
by the 4-year recurrence interval).
The longer the number of years of flood records,
the higher the probability is that very large floods with
Flood Frequency in the Seattle/Tacoma area
Questions 1-11 investigate recurrence intervals, 100-year
floods, and changing flood frequencies for two watersheds
in the state of Washington. These data are slightly modified
from U.S. Geological Survey information. Population in the
Puget Sound area is growing rapidly, and humans have
made many changes to rivers and drainages.
Pick
one
of the four data sets (Mercer Creek 1, Mercer
Creek 2, Green River 1, or Green River 2) in Table 10.1. Be
sure, if you are working in a laboratory or class, that one or
more students select each of the four data sets. Each data set
spans 11 years of record along Mercer Creek or the Green
River. Use your chosen data set to estimate the likely dis-
charge for a 100-year flood. Follow the steps below.
1. Rank the peak flood discharges for the data set you have
chosen in order of magnitude, starting with 1 for the largest
and ending with 11 for the smallest. Write these results in the
"Rank" column.
