FLOOD04361

• County: Statewide
• Basin: Statewide
• Title: Techniques of Water-Resources Investigations of the US Geological Survey Measurement of Peak Discharge by the Slope-Area Method
• Date: 1/1/1984
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

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TECHNIQUES OF WATER-RE80URCE8 INVESTIGATIONS

Variable Discharge

Sometimes the best possible reach might have
variable discharge, such as when a tributary
enters within the reach, or when water leaves
through a break in a levee along th8 bank. It
is necessary first to compute independently the
discharge added or diverted, such as by a slope-
area measurement for the tributary How, or by
computing the How over the levee embankment.
The table at the bottom of the computation
form (fig. 7) can then be used to compute the
discharge as follows:

1. Assume a discharge Q, in section 1, and use
it to compute the upstream velocity head.
2. Add Q. and Q. (HllW in the tributary or
diversion) to obtain Q., and use this to
compute the downstream velocity head.
3. The computed Q should be the average of
Q. and Q.; if not, assum8 a new Q. and
recompute.

Evaluation of Results

The resulting discharge should be examined
and e,'aluated on the basis of the intrinsic merits
of the computation. If the state of How changes
from tranquil (F<l) to rapid (F>1) or vice
versa, there is cause for further examintJ.tion of
the base data. A change from rapid to tranquil
How indicates the possibility of the presence of
a hydraulic jump, with its uncertain energy
losses, Such a reach would be suspect. A
change from tranquil to rapid How might in-
dicate a sharp contraction within the reach,
with attendant contraction losses which have
not been evaluated, or might indicate the
pres8nce of "free fall," caused by a series of
ri1Iles, which means a discontinuous water-
surface slope not related to the discharge as in
the Manning formula. Examination of the

high-water profiles might show sharp drops
which bear out either of the two latter possi-
bilities, and the computed discharge would then
be known to be at fault. On the other hand. a
gradual transition from tranquil to rapid How
is possible; a continuous water-surface profile
would bear this out, in which case the discharge
computations may be accepted as valid.
The consistency of results from separate sub-
reaches is some indication of the reliability of
the answer. If the spread in discharges exceeds
25 percent, the results would be classified as
poor.
Adequacy of the high-water marks, amount
of fall, presence of bends in the reach, and the
magnitude of the velocity head in relation to
the fall are other factors which should
be examined in rating the accuracy of the
measurement.

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Example

The computation sheets for a slope-area
measurement of the Hood of February 21, 1956,
on Snake Creek near Connell, Wash., are shown
on figures 2-7. This example illustrates the
sheets used in plotting and computations, and
how the results of a slope-area measurement
should be presented. A study of this example
will further clarify the entire procedure used in
making this type of measurement.

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Selected References

Benson, M. A" and Dalrymple, Tate, 1967, General
field and office procedures for indirect discharge
measurements: U.S. Geo!. Survey Techniques
Water~Resource8 Inv., Book 3, Chap. Al (in
press) .
Chow, V. T. , 1959, Open-channel hydraulics: New
York, McGraw-Hill Book Co.
Houk, I. K, 1918, Calculation of flow of water in open
channels: Miami Conservancy Dist., Dayton,
Ohio, pt. 4.

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