Lawrence democrat. (Lawrenceburg, Tenn.) 1884-1925, 1904-07-29
STREAM MEASUREMENTS.
Phenomena of "Pulsation of Moving
Water" Interferes Greatly
with Accuracy.

SeOlOBlcdl survey puDlisneu WdIeT-
supply and irrigation paper, No. Ga,
entitled THE Accuracy of Stream
Measurements" considerable addi-
tional data has been collected in re-
gard to this subject. Among these
are the results of Investigations made
by Mr., Edward G. Murphy in the hy-
draullc laboratory Of Cornell univer-
sity, concerning the flow of small and
moderate sized streams and the re-
suits Of highwater measurements made
by him near Oswego, and at Bingham
ton. This paper also contains report
on a series of vertical velocity measure
ments made under ice on streams in
the Catskill mountains, which are of spe-
cial interest, as they are the most ex-
tensive series of measurements of this
kind which have been made.
The accuracy Of the measurement
of stream depends largely upon the
accuracy with which the cross-sec-
tional area and the velocity are meas
ured. There is no special difficulty in
measuring the first factor, but the Sec
ond factor Is very difficult to deter
mine, chiefly because it is constantly
changing. The velocity varies not
only from the surface to the bottom
of the stream, and from one bank to
the other, so that it is necessary to
measure it at many points, but is con-
stantly changing at every point, even
when the cross-sectional area and the
discharge remain constant.
Several experimenters have observed
the phenomenon of "pulsation of NOV
ing water" and a few have tried to
measure it, but as yet little is known
OF the magnitude and frequency Of
the pulsations or of the laws governing
them A knowledge of such phenomena
is evidently of vital Importance in MAX
ing and computing stream measure
nents. If only few observations of
velocity are made, these may all, or
nearly all, be made at time of maxi
mum impulse, and thus the measured
mean velocity be too large; or it is pos
sible that most of the observations may
| be made at time of minimum impulse,
and thus the mean velocity be too
| small.
The Investigations made have also
an important bearing on the kind Of
instruments best suited to measure
the velocity, as some-the float rod,
for example-give the velocity of
single impulse, while others-as the
current meter-show the average We
locity due to all the impulses during the
observation.
The motion of water in an open chan
mel is not, however, simply succession
of impulses. On the contrary, it is ex-
ceedingly complex, and is very different
from the uniform flow in parallel
straight lines that is assumed in deriv-
ing the ordinary hydraulic formulas. I'm
der close observation the water of the
most undisturbed streams is seen to
contain some particles that move up,
others that move down, and still others
that move across.
The accuracy of discharge measure
ment also depends much upon the phys
ical features of the stream at the dis
charge section or point of measurement
Seldom are all the conditions favorable
for the most accurate work.
Discharge measurements of streams
flowing in natural and artificial chan
nels have been made with various
kinds of instruments and in many ways
The earlier ones were made with crude
instruments, and in some cases the sur
face velocity only was observed, the
mean velocity being computed from a
formula which we now know is not cor
rect. Very little seems to have been
done in the way of determining the de-
4 gree of accuracy Of the measurements
Even when the experimenter has dsed
two or more instruments to measure We
locity, he does not appear to have made
simultaneous measurements with diff
ferent instruments, or to have employed
different methods with the same instru-
ments, in order to test the accuracy of
the results.
These matters are all discussed in de-
tail in Mr.. Murphy's paper, which is list
ed as Water Supply and Irrigation Pa
per No. 95, and may be obtained on APP
plication to the director of the United
States geological survey, Washington,
D. c.