6 
Colorado Experiment Station 
Venturi principle, but the water passes the contracted section 
without pressure ; that is, exposed to atmospheric pressure only. 
The Venturi principle, as applied to an open flume, manifests the 
same characteristics as when the flow is confined in a tube ; namely, 
there is a reduction in the head at the contracted section. 
The floor of the Venturi flume is level and placed at the eleva- 
tion of the grade of the channel. The short throat section forming 
the contraction has been arbitrarily taken as 1 foot long for the vari- 
ous sizes calibrated, and midway along the axis of the throat the 
head is observed. The head in the converging section is observed at 
a point on the side of the flume. This point is twice the width of the 
throat from the upper edge of the throat, and this distance is meas- 
ured along the longitudinal axis of the flume. This distance was 
arbitrarily taken so as to give a point well within the converging 
section, and to be so situated as to correctly register the head at that 
point. The flow through the flume will be a certain function of 
these two heads, and the width of the throat. 
In the preliminary investigation of the Venturi* flume, various 
slopes of the converging and diverging sections were tried out, and 
from the standpoint of economy and the general behavior, it was 
decided to base all the calibrations of the various sizes of rectangu- 
lar flumes on the general plan that the total length of the flume 
would be six times the width of the throat, plus 1 foot, and that 
the width of the flume at either end would be three times the width 
of the throat, but more recent observations prompted the suggestion 
that this originally assumed plan should be somewhat modified. 
Our investigation so far has been confined very largely to the 
rectangular flumes; that is, those where the throat section is defined 
by two vertical faces, each 1 foot in length, measured along the 
axis of the flume and placed parallel to and equidistant from this 
axis. The V-notch and trapezoidal flumes; that is, those where 
the throat sections are defined by two sloping faces, have been 
tested, but the inadequate facilities for making the tests prevented 
calibrating the flumes of this type for large discharges. 
Field calibrations of trapezoidal flumes have been attempted 
but with little success. As a usual thing, the field installation pro- 
vides little or no regulation for the purpose of extending the obser- 
vations over any considerable range. It is hoped that more com- 
plete information may be obtained on a series of trapezoidal sec- 
tions which would permit of establishing the law of flow through 
