240 



THE IRRIGATION AGE. 



other things being equal, the measurement should be 

 made in a section where the velocity of flow is quite 

 uniform, where the channel is straight and banks free 

 from indentations. Measurements made in flumes which 

 are in good order are, as a rule, more accurate than 

 those made in the natural channels. Whether the chan- 

 nel be a flume or an earthen section, the measurement 

 should be made where the bottom of the channel is on 

 or slightly above the general grade of the channel. An 

 accurate measurement can not be made where there is 

 dead water space in the section. 



The various ways in which the meter is manipulated 

 in determining the mean velocity of a section are shown 

 in Fig. 5. Sections 1 and 2 represent the Point method. 

 Sections 3 and 4 represent the Intergration method. 



THE POINT METHOD. 



Section 1. The meter is held at one point in the 

 section at which it is supposed or assumed the velocity 



Either method will give accurate results, depending, of 

 course, upon the number of observations made. 



THE INTEGRATION METHOD. 



Section 3. The most common way of taking ob- 

 servations is to slowly move the meter up and down on 

 the center line of the section during the period of ob- 

 servation. In this way an integrated value representing 

 the mean velocity of the section is obtained. The mean 

 velocity of the section may also be determined by slowly 

 moving the meter in a zig-zag path over the entire area, 

 as shown in Section 4. In any measurement, the same 

 method, of course, should be used for all sections. 



In dividing the cross-section of natural channels 

 into sections much more care is required than where the 

 measurement is made in a flume ; for in the open channel 

 the beds and sides of the channel being more uneven in 

 character cause the motion of the water to be much 

 more disturbed. As the variation velocities between the 



Method ol Measuring Stream Flow by Wire Cable and Car. 



is the mean of the entire section. Where the flow in 

 the section is uniform, as in the case of the flume under 

 consideration, this point will be found at the center of 

 each section and at a point about six-tenths of the depth 

 below the surface. It will be seen that by using this 

 method but little time is required to determine the mean 

 velocity in each section, but that the accuracy of the 

 measurement depends upon the success with which the 

 point representing the mean velocity is chosen. 



Section 2. A more accurate result will be obtained, 

 if, instead of determining the velocity at one point in 

 each section, observations be taken at a number of points 

 in a section. These may be located at intervals in a 

 vertical line through the center of the section or may be 

 taken at random over the entire area of the section. 



center and the side in the natural channel is much 

 greater than in the flume the sections generally should 

 be made smaller and their area determined with greater 

 care. 



MAKING A MEASUREMENT OF A STREAM IN NATURAL 

 CHANNEL. 



The observer, having chosen the point at which a 

 stream is to be measured, proceeds in the following man- 

 ner to determine its discharge. Wherever possible, some 

 structure, such as a bridge, plank, or log. is used as a 

 platform from which the observations and measurements 

 are made. Where the stream is small and great accuracy 

 of measurement is not required the observer may get 

 along without such a structure by simply wading in the 

 stream. Where this, however, is done, it is very neces- 



