12 



THE IRRIGATION AGE. 



number of points, called measuring points, are measured 

 off above and in the plane of the measuring section at 

 which observations of depth and velocity are taken. (See 

 Pi. I, A.) These points are spaced equally for those parts 

 of the section where the flow is uniform and smooth and 



Plate ]A. Showing Method of Measuring Cross Section of River. 



are spaced unequally for other parts according to the dis- 

 cretion and judgment of the engineer. In general the 

 points should not be spaced farther apart than 5 per cent 

 of the channel width, nor farther apart than the approxi- 

 mate mean depth of the section at the time of measure- 

 ment. 



The measuring points divide the total cross section 

 into elementary strips at each end of which observations 

 of depth and velocity are made. The dis- 

 charge of any elementary strip is the prod- 

 uct of the average of the depths at the two 

 ends times the width of the strip times 

 the average of the mean velocities at the 

 two ends of the strip. The sum of the 

 discharges of the elementary strips is the 

 total discharge of the stream. 



Depths for the determination of the 

 area are usually obtained by sounding with 

 the current meter and cable. In rough 

 sections or swift current an ordinary 

 weight and cable are used, particular care 

 being taken that all observations shall lie 

 in the plane of the cross section. 



Two methods of determining the ve- 

 locity of flow of a stream are in general 

 use the float method and the current- 

 ititer method. 



The float method, with its various 

 modifications of surface, sub-surface, and 

 tube or rod floats, is now considered obso- 

 lete in the ordinary practice of the United 

 States Geological Survey. The use of this 

 method is limited to special conditions 

 where it is impracticable to use the cur- 

 rent meter, such as in places where large 

 quantities of ice and debris which may 

 damage the meter are flowing with the 

 current, and for miscellaneous measure- 

 ments or other work where a high degree 

 of accuracy is not necessary. Tube floats 

 are very satisfactory for use in canals with 



regular bottoms and even flow of current. Measurements by 

 the float method are made as follows : The velocity of flow 

 of the stream is obtained by observing the time which it takes 

 floats set free at different points across the stream to pass 

 between two range lines about 200 feet apart. The area used 

 is the mean value obtained from several 

 cross sections measured between the two 

 range lines. The chief disadvantages of 

 this method are difficulty in obtaining the 

 correct value of mean area for the course 

 used and uncertainty regarding the proper 

 coefficient to apply to the observed velocity. 

 The Price current meter is now used 

 almost to the exclusion of other types of 

 meters by the United States Geological 

 Survey in the determination of the velocity 

 of flow of water in open channels, a use 

 for which it is adapted under practically 

 all conditions. 



Plate II shows in the center the 

 new type of penta-recording current meter 

 equipped for measurements at bridge and 

 cable stations ; on the left the same type 

 of meter is shown equipped for wading 

 measurements, to record by the acoustic 

 method ; on the right the meter is shown 

 equipped to record electrically. (See PI. 

 I, B.) Briefly, the meter consists of six 

 cups attached to a vertical shaft which re- 

 volves on a conical hardened steel point 

 when immersed in moving water. The 

 revolutions are indicated electrically. The 

 rating, or relation between the velocity of 

 moving water and the revolutions of the 

 wheel, is determined for each meter by 

 drawing it through still water for a given 

 distance at different speeds and noting the 

 number of revolutions for each run. From 

 these data a rating table is prepared which 

 gives the velocity per second of moving 

 water for anv number of revolutions in a 



given time interval. The ratio of revolutions per second, to 

 Velocity of flow in feet per second is very nearly a constant 

 for all speeds and is approximately 0.45. 



Three classes of methods of measuring velocity with 

 current meters are in general use multiple-point, single- 

 point, and integration. 



The two principal multiple-point methods in general 

 use are the vertical velocity curve and 0.2 and 0.8 depth. 



Plate IB. Electric Recording Current Meter. 



