106 



THE IKRIGATION AGE. 



TO MEASURE THE VELOCITY OF WATER. 



Miner's Ditch, Weir Dam and Cross Section and Velocity 

 Methods Described. 



Measuring the flow of water so as to determine the 

 velocity in feet per second is an interesting process 

 which is accomplished by three different methods : The 

 Weir dam method, the Miner's inch method and the 

 method known as "Measurement by Cross Section and 

 Velocity." The latter is only used when it is desired to 

 approximate the water quantity and is not very . ac- 

 curate. The method is as follows: 



MEASUREMENT BY CROSS SECTION AND VELOCITY. 



Select a stretch on the stream or ditch which will 

 afford as straight and uniform a course as possible. If 

 the water is at any point carried in a flume it is better 

 to measure at this point. Lay off a distance of, say, 

 300 feet; measure the width of flowing water at about 

 six different places in this distance, and obtain the 

 average width; like- 

 wise at these same 

 points measure the 

 depth of water at 

 three or four places 

 across the stream, 

 and obtain the aver- 

 age depth. Next drop 

 a float in the water, 

 noting the number 

 of seconds it takes to 

 travel the given dis- 

 tance. From this can 

 be calculated the ve- 

 locity of the water in 

 feet per second. The 

 quantity is the prod- 

 uct obtained by mul- 

 tiplying the average 

 width in feet by the 

 average depth in feet 

 by the velocity which 

 (if in feet per sec- 

 ond) will give the 

 flow of the stream in cubic feet per second. From 

 the figures so obtained it is advisable to deduct about 

 20 per cent, as surface velocity of the water is in excess 

 of the actual average velocity. 



When the stream is of sufficient depth three feet 

 or over the average velocity can be more closely ob- 

 tained by using a pole, to one end of which is attached 

 a stone or piece of lead of necessary weight to allow the 

 pole to sink nearly to the bottom. In this way the 

 velocities at the surface and the bottom of the stream 

 counteract one another and a closer approximation of 

 the average velocity is obtained. 



THE WEIR DAM METHOD. 



To measure by the Weir dam method is the most 

 accurate and reliable and should 'be employed for close 

 calculations. It is as follows: Place a board or plank 

 in, the stream, as illustrated, at some point which will 

 allow a pond to form above. The board should have a 

 notch cut in it with its edge beveled toward the 

 intake, as shown. The length of notch should be at 

 least four times its depth for small quantities, and 

 longer for larger quantities. In addition to the above 



it is advisable for very accurate measurements to have, 

 when possible, the length of the notch about two-thirds 

 of the width of the water on the up-stream side; it 

 may be necessary to experiment somewhat in order to 

 obtain this condition. 



The overfall or vertical distance the water falls 

 on the down-stream side should be about twice its depth 

 that is, twelve inches if the notch is six inches deep, 

 and so on. 



In the pond not less than three feet, and pref- 

 erably six feet, from the weir, drive a stake near the 

 bank, with its top precisely level with the bottom of 

 the notch in the weir; then measure carefully with a 

 rule the depth of water over the stake. This will give 

 the theoretical depth of flow, corresponding to that in 

 the table on this page. 



In order to obtain accurate results it is essential 

 that the velocity of water in the pond be extremely 

 slow, and if the weir be constructed of proper propor- 

 tions this condition will exist. 



The following ta- 

 ble for weir measure- 

 ment gives the cubic 

 feet of water per 

 minute that will flow 

 over a weir one inch 

 wide and from % to 

 20y inches deep. 



For example: Sup- 

 pose the weir to be 

 sixty-six inches long, 

 and the depth of wa- 

 ter on -it to be 11% 

 inches. Follow down 

 the left-hand column 

 of the figures in the 

 table until you come 

 ,-to eleven inches. 

 Then run across the 

 table on a line with 

 the eleven until un- 

 der five-eighths on 

 the top line, and you 

 will find 15.85. This 

 multiplied by sixty-six, the length of weir, will give the 

 result. 



TABLE OF WEIR, MEASUREMENTS, GIVING THE CUBIC FEET 



OF WATER PER MINUTE THAT WILL FLOW OVER 



A WEIR FROM % TO 20% INCHES DEEP. 



EXPLANATION OF MINERS' INCH MEASUREMENT. 



