26 



2nd. The velocity varies in different portions of the channel, being 

 smallest near the bottom and the sides and greatest on the center line of 

 the channel just below the surface. 



3rd. In a given channel the velocity increases with an increased volume 

 of water in the channel. For instance a flume 3 feet wide carrying water 

 to a depth of 1 foot with an average velocity of 2 feet per second gives a 

 discharge of 6 cubic feet per second. When it carries a depth of 2 feet the 

 area of cross section is doubled and the velocity will be increased to about 

 2.55 feet per second giving a discharge of 15.80 cubic feet per second which 

 is more than double the first amount. 



To obtain the velocity the following methods are commonly used: 



1st. Surface floats of chips or other material. 



2nd. Rod floats of bottles, tubes cr rods. 



3rd. Current meter. 



To obtain the velocity with surface floats or rod floats the procedure is 

 the same. It is carried on as follows: Select a straight section of canal at 

 least 50 feet long where the cross section is uniform and the flow not affect- 

 ed by obstructions. Lay off the course by placing two wires across the 

 canal or simply stakes at the desired distance apart which may be 50 or 

 100 feet. Place the float above the upper wire and time the float for its 

 travel over the course. The distance in feet divided by the time in seconds 

 gives the velocity of the float in feet per second. When a rod float is used it 

 must be weighted at the lower end so that it will float vertically. This may 

 be done by placing weights in the bottle or tube or by wrapping lead or 

 wire at the lower end of the rod or simply tying a stone to the end of a 

 stick. Rods 1% to 2 inches in diameter should be used. The length of 

 the rod should be such that it will nearly touch the bottom. The velocity 

 obtained with a rod float will represent the average velocity, but the velocity 

 obtained with a surface float is 20 per cent, greater than the average 

 velocity and must therefore be multiplied by 8-10 to give the average 

 velocity. 



To obtain the discharge accurately it is necessary to subdivide the cross 

 section into partial areas bounded by imaginary vertical lines extending 

 from the bottom of the canal to the top of the water surface and obtain the 

 velocity for each one (Fig. 22). The widths of these areas are marked by 

 tags placed on a wire or marks on a plank spanning across the canal and 

 the depth of water at each tag is obtained by soundings or measurements 

 with a graduated rod or rule. The width in feet multiplied by the average 

 of the two depths for each section will give the number of square feet in 

 the section. This product multiplied by the velocity in feet per second will 

 give the discharge for each partial area and the sum of discharges of each 

 partial area will give the total discharge. 



Fig. 22. Subdivision of oaual cross section for measurement of discharge. 



