13 



water is delivered, and the time to fill the tank is recorded. Knowing the 

 volume in cubic feet and the time in seconds, the rate of flow in cubic feet 

 per second may be obtained by dividing the first by the other. For in- 

 stance, if the tank is 2 feet wide, 4 feet long, and 2 feet deep, it holds 8 

 cubic feet of water and if it takes 40 seconds to fill the box the rate of flow 

 is 8 cubic feet in 40 seconds or 1-5 'of a cubic foot per second or about 7 

 miners' inches. 



2. Weirs. 



The weir is the most generally used measuring device found on irri- 

 gation systems, for the reason that it is simple to construct and use and 

 will give accurate results when properly installed. The weir is generally 

 limited to moderate quantities of water and requires sufficient fall or grade 

 for its installation and for these reasons can not always be used, but in 

 British Columbia the quantities of water to be measured are usually small 

 and in most cases there is ample fall. These conditions are very favorable 

 for the installation of a weir. 



The term weir is applied to any dam or barrier across the stream 

 over which the water flows. The weirs used in irrigation consist of a board 

 or barrier into which a notch is formed through which the water flows. 

 The volume of water passing through the notch is obtained by knowing the 

 length of the notch and measuring the depth of water passing over it. The 

 form of notch is generally either trapezoidal or rectangular and in some 

 cases a triangular or V shaped notch is used. 



The trapezoidal weir is known as the Cippoletti weir and is more com- 

 mon than the other forms. It consists of a horizontal crest and the two 

 sides, each sloping outward one inch for every four inches rise. The rec- 

 tangular weir has vertical sides. The first form has the advantage that the 

 flow may be computed by means of a simpler formula. The second form is 

 'a little easier to construct and is more accurate because its formula has 

 been derived from a larger number of experiments. In either case the flow 

 can be obtained by referring to the tables given farther. 



The weir may consist of a simple board placed across the ditch or of a 

 board set in a short section of flume or box, in which case it is called a weir 

 box. The weir board may be of wood, metal or concrete. When made of 

 wood or concrete it is desirable to use a metal plate to form the edges of 

 the crest and the sides. When no weir box or flume section is used, the 

 weir board is placed directly across the canal, and sufficiently braced with 

 posts. A weir box or flume box is generally used. The box is a short flume 

 section whose length is not less than 8 to 12 feet. The width and depth 

 must be at least sufficient to give the required dimensions to the weir 

 board. The weir board should be placed at a distance from the upstream 

 end equal to two-thirds the length of the box, the lower third forming a 

 floor for the falling water. The depth measurement should be made at 

 least 4 to 6 feet upstream from the crest, from a post or scale fixed on the 

 side of the flume with the zero point level with the weir cre^t. To obtain 

 accurate results the following rules for the dimensions of the notch should 

 be observed: 



1st. The greatest depth of water which should be allowed on the crest 

 of the weir should not be more than one-third the length of the weir and 

 the least depth 1 inch. The depth is usually controlled by the fall avail- 



