14 



able. Where the fall available is small, a large length and small depth are.- 

 necessary. 



2nd. The distance from the crest of the weir to the bottom of the canal, 

 or floor of the weir box should be at least three times the depth on the weir. 



3rd. The distance from the edges of the weir notch to the sides of the- 

 canal or of the weir box should be at least twice the depth on the weir. 



4th. The upstream side of both the crest and the edges of the weir 

 notch should be brought to a knife edge or to a sharp corner; the bevelling, 

 should be on the downstream side. With a sharp corner and a thickness of 

 crest not greater than one-half the minimum depth of water the discharge 

 will be the same as for a knife edge. 



In placing the weir in position the following directions must be fol- 

 lowed: 



1st. The weir when used on a ditch should be placed in a section of the 

 ditch which is straight for at least 50 feet above the weir and the center 

 line of the ditch should be perpendicular to the weir board and pass, 

 through its center. The cross section of the channel should be not smaller 

 than the cross section of the weir box, in order to have slow velocity and 

 fairly calm water above the weir. If the weir box must be placed near the 

 takeout gate the velocity must be made uniform by means of baffles. 



2nd. The weir must be set high enough to give to the overflowing 

 sheet a free fall on the downstream side. A common rule is to make the 

 level of the water on the downstream side lower than the crest by not less 

 than one-half the depth of water on the crest. To obtain the free fall it is 

 best to select a section of the ditch which has considerable grade. 



3rd. In letting water in a weir box through a pipe it should discharge 

 at the bottom of the box and the depth of the box should be sufficient to 

 produce a calm body of water on the upstream side of the weir. In some 

 cases this requires the use of baffle boards to break up the velocity of the 

 approaching water. 



4th. The crest of the weir should be level from end to end. 



5th. The measurement of head should show the true elevation of the 

 water surface above the weir crest. Directly at the crest and for a short 

 distance above it the water surface curves down. This requires that the 

 water be measured a certain distance upstream. 



Measurement of discharge. 



When the weir has been installed the only measurement to take is the 

 depth of water or head over the crest of the weir. To make this measure- 

 ment it is necessary to provide a reference point level with the weir crest, 

 from which the depth is measured. This point must be at least 2 feet up- 

 stream from the weir crest for a small weir, and preferably 4 to 6 feet. 

 For a weir box this reference point may be a nail driven part way in the 

 side of the box at the level of the weir crest, or a bracket or support formed ' 

 by nailing a strip to the side of the box. For a simple weir board a stake 

 may be driven into the ditch. The depth of water above the reference 

 point can be obtained sufficiently close for ordinary purposes by using a 

 carpenters' rule and reading the depth to nearest one-eighth of an inch. 



Knowing the length of the weir crest and having obtained the depth of 

 water, the discharge in cubic feet per second or miners' inches may be ob- 

 tained by referring to the following tables: 



