December, ipip 



BETTER FRUIT 



Page p 



Definitions. — "A bulkhead or wall 

 built across a ditch or stream, with an 

 opening cut in the top of the wall 

 through which the water is allowed to 

 pass, is called a 'weir' and the opening 

 is called the 'weir notch.' The bottom 

 portion of the ditch immediately up- 

 stream from the bulkhead is the 'weir 

 box' or 'weir pond.' The height of 

 the water surface in the weir pond 

 above the weir crest is the 'head.' 

 When the water flows over the crest 

 into the air before it strikes the sur- 

 face of the water in the ditch down- 

 stream from the bulkhead, it is said 

 to have 'free fall,' and when this over- 

 pouring stream of water touches only 

 the sharp upstream edge of the crest, 

 the weir is said to be 'sharp crested.' 

 The weir notch must have a definite 

 shape and size. The horizontal dis- 

 tance from the end of the weir crest 

 to the side of the weir box is called 

 'end contraction,' and the vertical dis- 

 tance from the crest to the floor of 

 the weir box is called 'bottom con- 

 traction.' When these contraction dis- 

 tances are great enough to cause a 

 pondlike condition, which permits the 

 water to approach the weir notch 

 without any appreciable velocity, the 

 weir is said to have 'complete con- 

 tractions.' " 



Advantages and Disadvantages of 

 Weirs. — The advantages of weirs for 

 water measurement are: (1) accuracy, 

 (2) simplicity and ease of construc- 

 tion, (3) non-obstruction of moss or 

 floating material, and (4) durability. 



The disadvantages are: (1) the re- 

 quirement of considerable fall of the 

 water surface, or loss in head, which 

 makes their use in sections having 

 level land impractical, and (2) the 

 collecting of sand and silt above the 

 weir which prevents accuracy of 

 measurement. 



Weir Box and Pond. — In the use of 

 either of the weirs above described, 

 the ditch or canal must be made wider 

 and deeper than the average section 

 of the canal for some distance up- 

 stream from the weir. This is to 

 make the water approach the weir 

 very slowly by flowing through a rela- 

 tively large channel. The enlarged sec- 

 tion of the ditch should be gradually 

 tapered to the natural size about 40 to 

 50 feet above the weir. Cross currents 

 just upstream from the weir must be 

 prevented. 



The weir may be placed in a weir 

 box built of lumber or concrete, as 

 shown in Fig. 1, or it may simply be 

 placed in an enlargement of the ditch. 



Less room is required when a box 

 is used but cleaning is made more 

 difficult. For temporary use the plac- 

 ing of a weir in the open ditch as in 

 Fig. 2 is the more economical method. 



Cleaning is also less expensive in 

 the open ditch as a scraper may be 

 used. The ditch downstream must be 

 protected with loose rock or other 

 material to prevent washing by the 

 falling water. 



Table 1, taken from I-'armers' Bui. 

 813 by V. M. Cone, Irrigation Engineer 

 for the U. S. Department of Agricul- 

 ture, gives the sizes of weirs best 

 adapted to measuring streams of water 

 varying from yi to 22 cubic feet per 

 second, and also the proper dimen- 

 sions for each size of rectangular, 

 trapezoidal and 90-degree triangular 

 notch weirs. 



Table 1. — Weir-bo.r dimensions for re 

 ctangular Cipolletii, and 90-degrce tri- 

 imgular notch weirs. 



leaving this rather rough edge to serve 

 as the crest and sides of the weir 

 notch, it is better to make the opening 

 in the bulkhead at least one inch 

 deeper and two inches wider than the 

 desired size of weir opening. This 

 will permit attachment of crest and 

 side strips to the bulkhead so as to 

 project about an inch all around, mak- 

 ing more perfect edges, and the over- 

 pouring sheet of water will not touch 

 the bulkhead." 



On permanent installations it is de- 



Ihe heads of the columns in this table refer 



(All dimensions are in feet. The letters at 

 to Fig. 1.) 



RECTANGULAR AND TRAPEZOIDAL WEIRS WITH END CONTRACTIONS 



•This distance allows for about % foot freeboard above highest water level in weir box. 

 tEquals distance from crest upstream to gage. 

 §Equals distance from end of crest over to gage. 



'Cone, V. M. "Construction and Use of Farm Weirs," Farmers' Bulletin 813, U. S. Depart- 

 ment of Agriculture, p. 9. 



The weir dimensions in Table I, 

 illustrated in Fig. 1, as given by Cone 

 ai-e a little smaller than what would 

 be necessary to obtain rigid accuracy, 

 but boxes of these sizes will give re- 

 sults within 1 per cent of the correct 

 ^alues. Cone outlines the conditions 

 necessary for weir crests and sides as 

 given below.* 



Weir Crests and Sides. — "W'eir 

 crests and sides should be true, 

 straight and rigid. The crest must be 

 level, the sides must be set to the 

 proper angle with the crest, and care- 

 fully spaced to give the correct length 

 of crest, as indicated by 'L' in Fig. 1 

 and Table I. The 90-degree triangular 

 notch has no length of crest because 

 the sides meet at a point. 



"It is not necessary that the sioes 

 and crest be sharpened to a knife edge, 

 but the edge of the crest on the up- 

 .•;fream side must be sharp in the sense 

 ihat it is not rounded. If a depth of 

 water not less than 3 inches is to be 

 run over the weir, the crest thickness 

 on the edge may be as great as one- 

 fourth inch without the water adher- 

 ing to the crest, provided the inner 

 edge is sharp. However, if the crest 

 is beveled, this bevel nmst be plai-cd 

 on the downstream side, for the up- 

 .stieam face of the crest and of t]ie 

 bulkhead which holds the crest iiuist 

 he even and in a vertical position. The 

 downstream face of the opening in 

 the bulkhead must be beveled outward 

 and downward about 45 degrees to in- 

 sure free passage of air under the 

 sheet of water as it flows over the 

 weir. 



"Instead of cutting the notch in the 

 bulkhead to just the size desired and 



sirable to make the crests of metal 

 heavy enough to avoid warping. On 

 small weirs the crest may be cut out 

 of a single sheet but on large ones 

 separate strips are necessary. Angle 

 iron is now frequently used, one face 

 of which is bolted into the bulkhead 

 opening in such a position that the 

 other face is flush with the upstream 

 face of the bulkhead. 



For temporary wooden weirs, the 

 wood of which the weir is constructed 

 may well form also the weir crest and 

 sides. Since wood warps easily and 

 the sharp edges become worn and 

 splintered, its use for crests and sides 

 is seldom desirable. 



Measurement of Head or Deptli on 

 Weir Crest. — The measurement of the 

 head or depth of water on the weir 

 crest is obtained with a specially con- 

 structed scale or a carpenter's rule. 

 The special scale called the weir 

 gage must be set upon upstream above 

 the bulkhead a distance no less than 

 four times the depth of the water "H" 

 flowing over the crest. This is made 

 necessary by the downward curvature 

 of the water surface near the crest. 

 A scale marked off into feet, tenths 

 and hundredths of a foot on hard 

 wood is satisfactory. The zero point 

 on the scale must be set level with the 



Rcctiuigular weir. 



