Page 10 



BETTER FRUIT 



December, ipip 



crest of the rectangular or trapezoidal 

 weir, or witli the vertex of tiie trian- 

 gular weir. If an open weir pond of 

 sufficient width is used the scale, or 

 a lug upon which to place a rule, may 



Fig. 4. TrinngLihir notch weir. 



be fastened to the bulkhead at a lateral 

 distance from the end of the notch of 

 not less than twice the greatest depth 

 of water "H" over the crest. To get 

 the zero point of the scale or the lug 

 level with the crest, a carpenter's level 

 and straight edge or a surveyor's level 

 must be used. Allowing the water to 

 flow into the pond and slowly rise 

 till it flows over the ^vei^ crest is in- 

 accurate, since the water surface will 

 rise appreciably above the crest before 

 flow over the crest begins. Small er- 

 rors in reading "H" causes relatively 

 large errors in the discharge determi- 

 nation. 



Rectangular Weir. — The rectangular 

 weir, named from the shape of its 

 notch, is the oldest weir in use. Its 

 simplicity of form, ease of construc- 

 tion and accuracy make it a desirable 

 device for measuring water. 



Because of the vertical ends and the 

 complete end contractions the middle 

 part of the weir discharges more water 

 than the same length of weir near 

 either end, whereas the trapezoidal 

 weir with sloping ends is commonly 

 believed to discharge the same amount 

 of water near either end as it does in 

 the middle. However, extensive ex- 

 periments conducted by the United 

 States Department of Agriculture indi- 

 cate that four times the discharge over 

 a 1-foot trapezoidal weir is greater 

 than the discharge under the same 

 head over a 4-foot trapezoidal weir. 



Ninety - Degree (90°) Triangular 

 Notch Weirs. — The triangular notch 

 weir is especially adapted to the meas- 

 urement of small quantities of water, 

 varying from a very small fraction of 

 a second-foot to 2 or 3 second-feet. 



Cone has demonstrated that very 

 small crest lengths in the rectangular 

 and trapezoidal weir, e. g. K' foot, do 

 not follow the laws of discharge for 

 lengths of 1 foot and above. There- 

 fore for the measurement of streams 

 of 1-3 of 1 c.f.s. or less, which are too 

 small for the 1-foot rectangular weir, 

 the triangular-notch weir is especially 

 valuable. The 90-degree weir should 

 be so placed that each side will make 

 an angle of 45 degrees or half pitch 

 with the vertical. 



Trapezoidal or CipoUetti WeiVs. — 

 The trapezoidal weir, called also the 

 CipoUetti weir after the Italian engi- 



neer who designed it, is equally accu- 

 rate but more difficult to construct 

 than the rectangular weir. Its sides 

 are made on a slope of one inch hori- 

 zontal to four inches vertical. 



The conditions of installation out- 

 lined above apply to this weir. 

 Submerged Orifices. 



Submerged orifices as used in the 

 measurement of irrigation water may 

 be divided into two general types. A 

 description of the type of orifices 

 most commonly used — that with fixed 

 dimensions — is here given. The other 

 type is that in which the opening may 

 be varied. 



Definition. — A .submerged rectangu- 

 lar orifice having four sides which 

 are covered with thin edged plates, 

 and which are so far removed from 

 the top, the sides and the bottom of 

 the water channel, as to cause com- 

 plete deflection of the water filaments 

 as they pass through the orifice, is 



Fig. 5. Trapezoidal or Cipoletti weir. 



classed as standard submerged orifice. 

 The sides of the orifice may be made 

 of properly sharpened planks, but it 

 is best to use a thin metal plate. 



Conditions for Accuracy. The U. S. 

 Reclamation Service has outlined the 

 following conditions as necessary for 

 obtaining accurate measurements: 



"(a) The upstream edges of the ori- 

 fice should be sharp and smooth and 

 the distance of each from the bound- 

 ing surfaces of the channel both on 

 the upstream and on the downstream 

 side should preferably be not less than 

 twice the least dimension of the ori- 

 fice. 



"(b) The upstream face of the ori- 

 fice wall should be vertical. 



"(c) The top and bottom edges 

 should be level from end to end. 



"(d) The sides should be truly ver- 

 tical. 



"(e) The head on the orifice that 

 should be measured is the actual dif- 

 ference in elevation between the wafer 

 Mirface on the upstream side of the 

 orifice and the water surface on the 

 do vnstream side theicof. 



"(f) The cross-sectional area of the 

 water prism for 20 to 30 feet from the 

 orifice, on the upstream and oji the 

 downstream side thLreof, should be 

 at least six times the cross-sectional 

 area of the orifice." 



Advantages and Disadvantages. — 

 The greatest advantage in the use of 

 submerged orifices is found in rela- 

 tively level sections where it is diffi- 

 cult to obtain fall enough for weir 

 measurements. They have in addition 

 to the above, the advantages already 

 enumerated for weirs. 



The more important disadvantages 

 are (1) occasional collecting of float- 

 ing debris, and (2) collecting of sand 

 and sediment above the orifice, thus 

 preventing accurate measurement. 



As in the use of weirs, the ditch or 

 canal immediately above the orifice 

 must be wider and deeper than the 



Continued on page 34. 



Fig. 6. Submerged orifice used by the U. S. Reclamation Service. 



