THE IRRIGATION AGE. 



In California the equivalent of the inch in common 

 usage in the southern part of the State is one-fiftieth of 

 a second-foot, while in the statutes of California* the 

 equivalent is laid down as one-fortieth of a cubic foot 

 per second. 



The reader will readily see that the conditions gov- 

 erning the volume of an "inch" are so numerous that a 

 discrepancy might very easily creep into the measure- 

 ment of a stream by this method, even though much care 

 be exercised. Not "the least among the many difficult 

 conditions to fulfill is to maintain a constant head over 

 the orifice. Former State Engineer of Idaho, Mr. A. D. 

 Foote, designed the measuring box shown in Fig. 1, by 

 means of which it was possible to maintain a fairly con- 

 stant head on the opening. The flume structure is placed 

 in the stream channel as shown and a portion of the flow 

 is admitted to the chamber on the lower side of which is 

 the measuring slot. The excess above the amount to 

 be measured spills back into the main channel over the 

 long crest, which is of sufficient height above the orifice 

 to give the desired head. By regulating the flash boards 

 in the main channel and the gate to the chamber, the 

 approximate amount to be measured can be admitted; 



'.' ' - * .- 



Fig. 2. Trapezoidal Weir-box. 



then should any variation occur in the flow in the main 

 channel, the long crested weir prevents any excessive in- 

 crease in head on the orifice. The Foote box, when 

 properly installed, gives quite satisfactory results in the. 

 measurement of small streams. 



The most serious fault to be found with the miner's 

 inch system of measurement, however, is that its field of 

 usefulness is limited to the measurement of a compara- 

 tively small volume of water. A stream of two or three 

 cubic feet per second could be measured with ease, but 

 a box to measure the flow of one of the large canals of 

 the State carrying, say, 600 cubic feet of water per sec- 

 ond, would, if all conditions were observed, require a 

 six-inch slot nearly 417 feet long, which would be 

 practically impossible. 



THE MEASURING WEIR. 



By far the most satisfactory and accurate method 

 of measuring small streams, where all the conditions 

 imposed can be fulfilled, is by means of a weir. The 



Statutes of California (approved March 23, 1901) Sec. 1. The 

 standard miner's inch of water shall be equivalent or equal to one and 

 one-half cubic feet of water per minute (.035 cubic feet per second), 

 measured through any aperture or orifice. 



ones most commonly used in irrigation practice are the 

 rectangular weir and the Cippoletti or trapezoidal weir. 

 Both take their names from the shape of the notch 

 through which the stream of water is discharged. In Fig. 

 2 is shown a weir-box having a trapezoidal weir notch 

 while Fig. 3, a section passed longitudinally through - 

 the weir-box, shows the manner in which the water flow& 

 over the crest "C." The theory upon which the formula 

 for expressing the discharge is based is quite similar for 

 the two forms of weirs. It is, however, not within the 

 province of this paper to take up the theory of weir 

 discharge. Suffice it to say that the weir notch is con- 

 sidered as a large orifice through which the water flows. 

 The head, on the bottom of this orifice, is equal to the 

 depth of water flowing over the crest, while the head on 

 the top of the orifice becomes zero by virtue of the posi- 

 tion of the notch. 



The formula for expressing the discharge over rec- 

 tangular weirs having complete end contractions was 

 developed by J. B. Francis, in 1854, from an exhaustive 

 series of experiments made at Lowell, Mass. As com- 

 monly written it is, 



Q=3.33(L .2h)h 3 / 2 



In this Q is the discharge in cubic feet per second 

 and h is the head of water on the crest which is measured 

 in feet some distance back of the weir plate as shown in 

 Fig. 3, L is the length of the weir crest in feet, while 

 3.33 is a constant derived by experiment. 



The formula for the trapezoidal weir as derived by 

 Caesare Cippoletti, in 1887 is, 



Q=3.367 (LxhV.) 



In this Q, h and L represent the discharge, head on 

 crest, and length of crest as in the Francis formula. 



While the discharge over either form of weir may be 

 computed by the formulae just given, it is not entirely- 

 necessary, however, as weir tables may be had which have 

 been computed by these formulae and which give the 

 discharge for any depth over weirs varying in length 

 from one to twelve feet. 



By observing the following instructions regarding 

 the installation and operation of either of the weirs 

 just named, one may be confident that the error in meas- 

 urement will not be greater than one per cent. The con- 

 ditions as here given are taken from Prof. L. G. Car- 

 penter's valuable paper on "Measurement and Division 

 of Water." which appeared as Bulletin No. 27 of the 

 Colorado Experiment Station in 1894. 



CONDITIONS FOR THE WEIR, EITHER RECTANGULAR OR 

 TRAPEZOIDAL. 



1. That the channel leading to the weir be of 

 constant cross-section, its axis passing through the mid- 

 dle of the weir and perpendicular to it. This straight 

 reach to be of such length that the water flows with uni- 

 form velocity, without internal agitation or eddies. This 

 should be not less than fifty or sixty feet, more if pos- 

 sible. 



2. Only by making the contraction complete on 

 both sides and bottom can the constants in the formula 

 have a value free from uncertainty, and to secure com- 

 plete contraction, it is necessary : 



a. That the opening of the weir be made in a plane 

 surface, perpendicular to the course of the water; 



b. That the opening itself have a sharp edge on 

 the upstream face, and its walls cut away so that their 

 thickness at the point of discharge shall not be above 

 one-tenth the depth for depths below five inches, nor 



