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Show Duty and Measurement of Water npv By DON H. BARK, U. S. Engineer, in Charge of Irrigation Investigations Investi-gations in Idaho. Oregon Short ' Line Railroad Ik'in-onstratlo.i Ik'in-onstratlo.i Train Lecture. Tlie greater part of the land in the west would be practically worthless without water with which to irrigate it. yet there is far less attention given to the measurement of this water than there is to the measurement measure-ment of the land. Under the Carey Act the land is sold to the settler for fiO cents per acre and would hardly be worth that without the water, while water costs the settler all the way from $25 to $100 per acre. Miner's Inch. Wilier was first used in Idaho and other western states for mining purposes, pur-poses, the common method of measurement meas-urement being called the miner's inch method. The miner's inch was the unit upon which such measurement measure-ment was based, being the amount of water that would flow through a slrarp edged orifice one Inch square under a given pressure. The quantity quan-tity called for by a miner's inch, however, varied in different states, due to the fact that the pressure over the orifice was not the same. Cubic Foot Per Second. The cubic foot per second, which represents a definite tangible amount that is easily understood, was adopted adopt-ed as the legal standa-d for the measurement of water by the Idaho legislature In 1899. It Is commonly known as the "second foot" and represents rep-resents the flow of water which will exactly fill a vessel containing one cubic foot each second of time for as long a period as it is allowed to flow. Hence, a flow of one cubic foot per second delivers 60 cubic feet per minute, or 3,600 cubic feet per hour, or 86,400 cubic feet in a day of twenty-four hours. It is found that one cubic foot per second equals a flow of almost exactly 60 Idaho miner's inches, or 450 gallons per minute. A flume one foot wide and one foot deep, if filled with water that is flowing at the rate of exactly one foot per second, will carry one cubic foot per second, and otner flumes or ditches in the same proportion. propor-tion. The quantity discharged depends de-pends upon, the velocity of the flow and the area of cross section of the advancing stream of water. These two factors are taken into consideration consider-ation when determining the flow of large streams and canals, it being only necessary to determine the area of the cross section and the average velocity, which two amounts multiplied multi-plied together gives the discharge. The cross section is found by multiplying multi-plying the average depth of the stream by the width. The average velocity is found by measuring the rate of the same either with floats or with a current meter especially constructed for the purpose. A close approximation of the velocity can be secured by noting the time that is required for a surface float to advance ad-vance through 100 or more lineal feet of the ditch. This gives the surface sur-face velocity, and to find the average velocity one must multiply the surface sur-face velocity by .8, since the average aver-age velocity that much slower than the surface velocity, owing to the friction on the sidi?s and bottom of the channel Acre Foo,t. Where large volumes of water are to be considered the expression of the amount in cubic feet would in-, volve the use of such large numbers that the same would be cumbersome. In order to simplify tlise expressions expres-sions the term "acre-foot" is used, which represents enough water to cover an acre one foot in depth, or 43,560 cubic feet. The use of this term has the additional advantage of being easily compared with the acreage; acre-age; as for example, a reservoir containing con-taining 50,000 acre feet of water would furnish a depth of two feet for 25,000 acres of land. A cubic foot of water per second flowing continuously con-tinuously for twenty-four hours furnishes fur-nishes almost exactly two acre feet of water. Hydraulic Equivalents Which Will be Found Useful to Irrigators. 1. Idaho miner's inch equals approximately ap-proximately 1.50 of a cubic foot per second, or nine gallons per minute. 2. The flow of a cubic foot per second equals approximately 50 miner's min-er's inches, or 450 gallons per minute. min-ute. 3. One cubic foot per second for 24 hours equals approximately 2 acre feet. 4. One acre foot equals enough water to cover an acre exactly a foot in depth, or 43,560 cubic feet. 5. One miner's inch per acre for 100 days equals 3.97 feet deep on the land. 6. One miner's inch per a -re lor 130 days equals 5.95 feet deep on tho land. 7. Five-eighths miner's inch lie acre for loo days equals 2.48 feet deep on the land. 8. Five-eighths miner's inch per acre for 150 days equals 3.72 feet deep on the land. ' 9. One-half miner's inch per acre for 100 days equals 1.9S feet deep on tho land. 10. One-half miner's inch per acre for 150 days equals 2.98 feet deep on the land. Weirs and Weir Measurements. The most accurate, practical and economical method of water measurement meas-urement that has been devised for the measurement of comparatively small heads of water is the weir I measurement. The weir consists essentially es-sentially of a thin notch of a specific shape, which the water is caused to flow over. The amount, of flow depends de-pends upon the depth of water flow-ng flow-ng over the crest, as the bottom of the notch is called. The weir has been used for the measurement of water for hundreds of years and the method of its installation and the discharge of water can be measured over it with an error of less than one per cent, if care is used. There are several forms of weirs, the name of each designating designat-ing the shape of the notch. Cippo-letti, Cippo-letti, an Italian engineer, evolved and perfected the weir which bears his name, many years ago, this being be-ing the weir that is now most generally gener-ally used in the west. Weir Box. In order to maintain the weir in a proper and constant position and to prevent leakage around and under It, and in order that all water that is to be measured should be conducted over it, it Is usually necessary to construct some sort of box or frame to hold the weir In place. A common form or type of this box which is recommended recom-mended for a one foot Cippolettl weir is nine feet long, two and a half feet deep and three feet wide inside measurement, and with a one foot weir will measure with accuracy amounts ranging from ten to fifty Idaho miner's inches. Where a weir box is built, it is necessary nec-essary that it be made of sufficient size and depth in comparison to the size of the weir notch to eliminate all excess velocity of approach. 1-f the box is built too narrow or too shallow it will add to the velocity of approach to such an extent that correct cor-rect measurement can not be secured. se-cured. It is but a mere matter of convenience, however, to have a box of the exact length prescribed, for that part of the box above the weir may be omitted if a settling basin or. pool of the same size is constructed in the ditch above the weir. The weir boxes and pools which should be constructed for weirs of larger sizes should be in the same .proportion .propor-tion with respect to the size of the weir as for the one foot. The size of weir box that is required in order to eliminate velocity of approach has been found, by experiment, to be approximately ap-proximately seven times the cross section of the weir notch. Discharge. The discharge of weirs has been accurately ca'culated by various engineers who have carried on hundreds of experiments, cover- ling years of time, and it is found that under like conditions the same depth always produces the same discharge dis-charge over the same size of a weir. I The conditions given for the construction con-struction and installation of weirs must be rigidly adhered to, however, if accurate measurement is to be made. The formula which has been evolved and ''which gives the discharge dis-charge of accurately constructed Cippoletti weirs is Q equals 3.367 L H 3-2. Where Q equals the number of cubic cu-bic feet per second. L equals the length of crest in feet and H equals the depth of water on the crest in feet, provided the same is measured from a point level with the crest and up stream from it at a distance equal to the length of the crest. 1. The weir box should be set with its floor even with the bottom of tha ditch and should be level in all direc tions. The weir itself should be exactly ex-actly level and perpendicular. 2. The channel leading to the weir should be of uniform cross section, o? what is still better, should gradually enlarge as the weir is approached. The axis of the stream should pass through the weir and perpendicular to it; or, in other words, the weir should be located at right angles to the middle of the stream. The advancing ad-vancing stream should be free from internal cross currents or eddies, as these have an influence upon the dis charge. 3. The water should be brought as I nearly as possible to a state of rest; before it enters the weir. An excess velocity of approach due to the vei locity of the advan-'.ng current will affect af-fect (increase) the discharge more than almost any other one thing This velocity can be reduced by wid: ening and deepening the box or pool above the weir. It is calculated that weirs three feet long with a depth of, water of twelve inches, should not have a greater velocity of approach than six inches per second, which amount may be allowed to increase very slightly where greater depths over wider weirs are used. By constructing con-structing the box or pool above the weir with a cross section at least seven times that of the weir notch, a sufficiently low velocity of approach is usually secured. (To be Continued) |