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<br />GENERAL PROCEDURE FOR GAGING STRE~ <br /> <br /> <br />r <br /> <br />Figure 3.-Concrete artificial control on Mill Creek neor <br />Coshocton 1 Ohio. <br /> <br /> <br />Figure 4,-Artificial control on Delaware River near Red <br />Bluff, N. Mex., with shallow V.notch in the broad-crested <br />weir. <br /> <br />should not be subject to obstruetion by debris <br />and ice 01' to deposits of sand, grn vel, or silt. in <br />its ilnmediate yicinity, either upstream or down- <br />stream. <br /> <br />Measurement of Stage <br /> <br />The stage of a stream is the height of ,the <br />waleI' surface abo\'e an established datum plane. <br />)[easurem€'llts of ~tl'ealll stage are used in de- <br />termining records of stream discharge. In ad- <br />rlit.ion, l'et'ords of ~trealll stage are nseful in <br />t.hemsl'h'es, snell as ill the design of structures <br />alfeded by stream elC\'alion or in the planning <br />of the use of Hood pia ins. <br />A record of stage "an be obtained by system- <br />"tie ohserrations of a llolll'ecol'ding gage. In the <br />l'llrly (lays of the.Crt:"olog-i('al Survey, this was <br />the mt'alls g:eul'l'ally n~('d. hut. HOW the water- <br />stag-e l'l~l'ol'dpr is use(l at prad ically all gaging <br /> <br />5 <br /> <br />stations. The advantages of the nonrecording <br />gage are the low initial cost a.nd the ease of in. <br />stallation. The disadvantages are the need for <br />an observer and the lack of accuracy of the esti- <br />mated continuons stage gmph sketched through <br />the points of observation. For long-term opera- <br />t.ion the advantages of the recording gage far <br />outweigh those of the nonrecording gage, and <br />-thus the use of the nonrecording gage is no <br />longer considered a feasible method of obtain. <br />ing a stage record. <br /> <br />Methods of sensing stage <br /> <br />Stage is usually sensed by a float in a stilling <br />well that is connected to the stream by intake <br />pipes. The stilling well protects the lloalt and <br />dampens the fluctuation in the stream caused by <br />wind and turbulence. Stilling wells, though <br />usually placed in the bank of the stream, are <br />often placed directly in the stream as in <br />figure 4, attached to a bridge pier or abutments. <br />The bottom of the stilling well should be below <br />the minimum anticipated stage and its top <br />above the maximum anticipated stage. The in- <br />take to the well must be of proper size and loca- <br />tion to prevent lag during periods of rapid <br /><>hange in stage and to prevent velocity-head <br />effects at its end. <br />Stage may also he sensed by a gas-purge sys- <br />tem known as a bubble gage. This system does <br />not require a stilliug well. A gas is fed through <br />a tuhe and buhbled freely through an orifice <br />that. is permaneutly mounted iu the stream. The <br />gas pressure iu the tube is equal to the piezo- <br />metric head on the bubble orifice at any gage <br />height. The pressure in the tube is measnred by <br />a zero-displacement mercury manometer with <br />the associated electrical components to drive a <br />stn.ge recorder. <br />The bubble gage is used primarily at sites <br />where it would be expensive to install a stilling <br />well. H is also used on "'-\lid-channel streams be- <br />cause the gas tends to keep the orifice from be- <br />ing covered with sand and the tube may be <br />,'asily extended to follow a stream channel that <br />shifts its location. However, the float stilling- <br />,,"PI! installation is cheaper ,to install at most <br />sites., and its performance is more reliable than <br />is that of the hubble ga/-,,,,. The'l,wo systems have <br />ahout, the same accuracy-:!:O.Ol foot. The <br />