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<br />
<br />,i,i ,ull.l,'i r~:ci,l, ,prrlllyd
<br />~,~ul.,. ~ :~! Iluw cun,litnnu occur and
<br />I\\,~ \1';Ili i ili I~iII Ili .I5ll1'e lllrlllti ;II'e rrll llllrll l(1
<br />\;: t.•Inunr the r;ue ut Iluw.
<br />I'.~nh.lll Iluntcs ran be tlcsicncd to mrasurc
<br />tluo fntw ILUI cubic toot per second (efs) to
<br />3,UUU cl's. 'I his di,russion will be limitrJ to
<br />small I'arshull (luntcs having frcc-IIoW
<br />c.lp.~, ilics of O.US to 139.5 cl's. "these
<br />Ju; h;uges correspond [o Ihrua[ widths of 6
<br />in:hcs lhruut;h 8 Icot. For G- and 9-inch
<br />Iluul.•s, tllc nl,lximunl deprcc of subnu•rgcnec
<br />I Ill' latiu, sir fig. S-6) fur Ircr-flow Jisrhargc is
<br />II
<br />J
<br />hll prlerul, wltrrcas, fur flunus I through ti
<br />lint in wiJ[It, [he maxinuun Jrgrcc
<br />ut wbmrr~rncc Ibr frcc flow is 70 pcrccnt
<br />(I (.= Gage zeros for both H and lib arc seta[
<br />the tlumr crest elevation soJ that water depth
<br />mrasurcnlrnts arc depths above the crest.
<br />Curf.•ri crruing :utd reading oC the gages is
<br />nrerssary for accurate results which is usually
<br />within .Ibuut 2 pcrccnt for frcc flows and
<br />about 5 pcrccnt forsubmergcd tlows (2(.
<br />Parshall flumes can be constructed of
<br />concrete, wood, galvanized metal or any other
<br />ronstnlctiun material that can be built to the
<br />given diutcnsions in the field or prctabricated
<br />in a shop. Care should be taken in construction
<br />su that flu structure is built as closely as
<br />possibly to the standard dimensions given in
<br />figure J-7 with one exception; Utc wingwalls
<br />utay be Icngthcncd us required. Also, it is
<br />tut utrtant to hav_c t_he floor of the conycr:;in~
<br />~cctwu facet sc that the same amount of water
<br />iris p;lsutg aver each increment of throat wiJ[h.
<br />An .ulglc~irun is usually embedded flush wtt t
<br />the Iluur ;utJ perpendicular to the flow at the
<br />Juwnslre:.m end of the crest to prevent cJrst
<br />rrus~uu ,u.J ro provide a smooth surface for
<br />salting ~a~r zeros. Sidewallsof the throat must
<br />IL• p,uallrl ,Ind vertical.
<br />I',I;shall Iluntcs should be located only in
<br />s(r,lit•ht scruons of channels where the Ilow !s
<br />lil,,tiv.•ly ,uwo[h and uniform. They should ,k
<br />n.\cr br lur;tlal on a curve or at right angles [o
<br />f!Ir rural Iluw, as in a turnout, unless the Ilow
<br />7n umber u. br~c beta refer to ilcma in the bibliugraph Y. sec
<br />~.x~,.
<br />• 24:,
<br />h,n been straiglltcnrJ :InJ unlfurmly
<br />rcJlstribulcd. 1':ushall Iluntcs should be IucatcJ
<br />,:IS close as possibly to c:utal discttargt regulating
<br />g;urs fur cunvcnicncc of operation, but far
<br />enough away fraru the gates so that the (low is
<br />uniform and frcc from eddies, turbulence, and
<br />waves.
<br />5-3. Advanruges.-Parshall Ilumcs arc
<br />rocot;nizal as accurate and pliable flow
<br />measuring structures and have the following
<br />adv;utlagrs: (1) c:gtahlc of mc;tsunng r;dc of
<br />Ilow with relatively small head loss, (3)
<br />capable of measuring awide rangy cf trtr-flow
<br />discharges with relatively high tailwater depths
<br />using a single water depth mrasur_men[, (3)
<br />c;lp,iltlc of measuring submcrgerl flow using
<br />t\vo water depth ntcasurcmcn[s when Chr
<br />drgrrc of submergence impeJes tL~: frcc-flow
<br />discharge, (4) virtually a sclC-cleaning structure
<br />berallSe of Iume geometry and the dtroat
<br />velocity, (5) can not be easily altered to obtain
<br />unallocated water, and (6) unatfcc[\°d by
<br />velocity of approach, which is au omatically
<br />controlleJ, when tltc Ilunt,u is built to the given
<br />standard dimensions and used where the
<br />incoming flow is uniform, evenly distributed,
<br />and froc of turbuluuc.
<br />5-4. Disurlvantuges. -Parshall flumes: (1) are
<br />usually more expensive to construct than weirs,
<br />(~) cannot be used in close combination
<br />with turnouts because the inflow must be
<br />uniform and the water surface relatively
<br />smooth, and (3) must be ronstructrd carefully
<br />and accurately for satisfactory performance.
<br />5-5. Size Selecrios.-Parshall fhutc sizes are
<br />designated by their throat width:. 8cfor: a
<br />Parshall flume can be selected, tae channel
<br />cross section, the range of disch~rgcs [o be
<br />measured, allowable head loss through the
<br />Ilunm, and the normal depth of the tlow in the
<br />channel must br known. Minimizing
<br />construction costs is an hnportant factor.
<br />A particular discharge or discharge range can
<br />be measured by any unc of sevcrtl diffemnt
<br />,ice flumes. Filial selection is ba>ed on the
<br />Iluntc width which best fits the canal
<br />dimensions and hydraulic properties. As a
<br />;:rncral rule, [hc width of the Parshall flume
<br />should be about one-third to one-!tall the
<br />width of the upstream canal water surfaw at
<br />design discharge :utd normal depth. With these
<br />
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