dethridge meter pages

8/3/2019 Dethridge Meter Pages

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0 IRRIGATION AND DRAINAGE PAPER

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TABLE OF CONTENTS

VOLUME I

Page

PREFACE

1. INTRODUCTION

2. IRRIGATIONNETWORKOPERATION2. 1 Gen eral Consideratio ns2.2 Ent irely Manually Operated Systems2.3 Hydro-Mechanically A utomated Flow Control Sy st em s2.4 Electrically-Electronically Automated Flow Control2.5 Manual Ve rsu s Automated Control2.6 Water Distribution on the Farm

3. INTAKE STRUCTURESIntroduction 33Intakes of Small Canals (Punjab Type) 38Silt Selec tive Head Intake 55Constant-Head Orific e (CHO) Intake 63Neyrpic Ori fice Module Intake 8 1Double Ori fice Module Intake 90Open Intake Structu re made of P re -C as t Reinforced Concrete(U. S.S. R .) 9 1Intake Str uct ure D ischarging into a Flu me Channel (U. S. S. R. ) 107Pip e Regulator with Crossin g mad e qf Pre -Ca st ReinforcedConcrete (U . S. S. R. ) 110Intake Str uct ure on Secondary Can als (Calomb ia) 119Gate Valve Intake (Czechoslovakia) 128Ven turi Head Intake 134Squ are Head Intake 138Dupuis Canal Intake 142Intake w ith S tone ' ~ e s h eir ( ~ u s t r a l i a ) 151Groyne Intake and Ancillary Works (C yprus) 158King's Silt Vanes 162Gibbl s Groyne 166


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Table of ContentsCont'd.

PageINTAKE STRUCTURES (Cont'd . )3. 19 Curved Wing with Silt Vane@3. 20 Silt Pl at fo rm s3.21 Rev erse Vanes3.22 Vortex Tube Sand Trap3.23 Sloping - Sill Sand Screer

4. FLOW DIVIDING STRUCTURES4. 1 Introduction4. 2 Fixed Proport ion al Diviso rs4. 3 Stru ctur es with Adjustable Spli t ter4 . 4 Propor t ional D is t r ibutors4 .5 Division o r Diversion Boxes

5. OUTLETS OR FARM TURNOUTSIntroductionConstant-Head O rific e Fa r m Turnout (U. S. A. )Neyrpic Orifice Module (France)Double O rifi ce Module o r Siphon ModuleDethridge Meter (Austra l ia)Pl as ti c Siphon Outlet fitted with an Intake Tube (Turke y)Open Flu me Outlet (India and Pak istan )Adjustable Orifice Semi-Module (India and Pak istan )Ja m ra o Type Orif ice Semi-Module (Sind, Paki stan )Pip e Semi-Module (India and Pak ista n)Fayoum Standard Weir F a r m Outlet (Ara b Republic of Egypt)Scratchle y Outlet (India and Pak ista n)Pip e Outlet (India and Pak ista n)F a r m Outlet (U. S. S. R. )Pr e- Cas t Fa rm Turnout (Turkey)Adjustable Weir Fa r m Outlet (Malaysia)PVC Pipe T urnout (Republic of Korea)Pip e Outlet (Philippines)Gated Pipe Out le t (F er ra ra Type, I ta ly)Outle t S t ructures on the Fa rm


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LIST OF REFERENCES

NOTATIONS AND SYMBOLS

Table of ContentsCont'd.

Page


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- ea se of adjustment- abi l i ty to wi thdraw s i l t- immuni ty f rom tamper ing .

I t shou ld a l so be no ted tha t modules wi th moving p a r t s com pr i s e m or eo r l e ss compl ica ted mech an i s ms wi th the re su l t ing poss ib il i ty of the moving pa r t sbecoming jammed.

In conc lus ion, t he in forma t ion and da ta on fa rm ou t l e t s p resen ted inthis ch apter wi l l , i t i s hoped, be of he lp to the designer in making the bes tpossible se lec t ion of the type or types of fa rm o ut le ts he should adopt ford i s t r ibu ting cana l s to su i t mos t cond it ions and requ i rement s . No c l a im i s madea s to the c omple ten ess of th is chapter and i t i s hoped to mak e up any def iciency inthe r ev is ed edit ion of this handbook.

5 . 2 CONSTANT-HEAD ORIFICE FARM TURNOUT (U. S. A. )See Section 3. 4 of Ch apte r 3.

5 . 3 O R L F I C E M OD ULE ( FRA N CE )

See Section 3.5 of Chapt er 3 .

5 . 4 DOUBLE ORIFICE MODULE OR SIPHON MODULESee Section 3. 6 of Ch apt er 3.

5 .5 .1 BackgroundThe De thr idge Mete r i s a se l f - in tegra t ing mea sur in g s t ruc tu re used to

del iver w ater to f ar m er s and to check the volume of water suppl ied for appl ica t ion

" ased on informat ion suppl ied by the Aust ra l ian Nat ional Com mit tee , ICID.


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Cyclometer f~x e d o the wheelWelght of wheels:

W he el Old Type Counter Lorge meter outlet not golvonised--187Cylinder ond vones of Lorg e meter ou t let golvonised-- . .- I9414 gouge hot dip golvonised Smo ll meter o utle t not golvonised.10 1Smoll meter out let golvonised--- - . I07mild steel.

" I,'/e d iameter hi.S. Spokes, ,,Diameter of cy l inder 3 - 4, ( 2 - % )

Depth of vones rodlo l ly 10 (7%)Outs id e dio,me/!er ,of,,wheel to t lps Anthony beorrngof von es 5 - 0 (4;0),, , ,,W i d t h o f w h e e l 2 - 6 ( 1 - 8 )Cleoronces between wheel ondemplocernent : I/ I#

Cost Iron counter housing fo rwire conne ction type counter

Concrete In emplocernent IS 4 thlck(except where fl\!eted) ond re ln fg rce dby o g r ~ d f v4 bors spoced 4 both woys,for both lorge ond smol l metersNote: D/mensions shown ore for both

Meters lLorge and Smolll Thosein brockets ore for Smoll MeterOutlets.

FA0 - l C l D*

D E T H R I D G E M E T E R O U T L E T D E T A I L S

P r o j e c t , Region, CountryA u s t r o l i oF igu re No. 5-1


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of water charges .

The device i s widely used in the Sta te of Victor ia and other S ta te s inAus t r a l ia and to som e ex tent in the U. S. A. and in Asian cou ntr ies . Approximately1 5 ,5 0 0 l a r g e m e t e r s a nd 7 , 000 sm al l me te r s a r e in opera tion in V ic toria .

The Dethr idge M eter was invented in 1910 by the Comm iss ion er a t that t ime,the la te J . S. Deth r idge, of the Sta te Ri ve rs and Water Supply Comm iss ion,V ic tor ia , and adopted by the Commiss ion a f te r t e s t s fo r th r ee ye ar s under f ieldcondit ions . The or iginal Dethr idge Met er was a s imp le me asu r ing device givinga posi t ive meas urem ent of vo lume d ischarged , and r ecord ing i t d i r ec t ly in ac re -feet , with an acc ura cy of 2 3. 5 pe r cent for f re e outfa l l condi t ions over a range

3of d i scharg es f rom 42 11s to 140 11s (1 .5 f t / s to 5 f t3 / s ) . A s m a l l m e t e r f o r3 3capac i t i e s f rom 14 l / s to 70 11s (0.5 f t / s to 2. 5 f t / s ) was developed in the

1920's .

The cons t ruc t ion of the De thridge Mete r has r emained bas ica l ly the sam eove r t he ye a r s , i . e . concre te emplacement w i th mino r va r ia t ions to head wall andt r ans i t ion shape . Labora tory and f ield te s t s have r es u l ted in a s tandard se t t ingof the wheel in re la t ion to channel flow level and mech anic al improv eme nts for th ewheel and f i t t ings , e . g . whee l w ith a mi ld s tee l p la te d rum , vanes and wate r p ipeaxle , or iginal ly with t im ber spokes and r ivet te d, now al l s teel and welded; t im be rbear ing b locks now rep laced by a mor e robu s t sea led uni t ; s tee l ga te in a t imb erf r am e , now rep laced by neoprene gu ides se t in concre te .

S t ruc tura l Des ignThe gene ral form and main d imensions of the two s ta nda rd s iz es of me te r

a re shown in F igure 5- 1 . The wheel i s mad e up of a cylinder of 14 gauge(2 . 03 m m o r 0.08inch th ickness) mi ld s tee l shee t , bea r ing e igh t ex te rna l vanes ofthe same mate r ia l , and in te rna l ly braced by th ree c ros sed pa i r s of s tee l spokesplaced a t the midd le and both ends of the cyl inder . The ax le i s a 2 .54 c m (oneinch) diam ete r galvanized pipe welded to the spokes . The cylin der, vaneat tach men ts and spokes ar e fabr ica ted by welding.

The vanes a r e "V" shaped a s shown (F i gur e 5- l ) , with the ap ex of the "V"leading in the direct ion of rota t ion. At the base of each vane, and a t the apex of


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the "V", the re i s an a i r vent to fac i l i ta te the f i l l ing and emptying of adjacent com-par tm ent s a s they en te r and leave the s t re am of wa te r pass ing under the whee l.The outer c or ne rs of the vanes a re cham fered to sui t the f i l le ts a t the junct ion ofthe wal ls and f loor of the co ncre te em placement .

The co.mple te wheel uni t i s ga lvanized for protec t ion again st corros ion. Thewheel unit i s supported accur a te ly in the em placem ent with the ends of the axleres t ing on bal l bear in gs in a Delr in r ac e f ixed on the wal ls of the f lume. ( ~ i m b e wbear ing b locks were o r ig inal ly used . )

A pendant ac tuated sea le d cyclometer uni t i s r ive t ted to the wheel cyl inderand rota tes wi th i t . The o lde r m e t e r s have a cas t i ron housing a t tached to thef lum e wall for the revolution co unter , which i s connected to the end of the axle bya f lexible wi re l ink.

The shap e of the f lume can bes t be desc r ib ed by consider ing i t in th reep a r t s . Upst ream of the wheel it i s of s im ple rec tangular sec t ion, wi th levelfloor in the vicinity of the w heel . While the w al ls rem ain plane and para l le l , the

0f loor i s indented to accommodate an ar c of approximate ly 70 of the wheel ' sc i rcum ferenc e . And immedia te ly down stream of the wheel the wal ls a r e splayedoutward and the f loor i s s loped up to a l ip a t suff icient he ight to e nsu re drowningof the passa ge swept by the vane s under the wheel .

At the en t ry to the f lume a cut-off wal l extends to e i th er s ide into the canalbank, and downward below the natura l su rfac e of the ground to prevent seepagearound the s t ruc tu re . The galvanized stee l s luice ga te to control disch argethrough the me te r i s f i tt ed in neoprene gu ide s t r ips se t in re ce ss es so tha t theya r e f lush wi th the wal ls and f loor a t the en t ry f ro m the channel .

The f lume and cut-off wal l a r e const ructed of re in forced co ncre te , a l thoughothe r ma te r i a l s may be employed. In ea r ly t imes of cement shor t age , t imbe rwas used, but proved unsat isfac tory because of leakage between the t imb ers anddi s to r t ion .

Al l c oncre te i s of high qual ity and 10 cm (4 inch) thickness (except wheref i l le ted) , and re inforced wi th a gr id of 6 m m (0 . 25 i nch) d i ame te r ba rs spaced10 cm ( 4 i nches) apa r t bo th w ays . The s t ruc tu re may be e i the r cas t i n - s i tu o r


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a sse m ble d f rom p r e - c a s t un it s . The pre -ca s t uni ts provide a mor e accura teemplacem ent with better concrete quali ty. (See Figure 5- 2)

FIGURE 5-2. - P r e - c a s t l a r ge m e te r e m plac e m e n t w ith whe el sinsta l led; note re inforcement to t ie into cut-offs and pr e- ca sthead wall .

Ri p-r ap protection i s placed on the bed and bat ter s of the fa rm ditchimmed iate ly downstream of the me ter to prevent scouring.

F o r la r ge cana ls , where continuous a cce ss i s requi red a long the cana l bank,a pipe outle t is in sta l led through the bank and the me te r i s er ect ed outside thebank a t the end of the pipe.

Fi gu re s 5- 3 (a) , 5-3 (b ) , 5-4 ( a) and 5-4 (b) show detai ls of both lar ge ands m al l m e t e r s .

Fig ure 5-5 shows a lar ge Meter Outle t in operation with f r eeoutfall.


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250.

Groa* to be formed in wal l 1-0 above I1010 bndi~otemoiimum ollowoble toilrmter leve

-----

y level 10 be painted block

Earth plat form level wl th lop01 side wall en bot h s~dcs

C d i o . b o l h G o n g . s et 5 s blblock-OUI 01 8 centres forAnthony brorinqr. For lumber9" locks 3bmdto bolts o n

Anthony bearing

G01.s - Go1vonosed, mqether w t h 0 x 1 ~ L A R G E D E T H R I D G E M E T E R OUTLET

R81hg& hrge meter cutletT h e od op le d r o li nq i s 2 9 . 0 4 l ? l r e v o ~ l l ~ nqu i vo l rn l l o 1 ,500 rev ~ lu t i o n r 1 ocre-foot basedon col ibrot ion Ierls wi th free ~ l f f o l l M the end s11H i i r loilwolerr incrcosc the volume porredThe cyc lomet ers c l od d ~rcc l t y n oc r l I t I r p m . orrr-lootl doy opproxlm olely AustraliaCkor0nc.s between the wheel ond cmplocement should be occ urolr ly mointo~n ed nd the wheel.hod4 (DIe 0110wed to turn ol less thon 3 r p.m.


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16 1

&l:block o ut {long for Anthony beorlngs,1l"long for tlmber blocksWlngwall shape when o ccers culverl.-eaulred or road crosslnq

er 6 feet transition, ,,7 - 1 0 - .- -- --

S e c t ~ o nC C (s ee F lgu re 3 (0 1P

U p s t r eo r n V i e w o f D r u m

S e c t ~ on D D

N be~ngcons~deredDev e lopms n t o f Vane

M e t e r W h e e l D c t o i l(See note 7 ond B on Ftqure 3(01

P o s ~ t ~ o nf W h e a l In E m p i o c e r n e n tT~ mb er x l e B loc k [See note 4 on F~gure3 ( 0 )Block made In halves from seasoned R G or other durabletlmber dressad to dlmens~onsshown Slot ond houslnpbolt hdes rrqulred on one set only

'/:dm ec en tr es ln centre of plonk ?g dto Q e c e n t r e ri~ 1 / 2 1 - - ~ ~ + jT

S e c t i o n E EP r e c o c t R C F o o t P l o n kTimber plank may be used as olternot~ve

F A 0 - C I DSECTION CC OF FIGURE 3(0) N D

DETAILS OF METER WHEELOF LARGE DETHRIDGE METER OUTLET

P r o j e c t , R e g ~ o n ,CountryA u s t r o l ~ oFlgure No 5-3 (b )


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Groove to be formed in wl l 9 obove f lwr I Top of gote nom~nolly evel with tap of heodwolllo indicote moximum ollowoblc toilwo ter level Gotes ow~loble ore 1-07 2I6: $-9" or 3'-0"high.Select gate rile neorest to required height.See notes 2 ond 3.

C--- -- --I Groove to be pointed blockI with chlorlnoted rubber pain

H a l f S e c t ~ o n A - A / Half S e c t i o n 8-8

Notes P l a nIf the derlgned rnlnlmum free board l o the lop of bonks 1% less than < / F B ~ ~ /4, 1 11 0s

A u s t r o l i a

F i g u r e No 5 - 4 ( 0 )

equal to or greater thDn 2 then /FB,,/ = 12 "The helght of the heodwaliond gote lo ttxed from Ihe D 0 L of the channelThe moilm um h eq N ot gote su8toble for hond lhfttng IS 3'-0'where larger gate s necessary,type 'C' lhfttng gear s hould be usedAnthony bearongs ore adopted as stondordConcrete tor emplacement should be of mcsmum com pnss se strength 3 ,500 p n I o t 2 8doys fo r du rob l l~ tyBackfoll should be well compacted around the heodwoll ond sfde wallsProtectton of metal work on currenl useGoter - Galvobond plate Wheels- Golvon~ rcd, together w~t h oxleAll bolts galvonlsedFeld tests are on hond for c old po tnl ~ p p l t c o l l ~ n rn gates e g,Galvo frotd woth Mvconoxre01 coat ond for olumlnlum wheelsRofmg of smo// meter out/etThe odopled rotlng IS I 2 45 f?/revolutnon cquwalent to 3 500 revclvttont = I ocre-footbosed on collbrattan tests w ~l h ree o utfoll owr the end 91;Hfgher to#lwatcrs lncreose the volume possedThe cyclometers read directly In ocre-fecl/r p m = 3 ocrt-tcet/doy opproxlmatelyClearances between the wheel ond emplacement should be accutotel y mo~nto8ned ondIhe wheel should not be ollowed to turn ot less than 3 r p m

F A 0 - I C I D

SMALL DETHRIDGE METER OUTLET

P r o j e c t , R e g i o n, C o u n t r y


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On@ vonar hown-

S e c t i o n C- CSee Figure 3 ( 0 )t

Dovies Shephordpendant weight counter

fi- a+

\6ntc rnol diameter stondordrned~urn lpe (not golvonlsed.not vornished, not oil painfed)- dio. M.S.apokes welded lo drum

I S e c t l o n D - DU p s t r e o m V ~ e w o f D r u m

S i d e E l e v o t l o n,vote use of o/um~n~umheels of whfed or

nveled construmon IS &IW consd8nd

7-0rodlurO e v e l o p m e n t o f Vans

M e t er W h e el D e t o ~ lSa@ note 7 on d 8 on F~ gu re (0) POSI~IO~ f Wheel ln Emplocemenl

'/;; C S acers for cover plateof C I housing (on counter $~de bolts ~n centre o f blocksonly F A 0 - I C I DBlock mode In halves fromseasoned R G or other durablettmber dressed to dlmens~ons SECTION C-C OF FIGURE 5 4 a) ANDshown Slot and hous~ngboltholes nqul red ~n one set only D E T A I L S OF METER WHEEL

OF SM AL L DETHRlDGE METER OU TL ET

woter plpe - Pro ject , Region, Countryd x 2 % " x l i T l m b e r A xl e B l o ckSea note 4 o n F ~ g u r e ( 0 )

Scale ? ? ? 9 InchesAustral to

Flgure 5-4 (b)


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FIGURE 5-5. - La rg e Dethridge met er out let in operation withfre e ou t fa l l.

The s t r uctu re i s robus t in cons truct ion and reasonably r es i s ta n t to i l legaltampe ring. If ne ce ssa ry, gates can be padlocked in posit ion and lead sea ls f ixedto counters . The counter has a ra tchet ar rang emen t to prevent winding back ifthe wheel i s rotated in the opposi te direct ion ( im portant against tampering ). Onevane i s painted so that the canal at tendant can see fr om a d is tance whether thewheel i s turning cor rec t ly. The me te r wil l pa ss a fai r amount of f loat ing deb risin the wate r without damage to, o r stoppage of, the wheel.

Once ins ta l led c orrec t ly in an accura te emplacement the m ete r does notrequi re ad justment. Regulation of disc harge through the met er i s c ar ri ed out bysim ple adjustm ent of the ga te opening.

Hydraulic Characte r i s t i csI t i s impor tan t tha t the me ter be ins ta l led a t the c orre c t l evel in re la t ion to

the designed discharg e level of the wa ter in the can al , so as to make the bes t useof the gen eral ly l imited head avai lable while at the s am e t ime ensuring sufficienthead and yet avoiding drowning of the wheel by wate r that may back up fro m thefa r m d i t ch. The s tandard se t t ing for the lar ge me ter , a s shown in F igure 5-6 ,


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i s to have the floor of the flume, at entry, 38 cm (15 inche s) below supply levelin the canal. Fo r the smal l me ter this depth i s 30.5 cm (12 inches) .

The supply leve l of the canal i s fixed by the designed upstre am level of eachcanal regulator and i s taken a s a horizontal pool at this level extending upstrea mfro m the regulator to the next regulator . This level i s fixed in relation to theland to be supplied.

The commanded land i s defined as the land 15 cm ( 6 inches) or m ore belowthe supply level making al lowances where nece ssa ry for fa rm canal g rade s andstru ctu res . This al lows about 7.5 c m (3 inches) drop through the me ter to themaximum al lowable level in the fa rm er 's di tch and 7.5 cm ( 3 inches) depth ofme te r over the highest ground. As the canal running level is above thehorizontal supply level , me te rs , set some dis tance upstre am of a regulator , havea water depth over the floor of the flume in ex ces s of the stan dard 37.5 cm (15inches) or 30.5 cm (12 inches) respect ively and the gate and head wal l levels m ustbe raised accordingly to provide the required fr ee board. As far a s possible,regula tors a r e located so that met er s a re wi th in a shor t d is tance ups t ream.

If t he re we re no nece ssi ty for clear anc es between the wheel and the f lume,the met er would give an exact positive me asu rem en t of the water p assing throughit, a s each revolution of the wheel would pa ss an inva riable quantity of the waterbetween the vanes and the cylinder swept through the emplacemen t. With theprovis ion of the nec ess ary clear anc e (se e sect ion 5.5.4) however, leakage occu rsthrough the clea ranc e space a t a r at e dependent not only on the ra te of rotation ofthe wheel but also on other fa ctors , such a s the difference in water le velsimmediately u pst rea m and downstream of the wheel, and the depth of submergence.The quantity of water pa sse d per revolution of the wheel does, the ref ore , vary tosom e extent under o perating conditions. Fo r th i s reason , the c learan ce i s kepta s small a s p rac ti ca l.

5 .5 .3 .1 Relationship between revolutions and dischargeFo r the conversion of revolut ions to acre -fee t , constant rat ios a r e

assumed: 1,500 revolut ions per acre-foot for the larg e wheel and 3,500revolutions pe r acre-fo ot for the sma ll wheel. The dimensions of the wheelshave in fact been designed to provide these s imple rat ios , and the e r ro r is not


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m or e than 5% over the rang e of norm al opera t ing condit ions for e i the r me te r .This degree of accu racy i s cons idered qui te reasonable for the measure men t ofwate r de l ive r ies for i r r iga t ion .

S imi la r ly revolu tion counte rs a r e geare d in the ra t io of 15 : 1 f o r the l a r gem e te r , a nd 35: 1 f o r the sm a l l m e t e r , and so the d ia l read s d i rec t ly in ac re - fee t .The counte rs a s manufac tured rea d to two dec imal p laces , but i r r iga t ion rec ord sa r e rounded off to one de cima l place.

3A d i sc ha r ge r a t e of one a c r e- f oot pe r da y (14 l / s o r 0 .5 f t / s ) c o r r e sponds1closely to one revolution p er minu te of the lar g er wheel o r 2 7 revolutions per

min ute of the sm all wheel. These re la t ionships provide a very convenient guideto the cana l opera tor o r wa te r -ba i li f f in se t ting a met e r ga te to g ive whatever f-d i s ch a r g e r a t e i s r e q ui r e d.

Design ra t ings( a ) Ratings and set t in gs adopted in re la t io n to the supply canal levels fo r

the two me te rs a r e a s indica ted below:Rat ings Larg e Mete r Out let Smal l Mete r Out le t( i ) F r e e o v er f al l 820 l / evolution

or 29 .04 f t3( i i ) Ta i l wa te r a s in ( i i i ) 860 l / evolution

or 30 .38 f t3

349 l / revolutiono r 12 .45 f t3

( i i i ) Ta i l Wate rAbove down stream 1s i l l l e ve l 17 .8 c m o r 7 inc he s 1 3 . 3 c m o r 5 3 i n ch e sAbove upstreamsi l l leve l 30 .5 c m o r 12 inc he s 22.9 c m o r 9 inc he s

( iv) Designed supply levelAbove ups t reams i l l l e ve l 38 .1 c m o r 15 inc hes 30 .5 c m o r 12 inc he s

(v) Clea ranc e betweenthe Meter wheel andthe Qut let S t ruc tu re 1 1B ottom 0 .64 c m o r q inch 0. 64 cm o r 2 inch

3 1Side 0 . 9 5 c m o r 8 inch 0.64 c rn or 7 inch


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5. 5 .4 Insta lla t ion and OperationCanal regul ators a r e located to regulate supply levels for the me ter outle ts

a s shown in Fig ure s 5-6 and 5-7.

FIGURE 5-6. - Typical setting of Dethridge m et er just up st rea mof a regulator. The supply level of the ca nal i s indicat ed by thebottom of the slo t in the wall on the righ t hand s ide of the re gul ato r.

Generally one me te r i s provided for each farm , a sm all one for are as up to40 a c r e s a nd a l a r ge one f o r l a r ge r f a r m s . If the farm area commanded bygravity and suitable for ir r igati on exceeds 120 ha (300 acre s) , a second la rg em et er may be provided. These general rul es may be modified, i f ne c e ssa r y ,because of par t icular fa rm layouts and topography.

The canal a t tendant reg ulates f low to the fa rm through the me te r a t requiredinterv als dur ing the ir r igation season to provide f lows as requested by thei r r i g a t o r . Regular readin gs of the coun ter a r e taken to keep check on the totalvolume supplied.

The flow i s regulated by adjusting the gate to the app ropria te opening a sindicated in Fi gu re 5-8. In actual operation the flow i s readil y checked by


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Ea r t h f r e e b o a r dannel regulator- I t- Min . d rop 0 .2 0-L L!7f +- - -L .M.0 F loo r leve l ( 1 )B e d

Capaci tyinf t 3 / s0 - 3 0

3 0 - 4 9 0> 4 9 0

3 maximum str ippingo f d rop bars

E a r t h b a n k

T yp i co l Longitudinal Sect o nD.D L . Designed discharge level

Flow profile for maximum discharge with roughness and grade selectedS L. -Supply leve l for s et t ing m ete r ou t le ts and spur o f f tak es

/I

( I 1 For SM O se t t ing o f f loo r i s 12 below supp ly leve l

* The required crest width may be larger thanthe min imum to meet perco la t ion grad ientrequirements or const ruct ion equipmentrequirements.

S t r u c t u r e sMin jmum*c r e s t w ~ d t h n ee t6812

If access is required along the bank atminimu m c re st width of 12 is required.

* * ~ e a s u r e d f r o m D. D.L. a t a b u t m e n t s t otop o f p res t ressed slab .

i

Minimum concretef ree board

] 9"12"Minimumfree board in feet

1'- 6"1 6"2'- 0"

.

F A 0 - C I DCHANNE L DESIGN, FREE BOARD

AND SETTING OF DETHRIDGE METER OUTLETS

P r o j e c t , R eg i o n, C o u n tr yAus t r o l i o

F i g u r e N o . 5-7

Prestressed slabb r i d g e s * *

1


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counting the revolutions p er m inute of the.whee1 and ref ere nc e to the table i s notnecessary .

Acc urate rat ings have been obtained in the laboratory to give graphs forvolume pass ed per revolution of the wheel over a range of dis ch arg es and fo rdifferent canal levels and tai lwater le vels. Other se ri es of tes ts have beencarr ied out for non-standard clearan ces. These rat ings a re not required foroperations o r norma l design purposes because averag e rat ing s have been adoptedfor the counter gearing. Detailed rat ings ar e, however, available for specialinvestigations.

5.5 .4.1 Costs and quanti t ies of ma ter ial sDirec t labour and ma ter ia l costs of the instal led pre- cas t out let

(excluding fittings) a r e in A us tra lia approximately:smal l meter ou tl e t $ A 350larg e me ter outlet $A 450These costs vary depending on site conditions and access. The costs

of fi t t ings, (wheel, gate, guides, be arings , cyclom eter) a re $ A . 43 for a smal lmeter and $A . 63 for a l arg e met er and a r e included in the above cost .

Approximate concrete volumes for pre -cas t uni ts are :smal l mete r ou t l etlarg e meter outlet

MaintenanceThe main i te m of r egu lar m aintenance for the old type of m et er was

recoating the wheel with ta r, for protection against corrosion. This was doneeach year in the winter months, when the re was no irrigation. This h as now beeneliminated by using galvanized meta l pa rts. When the canal oper ator takes areading of the counter or adjusts the discharge, he should check that the wheel,the bearings and counter a r e operat ing correct ly . Any damage to the galvanizingshould be treated with a cold zinc- rich epoxy paint. The fa rm er 's di tch mu st bechecked to ens ure that i t i s clean of weeds which would inc rea se the wate r leve land cause high tai lwater le vels on the me ter .


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--(Large Meter Outlet) 12 Inches tallwater

Dfscharge In ft3/sApproximate No of r p m of wheel given in brackets

Marks on precast fromupstream water depth

(Small Meter Outlet 9 Inches tallwater

angle"^" angle^"

F A 0 - I C I DG A TE C A LI B R A TI O N TA B LES

FOR L M.O. AND S.M.O.RIVETTED GALVABOND TYPE GATES

P r o j e c t , R e g i o n , CountryPrecast frame wlth gate locked In closed po s~ t~o n pstream vtew A U S ~0li0angle"^" ~ n g l e ~ ~ *

F ~ g u r eNo . 5 - 8


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Numerical ExampleDesign a Dethridge Met er Outlet under f re e overfall conditions for standard

3ratings and set t ing to deliver dur ing par t of th e ir r igati on season 2.5 f t / s and in3the other par t of the season 3.8 f t / s .

Refer to the Tab les on Figure 5-8, which show that for s tan dard ra t ings and3se t tings ( see 5 .5 .3 . 2 ) , the lar ge met er outle t wil l give a discharge of 2.5 f t / s

5 3with a gate opening of 5g inch and a dischar ge of 3.8 ft / s with the gate fully open.

Other dimensions and detai ls a r e as given in 5.5. 3 and 5.5.4.

Fu r th er Res earch to Improve the S t ruc tureExperime nts have been ca rr i ed out for some t ime to try and find an arran ge-

ment with better accuracy at low discharge. Te sts have included rubber f laps onthe wheel to seal the c learances. A modif ied emplacement i s a t prese nt underte st with the downstream section of the f lume the s ame width a s the upstrea msection, inste ad of flared, and the floor horizontal. A gate is in sta l led on thedow nstre am end to con trol the flow of wat er and to ens ur e filling of the volumebetween the vanes and drum.

A somewhat si mi lar modified arr ang em ent is used in New South Wales.

Tr ia l a luminium wheels, r ivetted and welded, *have been in stalle d but a r emuch mo re expensive than galvanized steel . Fibre glas s wheels have been tr iedin New South Wales.

SummaryThe Dethr idge me te r outle t can operate with a small head loss , a s low a s

37.6 cm (o r 3 inches) for disch arges up to 112 11s (or 4 f t 1 s ) f o r a l a r ge m e te r .A head loss of a t least 17 cm (or 7 inches) i s required for best accu racy.

The device me as ur es by positive action. The measurement i s se lf -integrating. Accuracy of the me asure men t i s around 5% which i s well withinreasonable l imits for ir r igation deliver ies.

The accurac y dro ps off sharply with high tailwater le vel s. The water levelin the fa rme r ' s d i tch must be contro l led to keep i t a t leas t 7 .5 cm ( 3 inches) below


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