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HY 5 7

UNITE ST TESJEP RPMENTOF THE NTERIORBURE U OF RECLAMATION

H Y D R A U L I C M O D E L S T U D I E S O F T H EF B N T E N E L L E P O W E R P L A N T D R A F T T U B EA N D T A I L R A C ES E E D S K A D E E P R O J E C T W Y O M I N G

R e p r t No. Hyd-571

Hydraulics BranchDIVISION O F RESEARCH

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The information contained in this report may not be used in anyp ~ ~ i c ~ t i - s nci-.s-tising or other promotion in such a manners to constitute an endorsement by the United States Governmentor the Bureau of Reclamation either explicit or implicit ofany material product device or process that may be referredto in the report.

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UNITED STATESDEPARTMENT OF THE INTERIORReport No. Hyd-571Checked-andReviewed by: W E. WagneS12zaittedDy: H. M. Martin

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DRAFT TUBE ND TAILRACESEEDSKADEE PROJECT, WYOMING

This study was made to define and investigate hydraulic p roblemswhich could a r i s e while the Fontenelle Powerplant s used f o r flowdiversion and rese rv oi r control during rehabilitation of the ri ve routlet works stilling basin.

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The recommended dra ft tube modifications w er e obtained through thecombined effor ts of the Struc tural and Arc hitec tural Branch, the DamsBranch , and the Hydraul ic Machine ry Branch , Division of Design; andthe Hy drauli cs Branch , Division of Researc h. ; e Division of Engi-neer ing Geology was helpful in supplying topography and b edro ck con-ditions of th e ta ilr ac e. Photography was by W.- iJI. Eatts, M. P Einert,and S Rasrnussen.INTRODUCTION

Fontenelle Dam is the principal fea tur e of the Seedskadee Proje ctlocated in the Upper G reen Riv er B asin about 50 mil es northwest ofRock Springs, Wyoming, F ig ur e 1 The project is intended to provid eirriga tion water fo r about 60,000 ac re s along the Green River.The dam is an ea rth and grave l stru ct ur e approximately 5,003 feet longat the cr e st and ri s e s about 127 feet above the riverbed. The principalhydraulic fea ture s a r e the spillway, the ri ve r outlet works, and thepowerplant. The spillway is loca ted in the right abutment of th e dam.It is an uncontrolled, double sid e channel spillway with a c re s t lengthof about 300 fee t designed fo r a max imum dis ch ar ge of 20,000 cfs. Flowfr om the spillway pas se s through a 400-foot-long diverging rectangu larchute into a st il lin g '7asin. Fr om the stil l ing basin, the flow pas sesthrough an excavated channel into the Gr ee n River.The riv er outlet works, Fi gu re 2, located ne a r the cen ter of the embank-ment is designed fo r a maximum dis cha rge of 16,400 cfs and includes anintake struc ture, a triple -barr eled ups tre am conduit with 11-foot-diameterwat er passages, a gate chamber, th re e 14-foot-diameter downstream con-duits, a chute stillin g basin and an outlet channel to the Gre en River.

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The probl ems encountered at Fontenelle Dam began with a multipledrowning in the rese rvoir . After an extensive sea rch fo r the vict imsproved unsuccessful, the possibility ar o se that th ei r bodies might havebecome entangled in the r i ve r out let t ras hra ck struc ture. Thereforethe riverlirutlets we re closed to perm it an inspection of the tra sh ra ck sby dive rs. This closing of the ri ve r outlets resulted in a rapid de cr ea sein the downstream ri ve r elevation which in tur n caused the sa turatedbackfill ne a r the left end of the outlet wor ks stilli ng basin to s lid e intothe outlet wo rks channel and stilling basin. When re le as es thri+gh theoutlet works w ere resum ed, this m ater ial churned in the stil l ing basinand se vere ly abraded the conc rete su rfa ce s and exposed reinforcingbars .As soon a s the extent of the damage was realized, a contract was letfo r cleaning and r e pa ir of the basin. Rel eas es through the outlet worksw er e again stopped and a cofferdam was construc ted in th e excavatedchannel downs tream fr om the stilli ng basin. As unwatering of the bas inbegan, a second backfill s li de for med near-. the-right end of the ba sin anddepositing mor e ma ter ial within the stil l ing basin.As the cleaning proces s progressed, Fig ure 4, lar ge up stream inflowsfilled the re se rv oi r to within 2 112 feet of the maximum r es er vo ir ele -vation. At the peak flow, a s much as 10,000 cfs we re discharged ov erthe spillway. Th is operation norma lly would not have caused concern;however, a la rg e le ak suddenly developed in the right dam abutmentjust to the le ft of th e spillway. This leak was the resu lt of w ater se ep-ing through the bedrock underneath the ear thfill and it ca used consid-era ble ero sion of the downs tream fa ce of the dam. The erosion of thedam was s o extensive that the safe ty of the s tr uc tu re appeared to be injeopardy, Figure 5.To quickly lower the re ser voi r elevat ion, rel ea se s we re bgain made

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not stoppe d through an ;nti ely mechanical breakdown of the untestedrunriing parts, the turbi ne runn er will be removed. The turbin e shaftwill be replaced with a 10-foot-long, 2-foot 8-inch outs ide -di ame terir vent pipe. Th e bottom of the ir vent and the bottom.oLthe wicketgates will be a t the sa me elevation, Fi gu re 6. 5Th is study was made to provide in formation concerning flow conditionsin the draf t tube and tailrace at var ious discharges, rese rvo ir eleva-tions, and wicket gate openings when opera ting without the turb ine run-ner . The information is needed to prevent damage t o the powerplantta ilr ac e and downstream channel during the diversion period.

THE MODEL

Since the model was originally intended fo r basic re se ar ch studyconcerning su rg es in draft tubes, a homologous represen tation of theflow passag e from the penstock to the tai lra ce was not possible. Themajo r deviations fro m the Fontenelle design we re (1) the ang le betweenthe model penstock and the tai lrac e was 90 whereas the prototypepenstock and ta ilra ce a re in line; (2) the model sp iral ca se was rec-tangular ~ cr os s section, whereas the prototype spira l case is cir -cular; however, the cross-s ectio nal a r e a in the model at each spi ralcas e station was to scale; (3) for s tr u c b ra l reasons, the modelwicket gates wer e thicker than those in the prototype. In an attemptt o p rti lly compensate for the decsea3ed flow ar e a at a specific

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duced the tende ncyfor erosion in the tail rac e due -to he lowertailwater elevation. Disc harge s in t3 e model w er e mea sured with an .orifice Venturi meter. The total upstream head on the spir al case wasdetermingkl by adding the computed velocity head to th e m eas ure d pie-6-inch supply line.The floor:of the ta ilr ac e sec tion was m ade from concr ete and conformedwith the bedrock a s determined fro m profiles m easured in the field. Theprototype rip ra p was simulated in the model by 6.5-mm (millimeter)grave l placed ove r the concrete. The model grave l was graded uniformlybetween;:5.0 and 6 .5 mm, with only 10 percent fin er than 5.0 m m , Fig-u r e 10.: his gradation geometrically represented 4- to 5-inch ri pr ap ination of the prototype ri pr ap s notsi z e ranged between 1 112 and 5 inches.

specified, al l dime nsions in The Investigationrototype dimensions.

THE INVESTIGATIONrvatio ns of flow in the dra ft tube and tai lr ac e revealedow conc entrations at the outlets of th e dra ft tube whichhe ta il ra ce rip rap . Therefore,-+e-major portion oferned with var ious draf t tub$%ioaifications to reduceations. Means of protec tirg the ta ilr ac e ri pr ap

e als o investigated.Flow Conditions in Unmodified Draft Tube

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sp ira l becomes weaker. F o r la rg e s wir l angles and sufficidntly highReynolds numbers, the second o r weaker sp ira l may even disappear.Thus, the effect of intensifying one of the coun terro tating sp ir a lsthrough inlet swi rl is to concentrate the high ;relocity flow in a r e l a -t ively sm all cross-sect ional ar ea of the elbow. These sa me consider-at ions a r e applicable t o flow i n a draft tube since a draft tube is essen-tially an elbow whose a re a incr eas es in the downstream direction.The dependence of the flow concentration on the sw ir l angle was con-firme d qualitatively i n the model. With the wicket gate s open 144 per -cent, displaceme nt of the 4- to 5-inch r ock s n the tailrace began witha di sc ha rg e of 8 15 cfs. Thi s gate opening corresponded t o a swir langle of 45O48'. About 5 per cent of the flow discha rged throu gh the . .left flow passage of th e draft tube; a tendency existed f or flow inthe upstre am direction in the cent er flow passage; and the re main -ing flow left the dr aft tube through the r ight flow passage.With the wicket ga te s open 77 percent, 4- t o 5-inch ro ck s in the tail-ra ce began to be displ aced with a di sc ha rg e of about 710 cfs. Thisgate opening corresponded t o an inlet sw rl angle of 63O44'. Essen-tially all of the flow discharged through the right flow passage; aslight tendency existed, however, fo r ups tre am flow in the ce nte rpassage and fo r downstream flow in the left pa ssag e of the dra ft tubes.The 4- to 5-inch ro ck s in the tail ra ce began to be displaced with adis cha rge of 400 c fs when the wicket ga tes w er e open 19 percent.These conditions corre spon ded to an inlet s w ir l angle of 84-26'. Allof the flow was concentrated in the right halfiof the right flow passagewith up st re am flow in both the c en te r and left passag es. The upstreamflow can be attri buted to the Ventu ri effect of t he high velocit ies con-centrated in the. sp ir a l flow which discharged through the right passage. -

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Effect of Paving the Ta ilra ce with.20-ton Slabs

pre par ed to:t est the ca pacity of the powerplatit c ra ne and w e re available

> \Effect of T ri- van e Flow Splitte r in Dr aft Tube ConeExperience h as shown that flow sp lit te rs i n draft tube cones have beeneffective in attenuating draft tube sur ges . P a rt of this s undoubtedlydue to the dec re as e in the magnitude of the sw irl before the flo w en te rsthe elbow of the dr af t tube. By dec rea sin g the inlet swirl , ext rem e flowconcentrat ions at the draft tube exi t a r e reduced. F o r this reason, as e r ie s of te s t s we re conducted to det erm ine the effectiveness of flowspl it te rs in reducing the flow concentrations. This approach seemed

Two separ ate flow split ters, each consisting of thr ee s traigh t vanessep ara ted by 120, &w ere es te d in the cone of the dr af t tube. One flowsp lit te r wa s 2-112 eet long and the oth er was 5 fee t long. Eac h flow

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nificant effect on the flow distribution. However, the orienta tion wasfound to be decisive in establishing flow pa tter ns withimthe d raft tube.The effect of orien tation on the flow -conditions in the d raf t tube can beillust rated with the following example: With the wicket gatesof he van es inc rease d t he p ossibility of sealin g the a i r ventpipe when the flow sp li tt er s we re placed high in the dr aft tube cone.Although no adverse operating conditions were noted in the modelwith the highest placement of the flow splitt ers , undesirable ope rat-ing conditions might exis t at sm al le r gate openings than could betested i n the pre sent model. 2

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we re placed in the right and ce nte r flow pa ssag es of the draf t tubeupstream from the dr aft tube gate slot blockout, Figu re 12. Thisplacement allows the draft tube gates t o be closed and re qui res aminimum of unwaterin g during installatio n of th e baffles i n theprototype.The baffles in the model we re ad justable both in ove rall height and-indistance fr om the draf t tube invert. With the baffles placed on thedraft tube floor, ri pr ap was drawn from the ta ilr ac e and depositedagainst the downstream fac e of the baffles. Since this rip ra p move-ment would tend to s co ur the dra ft tube and its agglom eration wouldprevent the dra ft tube ga tes fro m being closed, thi s configuration wasrejected.The baffles w ere te sted in succes sively higher positions above thedraft tube inve rt until the flow under the baffles was sufficient to pr e-vent ups tre am movement of the rip rap . The optimum distance betweenthe dr af t tube in ve rt and the bottom of th e baffles was about 9 inches.A high er position of the baffle res ulted in la rg e flow velo citie s underthe baffle which eroded the tailrace.With the wicket ga te s open 144 perc ent and a 2-foot 6-inch high baffleplaced 9 inches above the dra ft tube floor, the tailra ce r ip ra p star tedto move with a discharge of 1 530 cfs . At a 77 perc ent gate opening.the rip ra p began eroding at 950 cfs. No rip ra p movement was observedfo r the 19 percent gate opening fo r discha rge s up to 520 cfs. Fo r eachof the se gate openings, the flow appeared to be concentrated in the ri ghtflow passa ge with s om e flow out the lef t flow passage. A tendency fo rthe deposition of r ip ra p at the entra nce of the ce nte r flow passage w asnoted.Effect of T ri-v ane Flow Split ter and Baffle Walls in Dra ft Tube

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To obtain a sat isfa cto ry distribution of flow with the baffle walls i n thedraft tube, the spl itte r vane orientation had t o be changed fr om its pre-vious optimum orientation. The new optimum orientat ion was with onevane placed t an angle of 7-112 counterclockwise f rom th e upstreamdirection, s seen from above. F o r this orientation the re was no re-tur n flow into the left flow passage. With othe r orientations, some re -tur n flow on the flo or of the left flow pass ag e was observed.Fo rc es on Baffle WallsMeasurements of the instantaneous pre ss ur es on the baffle walls wer eperformed t o determ ine the loadings fo r which the w alls mu st be designed.The te st s we re conducted with a 3-foot 6-inch long tri-va ne flow spli tte rin the dr aft tube cone rotated to its optimum orientation. The baffle wallswere 2 feet 6 inc hes high, extended ac ro ss the width of the ri gh t and cen-t e r dra ft tube flow passages, and wer e placed 9 inches above the drafttube floor. Pre lim ina ry test s were made with a piezometer located inboth the up str ea m and downstream fa ce s of a she et me tal baffle wall. Toinvestigate the effect of a thicke r wall, te st s we re mad e with a total of13 piezometers distributed o ver ll the flow su rf ac es of 10-inch-th ickwooden baffle wall. The more comprehensive measurements were madeon the right baffle wall only, sin ce this w all had the la rg es t indicatedpre ssu re differential . The measured p ress ure differential acr os s thesheet me tal wall was approximately the s am e as that ac ro ss the woodenwall. -=,l

pr es su re concentration was noted n ea r the c ent er of the wooden wall

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where the spec ific weight of wa terthe projected ar ea in the direction for whichthe force is being computed

The values of K in Fig ure 14 a r e from the maximum pre ssu re differ-entials obtained from an averaging circuit in the recording equipment.The instantaneous p re ss ur es exceed these ave rage values by about 30to 40 percent, Fi gu re 15. The magnitude of the instantaneou s pres -su re s was probably attenuated fo r frequenc ies higher than 10 Hz in themodel due to damping in the lin es connecting the ta p in the baffle wallwith the p re ss ur e cell. The se lin es consisted of about 1 foot of 1116-inch -inside-d iameter b r a s s tubing connected to about fee t of 114-inchTygon tubing. Data a r e not presently available to estim ate the amountof attenuation which occu rs with th is lead length configuration.The applica tion of Equations 1 and 2 is illustr ated in the followingexample:

Given: Wicket ga te opening 100 perc entRe ser vo ir elevation 6485Determine: The total horizontal for ce acting on a b ffle wallSolution: The dis ch arg e fo r the given conditions a s determin edf rom Figure 17 is 1,730 cfs. Fr om Figu re 14, aK value o 7.5 is obtained. Substitution of the di s-

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= a r e a of wicket gate opening, in fee t Fig ure 8)H = V2/2g P Y = total energy at centerline of thedistributor, in feet

The chara cteri ztic head, H, is defined a s the total energy at the ce nter-line of the distributor rathe r than as a differential head because thelar ge quantities of a i r entering the vent maintain near atmosph ericpr es su re on the downstream sid e of the wicket gates. Thus, fo r theunit operating without runner, the tai lwa ter elevation h as no effecton the d isch arge through th e unit. This p rem ise was substantiated bytests with the wicket gates open 144 percent and a discha rg e of 1 510 cfs.A tailwater vgriation from elevation 6393 to elevation 64 9 resulted ina 1 percent de cr ea se in the discharge coefficient. This change in dis-char ge coefficient is within the limi ts of experimental e r r o r and cantherefore be disregarded.The rating curves, Figure 17 were developed from the discharge coef-ficient curve in Figu re 16. Since the di sch arg e coefficient is based onthe total energy at the entrance of the distribuior, the energy lo ss be-tween the distr ibuto r intake and the r es er vo ir was added to the head atthe distributor to obtain the res er vo ir elevation. F o r these computa-tions, the energy lo s s was assume d equal t o one velocity head in the10-foot-diameter penstock.

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Unm odified dra ft tube 400 710 8 152-112-foottr i-vane flow sp li t te r L 770 820L5-foot tr i-vane flo w spl i t te r P 1,080 1,2252-1 12-foot b affle s .L. . 950 1 5302-112-foot baf fles plus 3-112-foott r i -vane f low spl i t te r * 1,975

*No e rosio n occurred f o r re se rv oi r elevations up to 6470.Recommended ModificationsTo insu re that di sch arg es up to 1,700 cfs can be diverted without erod-ing the tai lr ac e channel, the combined us e of baffle walls and a tri -v an e ..flow splitter is recomrncnded. The 'baf fles should be 2 feet incheshigh and installed 9 inches above the floor in the right and ce nt er draf ttube flow pa ss ag es . In addition, they should be placed 1 foot 9 inches 'upstream fro m the dra ft tube gate slots. The flow splitter should befeet 6 inch es long and installed with its top about feet 4 inches beiothe cente rline of the distribu tor. It should be oriented with one vanerotated 7-1/20 counterclockwise frox pointing upst ream a s viewed fro m

I,.iThe e rosio n tenden cies fo r vario us gate openings withathe recomm endedbaffles and flow splitter is shown in Figure 17.

-,\,

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F l y r e 4Report Hyd 571

Removal of remaining pervious backfill on west sideof ri ve r outlet works stillin g basin ugust 2 0 . 11165

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1View of unmodified model from downstream

F i g ure 7Re p o r t Hyd 571

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igureReport Hyd 571

View or unmodified model from downstream

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F IGURR E P O R T H Y D 5 7 1

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PORT HYD 571

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U S STANDARD SERIES CLEAR SQUARE OPENINQ*lo0 *SO 40 7 I '10's I 3h W lV2 3100

9 0

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FIGUR I IPORT HYD 571

I 6 ailrace

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839 FS

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FIGUR 6R I P O R T H Y D - 5 7 1

Q= Tota l d ischarge through dra f t tube in cubicf e e t p e r second.

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F I G U R E 7REPORT H Y O 5 7 1

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QU NTPPIES ND UNITS O SPACEMdtiply Y TOob

LENGTHil. 25. 4 exactly). lcronnches 25. 4 exactly). Mllllmeters2.54 exactly)*. Centimeterseet 30.48 exactly) Centimeters

0.3048 exactly)*. UetersO MXWOIIB exactly)* gilometersyardsards 0.9144 exactl MetersMiles ststute). 1,808.344 -tlyf* : : : : : : Meters1.608344 e-tlv) Kilometers

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m C T nslwmstudies of t b r a k tube a t Fontenalle ~ov ekpla ot how IIydreulie madel studies of the dra ft tube at mntannlie Pmcrplant shovtbet erosive flov concentrations can be reduced through t h e w a o f eitherw c~ ce nt rn ti on ac an e reduced throu(m the we of e i ther~ p l i t t e r n the draft tube thrcat or baffle vnlla in the a tr i-vane flov spl itt er in the draft tube throet or baffle v&a in thepasasg~awhen thi unit 1 operated without a turbine draft tibe now passages when tbe unit is omrated without a turbinepwtenaness ar e needed t o allw divers ion of 1700 ci s runner. Both appurtenancee are needed to a ll w diversion of 1700 cia

ntenelle Pouarplmt during rehnb ilita tion of thc river out- throuepl the Fontanelle Pwerplant during rehabilitation of the r i m out-ill in g baain. 3 ~ ffect of flow splitter 1bngt.h orientation let worka stil l ing in. The effect X n spl i t ter length or ientationand vert ica l poaition in reducing f lw coneantrstions is indicated. Bafnedlmnsiona a distancea above draft tube invert , well as forces on t lmbaff les a re given. Rovisionn l rati ng curves for the Fontanelle Powerplantare devclopd.

m me l %tudies 6f t he d ra ft tube a t Fontenellc Paverplant show Aydraulic model stud ies of the dr aft tube s t Footenclle Fwerplnnt abowthst erosi ie fl w ooncentrations can ha reduced through the use of eithera tri-vane flow r p l i t t e r in the draft tube throst or baffle w a l l i n t h edraft tube flow passages when the un i t la opmated without a turbinerunner. Both appurtensncee are needed t o allow divoraion of 1700 ci sthrough the Fontanelle Paverplant during rebabilitatioll of thc river out-l e t work~ t i l l i ng baain. ha effec t of f low apli tter length orlentationfoncentrationa i s indicated. Baffle and vert icnl position in reducing flow concentrations i s indicatad. Baffledimneions and distsncen above draft tube invert, as well force s on thebafrlee re ~ i v e n . Provisional ra t ing curves for the Fontcwlle Pa i e r p l ~ tare develomd.

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