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CONTEECs- -ContinuedFigure

~ e i l o w t a i l am- Location Map . . . . . . . . . . . . . . . . . . 1Yellowtail Dam axi3 Powerplant- Plan . . . . . . . . . . . . . . 2. YeU-ob*&iiii Dam and Powerplant..E le vatio n and Sect ions . . . . . 3Yellowtail Dam Spillway.. Pl an and Sect ions . . . . . . . . . . . 4' &Yellowtail Darn.. Spillway Crest and 'kurnel . . . . . . . . . . . 5Yellowtail Ihuu.. Spillway Intake Structure Plan and Sections . .Yellowtail Dam-5 0. by 61.66-foot Radial Gate GeneralInstal la t ion 6' P. . . . . . . . . . . . . . . . . . . . . . . . . 7Yel lowh~i l am- Powerplant Area.. ight Side Parking 'Area,Ou tlet St i l li n g Basin, and Retaining Wall . . . . . . . . . . 8Yelloutai l Darn- Ou tle t Works, Plans, P ro fi le s, and Sections . . 9 *Yellowta il Bowerplant.. General Arrangement- TransverseSections Center Line of Units and Outlet Structure . . . . . . 1 0Yellowtail Pow rplant.. eneral Arrawement.. Longitudinal .Section Through Center Line Units . . . . . . . . . . . . . . 11Spillway Outlets and Powerhouse Model . . . . . . . . . . . . . 12Spillway Model Construction . . . . . . . . . . . . . . . . . . 13Spillway Model Construction . . . . . . . . . . . . . . . . . . 14Out le t Works S t i l l i n g Basin Model- Basin One . . . . . . . . . . 15Yellowtail Dam.. Preliminary Spi'Uway Crest and Tunnel . . . . . 16Preliminary Spillway..A pproach ~hkn ne lDischarge 173,000Second-feet . . . . . . . . . . . . . . . . . . . . . . . . . 17Preliminary Spillway..A ppmach Channel Modifications . . . . . . 18Preliminiary Spillway- P i e r Nose and Gate Section . . . . . . . . 19Pre li ai nar y Spillway.. Capacity Curves . . . . . . . . . . . . . 20Preliminary Spillway.. Crest P r e s s w ~ s . . . . . . . . . . . . . 21Pre liminary Spillway.. Water Surface Pr o f i les Over Crest..Discharge 173,000 Second-feet .,. . . . . . . . . . . . . . 22. . . .Preliminary SplUway.. Right Bay Discharging into Tunnel 23

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PRELIMINARY STUDIES

the prototype, are covered with talus. The basi n reconmaended f o r proto-type .use, Figure 55, was developed, ~igures"39through 54, inclusive,i n the lar ger o u tl et works model and rete ste d I n the spil lway model t obe cer tain that the flow conditions i n the r iv er channel, Figure 80,w e r e satisfactory for combined spillway and outlet works flows.

Motion pictures were taken throughout construction of the model,te s ti ng of the 1:54 sc ale spillway model, and te st in g of the 1:28 scale- I t le t works model. These,,pictures are assembled in a 16-mm f i lm ent i t l e d" ~ y a r t u l i cModel Studies of Yellowtail Dam Spillway and Outlet Works."ArnOrnDGMENT

,The f i n a l plan s evolved from these st ud ie s were developd throughthe cooperation of th e s t a f f s of the Concrete Dams Branch and the HydraulicLaboratory Branch. These s tudi es were conducted during the per iod fromNovember 1950 t o January 1953. The f i n a l studie s began i n October 1960and w i l l be reported in ~yd-48 2L/ and ~yd-4.83.mTRoDuCTION

Yellowtail Dam i s a part of the Missouri River Basin Project.It i s located on the Big Horn River about 60 miles southeast of Billings,Montana, Figure 1. The dam, Figures 2 and 3, i s a concrete arch-typedam approximately 1,300 feet long and 500 f e e t high above the riverbed.The discharge s truc tu re s include a spillway, o u t l e t works and powerhouse.

The , spillway, Figures 4, 5, and 6, located i n the l e f t abutment,consists of two converging gate sections discharging into a concrete lined"horseshoe" tunnel. The spil lway cr es t i s a t elevation 3593, 13 f e e tabove the channel f loo r of -1;he sp il lway approach and 64 feet below themaximum design reservoir elevation. The spillway tunnel consists of atr an si ti on section i n which the tunnel changes from rectangu lar tohorseshoe, a 45' inclined section in which the tunnel tapers from a 49-foot-

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The ReservoirThe reservoir was contained i n a head box which allowed repro-,duction of the topography for 300 feet upstream f'rm the spil lway crestand approxhately 300 f e e t t o t he r i g h t and 100 fee t t o t h e l e f t of t h espillway center l ine . Topography in the reservoir area was modeled ofconc rete mortar placed on metal l a th which had been nailed over woodentemplates shaped t o th e ground sur face contou rs a s shown i n Figure 13A.The surface was given a rough fin ish t o simulate th e natural topographyof the prototype.

Spillway StructureGate section. The gate section con sist s of th e cre st , gates,and center pier. The crest was molded i n cement mortar, Figuse 19.Sheet-metal templates accurately cut and placed were used as guides,Figure 13B. Piezometers were in st al le d i n th e spillway cres t and con-s is te d of 1/16-inch inside-diameter bra ss tubes soldered a t r ight anglest o t he p r o f i l e shape of the template and f i l e d flush, Figure 13B.The ra d ia l gate s were construct ed of 16-gage sheet metal.Threaded rods with a crank handle were profl-ded fo r reg ul at in g the gateopening.The center pier for the preliminary design was constructed ofwood, Figure 1411. The p ie r was soaked i n l inseed o i l t o prevent warping.I n the recommended design th e por tion of t he pi er extending over th ecres t sect ion was constructed of sheet metal while th at portion extendinginto the tunnel w a s constructed of transparent pla st ic .Tunnel. In the preliminary design the upstream portion of thetunne l tr an si ti on containing the ce nter pie r, shown i n Figure 12, wssformed of shee t meta l, Figure 13C. The remainder of the tunnel, down-s tream to the t ran si t io n sect ion preceding the s t i l l i n g basin , was molded

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th e pi er were re bu il t of transparent p%tic, th e semr as the remainderof the tunnel.Hydraulic losses. Head ,losees due t o f r i c t i o n i n th e model cueusually greate r, proportionately, than indicated by the model scalebecause surfaces s uf fic ie nt ly smooth t o represent prototype sw faces t oscale do not exist. Therefore, t o maintain th e sc al e velo city of thaflow entering the s t i l l i n g basin, i t was necessary t o ei the r increase t heslope of the tunnel or reduce the horizontal length. To maintain p m p rgeometric similitude a t th e Junction of the tun nel and the st illi riri basin,

it was be tte r t o reduce th e tunnel leagth than t o increase the tunnelslope. For developing the design of +thestill* basin, it ,yas importantthat th e velocity of th e sweep-out discharge be cor re ct ly represented i nthe model. The model tunnel length reduetion was, therefore, calculatedf o r the ant ic ipated maximum capacity of th e s t i l l i ng bas in , ~6 ,000second-fee t , a t an entrance velocity of 81 feet per second. The reduction i nleng th was computed t o be 8.83 f ee t i n t he model o r approxilllately 477 f e e ti n t he prototype. This reduction was based on a p r o t o t m roughnesscoefficient "a" of 0.014 i n Manning's equa tion and a model roughnessco ef fic ient of 0.009, which repre sents a prototype roughness co ef fic ientof 0.0175. Based on th es e coeffi cien ts , th e tu nn el would have been sh or t-ened t o 10 fe et t o cor rec t fo r 173,000 second-feet.

Spillway s t i l l i n g basin. The preliminary basin had a concretefl oo r with sheet-metal sidewa lls. Sheet-metal templates accurately cutand placed were used as guides in molding the fl oo r t o correct elevation.In the recommended design, th e upstream port ion of th e bas in was moldedof t ransparent p la st ic while th e downstream proti on, which wa s open a tthe top, was constructed of wood presoaked in linseed oil.The Tail Water Area

Outlet works. In the preliminary design, th e ou tl et works con-si st ed of two 90-inch h~ ll ow -J etvalves discharging directly into the

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River channel. A length of river channel extending from t he-owerhouse t o approximately 1,000 f e e t downstream fran the spillwaypor ta l w a s simulated i n the model a s shown i n Figure 12. Topographywas f i r s t molded i n 3/4-inch gravel, then covered with a 3/4-inch layerof cement mortar.

Water SupplyWater was supplied t o th e model from th e l abo rat ory 's permanentsupply system. Al l flow ent ere d the head box and was measured by cali-brat ed Venturi meters. The portion of t h e flow t o be passed through t hepowerhouse and ou t l e t works en te red a supply pipe connected t o t he headbox and was measured by meens of a bend meter calibra ted i n place andshown in Figure 12. The differential. pressure between the inside w a Uof the bend and the outside d l as used t o cal ibr ate the bend meterf o r a range of discharge s. To again divid e th e flow and measure th eport ion t o be passed through the outlet works, a piezometer was ins ta l led

1diameter upstream from one of the hollow-jet valves. The dischargepressvae relationsh ip a t the piezometer was determined f'roln calibra t iont e s t s with the s tm ctu re and piping i n place. The valves could there-fo re be regulate d so th at proper division of the flow could be madebetween the spillway and the powerhouse and outlet works structures,then f'urther divided between the ou t l e t works.and th e powerhouse.Water-surface Elevations

The.reservoir water-surface elev atio n was measured by means ofa hook-gage-in-well shown i n Figure 12. Water-surface el evat ions i n theriver channel were controlled by a tailgate a t the downstream end of themodel. Two staff gages and a point gage were used t o measure t h e eleva-ti on of t he water surface i n the ri ver channel a t thre e locations. Staffgages were used a t the powerhouse and near th e spillway s t i l l i ng basinwhile t he poin t gage, shown i n Figure 12, was used near the downstream

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flow throughout the depth of the basin. The river channel topographywas molded i n sand t o provide a movable bed f o r studying erosionchar acte r is t ic s of the f low from the basin.Water was supplied t o t he model from a portab le ve rt ic al pumpthrough an 8-inch l i ne t o a manifold where it was divided between $he3-inch pipes supplying th e hollow jet s . A portable 8-inch orificeVenturi meter was used t o measure th e d ischarge. The piezometric headon the valves was s e t i n th e model by opening o r c losi ng th e hollow-jetvalves and observing the pressure 1diameter upstreem from each valvewhere a piezmeter and water manometer had been installed. Water-surfaceelev atio ns i n the riv er channel were regulated with a ta il ga te andmeasured by use of the t a i l water staff gage, shown i n Figure 15.

THE INVESTIGATIONPurpose and Scope

The primary purpose of the investigation was t o develop thehydraulic design of the spillway and ou tl et works s truc ture s and t odetermine the ef fe ct of operating the t hree discharge stru ctur es singly,i n p ai rs , o r all together. I n developink7the spillway design, it wasnecessary t o study the c hara cter ist ic s of the flow a s it approached andpassed through th e spillway a s well as the characterist ics of the flowas it entered and flowed through the river channel. Two spillways weretes ted ; the preliminary spillway ut il iz ed a 45-foot-diameter horseshoetun ne l while i n the recotmended design th e diameter was reduced t o 41feet. Each was tes ted with modifications t o the entrances and exi t s .In developing th e ou tle t works s t i l l ing .ba sin , a ft er preliminary ' .

tests on the 1 4 model had shown the need for an energy dissipator , itwas necessary t o s tudy the f low in the s t i l l i ng basin and i n the r ive r

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t r a i n i ng w a l l , and along the center l ine of th e l e f t bay fo r the designflow of 173,000 second- feet. The pro f i l e in the r ig h t bay was a l soobtaine d fo r discharges of 60,000, 125,000, and 173,000 second-feet byphotographic record, as shown i n F igu re 23. The flow profiles weresa t i s f ac t o ry .Tunnel

Flow through 'the tunnel i s shown i n F igu res 24 and 25 fo r d i s2cha rges of 20,000, 60,000, 125,000, and 173,000 second-feet . A standingwater f i n occurred i n th e incl ined tun nel anti was most prominent f o r th esmaller discharges. The f i n was caused by flow convergence after passingthe cnd of the center dividing pier . The pier, which extended well downin to th e in cl ined tunn el , could not be el iminated or shortened becauseit was needed for structural support of the tunnel roof. Pressures weremeasured a t thre e piezometers located on the ce nter l i n e of th e tunneldownstream from the p ie r , a s shown i n Figure 24. No subatmosphericpressures were found for any discharge. Flow through the entire tunnel,including the horizontal port ion, was sa t i s fac tory . Observations ofthe flow i n the converging sec t ion of the upstream tr an si t io n suggestedthe possibi l i ty that the convergence angle could be increased t o providea shorter t r an si t ion sect ion and reduce the ov eral l cost of t h is port ionof the str uctu re. Conferences with th e designers indicated th a t they 'were of the same opinion. I t was decided that the convergence anglecould be inc rea sed from 4' t o 6 or more.

Observations and measurements of th e flow depth i n th e tun nelindicated that the tunnel flowed only about two-thirds fu l l a t maximumdischarge. Since ins uf fla t io n of a i r and bulking of the flow a t t h i s .discharge would be neg ligib le i n th e prototype, and sinc e the mosteconomical spi llwa y pos sibl e was desired, it was suggested t o th edesigners that the cross-sect ional area of the tunnel be reduced. It wassuggested, since the tunnel was open for vent i la t ion a t both ends and

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undesirable condition since a dammed-up channel would probably reduce thepowerplant head and, t he re fo re , need t o be cleaned ou t. However, it i sintended, based onr'flood frequency curves, t h a t th e re lea se s w i l l seldombe more than ~O,O'OO second-feet, part of which w i l l be handled by thepowerplant ancl 'o~tlet~works. he spillway w i l l seldom be r equ ired t odischarge more than the s t i l l i n g basin capacity. I t should be realized,however, t h a t when sp ill wa y flows exceed 14,000 second-feet , he basinw i l l f l i p water in to the r iv er channel, and some cleanup of ta lu smaterial might become necessary.

I n the preliminary design, the s.t;illing basin apron w a s con-str uc ted downstream from the e x i t po rta l of th e tunnel. Af t e r t he f i r s tt e s t s it w a s suggested t h a t th e basin fapro n be moved upstream so t h a t thetunnel i t s e l f could be used-as par t of the basin, resul t in g i n a moreeconomical struc tur e with the ov era ll length of th e spillway struc turereduced.The dis tance t h a t th e bas in apron could be moved upstream w a sdependent upon the waker-surface pr of il e i n the basin . If the apron wasmoved too fa r upstream, th e water surface would st ri k e the roof of th etunnel porta l . Therefore,water surface profiles were measured for thedesign flow of 173,000 seco~ d- fee tand for the s t i l l i n g basin capacityof 14,000 second-feet, Figure 29A, since for these two discharges thewater surfac e would be higher than a t any oth er time. The ~ r o f i l e showt h a t it i s pos sib le t o move t he ba sin upstream; however, th e bas in wasfu rt he r developed before i t s loca tio n was changed i n t he mcdel.Modification of preliminary basin. To p r o ~ i d e moreecor.omical basin, th e bas in apron and th e excavated discharge channel

bottom were elevated 10 f ee t t o e leva tions 3145 and 3170, respectively.This modification reduced the amount of excavation required and alsoreduced the length of the tr an sit io n t o the basin. The ~erformancewassimilar t o the preliminary basin even as t o the quantity of f l o w required

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approximately 40 feet downstream frm the start of the transition.Pressures were therefore satisfactory.Testing was continued with this basin moved upstream into thetunnel after the recommended tunnel was installed in the model. Thefinal tests are discussed in "The Recommended Spillwayt' ection of thisreport. *

Powerplant Tailrace and Outlet WorksThe preliminary design of the powerplant tailrace and outlet 3

works is shown in Figure 31. The powerplant contains four turbines andthe outlet works two 90-inch hollow-jet valves. The turbines aredesigned to discharge approximately 1,600 second-feet each and the hollow-jet valves approximately 4,000 second-feet each. The hollow- et valvesdischarged directly into the river channel from a valve house on the rightbank downstream from the powerplant. No stilling basin was provided.The powerplant tailrace contained a concrete :weir to maintain the propertail water depth above the draft tubes.With reservoir elevation 3614 and the outlet valves dischargingtheir combined maximum flow of 8,200 second-feet the poweqlant dis-charging 3,200 second-feet, and the spiilway discharging 8,600 second-feet to produce a river discharge of 20,000 second-feet, as shown inFigure 32A, wave action along the right bank of the channel was rathersevere. With the spillway not discharging, the Jets from the valves

washed high on the left bank, particularly when the powerhouse was not inoperation, as shown in Figure 32B and C. Either case is undesirable forprototype operation because of the danger of washing talus material intothe river channel and causing a rise in tail water elevation in thepowerhouse tailrace. Reduction of head on the powerplant or frequentcleaning of the channel was not desirable.

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directly downstream could not be adopted because of s tm c tu ra l reasons.A stj11rzZg basin was then suggested.

Ihe idea was first exglored in the model by s ~ p a r i n gnerodib le bed in which the jets f rom the horizontal outlet valves couldscour a naturs l basin. B e scour hole would, thereby, indicate thenatural location and depth t o which a basin could be excamted.To determine the extent of t h e basin, two erosion t e s t s were

ruxl as ahawn in figures 9 nd 35. Each began with .th e parezhouse &is-charging 3,200 second-feet and the t ai lr ac e water surface at the expectedelevation 3179. lChe tail. water cont rol &ate set ti ng was not changedduring each te s t, but th e discharge i n the channel w a ~ncreased over aperiod of a p p m m t e l y 30 minutes t o 20,000 second-feet by adding theoutlet and spillway discharges i n tno probable aperating sequences asfurnished t o th e laboratory by t he designers. For each test, the eleva-ti o n of t he bar of deposited material was a lso measured and found t o bea t elevation 3181. Ihe average depth of scour was t o elevation 3145.%!his ccurred quite some distance downstream from the oublets. 'Parardth e end of each t e s t , as the scoured hole was formed, th e appearance ofthe flaw i n th e puwerhouse t ai lr ace and ri ve r channel was only slight lyimproved. It was, therefore, found that an excavated ba sin would n e ebe very large and would s t i l l not quiet the flow very sati sfa ctor ily.

It was als o noted that af t e r the bar was formed and when onlythe puwerhouse was discharging, th e elevation of the water surface i n thepowerhouse tailrace was approldmately 2 feet hi @er than th e elevation a twhich the tests wen? begun. Thus, these tests also shared that it isimportant that a bar of deposited material does not form i n th e ri ve rchannel. As a result of these tes ts, it was thought that t i l t i n g thea v e s downward m i & t r e d k e the s i ze requirement of the excavated basin,but that t o quiet the flow sat isfactori ly i n as small a space as possiblean enclosed concrete basin similar t o the one recommended f o r use a t

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The plastic model hollow -jet valves i n the 1:54 scale model were reusedfo r these preliminary basin t e s t s and, therefore, represented 90-inchvalves fixed a t 100 percent open.

The preliminsry basin, shown i n Figure 36, was f i r s t ins ta l led 'i n the spillway model and operated as shown i n Figure 37 for dischargesof 8,000 and 5,000 second-feet. The roof of the basin was omitted inthe model, and th e top of the center and l e f t trai ning walls uae a televat ion 3184 ins tead of a t eleva tion 3206, a s shown in Figure 36.For 5,000 second-feet, it appeared that the preliminary basin

could be reduced i n s iz e but that the converging w a l l s on the slopingfl oor appeared t o be to o low t o canpletely contain th e jets betweenthem. The water surface was quit e smooth, but, because th e model valveswere oversized and could not be regulated to produce the design head.atth e valves, the water surface i n the model basin probably appeared t o besmoother than if true model valves were used.

To develop th e s t i l l i n g basin design it was decided t o constructa larger independent model with operating valves and a transparent sidew a l l on the basin. The model constructed for this study i s shown inFigure 15. In th i s model, too, the roof of th e basin was ani tt ed and th eupstream portion of the center training w a l l did not extend t o th e f u l lprototype height. The basin utilized two 3-inch model brass valves thatwere re ad lly availa ble i n th e laboratory. One w a U of the basin wasconstructed with glass for observing f l o w cha rac ter ist ics throughout theen ti re depth of th e basin.Basin One

me design and construction of th e model began before thedesigners had specif ied &-inch valves t o rep lace the 90-inch valves.The +inch model valves were t o have provided a 1:30 model scale. How-ever, because the prototype valve s ize was changed t o & inches, the

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Basin Three i s shown discha rging 5,00O*second-feet with re se r-voir elevation 3640 i n Figure 4 1 ~ . ~ e r f o r m k c ewas very .good. Theucstream portion of basin was well ut il iz ed , and the water surface i n th ebasin and leaving the basin was quite smooth. The energy-dissipatingaction was moved upstream s uf fi ci en tl y t h a t th e length of bas in could bereduced. Fur the r te s ti n g was done t o reduce the amount of spray occurringa t the converging walls.Basin Four Recommended

Using the same height of converging walls and gap width asused i n Basin Three, the walls were lengthened t o provide a more gradualconvergence s ta rt in g a t th e top of th e slope. The w a l l lengths were thusincreased' to 46 fe et . Tfie performance of th e basin i n di ss ip at l:~ g energyw a s very good, as shown i n Figure &A,.. for the design discharge of 5,000second-feet at reservoir elevation 3640. The spray a t the convergingwalls was al so eliminated. I t appeared th a t t h i s basin could be reduced37 f e e t 3 inches i n length. Note ' i n Figure 42A that the bottom currentshave l e f t t he flo or well upstream from the end of the paved apron.

Pressures were measured along the downstream edge of the con-_ verging walls at th e t hr ee piezometers shown i n Zigure 4 2 ~ . For thedesign d ischarge, reservoir elevation and t a i l water elevation allpressures were above atmospheric.Tes ts with the t a i l waBer below normCL elevation were made t odetermine the factor of safety against sweep out i f the t a i l watereleva tion i n the prototype should be lower than expected. Figure 43Ashows the b as in t o perform well i f the t a i l xater elevat ion. - is 2 f e e t

below th e expected normal eleva tion, and not un t i l the t a i l water eleva-tion reache& 7 feel; below the expected normal did the flow sweep out,Figure 4 3 ~ . The f ac tor of safety aga inst sweep out.was theref oresat isfactory. This basin with the length reduced 37 f e e t 3 inches was

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Lowering the tai1,water levation 1 foot for 5,000 second-feetreduced the pressure at Piezometer 2 to about 2.5 feet below atmospheric;other pressures were still above atmospheric. Maintaining the tail waterat elevation 3179, but increasing the discharge to 6,000and then to8,000 econd-feet reduced the pressures at Piezometers 2 and 3 tosubatmospheric. For 8,000second-feet with tail water elevation at 3179,the pressure at, Piezometer 2 was 21 feet of water below atmospheric andfor 6,000 second-feet was 3 feet of water below atmospheric. These, ofcourse, are2not ontemplated operating conditions, but the tests indicatethe pressure trends.

It was ~ttempted o determine the tail water elevation at whichthe flow would sweep o ~ tf the basin. The tail watt%= as lowered toelevation 3168, which was as low as possible in the model, but the flowwould not sweep out. From a hydraulic standpoint this basin performssatisfactorily, and with the length reduced 65 feet would provide aneconoxical structure. However, since the effectiveness of t3e basin isbased on impact action on a s mdl area, it was felt that at high headsthe concrete walls and floor of the basin night rapidly deteriorate undercontinued operation.Basin Nine

In Basin Nine, shown in Figure 45~,he sill used in BasinEight was tilted upstream so that its upstream face was at an angle of45' with the floor. More stilling action occurred &OM the sill than .in Basin Eight, and it appzared that the basin could7be ade another 5or 10 feet shorter than Basin Eight. The stilling action in the upstreamportion of the basin created some water surface roughness there, but thewater surface was smooth as it left the basin.Basin Ten

In Basin Ten, Figure 47, tile elevation of the floor was raised

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3640 ankl tail watez elevation 3179. The basins were, therefore, testedfor 6,000 as well as 5,000 second-feet. After completion of these tests,however, the designers decided to limit the maximum capacity of theoutlet works to 5,000 second-feet.

Basin Four is shown discharging 5,000 second-feet in Figure 5Mand 6,000 second-feet in Figure 51B. Performance of the basin was not asgood for 6,000 as for 5,000 second-feet, The capacity of the basinappeared to be exceeded with 6,000 second-feet; and if the tail waterelevation was lowered 2 feet below normal, the flow was on the verge ofsweeping out of the basin, as shown in Figure 51C. Basin Four, therefore,is not recommended for flows exceeding 5,000 second-feet. Basin Four wasalso tested for other anticipated operating conditions. Tests were madefor small ?lows and for 5,000 second-feet with the reservoir at elevation3337. For these tests the basin was found to be very satisfactory, aspreviously described in the tests for Basin Four.

Basin Ten was tested for the same conditions described forBasin Four. Discharging 5,000 and 6,000 second-feet, Figure 52, BasinTen performance was excellent, and the water surface of the flow leavingthe basin was smoother than in Basin Four. Compare Figures 51 and 52.For 6,000 second-feet the tail water elevation could be lowered at least6 feet below normal withou-b the flow sweeping out. Figures 48 and 43also show that Basin Ten performe better than Basin Four w i t h below nopmaltail water. However, for low tail water, subatmospheric pressures occuron the sill of Basin Ten as previously pointed out in Figure 49.

For small discharges with high reservoir elevations, Basin Fourperformed better than Basin Ten. The sill produced some water-surfaceroughness that did not occur in Basin Four. However, this rou&mess wasnot considlered to be objectionable since it did not persist downstreamfrom the basin.

Pressures were measured in both basins at locations considered

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Long and short ra di i f i l l e t s were tes ted in the model, a l l of whichperformed satisfactorily. The purpose of the-fillet was t o reduce impacteffects on the basin floor and to provide positive cleaning action ofaccumulated debris when the valves are f i r s t opened. In the mde l thisf i l l e t did not appear to be necessary to improve the cleaning actionsince grave l deliberat ely placed i n th e corners quickly washed out whenthe valves were opened. In the prototype, however, tbe f i l l e t s m i g h ta id in flushing o ut more densely packed deb ri s and thezeby reduce thepossibility of abrasive erosion of the concrete.

Tests were then conducted to determine whether the valvesmight become submerged i f it ever became necessary to discharge w i t habove noanal ta i l water conditions. The model was operated w i t hextremely high t a i l water elevations, but i n no case were the valvessubmerged. A t a l l times the jets were open to the atmosphere and properventi lat ion of the jets occurred.

It was evident, however, t ha t the basin did not perform a swell with high ta i l t e r a s with normal ta i l water whe.n the basin wasdischarging 5,000 sernd-feet, a s shown i n Figure 54. More sir i sentrained in the water in the basin and higher waves occur in the basinand downstream than when the ta i l water i s a t normal eleva tion 3179.Ta i l water el evat ion 3185.3, shown i n Figure 54, v i l l occur when thespillway and powerhouse ar e discharging i n ad dition t o the ou tl et s t oproduce a t o t a l of 20,000 second-feet, Figure 27. Since tai l waterelevations higher than eLevation 3179 w i l l occur only occasionally, thischaracterist ic of Basin Bur was not considered objectionable.

The recommended Basin Four, Figure 55, was t hen i n sb l l ed inthe 1:54 scal e model along w i t h the recommended 41-foot-diameter horse-8koe spillway tunnel. Basin Four was found to operate a s satisfactorilj-in the 1:54 model a s in the 1:28 sca le model. Aft er completion of themodel s tu die s the recommended basin was al t e red s l igh t ly a s shown onFigures 8, 9, and 10.

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str uc tur al reasons and s t i l l further modifications were made af%ercompletion of the model study t o simplif y prototype cons truction asshown i n Figures 5 and 6. i

The o u tl et works basin developed i n the 1:28 sc ale model te s t s , Yand most of the other modifications recommended during the preliminaryt u m e l te s t s were al so incorporated i n the .model. The en ti re revis eddesign was then tested, and further modifications were made before thest ru ct ur es were recommended fo r proto type use.Spillway Approach

The spillway approach channel, a s developed i n Figures 17 and18, was modified by th e designers t o provide clearance f o r con structio nof the r ig ht and le f t spi llway piers . For co nstruc tion reasons th edesigners found it was desir able t o cut in to t he topography a t both ther i g h t and l e f t p ie rs . These cuts as f i r s t designed, Figure 57, wereins tal led i n the model, Figure 58, and tested.Flows up t o about 100,000 second-feet were s at is fa ct or y, a s

shown i n Figure 5 8 ~ , , and D, but f o r discharges of 100,000 t o 173,000second-feet a boi l occurred i n the cutout area on the ri gh t. Other flowcha ract eris t ics do ng the righ t and l e f t banks were satisfactory. Toeliminate the boil , the extent of the cuts was reduced a t both the rightand l e f t pie rs, as shown by th e recommended re vi sion i n Figure 57.The center p ie r nose shape i n the preliminary design was a l sore vi se d fo r th e recommended des ign . The recommended p ie r nose was shapeda s shown i n Figure 59. This nose i s more pointed than the-preliminary

one; and, th er efo re, reduces th e water-surface drawdown e ff ec t t h a t wasexperienced i n Figure 19. The recommended nose performed very well a sshown i n Figure 58.

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obtained f'rom simple curves rather than from the usual parabolic curve. ' .J-U tera tion s th re e and fou r were made t o provide a deeper pool a t thedischarge end of t he tun nel so t h a t th e discharge channel could ac t aspa rt of th e s t i l l i n g basin when the bucket was not operating as a f l i pbucket.

Operation with this basin was poor; th e high downstreamtopography in t he discharge channel prevented proper e x i t co nditions f o r .the flow. I t was evident that better performance would be obtained i ft he 1on 5 slope was replaced with a hor izo nta l channel flow. Therefore,the slope was removed without f'urther t e s t i n g of Basin I. (PBasin 11. Basin 11,shown i n Figure 67, was the same as BasinI except t hat th e topography i n th e discharge channel was removed t oelemi tion 3170. This made it possible t o perform t e s t s on the basini t s e l f which could not be made with th e highe r topography blocking thedownstream flow.The 4:l sloping s i l l produced a clean je t; however, the f l a t

sLll slope did not project the flow downstream as fa r as was considereddesirable and t;he bas in d id not hold th e hyd rauli c jump i n th e basin aswell as th e 2-1/2 sloping s i l l in the preliminary basin te st s. Nofurther tes ts were made with the 4:l slope.

Basin 111. Basin III,, Figure 67, was tes t ed with rr 2 : l slopingend s i l l . The jet leaving the s i l l appeared unstable and ragged for freeflow with th e basin acting as a f l i p bucket. The hy draulic jump sweptout of the basin a t about 14,000 second-feet which was the same as f o rth e basin having the 2-1/2:1 slope in th e preliminary basin. The s i l lappeared t o be too steep , t her efo re, furthe r t e s t s were made using othe rs i l l types.

Basin I V . In Basin I V , Figure 67, a curved end s i l l was tes ted. ,

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Basin V. To reduce the flow concentrations, a curved floor wasplaced at the intersection of the transition floor and the basin apron,Figures 67 and 69 . Flow concentrations were still evident as previouslydescribed. The curved fillet was removed from the model before further ,tests were made.

Basin VI. The curved-face end sill in Basin V was replacedwith a 2 - l m p i n g ill, and an additional 45' sloping sill 4 feethigh at the downstream end of the 2-1/2:1 slope, Figure 67. 'The adriitional.sill was added to extend the discharge limit for which the hydraulic jumpwould remain in the basin.!The jump remained in the basin for discharges up to approximately15,000 second-feet. However, the 45' sill caused a boil on the watersurface when the Jump was in the basin; and the flow, after leaving thesill, appeared to pass through critical depth to form a second hydraulic

jump. When the basin acted as a flip bucket the jet was clean, especiallyfor the design flood of 173,000 second-feet. The jet arched high andspread to the f.'ull width of the river channel. Because of the poorperformance with the jump in the bucket, the secondary sill was given nofurther consideration.

Basin VII. Basin VSI was the same as Basin VI except that the4-foot sill was removed, Figure 67 . This basin produced a satisfactoryjet when it acted as a flip bucket, as shown in Figures 70,71, and 72,for discharges of 40,000,100,000,and 173,000 second-feet, but the basindid not hold a jump for 13,000 second-feet as shown in Figure 73.The basin design showed promise, however, and investigationswere made upstreem f'rom the basin to improve the performance of the basinitself.The transition section to the stilling basin was investigatedfor pressures along its invert. Since the flow entering the tunnel

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PRELIMINARY STUDIES ,

A higher s i l l also provide^ more jump capc i ty as shown by the data. Byinterpolation between curves i n 45gure 77, the mst accrnamical basin t o .cansfruct, ha- a ,IIW capacity sf 12,000 second-feet, i s th e oneshown i n Figure 78. me recormnended basin is 130 f e e t long at apronelevation 3145 and has a s i l l at elevation 3170. A fac tor of safety of500 skcond-feet was used so th at the model data actua3l.y indicated thatthe jump fo r a flow of 12,500 second-feet would be reeained in the basin.Ihe average velocity of flow imnt3diate.y down8tream from the basin inthe discharge chamel was messured to be approximately 19 fee t persecond.

With the apron a t el em ti on 3145, instead of a t 3135, therequired excavation was l e s s and the length of the t ransi t io n andtraj ecto ry curve was reduced by approximately 30 feet. However, withthe b a s h apron at elevation 3145, it was desirable t o raise the roof ofthe basin 4 feet t o elevation 3196.A water-surface prof i le measurement for 12,000 second-feet,Hguse 79, shared t h a t it was necessary fo r the top of the basin wallst o be raised t o elevation 3195 or hi(?per to contain the waves within thebasin. !be clearance between t he wa t e ~ urface and the portal roof wasapproximately 14 feet. It was f e l t that this clearance, o r more, shouldbe provided f o r adequate verrti lation of t he tunnel when the mcuclnnrmwater-surface eleva tion occurred w i t h the hyrh~~ulicu q in th e basin.When th e basin perfornred as a fllp bucket, it was also neces-sary that the tunnel have f r e e a ir passage from the ga te section t o thepor ta l t o reduce the poss ib il it y of subatmospheric pressures developingin the tunnel. !The reconmended design provides erufficient clearance fo ra l l flows up t o end including the design flow of 173,000 second-feet.Model @mtopphs of t he reconmended spillway s t i l l i n g basindischarging c813pot be presented because the tunnel portaJ, cram andthe basin walls were never actwdly constructed t o t h e recommended

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used w l t h the recormended design provided better flow conditions at thebasin entrance than the transition 150 feet long used w i t h Basin VII.River Channel

The ri ver channel discharging 20,000 second-feet i n Figure do,12,000 through the spillway and 8,000 through the ou t le t works, showst h e flow conditions t o be expected ih the river channel just before .sweep out occurs i n the spillway st i l l i n g basin. The basin shown var iesfrom the r ec mend ed design i n th a t th e crown of the portal i s a tel ev at ion 31% in stead of 3196, and the t ra ining walls are at elevation w3190 ina&ad of 3195. The clearance between water surface and portalroof, therefore, would be 4 f e e t more than i s shown and the freeboardon the t raining walls 5 feet more.

, *Flow downstream from the powerhouse and out le t works, shown i nFigure 80, has a much rougher water surface than would be encountered i nthe prototype because 8,000 second-feet i s being discharged by the out-l e t s and none by the powerhouse, whereas, i n the prototype the out le tworks discharge wi l l not exceed 5,000 second-feet. NoFmslly, the power-house would discharge the addi tio na l 3,000 second-feet. The ou tl et sdischarging 5,000 second-feet produced good flow condi tions i n thechannel e i ther w i t h o r without the powerplant discharging.

Tests were also conducted t o determine t he ef fe ct s o f t h espillway discherge in producing a drawdown i n the powerhouse ta i lr acechannel. With th e spillway alone discharging 12,000 second-feet, thet a i l water sur face a t the powerhouse was at approximately elevation 3182.With an additional 8,000 second-feet discharged through the powerhouseand ou tl et works, the t a i l water surface at the powerhouse was atapproximately elevat ion 3186. When th e spillway discharge was increasedt o 13,000 second-feet, th e jump swept out of the s ti l l in g basin and thet a i l water surface a t the powerhouse dropped approximately 3 f e e t i n

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A. Spillway*nd t h e 1pier

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B. 125,000 eecond feet

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C. 173,000 eecondfeet

A. 14,000 second feetjust before sweepout

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5,000 mecond feetPowerborue - 3,000 mcenrd feet

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B. 6.000 eecond feet - &a. Elev. :3640 - Normal T.W. Rev. 3110 :

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Bas in Four --Recommended Basi n Ten

~ k s i o nattern after 5,000 second fe et discharged for one hour in the modelwith reservoir at elevation 3640

YELLOWTAIL DAMOUTLET WORKS BASINS FOUY AND TEN--EROSION TESTS1 :28 SCALE MODEL

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-.,.9;..,.;..:.v- . Ia z s : , :~ : ; . : * . - .~ . ; ; :~ ; ;~ . : : .~ . .> ; ; ::j?,:.:.:..:....:... ):..::&.::* ...;+:.:$::.-.:b:::.!---

AA A

-P L A N _,,----Note: A ft er co mp le tio n of th e,-El. 3210.0 ,.*-7v - ,,' )'

model study, bosin roof wasi-4'-0yw - p:.b;::,s..;&:*:. ; :::,%..;... 2.5 :I--\,-El. 3146.0

@ Block 13Sca l e : I" = 0' S E C T I O N A-A

Y E L L O W T A I L D A MREGOMMENDED OUTLET WORKS STILLING B A S I N

I\, 7 -..

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YELLOWTAIL DAMRECOMMENDED SPILLWAY A P ~UTL ET W ORKS--DISCHARGING INTO R N E R CHANNEL1:54 SCALE MODEL