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S T U D I E S O F T H E I N TA K E S T R U C T U R EI F O R T H E O U T L E T W OR KS O FP A N O C H E C R E E K D E T E N T I O N D A ML U IS U N I T . C E N T R A L V A L L E Y P R O J E C -C A L I F O R N I A

Report No. Hyd-560

HYDRAULICS BRANCHDivision of Research

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DEPARTMENT O F THE INTERIORBUItEAU O F RECLAMATION!Office of dhi ef Engineer Repo rt No. Hyd-560Division of Research Author: T. J. RhoneHydraulics Branch Checked andDenver, Colorado Reviewed by: W. E . WagnerJune 1, 1966 Submitted by: H. M. Martin

HYDRAULIC MODEL STUDIES OF THE INTAKF. STRUCTURE FORTHE OUTLET WORKS O F LITTLE PANOCHE CREEK DETENTIONDAM--SAN LUIS UNIT, CENTRAL VALLEY PROJECT, CALIFORNIAPURPOSE

The purpose of thi s study was t o verify th e hydraulic de sign of thevert ical drop intake s tructur e to insu re a smoothly operating struc-tu re capable of dischar ging the required flows within the designre ser vo ir elevations.

UNITED STATES

CONCLUSIONS

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The studies described in this r epor t w ere accomplished through thecooper ation of the Spillway and Outlet Work s Section of th e D am sBranch, Division of Design, and the Hydraulics Branch, Division ofRes ear ch. Model phoiography w as by W. M. Bat ts, Office Ser vice sBranch.

INTRODUCTIONLitt le Panoche C reek Detention Dam, a pa rt of th e San Luis Unit of theCentral Valley Pr oject, is an earth fill dam located on Little PanocheCr eek about 20 m il es south of L os Banos, California, Figure 1.The purposes of the detention dam a r e to provide a sediment tra p andprevent flooding of cana ls downstream fro m the dam. The dam is ap-proximately 1,440 feet long at the c re s t and r is e s about 120 feet abovethe cree k bed. The principal hydraulic feat ures of the dam ar e amorning glory type spillway and an outlet works, both located ne ar theright abutment, Figure 2.

ACKNOWLEDGMENT

The spillw,ay inl et is a 30-foot-diameter mbrning glory str uct ure with thecr es t a t elevation 641.50. Its maximum discharge capacity is 3,220 cfsat res er vo ir elevation 670.40. The spill

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THE MODEL,.. :!(-The model, bu i l t to a scale rat io of 1:15, included the intake s t ~ c -t i r e y i t h the trashrack and stoplog superstructure, the vert ical bend,and a sh or t length of n ea r horizontal conduit. The structu re wasmounted i n the cent er c!f a 6 - by 10-foot shee t me tal lined box. The'stilling basin wasnot included in this investigation. The intake stru c-ture and vertical bend were constructed of transparent plastic, heatformed over wood mol'ds. The rash rack and stoplog superstru cture,stoplogs, and guide vanes in 'th e elbow we re made fr om wood:paintedto re si st swelling. The horizontal conduit and theflow.d eflector. . ..~ nithe elbow we re made jfrom sheet metal, Fig ure 4.

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~ .

A 51-foot-high reinforced concrete tra shr ack and stoplog str uct ureis located on top of the intake structu re. Th is struc ture consi sts ofa 5-foot 6-inch squ are column at each corn er reinforced with ho ri-zontal be am s between columns at 10-foot inter vals, Figu re 3.Elbow Studies

.? . ?Vertical flow deflector. --The purpose of the ve&ical flow deflectorwas to provide smo oth~ flow -inth e orizontal conduit by directing theflow to .~ t h~ i zve r t ' o fhe elbow and to act a s a con trol to establish a-- discharge res erv oir elevation relationship. The ai r vent was placedunder the deflecto r to in su re full aeration and free.flow conditions inthe horizon tal conduit. Ideally, a vert ica l defle ctor should extendsufficiently down into the bend to estab lish the d es ire d flow conditions,but mus t stil l allow sufficient flow ar e a to pa ss the design discharge ,at the.de sign re se rv oi r elevation, (900 c fs at elevation 641.5. )The initial verti cal deflector was 72 inches long, which reduced th eflow are a in the conduit elbow from 23.77 squ are feet to 15.51 s qu arefeet. Th is defl ecto r provided ex cellent flow conditions in the elbow fo r ,=-al l disch arge s tested, except nea r 500 cf s when the flow tended to Gsp ira l over the crown of the tunnel. The 900-cfs design discharge wa snot obtained until the re se rv oi r reac hed elevation 663.0, indicating thatthe d eflector con stricted the tu nn el and reduced the flow.~ b rhe second t r i a l the de flecto r length was I'educed to 57 inches, which 'incre ased the flow ar ea to 18.12'square feet. The flow appearance wa s thesa me a s with the init ial deflector, but the design discharge was passedat re se rv oi r elevation 633.0, about 8.5 feet below the design level.

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Preliminary Intake Structure

of the intake structure by four piez6meters placed on the invert side and

ue .to submergence. '

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Aeration StudiesConsiderable flutter and instability of the nappe occurred at dischargesbetween 300 and 500 cf s with the stoplogs insta lled t o a height of 34 feetabove the crest , Figure 7 . This flutter caused an audible rumble thatcould possibly be a so urc e of vibration in the prototype str uc tu re , andthe normally smooth water su rface in the model res er vo ir showed sm allstanding waves emanating from the stru ctu re.Th ere was a 1-foot sp ace between the top of th e upperm ost log and thebottom of a st ructur al beam at this st o ~ l o gevel. The flutter occurredwhen orifice-t ype flow passe d between the top of the stoplog and the bot-tom of the horizontal beam in addition to wei r flow over the top of thebeam. The weir flow prevented adequate a eratio n of the or ific e flowwhich besulted in fluctua ting-p ressu res under t he nappe and caused thenappe to flutte r. The spa ce between the weir and ori fic e nappes w a sareate d through the stoplog slo ts and was at atmospheric pr es su re. Theflutter did not occur when the distance between the stoplog and beam wasmore than 18 inches o r less- than 6 inches o r when the tops of the stoplogsand beams coincided and all flow was ove r the top. The fluttering actionceased when air was admitted under the nappe of the orifice flow. Twoaltern ative methods of eliminating the flutter w ere cons idered: (1) adjust-ing the placement of the stoplogs to avoid the cri tic al spacing and (2) d-mitting a ir under the jet. Plac ing stoplogs to avoid cri tic al spacing wouldreq uir e a s much a s 4 feet additional depth in the s torag e pool. This wasconsidered to be excessive since a ll runoff f rom a s to rm is subject todownstream water r ights and i f almost normal runoff is not permitted theope rato rs might be subject to litigation. Therefore, the fi rs t method wasabandoned and methods of admitting air were investigated.Struc ture modification. --The fi rs t method of adm itting ai r under theorifice-flow jet was to chamfe r the inside c or ne rs of the four columns of

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Flow spl itters . --The te st s wer e directed toward developing a sati s-Tactory method of splitting the flow passing o ve r the Seam o r stop-log to allow a ir to e nter under the jet. A vert ical column centeredbetween the bea ms o r on top of the stoplogs split the flow a s desi reds o detailed investigations w er e made to develop the mo st satisfactoryshape and location fo r the column.An 8- by 8-inch angle iron, 5 feet.high wa s fastened t o the top ofeac h stoplog on the cen terli ne of the opening on all four sid es ofthe st ruc tur e. The apex of the angle faced ups trea m and the endof each le g was even with the downstream edge of the stoplog.Th e angle spl it the flow, alm os t eliminated the flutter , and did notreduce the discharg e capacity. Figures 8A and C show the flowconditions wi th th is arrange ment. The same angle fastened to thedow nstr eam face of the stop log provided even bette r flow conditions.When the angle ir on was reverse d, so that the vertex was on thedowns tream side , t he re was complete ae ratio n of the jets and theflutt er action completely disappe ared. A solid triangle-shapedcolumn al so provided good flow conditions when placed on top ofthe stoplogs with the verte x facing eit he r direction.

-.Although th is type90f flow s pl itt er was v er y effective, it would in-volve init ial prototype construc tion difficulties and its use ove r thelife of the str uct ure would be impr act ical since the top stoplogscontaining the spl it t ers would have to be removed from the struc tureeach time new log s we re added.

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on the .ce nte rli nes of the openings, extending 30 inch es above and18 inches below t he beam. Although the plates wer e v er y effectivein splitting the flow and allowing full aera tio n under th e jets, theyreduced the disc harg e capacity.Flex*, 8 - by 8-inch angle ir on s with the ver tex facing up st re am , w er eplaced vertically i n the opening between the beam s, Fig ure s 8 Ba nd D.The flow did not sp lit a s well a s when th e an gle s'w ere mounted on thestoplogs. The po rtion of the flow pas sing between the beam and th estoplog mer ged on th e flat s urf ace on top of the stoplog downstr eamfr om .t he angle, which prevented aer ati on under the nappe.The next flow sp litter was a vert ica l column with an equila teral tri-angle c r o s s section 12 inches on a side. The columnextended betweenbeams in th ec en te r of,the opening with a flat face upstr eam and the

ith equal. but not bet ter , re su lts .

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Figure 4Report Hyd-560

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Figure 7Report Hyd-560

A No stoplogs B

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A Discharge = 270 cfs B

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