hydraulic model study of chili bar dam spillway …

HYDRAULIC MODEL STUDY OF CHILI BAR DAM SPILLWAY

MODIFICATIONS

April 1990

U.S. DEPARTMENT OF THE INTERIOR Bureau of Reclamation

Denver Office Research and Laboratory Services Division

Hydraulics Branch

:PORT STANDARD TITLE PAGE 3. RECIPIENT'S C A T A L O G NO.

HYDRAULIC MODEL STUDY OF CHILI BAR DAM SPILLWAY MODIFICATIONS

7. A U T R. ~.%%e

9. PERFORMING O R G A N I Z A T I O N NAME AND ADDRESS

Bureau of Reclamation Denver Office Denver CO 80225

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Same -. - I

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April 1990 6. PERFORMING ORGANIZAT ION CODE

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16. ABSTRACT

A physical model study of Chili Bar Dam, located on the South Fork of the American River in northern California, was conducted to compare relative channel bed scour downstream of the spillway for different flip bucket elevations and geometries. The dam is owned by Pacific Gas and Electric Company, San Francisco, California. The prototype spillway consists of an ungated ogee crest with a three-level nonsymmetric ski jump on the downstream spillway face. Movable bed scour tests were conducted for the prototype spillway and for modifications to the three-level ski- jump design. The ski-jump design was modified to a short radius flip bucket. Bed scour tests were then conducted for different bucket elevations. These consisted of testing a single high elevation flipbucket, two-level flip bucket, and three-level flip bucket geometries. Limited testing of slotted and slit-type flip buckets were also conducted.

JUN 2 8 2091

17. K E Y WORDS A N D DOCUMENT ANALYSIS

a . DESCRIPTORS-- spillway/ channel scour/ flip bucket/ ski jump/ hydraulic model study/ slotted

bucket/ slit-type bucket , ,

b. I D E N T I F I E R S - - Chili Bar Dam/ Pacific Gas and Electric

c . COSATl F ie ld/Group COWRR: SRIM:

{ I S REPORT) I r n 18. D l S T R l B U T l O N S T A T E M E N T 19. S E C U R I T Y CLASS pl . NO. O F PAGE

(TI UNI

20. SE CLASSIFIED I 33

. C U R I T Y CLASS 22. P R I C E HIS PAGE)

HYDRAULIC MODEL STUDY OF CHILI BAR DAM SPILLWAY

MODIFICATIONS

R. A. DODGE 8. W. MEFFORD

Hydraulics Branch Research and Laboratory Services Division

Denver Office Denver, Colorado

April 1990

UNITED STATES DEPARTMENT OF THE INTERIOR * BUREAU OF RECLAMAT#)N

ACKNOWLEDGMENTS

Pete Julius, Hydraulics Branch civil engineering technician, and ArtStriefel, ACRM - General Engineering Branch (rotation engineer),were instrumental in model data collection and filming. WallaceEnglish, Pacific Gas and Electric, and Thomas Rhone, HydraulicsBranch Technical Specialist, provided technical review andguidance.

Mission: As the Nation's principal conservation agency, theDepartment of the Interior has responsibility for most of ournationally owned public lands and natural and culturalresources. This includes fostering wise use of our land andwater resources, protecting our fish and wildlife, preservingthe environmental and cultural values of our national parksand historical places, and providing for the enjoyment of lifethrough outdoor recreation. The Department assesses ourenergy and mineral resources and works to assure that theirdevelopment is in the best interests of all our people. TheDepartment also promotes the goals of the Take Pride inAmerica campaign by encouraging stewardship and citizenresponsibility for the public lands and promoting citizenparticipation in their care. The Department also has a majorresponsibility for American Indian reservation communitiesand for people who live in Island Territories underU.S. Administration.

This study was conducted by the Hydraulics Branch, Research andLaboratory Services, U.S. Bureau of Reclamation, in cooperationwith Pacific Gas and Electric Company under CooperativeAgreement No. 8-FC-81-12800.

The research covered by this report was funded under the Bureauof Reclamation PRESS (Program Related Engineering andScientific Studies) allocation No. SO-1, Stilling Basin Drainage.

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CONTENTS

Background. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Objectives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Description of the physicalmodel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Model similitude. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Flow similitude. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Bed material similitude. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Model performance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Model bed material. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Model test procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Tailwater elevations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Tests. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Bed scour tests of prototype ski-jumpspillway. . . . . . . . . . . . . . . . . . . . . . . . . . .Flip bucket modifications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Model tests of single-levelbucket spillway. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Model tests of two-levelflip bucket spillway. . . . . . . . . . . . . . . . . . . . . . . . . . . . .Model tests of three-levelflip bucket spillway. . . . . . . . . . . . . . . . . . . . . . . . . . .Model tests of three-levelflip bucket spillwaywith deflectorwall. . . . . . . . . . . . .Spillwayand overtoppingdischargecapacity. . . . . . . . . . . . . . . . . . . . . . . . . . . . .Crest pressures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Flip bucket pressures - three-levelflip bucket. . . . . . . . . . . . . . . . . . . . . . . . . . .

TABLES

Table

12

Spillwaycrest pressures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Flip bucket pressures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

FIGURES

Figure

12345

Dam elevations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Chili Bar Dam spillwaycrest pressure tap locations. . . . . . . . . . . . . . . . . .Prototype bed material survey. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Prototype bed material size distribution. . . . . . . . . . . . . . . . . . . . . . . . . .Model bed size distributions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

III

Page

1

2

2

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33356

7

7

7

8888999

101010

1010

1112131314

Figure

6789101112131415A15B15C15D1617

18

19

20

21

2223242526272829303132A32B

33343536

37

38

39

CONTENTS - Continued

Prototype channel bed contours. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Tailwater rating curve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Prototype bed scour after 10 percent of PMF ......................Prototype bed scour after 25 percent of PMF ......................Prototype bed scour after 50 percent of PMF ......................Prototype bed scour after 75 percent of PMF ......................Prototype bed scour after 100 percent of PMF .....................Prototype spillway bed profiles at toe of spillway. . . . . . . . . . . . . . . . . . . .Modified flip bucket geometry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Prototype spillway. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Single-level flip bucket spillway. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Two-level flip bucket spillway. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Three-level flip bucket spillway. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Single-level flip bucket, 25,000 fe/s (10 percent of PMF) . . . . . . . . . . . . . .Single-level flip bucket spillway -bed contours after

10 percent of PMF ........................................Single-level flip bucket spillway -bed contours after



75 percent of PMF ........................................Single-level flip bucket spillway - bed contours after

100 percent of PMF""""""""""""""""""'"Single-level flip bucket - bed profiles at toe of spillway. . . . . . . . . . . . . . .

Two-level flip bucket - 25,000 fe /s (10 percent of PMF) ..............Two-level flip bucket - bed contours after 10 percent of PMF ..........Two-level flip bucket - bed contours after 50 percent of PMF ..........Two-level flip bucket - bed profiles at toe of spillway. . . . . . . . . . . . . . . . .Three-level flip bucket, 25,000 fe /s (10 percent of PMF) . . . . . . . . . . . . . .Three-level flip bucket - bed contours after 10 percent of PMF .........Three-level flip bucket - bed contours after 25 percent of PMF .........Three-level flip bucket - bed contours after 50 percent of PMF

"""'"Three-level flip bucket - bed profiles at toe of spillway. . . . . . . . . . . . . . .Deflector wall geometry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Three-level flip bucket spillway with deflector wall

(25 percent of PMF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Bed scour next to tailrace wall, 168feet downstreamof dam axis. . . . . . . .Bed scour next to tailrace wall,218 feet downstreamof dam axis. . . . . . . .Bed scour next to tailrace wall,248 feet downstreamof dam axis. . . . . . . .Three-level flip bucket with deflector wall -bed profiles

at toe of spillway. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Three-level flip bucket - bed with deflector wall - bed contours

after 4 percent of PMF .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Three-level flip bucket - bed with deflector wall - bed contours

after 10 percent of PMF ....................................Three-level flip bucket - bed with deflector wall - bed contours

after 25 percent of PMF ....................................

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242525262627272828292930

30313132

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Figure

40

41

42

434445

CONTENTS - Continued

Three-level flip bucket - bed with deflector wall - bed contoursafter 50 percent of PMF ....................................

Three-level flip bucket - bed with deflector wall -bed contoursafter 75 percent of PMF ....................................

Three-level flip bucket - bed with deflector wall - bed contoursafter 100 percent of PMF ...................................

Spillway/dam discharge rating curve. . . . . . . . . . . . . . . . . . . . . . . . . . . . .Spillway crest pressures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Flip bucket pressure tap locations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

APPENDIX

Research studies on slotted and slit-type flip buckets. . . . . . . . . . . . . . . . .

v

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34

35

35363637

39

BACKGROUND

Chili Bar Dam, owned by PG&E (Pacific Gas and Electric) Company, is a concrete gravity damon the South Fork of the American River near Placerville, California. The dam and adjacentpowerhouse were completed in 1965. The dam is approximately 120 feet high and 375 feet wide.The structure contains an uncontrolled ogee crest spillway 170 feet wide located 31 feet below thetop of the dam (fig. 1). The downstream side of the spillway follows a 7 on 10 slope untilintersecting the ski jump radii. The spillway currently has three 45-foot-radius ski jumpsterminating at the horizontal. All ski jumps are located at different elevations and span differentwidths of the spillway. Looking downstream from right to left, the ski jumps are as follows:

Toe elevation (ft) Width (ft)

940930950

27.5100.042.5

The spillway exits into an unlined natural rock basin. The downstream channel near the dam has,in general, had all overburden removed to bedrock from previous spillway operation. Since the damwas built in 1965, the maximum daily average spillway discharge has been about 42,000 fe Is. Thespillway design capacity was 100,000 fe Is. The PMF (probable maximum flood) is currentlyestimated to be 250,000 fe Is. The dam must be overtopped by approximately 15 feet to pass thisnew PMF. Since the dam was completed in 1965, PG&E has reported a small amount of erosionat the toe of the dam due to spillway operation. Although this erosion is not currently serious,PG&E is concerned about continued erosion. Several alternatives that were investigated by PG&Eto repair the present erosion damage and prevent future erosion were:

1. To place tremie concrete to fill in the existing erosion holes. The bonding between theaquatic-vegetation-covered bedrock and the concrete was questionable for this alternative.

2. To build a cofferdam and dry up the dam toe to ensure a proper bond between the placedconcrete and bedrock. The cofferdam would have to allow for powerplant operation.

3. To not repair the existing erosion and install a flip bucket on the existing spillwaystructure which would halt further erosion.

PG&E investigated alternative 3 because alternatives 1 and 2, while repairing erosion, would notprevent future erosion from taking place. Alternative 3 was also estimated to be less expensivethan alternative 2 because the construction could be conducted without dewatering the dam's toe.A physical model study was necessary because the flip bucket design required did not fit intoexisting design criteria bounds for roller bucket radius or tailwater conditions. PG&E and theBureau of Reclamation were interested in research information that would allow for expanding theavailable design criteria for spillways similar to Chili Bar Dam. Therefore, PG&E and Reclamationentered into a cooperative research agreement to model study the proposed small radius flip bucketspillway.

OBJECTIVES

A 1:45 scale model was designed and tested to investigate flip bucket modifications in order toreduce stilling basin scour near the toe of the dam. A model of the structure was tested to:

1. Define relative scour patterns, particularly at the dam's toe, as a function of flip bucketgeometry and bucket elevation. Relative scour patterns were measured after passingdischarges of 10, 25, 50, 75, and 100 percent of the PMF. For the final bucket design, testsusing flows of 2 and 4 percent of the PMF were also conducted.

2. Determine if a deflector wall placed on the bucket adjacent to the powerhouse wouldreduce local scour near the end of the tailrace wall. The deflector wall studies were conductedonly on the final bucket geometry. A maximum design flow corresponding to the 65-year flood,62,500 fe Is (25 percent of the PMF), was used in evaluating scour along the tailrace wall.Protecting the wall from bed scour was not considered economically practical for larger flowsbecause of their low frequency. The present cost of repairing damage every 65 years isconsidered to be less than the cost to ensure that the wall is not damaged by major floods.Damage to the wall does not endanger the dam. Relative bed scour tests were conductedwith and without the deflector on the bucket.

3. Measure reservoir elevation versus spillway Idam discharge capacity, spillway crestpressures, and pressures in the flip bucket.

4. Conduct basic research on the hydraulic performance of slotted and slit-type flip bucketdesigns. Use the model results from the standard solid bucket design as a basis of comparisonto evaluate the performance of the nonstandard slotted and slit-type bucket designs.

CONCLUSIONS

. Overall, the modified flip bucket spillways tested produced less bed scour within the reachfrom the toe of the spillway to the end of the tailrace wall when compared to theprototype spillway design.

. Of the three modified spillway geometries tested, the three-level arrangement producedthe greatest deposition of fine bed material within the channel reach immediatelydownstream of the spillway and along the tailrace wall. Significant movement of bedmaterial upstream to the spillway toe occurred in the model for flows 25 percent of PMFand greater. Due to the large flows the spillway Idam is required to pass, moving materialtoward the toe of the dam is considered desirable. Although some abrasion to theconcrete on the downstream face of the lower flip buckets could occur for the higherflows, the abrasion action is not expected to be significant. Visual observations made ofthe movement of bed material in the model indicated the tailwater velocities beneath thejet and next to the bucket faces were generally low. The deposition of very fine materialalong the toe in the model also suggests the hydraulic action next to the spillway is notextremely violent.

2

. The implementation of the deflector wall in the right side bucket did not produce anoverall improvement in the local scour along the tailrace wall. Scour at several locationsalong the wall increased slightly with the addition of the flow deflector. The deflector didforce the edge of the jet out from the wall thus preventing the edge of the jet fromimpinging directly above the wall.

DESCRIPTION OF THE PHYSICAL MODEL

A 1:45 scale model of Chili Bar Dam including reservoir approach topography, dam, spillway,powerhouse, abutments, and about 500 feet (prototype) of downstream channel topography wasbuilt for the study. The downstream channel was modeled using an erodible bed material. The bedmaterial was contoured in the model by matching reverse templates representing channel crosssections spaced at about 40-foot intervals down the river channel. In the model, the dam abutmentsand the downstream canyon walls were considered nonerodible. They were geometrically modeledusing a wire mesh molded over plywood forms. A concrete mortar was placed over the wire to givea finished surface. The spillway crest and the ski-jump spillway were milled out of high-densityurethane. Six piezometer taps were placed in the crest to measure pressures. The taps werepositioned down the crest along the spillway centerline (fig. 2).

MODEL SIMILITUDE

General

For a model to truly represent actual conditions, it must be geometrically, kinematically, anddynamically similar to the prototype. If the model deviates from the prototype in anyone of thesethree areas of similitude, then care must be taken to properly interpret the model results. However,if anyone deviation is too large, or there are too many deviations, the model will not represent theprototype and no amount of interpretation will yield the correct results or conclusions.

Flow Similitude

Similitude analysis and model design are best started by finding valid homogeneous equations anddimensionless functional relationships that apply to both the model and prototype. Selection of aset of equations and functions includes the requirement that they be checked for their model andprototype application range and limits. Normalizing complete hydrodynamic equations for openchannel flow opposed to tractive shear or friction and then extracting dimensionless parametersresult in a Froude number squared (F2) or (y2/R.£) and a product of the Darcy-Weisbach frictioncoefficient (f) times Froude number squared as the required parameters for scaling flow. Thefriction coefficient is a function of relative roughness (I(.) and Reynolds number (R) expressed as:

f = t (I(., R)

where:

I(.R

= (k./4 Rh)(4RhV/u)

3

f = friction factorRh = hydraulic radiusV = velocityg = gravitational constant

~= function operatorIe. = rugosityv = kinematic viscosity

A geometric or length scale ratio (Lr) of 45 was selected because of limited laboratory pumpcapacity. Based on Froude law alone,

Length ratioVelocity ratioTime ratioDischarge ratioUnit discharge ratioTractive shear ratioPressure ratio

Lr = 45Vr = 6.71Tr = 6.71Qr = 13,584qr = 302tr = 45Pr = 45

Having selected the Darcy-Weisbach equation to normalize friction loss in the complete flowequation, the ratio of friction factors in model and prototype (fr) must be made equal to 1 toproduce similar vertical velocity distributions and secondary flows. The Darcy-Weisbach equationfor open channel flow in slope (S) form is expressed as:

S = f (1/4Rh) (yl /2g)

Since the friction factor (f) is a function of Reynolds number (4RhV /v) and relative roughness(kJ4Rh)' the modeler must work within Moody-type friction curves. Reynolds number cannot bemade the same for both a model and a prototype river. The model must be made to flow at watersurface elevations according to a tailwater curve and produce the proper corresponding velocity.The modeler must find the rugosity that produces an equal (f) for both the model and prototype.Putting the previous scale relations into the Reynolds number results in a measure of Reynoldsnumber distortion (Rdr) based on the selected model scale ratio expressed as:

Rd = L 3/2 = 302r r

A friction, Reynolds number, and relative roughness model to prototype comparison was done usingthe prototype tailwater curve and a Moody-type friction function for discharges from 5,000 to250,000 fe /s. These comparisons showed that Reynolds number was sufficiently large for bothmodel and prototype to be on the flat part of the friction function. Thus, model and prototype (f)are expected to be equal. It was also shown that relative roughness was sufficiently small (less than1/10) so that Darcy-Weisbach equation applies to both the model and prototype. Because Reynoldsnumbers are large and model and prototype friction coefficients can be made the same, verticalgeometric distortion of the model was not necessary. For example, for a prototype discharge of125,000 fe /s, the Reynolds number is 2.5 x 108prototype and 8.3 x 105model.

4

Bed Material Similitude

In general, attempts to scale the structural integrity properties of bedrock were unsatisfactory.Therefore, it was decided that the grain distribution analyses would be used to provide frictionscaling, indicate sizes that move and armor, and determine a qualitative comparison of the scouringpotential for flip bucket modifications relative to the existing spillway. Fluid shear at the bed, lift,drag, secondary flow, and turbulence are considered the main factors that initiate and maintain thetransport of bed material. Particle settling velocity, shape, packing, and submerged weight governresistance to motion. Simple Froude law and frictional scaling do not ensure that model sedimentwill similarly transport to the prototype sediment. Thus, further sediment entrainment andtransport parameters should to be considered. If transport rate needs to be scaled, a homogeneoustransport equation must be selected or a dimensionless transport function must be used.Reclamation uses Taylor's dimensionless sediment discharge parameter (q.fU*d.) denoted by (q. *)that produces a set of curves approximately parallel to Shields' entrainment curve for constantvalues of (q. *). Thus (q. *) is a function of Shields' parameters for grain diameter-shear velocityReynolds number Rg* or (U*d./v) and dimensionless shear T* or (1/('Y.-'Yw)d.) expressedfunctionally as

q.* = ~ (R/'

T*)

where:

q.* =d. ='Y. ='Yw =1

V

U* =

=

sediment discharge volume per unit width per secondsediment diameterspecific weight of sedimentspecific weight of watertractive shearkinematic viscosityshear velocity = (1/ p)1/2 = (SRhg)1/2 = V(f/8)1/2water densityslope

pS

==

Transport scaling is accomplished by finding a model sediment diameter and specific gravitycombination by trial and error that produces a model (q. *) equal to prototype (q. *). Analyses ofTaylor's function and homogeneous transport equations show that noncohesive transport scales by(Lr?/2. Using cohesive sediment transport equations, transport scales by (Lr). Some model andprototype checks were done of Taylor's parameters and compared to existing velocity-diameter data.These checks and comparisons indicated that scaling the river part of the model could be done.Since the prototype material at the plunge area is bedrock, detailed transport analyses were notdone. Reclamation experience indicates that scaling by settling velocity alone produces near (q. *)scaling, especially for large diameters. Scour in the plunge region is not expected to scale close toTaylor's relationship. Plunging jets scale closer according to pure Froude law for large particlesgeometrically scaled. Settling velocity is an important sediment parameter since it relates to whensediment will remain at rest and how long it will travel once lifted into flow. To size modelsediment, settling velocity is scaled by Froude law velocity scale ratio (L//2). Settling velocity forI-millimeter particles and larger are proportional to diameter to the one-half power. Therefore,these sediment model sizes scale settling velocity by Froude law and prototype diameters of 45millimeters and greater scale both geometrically and by settling velocity. Settling velocity scaling

5

has been successfully used for most of Reclamation's diversion dam model studies for relativecomparisons of different test arrangements.

Model Performance

The following are brief statements of the major results of the similitude and scaling analyses:

. In general, attempts to scale the structural integrity properties of bedrock areunsatisfactory. Therefore, the grain distribution analyses was used to provide frictionscaling, indicate sizes that move and armor, and provide a qualitative comparison of thescouring potential for flip bucket modifications relative to the existing spillway.

. Because of the large Reynolds number, model and sediment diameter, it was possible tomake the dimensionless parameters, Froude number, Darcy- Weisbach friction coefficient,and relative roughness the same for model and prototype. Therefore, an undistortedmodel was used for the river part of the model.

. Relative roughness (k./ 4Rh) was less than 1/ to, thus the Darcy- Weisbach equation appliedto both the model and prototype.

. Since the friction coefficient ratio (fr) could be made equal to 1, vertical velocitydistributions and secondary flow scaled.

. Some Taylor-Shields scaling checks were made and indicated good sediment scaling ofriver part of the model.

. Settling velocity scaling of sediment diameter generally produced close Taylor-Shieldsscaling.

. Settling velocity scaling and geometric scaling produced the same diameter distributiondown to I-millimeter material in the model, thus geometric scaling was good down to the20 percent passing size or the 45-millimeter prototype size.

. Verifying good riverflow and sediment scaling without vertical geometric scale distortionalso provided confidence in scaling parts of the model where Darcy-Weisbach equationdid not apply such as in the jet plunge region. Here, although scale effects weresignificant, scour scaled by Froude law. Thus sediment material too large to betransported by the normal riverflow was scoured by the impinging jet.

. Quantitative scour comparisons could not be made because loose bed material was usedto represent bedrock regions of the prototype, and scour of the loose bed material wouldbe affected by the more resistant exposed bedrock. It is believed that the model scouris conservative as compared to the prototype.

. The model is expected to provide good qualitative and relative comparisons of scouringpotential of flip bucket modifications because the scour form did not vary much in shapeand mainly changes of flip elevations were tested.

6

MODEL BED MATERIAL

PG&E provided four grain diameter distribution analyses of the river bottom material. The foursamples came from an area of deposited transport material 100 yards downstream of the spillwayplunge region. A field contour map of bars and sparsely distributed material remaining on bedrocknear the plunge area was also provided (fig. 3). The material on the bedrock near the plunge areaconsisted of 1- to IS-foot boulders.

Figure 4 shows the cumulative distributions of prototype sediment provided by PG&E. On thisfigure, samples and distribution curves are identified by circled numbers. Divers obtained fourdistribution samples by measuring and counting individual boulders within four sample areas.Samples 1 and 2 were combined into a single curve and samples 3 and 4 were plotted separately.Curve 1-2 was then weighted by two and averaged with curves 3 and 4. The average, plotted ascurve 5, represented the target sediment distribution to be modeled. Curve 6 defines the estimatedgrain distribution for isolated individual boulders near the spillway.

Prototype particle distributions were converted to model by geometric scaling and are plotted onfigure 5. Distribution curve 5, long dashed line, is considered the upper boundary for permissiblegeometrically scaled sediment for the model. The lower distribution curve 1-2, long dashed line, isconsidered the lower boundary. The mean or target distribution is shown as the light solid linecurve. The target distribution for settling velocity scaling plots on top of the curve for geometricscaling down to the I-millimeter model size then deviates as shown. Thus geometric scaling is validdown to the 80-percent retained size. The heavy solid lined curve indicates distribution of a pit runmaterial with some minor sieve separation used in the model.

MODEL TEST PROCEDURE

Prior to each new geometry tested, new bed material was placed in the downstream channel andthe bed was contoured based on a prototype channel survey conducted in March 1988 (fig. 6).For each spillway geometry, the model bed was allowed to progressively scour as increased flowswere tested. To prevent scouring of the bed during model startup, the model tailwater was slowlyraised before each test by filling the model tailbox downstream of the dam. Each spillway geometrywas tested for the same sequence of flows and for the same time of operation. After each flow thebed scour was contour mapped with string to show the scour pattern for photographs. In additionto the qualitative string contours, the templates used to form the initial bed were reinstalled andquantitative measurements of changes in the bed elevation were made. The measurements weremade in the same location for each test based on a preestablished grid pattern.

TAILWATER ELEVATIONS

The tailwater in the model was established for each test based on figure 7. The tailwater was setin the model artificially by adjusting the height of a downstream overflow weir.

7

LJBf1ARV

[J~N 28 -Je"",.,u ." ""'",;.,,,ltioo

Reclamati0: I SOfl/lC6

II

TESTS

Bed Scour Tests of Prototype Ski-Jump Spillway

The prototype spillway geometry was placed in the model to determine the scour patternsdeveloped for flows up to the PMF. Scour tests of the prototype geometry established acomparative base for evaluating future scour patterns created with modified spillway geometries.The three ski-jump levels on the spillway face often cause different flow conditions to occur beloweach level in the plunge basin. The 930-foot elevation ski jump generally operates in a tailwatersweep-out condition. Flow leaving the higher ski jumps to either side may fall as free jets or, ifsubmerged by tailwater, sweep out similar to the 930-foot elevation ski jump. The prototypeski-jump spillway geometry generally created a scour pattern with scour holes downstream of the940- and 950-foot elevation ski jumps starting about midway down the tailrace wall (figs. 8-12).During 10 percent PMF flows, fine bed material deposited against the spillway face. The materialwas drawn toward the spillway toe by return flow along the bed. Along the spillway face, materialgenerally moved from left to right (looking downstream). The flow moved the fines parallel to theface of the 930-foot elevation bucket, forming a deposition peak in front of the 940-foot elevationbucket. Flows larger than 25 percent of PMF progressively eroded the bed at the toe of thespillway (fig. 13).

Flip Bucket Modifications

The basic bucket designs tested in the model were developed by PG&E. The prototype ski jumpswere modified by reducing the radius to 35 feet and extending the arc 30° beyond the horizontal(fig. 14). The same bucket radius and arc length were used to modify the prototype geometry forall modified spillway tests. Using the set bucket geometry, model tests were then conducted forthree different spillway designs. Spillway modifications tested were Ilmited to designs which couldbe adapted to the prototype without requiring major concrete excavation of the existing spillway(fig. 15A). First, a single-level bucket at invert elevation 952.6 feet was tested in the model(fig. 15B). The 952.6-foot elevation was the lowest elevation at which a single-level bucket couldbe constructed due to the 950-foot elevation of the existing ski jump on the left side. For thesecond series of tests, the spillway was modified by lowering the right 127.5 feet of the bucket toelevation 942.6 feet (fig. 15C). Following testing on the two-level flip bucket spillway geometry, thespillway was changed to a three-level flip bucket design. The invert elevation for the middle portionof the bucket lying above the existing 930-foot elevation ski jump was lowered to elevation932.6 feet (fig. 15D).

Model Tests of Single-Level Bucket Spillway

The model spillway was changed to a single-level bucket 170 feet wide located on the spillway faceat invert elevation 952.6 feet. The lip of the single bucket (elevation 957.25 feet) was above thetailwater for flows less than about 25 percent of PMF. For these flows the jet leaves the spillwaybucket as a free jet (fig. 16). At higher spillway discharges the jet sweeps out with a partiallyaerated undernappe up to about 75 percent of PMF. At 100 percent of PMF the jet was fullysuppressed. Scour patterns produced were in general evenly distributed across the channel(figs. 17-21). The channel bed for the first 60 feet downstream of the bucket face showed only smallchanges in elevation from 10 percent PMF to 100 percent PMF. Just downstream of the toe of the

8

spillway the steep portions of the bed below the old 940- and 950-foot elevation ski jumps erodedto a near level profile (fig. 22).

Model Tests of the Two-Level 'FlipBicket Spillway

The two-level flip bucket spillway was tested for discharges of 10 percent PMF (fig. 23) and50 percent PMF. Scour patterns from flow over the two-level bucket were similar to the scourdeveloped by the single, higher elevation configuration (figs. 24-25). After the 50 percent PMFtest, an increase in the scour was noted along the tailrace wall accompanied by greater depositionon the opposite side of the channel as compared to the single-bucket test results. Greater scouralso occurred below the toe of the higher bucket (fig. 26).

Model Tests of the Three-Level Flip Bucket Spillway

Bed scour tests were run for the three-level flip bucket spillway at flows of 10, 25, and 50 percentof PMF. At 10 percent PMF, the outside buckets produce free impinging jets into the tailwater.The jet from the center flip bucket was partly suppressed (fig. 27). After the lO-percent PMF test,less scour had occurred along the tailrace wall as compared to the previous geometries tested(fig. 28). At higher discharges deposition of fine bed material increased downstream of the twolower buckets (figs. 29-30). Near the toe of the spillway the flows created only minor scour of thechannel bed on the steep slope below the high bucket (fig. 31). Fine material was carried fromthe downstream scour holes upstream to the toe of the spillway by the action of secondary returnflows. The eddies moving material upstream along the bed are driven by the overlying high velocitysurface jet. The material deposited in the model at the spillway toe was predominately very finematerial.

Model Tests of the Three-Level Flip Bucket Spillway With Flow Deflector Wall

A wedge-shaped deflector wall was placed in the model along the powerhouse wall (figs. 32A-32B).The deflector was tested to determine if it would protect the tailrace training wall from scour forflows up to 25 percent of PMF. The wall was designed to divert the jet from the 942-foot elevationflip bucket away from the tailrace wall. Without the deflector wall, the outer edge of the jetimpinges on the wall. Model tests with the deflector were conducted with 2, 4, 10, 25, 50, 75, and100 percent of PMF. A comparison of scour depths at several points near the tailrace wall showsthe deflector wall had little influence on scour in these locations (figs. 33-35). Near the toe of thespillway, material was generally deposited against the center bucket and small amounts removedfrom the steep slopes in front of the two outside buckets (fig. 36).

After 2 percent of the PMF, no definable scour of the bed was apparent in the model. A dischargeof 4 percent of PMF caused a small amount of scour across the channel near the end of the tailrace(fig. 37). The scour was local to the area directly beneath the point of jet impingement. Some ofthe eroded material was deposited in the upstream river channel area. Overall river channel scourwas nearly the same for model tests with and without the deflector wall (figs. 38-40). At 75 and100 percent of PMF, the three-level flip bucket spillway developed much less scour in the riverchannel between the toe of the dam and the end of the tailrace as compared to the prototypespillway (figs. 41-42).

9

Table 1.- Spillway crest pressures

Pressure ta) Discharae in cercent of PMF(ft. of water 10 25 50 75 100

1 13.5 18.5 22.1 24.4 24.22 3.9 0.3 -8.1 -19.0 -30.93 2.5 -2.4 -7.2 -14.3 -21.34 1.8 0.4 -5.1 -9.6 -13.85 1.8 1.6 -Q.1 -1.9 -3.36 2.6 1.5 1.4 2.9 4.5

1 22.8 27.9 59.62 25.4 47.6 74.73 23.9 33.5 56.04 21.8 14.5 19.25 24.2 12.0 18.76 23.6 35.6 62.97 15.7 44.1 80.5

10

Spillway and Overtopping Discharge Capacity

Reservoir elevation versus discharge was measured in the model (fig. 43). Although no attemptwas made to directly determine the maximum capacity of the sfillway before overtopping of thedam occurred, overtopping was estimated to occur at 115,000 ft Is based on the model data. Topass the PMF required a reservoir elevation of 1043.5 feet. Flow overtopped the dam by 15 feet.

Crest Pressures

Hydraulic pressures were measured on the spillway crest for discharges of 10, 25, 50, 75, and100 percent of the PMF (fig. 44). The pressures are listed in table 1. The pressures given aretime averaged values. For the PMF a maximum negative pressure of 30.9 feet below atmosphericpressure was measured on the top of the crest. Pressures remained positive for all flows tested atpiezometer location 6. The high negative pressures measured for 75 and 100 percent of PMF areat levels where intermittent cavitation may occur. Due to the very low frequency of the floodevents considered and the relatively short peak flow durations, severe cavitation damage is notprobable.

Flip Bucket Pressures - Three-Level Flip Bicket Spillway

Seven piezometer taps were installed in the model to measure hydraulic pressures at selected pointson the flip buckets and adjacent walls (fig. 45). These locations were selected to provide hydraulicload data for the structural design. Pressures were measured on the model during flowscorresponding to 25, 50, and 100 percent of the PMF. Pressures measured at each location and foreach of the flows are listed in table 2.

Table 2. - Flip bucket pressures

Pressure tar(ft of waterDischarae in cercent of PMF

25 50 100

rI~

IQ:!II.O

i''"

r"...~..

..g\

--~.' '.:.~>

.c5

.i::

.'.-.'-

~"I.0'

\

~>:~i"

"II

"i,~\~

...rQ.,Q

~.

,

"'.f"

...~'".,

~~

r\J

....!!

I..tI

II

II

!lifO

,,,,,,,,,,,I~

11

!...

;!

.

~'_},,,j,:

~\\!1

,-''':1t:IL-)

)i.;-L

.-,/,

'/'i

"-.

~~0i::.i

:0:~

"I

~':)~~......~

1..

~:.- ...."

enc:0~> Q)

CDECIS

0Q)

...:JC)

u:

CREST OF SPILLWAY

TAPSLOCATED AT 7.5' INTERVALS -II-- 6.7'

2..J.J"

2

4

5 P. T.

Figure 2. - Chili Bar Dam spillway crest pressure tap locations.

12

a 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375

+'4-225 225

-" ~200 200cOJII!

i5175 175:J::...

~150 1500a.

4-0125 125OJ"0.~VI

+'100 100

..c:

,

<1' 50%-100%

C'. 75BEDROCIC

/75

E0...4-OJ 50 50uc::

'"+'II! 25 25.~c

0 aa 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375

Distance from Da~ Axis - ft

Figure 3. - Prototype bed material survey.

0 100

20

I

40--

0>c:rJ)rJ)

~

I

//6 Estimate for

isolates near om::!l0

60/

//

//

.~~-~

/'/'"/

100I

//

/

I ",,,,'

.''"1,"'",~<D&[email protected] of spillway

/ ""'" midstream average of

.-,,;..:..:' -- / two samples

10 100Diameter - mm

Figure 4. - Prototype bed material size distribution.

80 @ 380 ft west. 50 ft.from southbank-

~~--~

1,000

13

80

60"CfI)c:

Ca;ex:

::!l

400

20

10,000

2510,45 ...In

100

90--

80

710,15 ...'n

HYDROMETER ANAL YSISTIME READINGS

GRADATION TEST SIEVE AHAL YSIS

I CLEAR S~UARE OPENINGS

U 3/8" 3/4" I~" 3"I

'200 liDO60 ...In 19 ...In I...'n4...'n

I~~Upper bound.

--1-1

IIII-- '"

70 --- __no--.---

...

-.j::o., 60z;;;

'"~~ 511

ZIIIU8i:..L 40 -----

30

20 --

10

- ---

III

---fIII

--L ,

-- -- --~-- I,II

II

--, --- _._-

III

-r.--II

-_.~

'I'-I---

Model target scaled by:

_g"i~trYI=t~

Lower bound.;

--~---~-- ~

-----

-- ----

u --

0

~001 002

0.42 2.0.074 .149 .297 .590 1.19 2.38

DIAMETER OF PARTICLE IN MILLIMETERS76.2 127 152.009 .Olt .037 4.76 19.1 38. I

Figure 5. - Model bed size distributions.

0

10

20

30

Q..,

z~...III.

58...z.,u....

60..

70

80

90

a 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375

.........

225 225

-""uOJ 200 200C1

OJVI:::J

1750 175:x:

~OJ:J:

150 1500c.......0

OJ 125 125

"0V>.... 100 100.c0>.~

~E 75 750

~.....

OJ 50 50u

'"'"....VI25 25.~

C1

a a0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375

Distance from Dam Axis - ft

Figure 6. - Prototype channel bed contours.

15

r

0,0

.II

51~..~I

...

8:.

....

....4

,..

iI:;.,I

...'

I:!...

".":

~2

..a

~,:

.".'.

..:,J

..-.

II

1.

.,

II.

t.I.

!

....uIII

:I8:CZU.i

I

II

II

I.,.

:.,.

II

II

IIIA

'I'nil

IIlO

lItA."

I.

IIII!

.I.....ugiz

,-.....I:..J:u...;

I

I..:.

II

16

a>~:JU0>c:'';:;ro..........(l)C

i;~"roI-,

:(l).....:J0>

u:

50 750 25

...225

~~u

200CIICI

CII<It

175;:,0::J:

'-CII 150~0a..

~1250CII

-0....VI

100....c0>iX 75e0'- ~50CIIuC'"... 25oni5

LLLLLL025 500

CII<It

175;:,0::J:

'- ~1500a..

~1250CII-0....VI

100....c0>iX 75e0

'- ~50

0 25

...~ 225

~u 200~

CIIUC

'"...oni5

25

00 25

125 200 225 0375250 275 0300 0325 0350150

-----931

I I I I I I I I I..L

5 200 225125 150 17f rom Dam Axi sDistance

ed scour after 10 percent of PMF.Figure 8. - Prototype b

325 350 375100

50 75 100 125 150 175 0325 0350 0375

50 175 200 225125 150Dam Axis

75

Distance from

.9 Prototype bed scour after 25 percent of PMF.Figure. -

17

225

200

150

125

100

75

50

25

0

225

200

175

150

125

100

75

50

25

0

0 25 50 75 325 350 375

....225 225....

~200u 200

~eLl

'" 175 175::00:c

'-r 150 1500Q..

It-1250 125

eLl"0....VI

100 100......cen

c;:75 75

E0'-....

5050eLlUc

'".... 25 25'"i5

0 00 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375


Figure 10. - Prototype bed scour after 50 percent of PMF.

0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 J75

~225225

~u200 200

~eLl

'"::0 175 1750:c

'- ~150 1500Q..

It-0 125

125eLl

"0....VI

100 100......cen.....

0:: 75 75E0'-It-

50 50eLlUc

'".... 25 25'"....c

0 00 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375Distance from Dam Axis - ft


18

0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375

~225 225

oW

~200 200acuIII::J 175 1750:x:

~~150 1500

"-'0-0 125 125

cu"t:J.....VI

100100.....c0>

iX 75 75E0

~'0-

cu50 50

uc

'"....25 25III

C;

0 00 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375



Prototype Three Level Ski-Jurno Spillway

950

940-.

930,--..,

~920Q)

~910"--"

~9000..-t-' 890ro:>Q)

880........

~- Up of S~llway870 = Original ed

~10 PMF~25 PMF

860 ~50 PMFtli.ij- 75 PMF..... 100 PMF

8500 20 40 60 80 100 120 140 160

Distance from right side (Feet)Figure 13. - Prototype spillway bed profiles at toe of spillway.

19

55°

30°

Figure 14. - Modified flip bucket geometry.

20

\ \\\\

Figure 15A. - Prototype spillway.

Figure 15C. - Two-level flip bucket spillway.

Figure 158. - Single-level flip bucket spillway.

Figure 15D. - Three-level flip bucket spillway.

21

Figure 16. -Single-level flip bucket, 25,000 ft3/S (10 percent of PMF).

0 2S 50 75 100 125 150 175 200 225 250 275 JOO 325 350 375

+''0-

.xuC1)

0

C1)"'~O

:I:

'-

~O

Q.

'0-0

C1)~

~+'.cOt

~

eo'-

'0-

C1)uc10

+'"'

C

Figure 17. -Single-level flip bucket spillway -bed contours after 10 percent of PMF.

22

225

200

175

150

125

loo

75

50

25

0 25 50 75 100 125 150 175 200 225 250 275 :300 :325 350 :375

....

225225

....

~200u 200GI0GIIII 175::s 1750:c

'- 0J 150150

00.

.... 1250 125GI"Q....V) 100

100.....s::DO

a: 7575

E0

'-.... 50GI

50

uc

'"....25 25III

C

0 0

0 25 50 75 100 125 150 175 200 225 250 275 :300 :325 :350 :375


Figure 18. - Single-level flip bucket spillway - bed contours after 25 percent of PMF.

0 25 50 75 100 125 150 175 200 225 250 275 JOO :325 :350 :375

....

225.... 225

~u200 200GI

0

GIIII::s 175 175

~'-

J 150 15000.

....0 125 125GI"Q....V)

100 100.....s::DO....

'"75 75

E0

'-....

GI50 50

uc

'"....25III 25

C

00 25 50 75 100 125 150 175 200 225 250 275 :300 :325 :350 375

Distance from DamAxis - ft

Figure 19. - Single-level flip bucket spillway - bed contours after 50 percent of PMf.

23

0 25 50 75 100 125 225 250 275 JOO 325 350 375

....225 225....

~200u 200

~GI

'" 175~1750%L.

J 150 1500

Go

....1250 125

GI~V;

100100.....s::;D>

;:;;:75 75

E0L.

....50GI 50

uc

'"....25 25

'";:;

0 00 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375



0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375

....225 225....

~u200 200

~GI

'" ~175 1750%L.

J 150 1500A.

....0 125 125GI

~...VI

100 100.....s::;D>...

a:: 75 75E0L.

....50 50GI

UC

'".... 25 25'";:;

~0 I I I I I II I I I I I I I I0

0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375Distance from Dam Axis - ft


24

975

965

955

---

~ 945

Q)

I:L. 935'-" ~

~I -===:::: ::--.~ 925

O

-+oJ 915ro

:>QJ 905

.-.

t::I:::I

895

885

875

-Lip of Spillwoy~ Ori9inol Bed~ .10.PMF~ .25.PMF~ .50.PMF-t..i:Jc.li .75. P!A FU.!.U PMF

, I I I' I' I '!-'..!-'.!-'.1-'0 20 40 60 80 100 120 140 160

Distance from Spillway Right Side (Feet)

Figure 22. -Single-level flip bucket -bed profiles at toe of spillway.

Figure 23. -Two-level flip bucket -25,000 ft31s (10 percent of PMF).

25

0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375

.... 225 225

....

oW

C 200u 200CII

CI

CII175'" 175:;,

r---0:z:

'- ({>150I 150

06..... 1250 125CII

"'0....100V)

100....z::0>iX 75 75

!'-.... 5050CIIuCto

25....25

'"....CI

00

125 150 175 200 225 250 275 300 325 350 3750 25 50 75 100


Figure 24. - Two-level flip bucket - bed contours after 10 percent of PMF.

0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375

....225.... 225

oWu 200 200

c!:

CII

'":;, 175 1750:z:

'-I 150 15006.....0 125 125CII"'0V;

100100

1;)....z::

(D0>iX 75 75!'-....CII 50 50uCto....

25'"

25....CI

0 00 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375


Figure 25. - Two-level flip bucket - bed contours after 50 percent of PMF.

26

975

965

955

"

,

~ 945Q)

~ 935"--'"

~ 9250

..-<-+-> 915ro:>Q) 905

I""""'t~895

885

8750

- Lip of Spillwoy00000 Original Bed~ .10.PMFtiKtdI' .50.PMF

20 40 60 80 100 120 140 160Distance from Spillway Right Side (Feet)

Figure 26. - Two-level flip bucket - bed profiles at toe of spillway.

Figure 27. - Three-level flip bucket, 25,000 fe /s (10 percent of PMF).

27

llewis

Placed Image

0 25 50 75 100 125 150 175 200 225 250 275 JOO 325 350 375

~225 225

oWu 200 200

~CIIIII::J 175 1750:x:

L.

J 150 1500

Go...0 125 125CII

~....on

100 100....&:I:ft

iX 75co

75'"E 0>

0L.

...

ell 50 50uc:

'"... 25III 25....0

0 J.J 00 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375


Figure 28. - Three-level flip bucket - bed contours after 10 percent of PMF.

0 25 50 75 100 125 150 175 200 225 250 275 JOo 325 350 375

~225 225

oWu 200 200

~CIIIII::J 175 1750:x:

L.

J 150 1500

Go

...0 125 125CII

~....on

100 100.....&:I:ft

iX 75 75E '-0L.

...

ell50 50

uc:

'"...25 25III

....0

0 00 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375



28

100 125 150 175 200 225 250 275 300 J25 J50 J750 25 50 75

~225 225

~200u 200c!:

CII

\75III::I 1750::I:L.

150I 1500a......

1250 125CII."U;

\00100...~0>

a: 7575E0L.....

50CII 50916u

c<IS

25...III 25C

00100 125 150 175 200 225 250 275 300 325 350 3750 25 50 75



975

965

955..-....

a:; 945Q)

i:r-. 935'-""

~9250.~

-+oJ915ro:>Q) 905.-

~895- Lip of S~iIlWOyQ.QWliI Original ed

885 66666 .1O.PMF~.2S.PMF

tdtt!kt> .50.PMF

8750 20 40 60 80 100 120 140 160

Distance from Spillway Right Side (Feet)

Figure 31. - Three-level flip bucket - bed profiles at toe of spillway.

29

rLIP BUCKET DEFLECTOR WALL

ELEVATION

1- 45'-2"

rB'L

PLAN

Figure 32A. -Deflector wall geometry.

!,:~

Figure 328. Three-Ievel flip bucket spillway with deflector wall (25 percent of PMF)

30

935.00

925.00

~

~ 915.004-'--"c0

+J

0 905.00>Q)

W

895.00

885.000.00

930.00

910.00~

+J4-

'--"c0

+J

0>Q)

890.00w

870.000.00

Original Bed Elevation-'-~ ----.

<>ea1Hl PROTOTYPESKI JUMP SPILLWAYTHREE LEVEL FLIP BUCKET SPILLWAY

~ THREE LEVEL FLIP BUCKET SPILLWAY WITH DEFLECTOR WALL

20.00 40.00 60.00Discharge (% of PMF)

Figure 33. - Bed scour next to tailrace wall, 168 feet downstream of dam axis.

80.00 100.00

Ori inol Bed Elevation

-- PROTOTYPE SKI JUMP SPILLWAYTHREELEVEL FLIP BUCKETSPILLWAY

~ THREE LEVEL FLIP BUCKETSPILLWAYWITH DEFLECTORWALL

20.00 40.00 60.00Discharge (% of PMF)

80.00 100.00


31

935.00

915.00, ,.

+-'4-'

/

c0

+-'0>Q)

895.00w

Original Bed Elevation--------

IHt<><tfJ PROT01YPE SKI JUMP SPILLWAYTHREE LEVEL FLIP BUCKET SPILLWAY

~ THREE LEVEL FLIP BUCKET SPILLWAY WITH DEFLECTOR WALL

875.000.00 20.00 40.00 60.00 80.00Discharge (% of PMF)


975

965

955;-.,

~ 945Q)

~ 935'--"'

~ 9250

...-.-+-J 915(1j

:><J) 9054

~895

885

8750

100.00

- Lip of SpillwayQQQQp Original BedX..2!2!.2U' .04. P M FAAAAA .10.PMF~ .2S.PMF'ti::ti.P .50.PMFti.:t..ti .7S.PMF~PMF

20 40 60 80 100 120 140 160Distance from Spillway Right Side (Feet)

Figure 36. - Three-level flip bucket with deflector wall - bed profiles at toe of spillway.

32

200 225 250 275 JOO ~25 ~50 ~75

0 25 50 75

...225

.... 225

--'\~...: ~~.. 200u 200c!:QI 175III::t 1750:z:

I-150

I 1500

CL.

....125

0 125QI"0.... 100V)

100...s:.go 75a: 75e0I- 50....QI 50 934Uc

'"25

...III 25;;;

0

0 150 175 200 225 250 275 300 325 350 375

0 25 50 75 100 125


Figure 37. - Three-level flip bucket - bed with deflector wall - bed contours after 4 percent of PMF.

a 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375

...

.... 225 225

...: ::\u 200QI ~2000QIIII .,'

(::t 175 1750

OJ

:z:

I-~150 150

0CL.

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33

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35

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DISCHARGE in 1000cfs

Figure 43. - Spillway/dam discharge rating curve.

Spillway Crest

LEGEND(Flow In % of PMF)

100 %=

75%= --50%= ""'.."

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Figure 44. - Spillway crest pressures.

36

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APPENDIX

Research studies on slotted and slit-type flip buckets

39

APPENDIX

CONTENTS

Conclusions of experimental bucket tests. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Model tests of three-level slotted flip bucket. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Tests of three-level slit-type flip bucket. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Bibliography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Table

A.l

Figure

A-IA-2A-3A-4A-5A-6A-7A-8A-9A-I0A-llA-12

TABLE

Comparison of slotted bucket tests. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

FIGURES

Slotted flip bucket design model tested for Cleveland Dam. . . . . . . . . . . .Slotted flip bucket design tested for Chili Bar Dam spillway. . . . . . . . . . .Slotted flip bucket spillway - bed contours after 10 percent of PMF . . . . . .Three-level slotted flip bucket, 25,000 fe Is (10 percent of PMF) ........Slotted flip bucket spillway - bed contours after 25 percent of PMF . . . . . .Slotted flip bucket spillway - bed contours after 50 percent of PMF . . . . . .Slit-type flip bucket design model tested for Chili Bar Dam spillway. . . . .Three-level slit-type flip bucket, 25,000 fels (10 percent of PMF) .......Slit-type flip bucket - bed contours after 10 percent of PMF ...........Slit-typeflip bucket - bed contours after 25 percent of PMF

""""'"Slit-type flip bucket - bed contours after 50 percent of PMF ...........Slit-type flip bucket - bed contours after 75 percent of PMF ...........

41

Page

43

43

44

45

44

464748484949505151525253

RESEARCH STUDIES ON SLOTTED AND SLIT-TYPE FLIP BUCKETS

Slotted and slit-type flip buckets are extensions of the standard solid flip bucket design. Slots orslits are designed ihto the bucket to expand the jet leaving the bucket, thus increasing the jet surfacearea and the stilling basin impingement area.

A hydraulic model design of a slotted ski-jump flip bucket for Cleveland Dam is shown onfigure A-I (Wilson, 1953). Wilson first studied the jet shape and measured downstream channelscour for smooth flip bucket geometries. The tests were repeated with the addition of splitterteeth to the bucket invert, herein referred to as a slotted flip bucket. Wilson observed the slottedbucket geometry distributed the jet over a larger area of the downstream river channel than thesolid bucket. Measurements of channel scour supported his observations. Less scour occurreddownstream of the spillway using the slotted bucket geometry. The apparent benefits of the slottedbucket design did not come without drawbacks. Wilson also conducted a limited study of thepressures on the sides of the slots. Low-pressure zones were found on the vertical edges of theslots which raised concerns about the slots acting as possible sources for cavitation. In general thelow pressure zones as found in the Cleveland Dam study can probably be corrected by improveddesign of the slot shape. Little additional work on slotted flip bucket spillways has been dOl.1ebyReclamation to date. The Chili Bar Dam model provided an excellent opportunity to conduct afurther study to compare the channel scour resulting from equivalent operation of ski-jump, slottedbucket, and solid flip bucket designs.

Conclusions of Experimental Bucket Tests

For a free jet, depth of basin scour from the jet passing over the slotted and slit type bucketstested were similar to scour depths for a solid bucket. The slot and slit geometries tested did notforce enough horizontal or vertical separation in the exiting jet to substantially alter the jet surfacearea or basin impact zone. This suggests the slots or slits should be designed large enough to passthe majority of the design flow between the bucket teeth, which in effect creates multiple jets.

Tailwater levels causing the jet to become suppressed, resulted in scour near the toe of the spillwaystructure. This resulted in greater bed scour immediately downstream of the structure thanoccurred for the solid bucket design.

Model tests of Three-Level Slotted Flip Bucket

The recommended three-level spillway bucket design was used to evaluate a slotted bucketgeometry. Each of the three buckets were modified to a include slots (fig. A-2). As the bucketwidth on each level varied, the number of slots and their widths at each level also differed. Theflip bucket geometry tested for the three level solid bucket was used for the slotted bucket teeth.The slots or grooves between the bucket teeth conformed to the original spillway surface. Theslotted geometry was therefore constructed by adding a series of rectangular teeth to the originalspillway surface. Slot and teeth widths were equal across a single bucket but varied between bucketsdue to the different widths of the three spillway segments on the prototype.

Tests were conducted on the slotted bucket geometry for flows of 10, 25 and 50 percent of thePMF. Like the solid bucket scour tests, the movable stilling basin material was contoured to theprototype topography before the lO-percent test. Scour was then allowed to progress for the threeflows tested. The flow versus test-length relationships were also the same as for all previous tests.Table A.1 lists the unit discharge, the ratio of the slot depth to flow depth, slot width to toothwidth, and entrance velocity to the bucket for the tests conducted. Estimates of the bucket flowparameters are also presented for the Cleveland Dam study.

43

Cleveland Dam Chili Bar Damq (ff~Is/ft) D/H V (ft/s) Sw/Tw q (ftaIs/ft) D/H V (ft/s) Sw/Tw

312 0.67 100 1.06 147 0.36 60 1.0537 1.11 107 1.06 367 0.80 61 1.0

735 1.43 67 1.0

Table A.1. - Comparison of slotted bucket tests

q = spillwayunitdischargeDIH = ratio of flow depth entering bucket radius to slot tooth heightV = velocity of flow enteringbucket radiusSw/Tw = slot width divided by tooth width

Basin scour during passage of 10 percent of the PMF occurred mainly downstream of the 952.6-footelevation flip bucket (fig. A-3). The scour pattern and .depths were vary similar to the resultsobtained for the solid bucket tests. The jet leaving the end of the spillway was saw-toothed inappearance, although the corrugated jet surface tended to blend together as the jet passeddownstream (fig. A-4).

During the 25-percent PMF test, large material was observed moving upstream along the bottomimmediately below the end of the 932.6-foot elevation bucket. At 25 percent of the PMF, the932.6-foot elevation bucket sweeps out with the lower jet nappe fully suppressed by the tailwater.Rocks of up to 3.5-foot diameter were thrust against the bottom of the spillway teeth by the eddydriven beneath the exiting jet. The size and speed of material impinging on the downstream faceof the spillway toe appeared much greater than was observed for any of the previous spillwaydesigns tested.

Following the 25-percent PMF test, the maximum depth of scour downstream of the slotted flipbucket was less than measured for the prototype structure and approximately the same as thatcaused by the solid flip bucket (fig. A-5). Although, the maximum scour depth (located downstreamof the 952.6-foot elevation bucket) was similar to the solid bucket scour, downstream of the lowerelevation flip buckets the scour was greater than for the solid buckets. This suggests that undersuppressed conditions the portion of the jet passing between the teeth again scours the basin flooras did the jet from the prototype spillway geometry.

The depth of maximum scour downstream of the 952.6-foot elevation bucket was less than foundfor the comparable solid bucket test after 50 percent of the PMF (fig. A-6). This scour wasprimarily caused by impingement of the free jet leaving the 952.6-foot elevation bucket. In contrastto the reduced scour measured, the influence of the slots at the free jet surface appeared small withthe large flows.

Tests of Three-Level Slit-Type Flip Bucket

Slit-type flip buckets are characterized by slots with converging walls. The slit geometry forcesgreater vertical and longitudinal spread of the jet. The slit convergence is important to theperformance of the bucket. In general, slits may converge along a single angle over the full bucketlength or the slits may converge following a small angle and then converge sharply near the bucketlip. The later is typical of several free discharge flip bucket spillways constructed in China. Thesharp convergence at the lip should be between 1:4 to 1:6 for Froude numbers between 4.5 to 10(Jizhang et aI., 1988). This style of slits is designed to pass flow predominately through the slits.Studies on Dong-jiang and Long-yang Xia Dams found the maximum scouring depths were reduced40 to 60 percent when compared to similar tests with solid bucket flips.

44

A continuous slit convergence of 1:16 was chosen for study on the Chili Bar model to provide acomparison to the slot and solid bucket studies. The previously tested slot geometry was alteredby adding triangular wedges to the existing slots (fig. A-7). Thus, the widths at the bucket lip ofeach of the previous slots were reduced by 5.4 feet.

Tests were conducted on the slit bucket geometry for flows of 10, 25, 50, and 75 percent of thePMF. Following the previous testing sequence, the movable stilling basin material was contouredto the prototype topography before the 10-percent test. Scour was then allowed to progress forthe four flows tested. The flow versus test-length relationships were again held the same as for allprevious tests of similar discharge.

During 10 percent of the PMF test, the free jet from the high bucket showed pronounced finsdownstream of each slit (fig. A-8). The formation of vertical fins were nearly washed out on thelower buckets which were partially submerged by tailwater. At higher discharges the height ordefinition of the fins on the jet continued to diminish. This suggests the height and convergenceof the slits must be large in relation to the flow depth to force significant vertical separation in thejet. To do so on the Chili Bar spillway was not considered practical due to the small radius of theflip buckets and the large flows being conveyed.

Scour depths and patterns measured after each test were similar to those incurred for the slottedand solid flip bucket geometries (figs. A-9 through A-12). Although the slit geometry producesstronger vertical spread of the jet, again as found for the slotted bucket, the bucket teeth were tosmall to create the needed increase in the jet surface area and impact zone.

Bibliography

Jizhang G., Qingchoa L., and Guifen, L., "A Study and Applications of Slit Type Flip Bucket,"Proceedings, The International Symposium on Hydraulics for High Dams, pp. 315-322, 1988.

Wilson, L. V., Hydraulic Studies of the Ski-Jump Spillway for Cleveland Dam, Bureau of ReclamationReport No. HYD-369, Denver, Colorado, 1953.

45

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, 5,.11..,

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CREST

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-17.69',

'9.16'

CHILI BAR SPILLWAYSLOT TYPE FLIP

PLAN VIEW

FLOWSPILLWAY CREST

CHILI BAR SPILLWAYELEVATION VIEW

Figure A-2. - Slotted flip bucket design model tested for Chili Bar Dam spillway.

EL.957.25EL.950

EL.947.25EL.940

EL.937.25EL.930

25 50 75 100 125 1~ 175 200 225 250 275 300 325 350 375"

225

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Figure A-3. -Slotted flip bucket spillway -bed contours after 10 percent of PMF.

Figure A-4. -Three-Ievel slotted flip bucket, 25,000 ft3/S (10 percent of PMF).

48

0 25 50 75 100 125 150 175 200 225 250 275 .300 0325 0350 0375

...225 225It-

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Figure A-B. - Slotted flipbucket spillway- bed contoursafter50 percent of PMF.

49

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0 25 50 75 100 125 150 175 200 225 250 275 JOO 325 350 375

~ 22~ 225

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000 25 50 75 100 125 150 175 200 225 250

Distance from Dam Axis -ft

275 300 325 J50 375

Figure A-9. -Slit-type flip bucket -bed contours after 10 percent of PMF.

51

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0 25 50 75 100 125 150 200 225 275 300 325 350

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....

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52

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*u,s GPO: 1990 773-182/20,001

53

Mission of the Bureau of Reclamation

The Bureau of Reclamation of the U.S. Department of the Interior is responsible for the development and conservation of the Nation's water resources in the Western United States.

The Bureau's original purpose "r provide for the reclamation of arid and semiarid lands in the West' today covers a wide range of interrelated functions. These include providing municipal and industrial watersupplies; hydroelectric power generation; irrigation water for agriculture; water quality improvement; flood control; river navigation; river regulation and control; fish and wildlife enhancement; outdoor recreation; and research on water-related design, construction, materials, atmospheric management, and wind and solar power.

Bureau programs most frequently are the result of close cooperation with the US. Congress, other Federal agencies, States, local governments, academic institutions, water-user organizations, and other concerned groups.

A free pamphlet is available from the Bureau entitled "Publications for Sale." It describes some of the technical publications currently available, their cost, and how to order them. The pamphlet can be obtained upon request from the Bureau of Reclamation, Attn D-7923A, PO Box 25007, Denver Federal Center, Denver CO 80225-0007.

hydraulic model study of chili bar dam spillway …

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