- ,. HYDRAULICS BRANCH OFFICIAL FILE COPY

HYDRAULIC STRUCTURE AT MOUNT ELBERT POWERPLANT

1 By Pe rry L. Johnson , A.M. ASCE and

2 Edward W. Gray, A.M. ASCE

INTRODUCTION

The Fryingpan-Arkansas Project is a multipurpose trans­

mountain diversion development. It will make surplus

water from the western slope of the Rocky Mountains of

Colorado (Colorado River drainage) available to inhabi­

tants of the eastern slope of Colorado (primarily in the

Arkansas River drainage). The water will be used for

municipal, industrial, and irrigation purposes. As

shown in figure l, the diverted water is transported

through a series of tunnels, reservoirs, canals, and

powerplants. Mt. Elbert pumped-storage powerplant

(figure 2) is one of two powerplants to be constructed

in the project. These powerplants will produce power -

from the water as it descends to the eastern plains.

Mt. Elbert pumped-storage powerplant will eventually pro­

duce 200MW of power with two units. Each unit will be

1Hydraulic Engineer, Hydraulics Branch, Division of Research , U.S. Bureau of Reclamation, Denver, Colorado

2 Supervisory Civil Engineer, Earth Dams Section, Division of Design , U.S. Bureau of Reclamation, Denver, Colorado

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WESTERN AND UPPER EASTERN SLOPE AREAS

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EXPLANATION

BUREAU OF RECLAMATION

COMPLETED AND AUTHORIZED WORKS

FRYI NGPAN - ARK ANS AS PROJECT

COMPLETED AUTHORI ZED

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CONDUIT

PUMPING PLANT

POWER PLANT

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3 fed f rom the 11,50 0 acre-ft (14,160, 000 m) upper or fore-

bay reservoir by a 15- f oot (4.6m) diameter penstock. The

length of each penstock will be approximately 3,000 feet

(940m). The forebay reservoir will be created by the

construction of a 90-foot (27.4m) high dam on a minor drain­

age. The forebay reservoir will have a compacted clay lining

which is necessary due to the pervious nature of the in­

place soils in the forebay area. The forebay reservoir

will receive water from the pumping action of the power­

plant and from the Mt. Elbert conduit. The Mt. Elbert

conduit will have a maximum flow capacity of 370 Ft3/s

(10.5 m3/s). The maximum water surface elevation of the

forebay reservoir will be 9,647.0 Ft (2948.6~) while the

minimum operating water surface elevation will be 9,615

feet (2930 . . 7m).

The lower reservoir for the plant will be an expanded

Twin Lakes (figure 2). The existing dam on the lower

lake will be replaced by a new embankment with a maximum

height of 52 feet (15.Bm). This will combine the two

8 3 existing lakes into a single 147,500-acre-ft (l.BxlO m)

lake at the top of active conservation water surface.

Twin Lakes has long been used as a recreational site. The

lower lake contains an outstanding self-sustaining lake

trout fishery. Twin Lake will receive water both from

the diversion and powerplant and from Lake Creek, the

mountain stream on which the -lakes lie. Large areas of

·/

MT. ELBERT FORE BAY

RESERVOIR

'

TWIN LAKE? RESERVOIR

I <f. Gate shaft----+---------2.994------------

Top of active nservation capacity . 9645.7 3:1

Top of dike, El. 9655.5

Top of inactive capacity, El.9615

3: I

Two 151

diameter steel penstocks

100 0 l ' I I I

100 I

SCALE OF FEET

300 I

Max. W. S. El. 9202.3

Top of active · conservation,

EI. 9200.0

Mt. Elbert pumped­storage powerplant

\ ·· PROFILE ALONG CENTERLINE OF PENSTOCKS SHOWING FEATURES

OF THE MT. ELBERT PUMP~D-STORAGE SCHEME

the lake bottoms are covered with glacial flour. Glacial

flour is finely ground rock which has no cohesiveness and

which is consequently easily disturbed by water movement.

The top of active conservation water surface of Twin Lakes

will be 9,200 feet (2804.2m) and the minimum active water

surface will be at 9,168.7 feet (2794.7m). The maximum

static head for the plant will therefore be 478.3 feet

(145.6m). The maximum discharge through each penstock

will be 3,600 Ft3/s (102m3/s) during the generating cycle

and 3,090 Ft3/s (87.5m3/s) during the pumped cycle. It is

expected that the first unit will be in operation by

January l, 1979. The second unit is to be constructed and

in operation by February 1980.

In this paper the authors intend to discuss unusual design

and construction hydraulic structure features of the power­

plant. These are:

1. A baffled chute which will dissipate the energy

in the canal water flowing into the forebay reservoir.

2. An upper inlet-outlet structure which will function

with acceptable head loss, vortex-free inlet operation

while yielding velocity head recovery and satisfactory

flow velocities past the trashracks during outlet opera­

tion.

3. Penstocks designed to interact with the

surrounding backfill when under pressure. Inclined

surge tank s, b uried in the side of the excavati on

required for the penstocks , were selected to r educe

visual impact.

4. A tailrace channel which will limit mixing affects

of the powerplant on Twin Lakes. This will minimize

the disturbance of the lake stratification and the

glacial flour sediment which will in turn help to reduce

the impact of the powerplant on the lake and the lakes

fishery.

Baffled Chute

The baffled chute or apron drop shown in figure 3 was

designed to dissipate the energy in the Mt. Elbert Canal

flow as the flow drops into the forebay reservoir. Since

the forebay reservoir water surface elevation can fluctuate

over a 30-foot (9.lm) range, it is desirable to have an

energy dissipator that will function satisfactorily no

matter what the tailwater elevation is. Baffled chutes,

because of their excessive roughness, are able to maintain

relatively uniform, low velocity flow over their entire

flow length. They can function with no tailwater although

under such conditions some scour will occur at their base.

They function equally well with higher tailwate~ and scour

is reduced or eliminated. Thus, the baffled chute is well

suited for this Mt. Elbert application.

Baffled chutes have been used as canal drop structures by

the Bureau of Reclamation for many years. They basically

lft-0.305m

LED CHUTE TYPICAL BAFF .

FIGURE 3

consist of a sloping apron, usually on a 2:1 or flatter

slope, with multiple rows of blocks or baffle piers equally

spaced along the chute. The flow passes over, around, and

between the baffle piers and appears to slow down at each

baffle pier and accelerate after passing the pier. The

multiple rows of baffle piers on the chute prevent excessive

acceleration of the flow and provide a reasonable terminal

velocity, regardless of the height of drop. The extent

of acceleration and the ultimate velocity at the base of

the chute depends on the discharge and height, width, and

spacing of the baffle piers. This type of drop has been

used in a wide variety of structures with success and has

been studied in detail by the Bureau of Reclamation's Hy­

draulics Laboratory. Presently used design standards are

available in several publications (1,3,4).

Of prime importance during construction were the considera­

tions to secure a smooth, dense finish for concrete surfaces

adjacent to high-velocity water flow. Cavitation damage

of such surfa~shas been traced to surface irregularities

or to poor quality of concrete in the surface.

Upper Inlet-Outlet Structure

The upper or forebay reservoir inlet-outlet structure (figure

4) functions to gradually accelerate or decelerate flows

which in turn results in reduced vortex tendencies during

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Structure

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-EL.95 74.00

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PR ELI MIN ARY INLET - OUTLET STRUCTURE .

FIGURE 4

the generating cycle and increased velocity head recovery

during the pumping cycle. The flow deceleration effect of

the structure also results in reduced flow velocities

through the structures trashracks. This is desirable in

that with the higher velocity flows a potential exists for

strong vortex shedding and thus the resulting vibration

which could result in fatigue failure of the trashracks.

Balanced against the above hydraulic considerations was

the desire to minimize the size, complexity, and therefore

the cost of the structure. In general a larger more gradually

expanding structure has better hydraulic characteristics

for both pumping and generating flow. During the genera-

ting cycle a slower flow acceleration would reduce the

potential for vortex formation. Vortices with sufficient

strength to entrain air are undesirable because of the

resulting potential for blowback and because the passage

of air through the turbine will result in rough operation.

During the pumping cycle a longer structure would allow a

more gradual expansion of the flow which would minimize

flow separation and thus minimize turbulence and high

velocity concentrations. This would in turn result in

maximum velocity head recovery and a more uniform, low­

velocity flow through the trashracks.

The structure length and expansion rate shown in figure 4

were determined by the designers to be economically feasible.

because it was realized that t h i s does not r epresent an

optimum hydraulic design, a hydraulic mode l study (2)

of the structure was undertaken to verify and improve

flow conditions as needed. The objectives of the model

study were primarily to insure satisfactory flow velocities

through the trashracks during the pumping cycle and to

insure against generating cycle surface vortices which

would have sufficient strength to draw air into the pen­

stocks. Head loss through the structure for both cycles

was also considered.

A 1:23.23 scale model was constructed. A 7.75-inch (197mm)

diameter plastic conduit represented the 15-foot (4.57m)

diameter penstock. The 3,600-Ft3/s (102m~/s) maximum

generating discharge for each unit was modeled by a flow

of 1.385 Ft~/s (39.2 1/s). The model was arranged so

that both pumping and generating flow could be studied.

In the model, velocity distribution data was collected

at the trashrack section and at the section containing

the stoplog slots. These data were collected for both

the pumped and generating cycles. Velocity distributions

during the generating cycle were quite uniform with only

slight fluctuations, and consequently these flow conditions

were determined to be satisfactory. The pumping cycle

however, proved to be a different story. Flow direction

and distribution were strongly influenced by the penstock

orientation. A typical trashrack section velocity distri­

bution is shown in figure 5. Note that not only were

severe localized high-velocity regions observed, but also

there was no flow passing through a significant portion of

the trashrack section. The structure expansion was too large

for the flow to follow. A deflector was then developed

(figure 6) which dispersed the flow and yielded trashrack

section velocity distributions similar to that shown in

figure6a· These velocities were considered to be low

enough so that trashrack failure would not be a concern.

The deflector structure was incorporated in the final design.

A combined Froude number-Reynolds number modeling criteria

was used to study the vortex potential. Direct Froude

modeling tends to underrepresent vortex strength. Use of

discharges greater than those determined by Froude modeling

will increase model vortex strength. The model was operated

at discharges of up to 200 percent of design discharge.

This created flow conditions that were felt to be as bad

as, if not worse than, actual prototype conditions. Under

the model operating conditions potential generating cycle

vortices with sufficient strength to draw air into the

penstocks were observed over a large range of reservoir

elevations (figure 7). At design discharge the model showed

vortices which would develop surface dimples but which

would not draw air (figure 8). A lattice wall vortex

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TRASHRACK SECTION VELOCITY OISTR I BUT ION

LOOKING IN DIRECTION OF GENERATING FLOW

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STOP LOG SECTION VELOCITY DISTRIBUTION LOOKI NG IN D IR EC TION OF PUMPED FLOW

STOP LOG SECTION

APPROXIMATE STREAMLINES

VELOCITY DISTRIBUTION TRASHRACK AND STOP LOG SECTIONS INITIAL INLET-OUTLET STRUCTURE

WITH CONCAVE FLOOR AND NO DEFLECTOR PUMPED FL OW

VELOCIT I ES SHOWN ARE PROTOTYPE V ELOCITIES IN FEET / SEC ON D

2 FOOT / SEC OND CON TOU R INTERVAL

FIGURE 5

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Gate Structure

Deflector iEL. 9580.00 • • ' ... • • • •. · . •..••• •' ... "l . : ' o •• ' • .' Penstock

----9a.oo·-·~~--·1 ..,._ ________ 228. 501---------....i

FINAL INLET-OUTLET STRUCTURE

FIGURE 6

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STOP LOG SECTION

APPROXIMAT_E STREAMLINES

VELOCITY DISTRIBUTION TRASHRACK AND STOP LOG SECTIONS INITIAL INLET-OUTLET STRUCTURE

WITH STRAIGHT FLOOR AND DEFLECTOR PUMPED FLOW

FIGURE .6a

VELOCITIES SHOWN ARE PROTOTYPE VELOCITIES IN FEET/ SECOND

2 FOOT /SECOND CONTOUR INTERVAL

~ . . · ~ .

VORTEX DEVELOPED BY PASSING 200% OF DESIGN GENERATING DISCF..ARGE

FIGURE 7

VORTEX DEVELOPED BY PASSING DESIGN GENERATING DISCHARGE

FIGURE 8

LATTICE WALL VORTEX SUPPRESSOR

FIGURE 9

suppres sor (figure 9) was developed which prevented a ir

core development for all flow conditions tested. Because

of uncertainty about the strength of vortices that will

occur in the actual structure, the inlet-outlet was designed

so that the lattice wall suppressor could be easily added if

operation shows that it is needed.

Head loss through the inlet-outlet structure and the con­

necting 124-foot (37.8m) length of penstock were evaluated

to be 2.36 feet (0.719m) of water for the pumped cycle and

2.31 feet (0.704m) of water during the generating cycle.

Penstocks and Surge Tanks

The penstocks were designed to take advantage of the com­

pacted backfill placed around them. There were no other

unusual design considerations for the penstocks except for

the surge tanks.

Studies conducted by the Special Studies and Testing Section

of the Mechanical Branch, Division of Design, in the Bureau's

Engineering and Research Center, indicated that control of

hydraulic transients was necessary to keep both upsurge

and downsurge within design limits for the penstocks. Using

characteristics of the model of the installed pump-turbine,

it was found that the design gradient could be seriously

exceeded in some portions of the penstock, and that a con­

dition of possible water column separation could occur due to

downsurge.

The obv ious a nd conventional solut i on was to place vertica l

surge tanks d irectly ove r each penstock. The tanks, however,

would need to be approximately 150 feet (45.7m) high and qM

30 feet (~ m) in diameter to effectively control upsurge

and to prevent water column separation.

Because the Mt. Elbert area is environmentally sensitive,

alternatives to the very visible, 150-foot (45.7m) high

vertical tanks were sought. After considering several

alternatives, sloping surge tanks located laterally from

the penstocks and buried in the side of a slope adjacent

to the penstocks were selected as the best option available.

This alternative had the advantages of the vertical tanks,

could be constructed in an unobtrusive manner, and would

relieve the upsurge and downsurge problems.

The basic dimensions of the tanks were determined from

the hydraulic transient studies. Separate tanks for each

penstock were selected to allow independent operation of

either pump-turbine unit. Each surge tank consists of a

17-foot (5.2m) diameter steel line approximately 370 feet

(112.8m) in length which leads to a vertical , reinforced

concrete tank about 50 feet (15.2m) high and 40 feet (12.2m)

in diameter. The steel lines will effectively act to con­

trol downsurge and will have maximum side support provided

by compacted backfill placed to the top of the pipe.

Because vertical risers from the steel pipes into the

concrete tanks would require an elbow which would impede

surge relief, direct side connections between the pipe

and the tanks were provided.

Powerplant Tailrace Channel

As previously stated, the bottom of Twin Lakes is covered

with glacial flour, a substance which can be resuspended

by minor flow disturbances. The lower lake also has a

self-sustaining fishery which is noted for the large lake

trout which it produces. The lake trout feed, during

certain life stages, on a fresh water shrimp (Mysis Relicta)

which was introduced to the lake in 1957 and which has

become very abundant. Potential environmental effects of

the pumped storage powerplant include: (1) Destruction

of the summer thermal stratification, (2) Resuspension of

the glacial flour with associated high turbidity, and

(3) entrainment and passage of fish through the pump tur­

bines. These factors could be expected to have direct

adverse effects on the fresh water shrimp and the lake

trout as well as direct or indirect effect on other biota.

Secondary effects on the resort and recreation industry

would result. Recognition of these potentially adverse

environmental effects led to initiation of a major study

of Twin Lakes. The study includes comprehensive biologi­

cal assessment of preoperation conditions with associated

studies of the lake thermal regime and lake hydrodynamics.

A Corp s of Eng i nee rs modifi cat ion of a mode l developed by

Wate r Re s ources Engin e e rs, Inc. was us ed t o s imulate t he r­

mal stratification in Twin Lakes. The model had been

previously verified using data from two very different

Bureau of Reclamation reservoirs. The model was applied

using climatological and hydrological data collected at

the site. Sufficient agreement between predicted and

actually measured temperature profiles was obtained. The

effects of plant operation were then incorporated in the

program. Additional inflow and outflow points were

designated to represent generating and pumping. The dif­

fusion coefficient was varied from that used in the verifi­

cation runs to show increased destratification effects

due to plant operation. Because the question remains as

to what diffusion coefficient value is truely representa­

tive of the effect of the plant, the mathematical model

analysis is continuing.

Physical mo~els were then used to evaluate the influence

of the pumped storage powerplant on Twin Lakes. Three

models were constructed (figure 10). The first, a dis­

torted (1:84 vertical, 1:6000 horizontal) thermally

stratified tabletop model was used to demonstrate and

approximate the possible destratifying effects of plant

operation.

A second distorted, thermally stratified model (1:100

vertical, 1:600 0 horizontal ) was used to study destrati-

TWIN LAKES MODELS

FIGURE 10

fication effects and circulation patterns in detail. The

model included both lakes, the connecting channel, the

inflow and outflow channels, and the Mt. Elbert plant.

Plant operation was controlled by a minicomputer which

also scanned water temperature monitoring thermistors.

Velocities in the model were measured with an electro­

magnetic current meter, and circulation patterns and

jet movement were determined by single-frame photography

of dye clouds.

A third undistorted (1:100), homogeneous model was

used to examine the near field characteristics of the

tailrace flow and to develop the design of the tailrace

channel. Velocities were measured with a miniature

propeller meter and the electromagnetic current meter.

Operation of the tabletop model showed that the jet

leaving the plant during the generating mode stayed close

to the west shore then turned eastward when it reached

the south shore of the lower lake. This model suggests

that, beginning with a well defined stratification, the

lake would be only weakly stratified after 30 days of

either 1- or 2-unit operation. Movement of glacial flour

was indicated.

Operation of the larger distorted model showed that the

effect of the plant on Twin Lakes is strongly dependent on

the relative tempe ra tur es of the gener ating cycle flow and

the water in Twin Lakes. If generating flow is warmer than

the lake water or the same temperature as surface lake water,

it will tend to stay on the surface and consequently not

affect bottom sediments. On the other hand, if the generating

flow is as cold as or colder than the bottom lake water, the

flow will tend to plunge and will quite likely disturb bottom

sediment. These findings indicate that efforts should be

made to use as warm as possible water for plant operation.

With this objective design efforts have been directed towardp

drawing only the warm surface Twin Lakes water into the plant

during the pumping cycle. The model indicated that the

temperature of the water drawn into the plant during the

pumped cycle roughly corresponds to the lake water temperature

at the elveation of the bottom of the tailrace channel.

Finally using the undistorted model, efforts were directed

towards developing the tailrace channel. Three channel

configurations were tested; one was a direct extension along

the plant centerline, the other two angled to the right toward

the west shore of the lake. Velocity distribution readings

taken during the generating cycle indicated that the configura­

tion of the draft tubes concentrated the flow toward the center­

line of the plant with very little dispersion. This gave the

flow a higher energy flux at the point of entry into the lake.

Bottom velocities seemed to be lower than intermediate depth

and surfa ce ve l ocities . On the ang l ed channe l s , the flow

impinged on t he left b ank before turnin g a n d fo llowing t he

channel alinement. There was also some return flow and dead

water areas, particularly with 1-unit operation. These

trends occurred throughout the length of the excavated channel.

Both 1- and 2-unit operation exhibited the same tendencies.

Flow distribution was much better during the pumped cycle,

particularly with the angled channels.

Based on the initial tests, an angled channel was studied in

both the large distorted and the undistorted models for further

investigation and refinement. The channel developed and in­

cluded in the final design (figure 11) angled 27 degrees to

the right of the plant centerline. The channel has a 60-foot

(18.3m) bottom width with 3-to-l side slopes. The invert is

at elevation 9150 (2790m). At the end of the channel, where

the channel enters the lake, a 5-foot (1.5m) high, 10-foot

(3.0m) wide berm serves as an underwater barrier. The barrier

encourages withdrawing water from a high level during the

pumping cycle and influences the inflowing water during the

generating cycle to have less tendency to move along the bottom

of the lake.

The velocity distribution measurements, in the undistorted

model, indicated that during the pumped cycle most of the

flow entered the channel from along the north shore of t he

lake . The flow was evenly distributed and t he velocity was

about 1.2 Ft3/s (0.37m3/s.

' / I ~

__ __J ( J ~ -

~·vv' £ /.? I \.r/ / /\/

TAILRACE CHAN1\1EL FIGURE 11

During the generating cycle flow was not evenly distributed

near the plant but had attained an adeq uate distribution

when it entered the lake ~here average velocities were

usually less than lFt/s (0.30m/s). Velocities were slightly

higher from mid-depth down.

Stratification was not affected by 2-unit operation although

there was a tendency for the ambient temperature to be affected

by the temperature of the generating flow. Over a two-week

operating period, warm water inflow increased the temperature

but the gradient remained essentially the same.

Time lapse motion pictures of colored water inflow showed

movement of the generating influent generally toward the

southeast and the pumping cycle intake generally originating

along the north side of the lake. There was noticeable

(but not appreciable) flow along the west side of the lake

during the pumping cycle. This was not apparent during the

velocity measurements in the undistorted model.

Gyetracers placed in the bottom of the lake in the vicinity

of glacial flour deposits prior to operation did not move

during the initial 2 or 3 days of operation but were gradually

assimilated into the surrounding water.

In summary, the angled channel wi ll provide satisfactory flow

conditions, minor bottom disturbance , small changes to the

natural stratification patterns, and a warming or cooling

trend to t he amb ient lake temperature, depending on the

temperature of the inflowing water.

Conclusions

1. Baffled apron chutes offer an energy dissipation

structure which yields good hydraulic performance and good

energy dissipation over a wide range of tailwater eleva­

tions. Consequently the structure is well suited to

dissipated energy of inflows into pumped storage reser­

voirs which have large water surface elevation fluctuations.

2. Pumped storage inlet-outlet structures should and can

be designed to yield optimum velocity head recovery, mini­

mum intake head loss, operation free of air entraining

vortices~ with uniform low velocity flow through

trashracks. These objectives can be sufficiently achieved

while still designing an economically feasible structure.

3. The penstocks and the steel pipe portions of the surge

tanks were designed taking into account the rigidity of

the compacted backfill. To reduce visual effect in an

environmentally sensitive area, the surge tanks were

located laterally from the penstocks, inclined and buried

within an existing slope, and terminated in vertical con­

crete tanks designed to blend wi t h the surrounding land­

scape. The surge tank arrangement provides adequate

protection for the expected hydraulic transients both

for upsurge and downsurge conditions.

4. At Mt. Elbert pumped-storage powerplant the tailrace

channel has been designed to minimize the environment

impact of the powerplant on its lower reservoir. The

channel has been designed to minimize destratification

effects of plant operation and to reduce the potential

for mixing of glacial flour lake bottom sediments.

...

Appendix . - Refe rences

1. "Design of Small Canal Structures," U.S. Department of Interior, Bureau of Reclamation, 1974, pp. 299-309.

2. Johnson, P.L., "Hydraulic Model St1.Jdies of the Forebay Reservoir Inlet-Outlet Structure for Mt. Elbert Pumped­Storage Powerplant, Fryingpan-Arkansas Project, Colorado," U.S. Bureau of Reclamation, Report REC-ERC-72-5, January 1972.

3. Peterka, A. J., "Hydraulic Design of Stilling Basins and Energy Dissipaters," Engineering Monograph No. 25, U.S. Department of Interior, Bureau of Reclamation, 1964, pp. 154-188.

4. Rhone, T. J., "Baffled Aprin as Spillway Energy Dissipater," Journal of the Hydraulics Division, ASCE, Vol. 103, No. HY12, Proc. Paper 13373, Dec. 1977, pp. 1391-1401.

5. "Third Workshop on Ecology of Pumped-Storage Research at Twin Lakes, Colorado and Other Sites," Compiled by J. F. LaBounty, U.S. Department of Interior, Bureau of Reclamation, February 1976.