UNITED ST ATES DEPARTMENT OF THE INTERIOR

BUREAU OF RECLAMATION

MODEL STUDY FOR THE REVISION OF THE STILLING

POOL APRON -- GILA VALLEY DESILTING WORKS

AT IMPERIAL DAM--GILA PROJECT , CALIFORNIA

Hydraulic Laboratory Repor t No. Hyd-344

ENGINEERING LABORATORIES BRANCH

DESIGN AND CONSTRUCTION DIVISION DENVER, COLORADO

January 30, 1952

CONTENTS

Page

Introduction . . . . . • • • . • • . • . . . . . . • . . 1 The Model . . . . • • . . . . • . . • • . • . • • . • 2 Existing Design . . • . . . . . • . . . . . . • • . . • ·2 Test Procedure . • • • . . • . . • . . • • • • . . .. • 3

Scheme 1 . . . • . • . . . . . • • . . . . . . . . 3 ··-.·,

Scheme 2 • . . • . • . . • . . • • . . . . . . • . 4

. Conclusions . . . . . . • . • • • . . . . . • . • . . • 5

Figure

General Plan of Imperial Dam and Gila Headworks • . • • 1 . Plan of Gila Valley Desilting Works • • • • • • • • • • • 2 Profile of Gila Valley Desilting Works • • • • • • • • • • 3 The Model . . . • . . . . . . . . . • . . . . • . . . 4 Tail Water--Jump Height Curve • • • • • • • • • • • • • 5 Photographs of Existing Design • • • • • . • • • • • • • 6 Photographs of Existing Design • • • • • • • • • • • • • 7 Scour Downstream from Existing Design • • • • • • • • • 8 Baffie Block Combinations • • • • • • • • . • • • • • • 9 Operation with Single Row of Blocks., 20, 000 cfs • • • • . 10 Operation with Two Rows of Blocks, 20., 000 cfs • • • • • • 11 Scour Downstream with Double Row of Baffie Blocks • • • • 12 Sc our Downstream with Double Row of Baffie Blocks and

Triangular End Sill • • • • • • • • • • • • • • • • • • 13 Small Cubic Block Combinations • • • • • • • • • • • • • 14 Operation with Cubic Blocks, 20, 000 cfs • ._ • • • • • • • 15 Scour Downstream from Recommended Design • • • • • • 16

UNITED STATES DEPARTMENT OF THE INTERIOR

BUREAU OF RECLAMATION

Design and Construction Division Engineering _Laboratories Branch

Laboratory Report No. Hyd-344 Hydraulic Laboratory Section Compiled by: Yin Feng. Checked by: J. N. Bradley Reviewed by: H. M. Martin

Denver, Colorado · January 30, 1952

Subject: Model study for the revision of the stilling pool apron--Gila Valley desilting works at Imperial Dam- -Gila Project, Arizona

INTRODUCTION

This report is an account of a model study performed to obtain information for revision of the sluiceway stilling pool apron at the Gila Valley desilting works.

. The Gila Valley desilting works is located on the left bank of the Colorado River at Imperial Dam in Arizona, Figure 1. In general, it consists of a gate structure, one large trapezoidal settling basin. and a combination canal intake and sluiceway structure at the downstream end. Water from the lake behind Imperial Dam flows slowly through the basin., depositing a portion of its silt load on the fioor. The canal in­take at the downstream end of the basin receives water from the upper level of the basin, while the sluiceway immediately under the canal in­take is used to fiush the deposited silt back into the river when neces­sary. The sluiceway does not operate continuously. A plan of the canal intake and sluiceway stilling basin is shown on Figure 2 and sections are shown on Figure 3.

The entire desilting works was operated as intended up until. 1944. At that time., the Army was permitted to practice amphibious,land­ing operations downstream from the dam. These operations· required considerably more discharge than the normal fiow of the river at this point.The Gila Valley settling basin provided a means of storing water. and the sluice way. offered a method of releasing it downstream at discharges much higher than normal for a short period of time.

Neither the settling basin nor the sluiceway stilling pool was de­signed for this type of operation. However. both structures served the purposes of the Army during and after the national emergency. but not without eventual detrimental effect. In 1949, the floor and side walls of the settling basin heaved in several places from unrelieved uplift pressures. and erosion downstream from the sluiceway stilling pool was reaching dangerous proportions from the large discharges. Holes 18 feet deep were

reported at the downstream end of the apron., even though the bed was originally heavily riprapped to resist-erosion. Continuation of either the Army or Bureau operations required immediate repairs.

The bafiin does not pass much over 1., 000 cfs during normal sluicing operations. However., the Army was using discharges of as much as 20., 000 cfs through this basin. As it was anticipated that the Army would continue operations., repairs were to be expedited and this meant that they would of necessity be of a more or less temporary nature. The object., therefore., was to revise the stilling pool so that it would handle intermittent discharges of 20., 000 cfs without doing fur­ther damage.

THE MODEL

The existing apron for the sluiceway stilling pool is 50 feet long by 158 feet wide. A sectional model was constructed of the exist­ing structure on a 1:30 scale. The model was 2 feet wide., which repre­sented 60 feet, or a little more than one-third of the prototype width (Figure 4). The stilling pool was constructed of wood a~d lined with sheet metal., while the piers, gates, and sills were of wood. Gates at· the upstream end made it possible to regulate the head. The model was installed in a laboratory fiume, having a glass side, through which the action of erosion could be observed. A pot-type gage was installed in the head box for setting the head on the gates, while a similar gage was installed in the tail box to observe the water surface in the river. The maximum discharge used in the sectional model was 1. 55 cfs.

EXISTING DESIGN

The tail-water curve for the river downstream from the dam is shown on .Figure 5. Superimposed on the same chart is the computed theoretical jump height curve, "A.,11 and a sweep-out curve, "B," for the existing design obtained from the model tests. A third curve, "C., I' represents 85 percent of the theoretical jump height which is permisslble in some designs which use an end sill and/or baffie piers. The jump height curve., "A, " and the tail-water curve intersect at a discharge of 8., 700 cfs which means that the tail water is less than would be desirable · for discharffes above this value. The 85-percent theoretical jump height curve., "C., intersects the tail-water curve at 'a discharge of 12, 000 cfs. This indicates that an acceptable jump may form. for nows up to 12, 000 cfs. The sweep-out curve., "B.," intersects the tail-water curve at a

.· discharge of 15., 300 cfs which means that a. jump cannot form for discharges above this value.

Figures 6 and 7 show photographs of the existing design for dis­charges of 8., 000., 12., 000, 16,000, and 20., 000 cfs. Figure 6-A shows good operation at a discharge of 8,000 cfs. Figure 6-B shows the jump partially in the pool for a discharge of 12., 000 cfs. Thus., more than 85

2

percent of the thoeretical jump height is required in this case for good operation at 12,000 cfs. The jump swept out of the pool for a discharge of 15,300 cfs. Figures 7-A and -B show the jump out of the pool for discharges of 16,000 and 20., 000 cfs, respectively. For a discharge of 20,000 cfs, the tail-water depth was only 65 percent of the theoretical jump height. Figure 8 is a plot of the scour obtained from the model of the existing design after a discharge of 20, 000 cfs for 1 hour (model time). The maximum depth of scour is 16 feet.

Since the prototype stilling basin apron had not been damaged by the Army operations, and since operation at discharges greater than 12, 000 cfs was unusual, it appeared unnecessary to disturb the present apron which is 3 feet thick. Lengthening the apron could not possibly increase the discharge range over which a jump would form because of the large deficiency in tail-water depth. The problem was, therefore, resolved into finding a means of dissipating the energy in the 20, 000-cfs dischar.ge on the present apron by adding sills and baffle piers.

TEST PROCEDURE

The plan to be followed was to place baffle blocks on the exist­ing apron in such a manner that a large portion of the dissipation of energy for discharges up to 20, 000 cfs would occur on the apron. Sev­eral schemes were tried, but only two of the most promising will be discussed here:

Scheme 1. --The use of comparatively large baffles to force a roller to form in front of the baffles, and additional baffles to step down the water surface from the high levels immediately above the large baffles to the tail-water elevation.

Scheme 2. --The use of blocks distributed over the entire apron to produce turbulence of the water with no intention of forming a significant roller or jump in this case.

Scheme 1

It was anticipated that baffles should be fairly large to force a roller to form with a shallow tail-water depth. A single row of baffle blocks 5 feet high., 3 feet 9 inches wide, and spaced 2 feet 11 inches apart was first tried (Arrangement 1., Figure 9). These baffles did not come up to expectations, as can be inferred from Photograph 10-A. The water was thrown up and over the large baffle blocks instead of forming a jump.

The baffles were then increased to 7. 5 feet in height with the same width and spacing as in the previous test. One row of baffle blocks was again used, positioned on the apron as shown in Arrangement 2, Figure 9. These blocks produced less disturbance than the former, as can be observed from the photograph on Figure 10-B.

3

Next, an additional row of baffies 5 feet in height, staggered · with respect to the first row of 7. 5-foot blocks, was installed as shown in Arrangement 3, Figure ·e. Figure 11-A shows some improvement over Arrangement 2, , but the downsweep, as water passed over th,e large baffies, was little better than the former arrangement with the single row of large baffies. A plot showing the scour after a 1-hour run at a discharge of 20,000 cfs is included as Figure 12. The maxi­mum depth of scour in this case was 7. 5 feet.

The next test was made with the same baffie arrangment ex­cept that a triangular sill was placed on the end of the apron as shown for Arrangement 4, Figure 9. Photograph 11-B shows the action at a discharge of 20, 000 cf s. There was no visual improvement in over­all performance.·· The gr·ound roller was strengthened by the triangular sill, however, and its effect can be observed from the plot showing scour after a discharge of 20, 000 cfs (Figure 13). A comparision of Figures 12 and 13 shows more scour with the ~nd sill installed, a maximum of 9 feet compared with 7. 5 feet without the triangular end sill.

Scheme 2

The object of Scheme 2 was to distribute smaller blocks over the entire apron length. In this connection, three rows of cubes 3 feet 9 inches on a side, with a 3-foot 9-inch spacing, were placed on the apron as shown in Arrangement 5, Figure 14. The jet of water was thrown up and over the second row of blocks for a discharge of 20, 000 cfs, with little dissipation of energy, as can be observed from Figure 15-A. This indicated that the front row of blocks should be smaller than the others. ·

In the ne~ test five rows of blocks, varying in height from 1 foot 10-1/2 inches at the upstream end to 3 feet 9 inches at the downstream end, were placed over the entire apron as shown in Ar­rangement 6, Figure 14. The action at 20,000 cfs, which is much improved over any arrangement tried, is shown on Figure 15-B. A plot of the scour downstream from the apron, which shows a maximum depth of 8. 3 feet, is plotted on Figure 16. This was by far the best arrangement tried to date. The water surface was quite regular, lack­ing the secondary jumps and rollers in the former tests. There was no apparent roller or jump, but the mixing action in the wake of each block was effective in dissipating energy.

An additional ·set of smaller blocks was finally installed up­stream from the first row at elevation 156. These were found ineffec­tive in improving the energy dissipation. They also proved detrimental in that they produced a backwater effect, causing the conduits leading from the gates to now full.

4

CONCLUSIONS

At this point in the testing, official word was received that the Army had decided to abandon operations at Imperial Dam, and plans for revision of the stilling basin apron were canceled. Testing was therefore discontinued on the basis that Arrangement 6 shown on Figures 14 and 15-B was adequate and little improvement was expected from further testing. This arrangement is recommended as a satisfactory and economical methofor revision which will allow intermittent nows of 20,000 cfs to be passed without endangering the structure.

The. settling basin was repaired, but the stilling pool apron re­quired no repairs for normal sluicing operations. Replacing. of riprap downstream from the apron, however, should.be done. It may be possible to postpone this operation unW deposition of river sediment.. by low dis­charges through the sluiceway, has time to partially refill the large hole. Should the Army return at a future date, revision of the stilling pool apron will be a necessity .

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SECTION A-A

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~.,;. •·:r;;:2o·t,m;t1:w I I !.&,1fJ7 • • ..:t- -- -·--- ~ I 1"'m; ~ ~ ,}: ..... :.,.~;r.;;,.~. ~"4·. ·. -·- ~- ~--_'---50'-o·---------~~R,prap

-ZO'Wooden sheet-pilin9.

REFERENCE DRAWINGS FOR LOCATION OF'SECTIONS.SEE 6E:NE:RAL PLAN DWG. SO·D-Z,,,.. DRAINAGE SYSTEM. SHEET PIUNG AND EMBANKMENTS DW&. 50-0-Z,,5

... ~, l)f/

,;,; !'/-22'-6'-----l Dentated sill. !/ ! ',· ... ~'-ff_,..,,,..-;! . , ------ -""=~ l ---~:'.--~~~'t7.00 , , i~---4--+--------,-1=,.,... ..... .i \ ; / - ' . : . ·,.;,, ~. ·-::: .

,.::--------- -, - --45 -o ------+-- ---33'-fft ---->F-T ;---Zit" ··- --~~/-- ··------4• Minimum af compacted r,w·· ~- .201 Woof!e_n

sheet-pihl}I/

i 9-Z6' 7',fes-·

' ' :.-----------------60-40' Pi7es -- ----------------- ~ SECTION a-o

OEPAATMaNT OF THE INTEII\IOA BUAEAU OF RECLAMATION

GILA PROJECT-ARIZONA

GILA VALLEY CANAL OESILTINGWOAKS BASIN 3 DIVERSION WORKS

PAAT PLAN ANO SECTIONS

I ::~~1;~;:~~;:;;~~~:-i 50-D-?"6

ti,

/ 3 0 -0 C ·;

(ad~jOjOJd ,09)

~-----.,1,a ------------

I I

I I H I \ I I I I I I

I I

I I I I 11 t-\ I \

I I I I

J

llg·i

I I I \

I I I I

\ ' ., e -= 0

... __ -

.. s· 1.,."

, ' N

N ,., !!! .::l

0 ,., ~­;;:; ,_ I

I _, -.., "': ~

.-------------¾ I I I I I I

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I

l I

--i--1 I I

I l

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!!! I I I I

l ' _ _t_

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l

---·l I I

=· "' r ' I

FIGURE 4

(/) ...I a.J

~o a:: 0 o2 ,: ...I

<(

(!) z z 0

- I-I- 0 ...J a.J 0

Cl) !'? (/)

~ q ILi 0 a.J

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>-z <( -o

ILi 31: Cl)

...J <( 0::

...J 0 <(

> 0

I-<(

<(

2 ...J a.J

::c (!) 0

Cl)

16

16 I

z 0 ;: 1·60

cf > UJ .J

. UJ

ll:: UJ 15 .... cf ~ .J -cf ....

B

15, "' -

15 "

,,....-

'

./

v"' ./

V v ...

./ /

v' /

' / ./ I-'-

L

V'" V -./

/ C .,"" 1,-

V"' ,. ""' -L..--" ,/

V V V .... _, _,..,. B v~ ,. i,,' ---~-

V / ---L _,.. -~ L ....

v"" i--- 1,r --- :,.,, / i,---~ / v· ---Tailwater curve-'.:!...--- / ./

---,---~

I

~-

/ I'" V ,. I,-' ..

.... - .. A - Theoretical jump height curve , L---"" 8- Sweep out curve _ .. V C - 85% of the theoretical jump height -/ _,..,..,..

i 10 12 l4 _16 18 DISCHARGE IN THOUSANDS OF CFS.

GILA VALLEY DESILTING WORKS SLUICEWAY STILLING BASIN TAILWATER CURVE

V V

,.. /""

/

/

I/ I-""'"'

20

V

~

...

'Tl

G)

C :::0 l"'I

UI

FIGURE. 6.

A Discharge a. 000 Second-Feet.

B Discharge 12. 000 Second-Feet

SLUICEWAY STILLING POOL - GILA VALLEY DESILTING WORKS

EXJSTING STRUCTURE

FIGURE 7

A Discharge 16,000 Second-Feet

B Discharge 20. 000 Second-Feet

SLUICEWAY STILLING POOL - GILA VALLEY DESD..,TING WORKS

EXISTING STRUCTURE

I ....---Existing sill

,X ~-"'"" ....... ....

\ ... _"-"-. -- .......

-, '

' Q =20,000 CFS I\ 1 Hr. run (model)

\ I\.

"' -

0 20

Left side ----- l of channel _____ Right side

....... ........

"-i.--...,,. ..... .......

~ / ~--' ..,,,. ~ ~ ,,,,- .. -.

" / '-/ ~ ' ''" // / .. -...........

i---..... ....... ·, .,,. / .v V ~

,_ i,..../

.,,.,., .......__ ·- ----

"' "'-"' ~ ~I'-........ / ~ .-

40 -60 80 100

DISTANCE FROM END OF APRON IN FT.

GILA VALLEY DESI L TING WORKS SCOUR PROFILES DOWNSTREAM

FROM EXISTING APRON

./ /

/ /

120 140

0

2

4

6

8

10

12

14

16

I-= u.. z 7: I-Q.. w C

"Tl

G> C ~ rn a,

.!.--[] "t--[J

.L[J "o,

"t--r-------------301

------- ----~----16' a"-----~ ' ' I ' I I I I ·,

~~ I I T I I

-~ I i ' I

'[D .,, __

t-[D _L[[J ·o,

I ·,--

NO. I

o::J _.L[D "o,

"t--,!. __ [J "r--o:::J

-1---[D -f--·co ,!-[[] "t---

t<------------30'-- _ ---------~-----,s' a"-----: ' ' I :i-- I I I

)" I -I :I I I <D I I

_1 I .....

,!.--IT] "f--[I]

*--ITJ =a, -.:, J--

NO. 2

o::=i

{;-[D -.., *--:;-·OJ ., N

CD

r-:- ---151- - ---~- - ----1 s'- ----:- - ---16_~ 811

-- - - ->: j--· l l !

n I"'" - --- -15'-----,fc-- ----JS~----->1,c---9'---M<--7!,?--..i 1'-10-lr-" ""'1 t<- I l : :

... -- ~ ~-1'-311 ., I I I

·c.o 1-· : I ·+ ·in I :

-----i'_ ' '

NO. 3

GILA VALLEY DESILTING WORKS

BAFFLE BLOCK ARRANGEMENTS ON

STILLING BASIN APRON

=~ x-J ' -! ! =er : i ="" ~ ~ LJ )I i~t·

I I --

ls~,01~ I I

~4'-41"1<--·f

N0,4

"Tl

G)

C ::0 I'll

co

FIGURE 10

A One Row of Baffle Blocks 5 feet high Discharge 20,000 Second-Feet

B One Row of Baffle Blocks 7. 5 feet high Discharge 20, 000 Second-Feet

SLUICEWAY STILLING BASIN - GILA VALLEY DESILTING WORKS

SCHEME A

FIGURE 11

A Baffle Blocks 7. 5 and 5. 0 Feet High Discharge 20,000 Second-Feet

B Baffle Blocks 7. 5 and 5. 0 Feet High and Solid Triangular end sill Discharge 20,000 Second-Feet

SLUICEWAY STILLING BASIN - GIL~_VALLEY DESILTING WORKS

SCHEME A

,-·Ex is ting sill 1--

X ~ \ Q= 20,000 CFS

1hr. run CModel

0

Left side ----t of channel ----- Right side

~-----~, - ~ ..... ......... - ---' r---.~ ---=--: r------- .. - ,----------' - -", --~ i----_ \ ~ -' ~---7

" I',.._

20

' " -· - -

_,,/ I,"

40 60 80 100 DISTANCE FROM END OF APRON IN FT1

GILA VALLEY DESILTING WORKS SCOUR PROFILES DOWNSTREAM FROM APRON

US I NG BA FF L E AR RAN GEM EN T N 0. 3

------ -----

120 140

0

2

4

6 ... IL

z 8~

I­Q. UJ

100

:!! Ci)

C ::0 m

I\)

,-Existing sil I Left side ' ' ' )< rTrlongulor ___ -- i of channel ' '~ 1 end sill _______ Right side

~ ~~ ~~ ~,

' ' ~-\ '

'\\ [\ V'"" .. Q= 20,000 CFS L..------"' "~-I HR. RUN (Model) \ ----~~ ' ·,~ i---- - -- -

'--- .,,,.,.,- v::=--- --. ' .,,.

/ ... -', .....

-· ~ i,.. -- ---~ r-:::..:::,__ -- -

0 20 40 60 80 100 120

DISTANCE FROM END OF APRON IN FEET

GILA VALLEY DESILTING WORKS SCOUR PROFILES DOWNSTREAM FROM APRON USING BAFFLE

ARRANGEMENT NO. 4 WITH A TRIANGULAR END SILL

.,, Ci)

C ;:u l'TI

-I (II

0

2

~ ~ ----- ,._ __

.... lu

4 lu LI..

z 1-

6 X .... Q. lu C

8

10

140

FIGURE 14

-.--D a:::::i

[] D 3•-9•!..._..,. _Lo [] [] -r·o [] 3'-9'!.--... _j __ a:::::J

~------------------------. --------- eo'-4"--------------------------~---1&' a" --- ~ 3'-9"---:f-- I i !

--T __ D a::::::::J

[] [] 3'-9!!--~

[I] [] .. ro [I] 3 -9---...

L [CJ

N0.5

D D CJ

D D D

I I S I I CO I I .,I I I

~ i : _J ___ , :

I I

~---------------------------ao'-4!?.-------------------------------------+---1&'-a'!!---·>1 11-10½1 : I f

- ...... ..,.- I : I o

N0.6

GILA VALLEY DESILTING WORKS

BAFFLE BLOCK ARRANGEMENTS ON

STILLING BASIN APRON

I I I o I o I o . . I I

I

FIGURE 15

A Three Rows of Blocks 3. 75 feet high Discharge 20. 000 Second-Feet

B Five Rows of Blocks 1. 875 to 3. 75 Feet High Discharge 20. 000 Second-Feet

RECOMMENDED DESIGN

SLUICEWAY STILLING POOL - GILA VALLEY DESILTING WORKS

SCHEME B

:-Existing sill I \

\

~ '~

i\.

\, \

0=20,000 CFS I Hr: Run (Model)

Left side ---iof channel -----Right side

I -r---

/ _.--- - -~~ ... ~ ::::::-- -7 ' ..... -----

"' '

20

- / .......... --.............. ___

7- ---- ---- --------

40 60 80 100 DISTANCE FROM END OF APRON IN FEET

GJLA VALLEY DESILTING WORKS SCOUR PROFILES DOWNSTREAM FROM APRON

USING BAFFLE ARRANGEMENT N0.6

--........... ........__

~ ~ -------

--~------ --

120 140

0

2

4

... w

6 W u.

8 z

:I: ... Q.

10~

,, Q C ::0 ITI

0,