CORPS OF ENGINEERS, U.S. ARMY

MISSISSIPPI RIVER COMMISSION

SPILLWAY FOR BENBROOK DAM

CLEAR FORK OF THE TRINITY RIVER, TEXAS

MODEL INVESTIGATION

TECHNICAL MEMORANDUM NO. 2-269

WATERWAYS EXPERIMENT STATION

VICKSBURG, MISSISSIPPI

MRC-WES-125-MARCH 49

MARCH 1949 FILE COPY RETf7 R .V TO

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1. REPORT DATE MAR 1949 2. REPORT TYPE

3. DATES COVERED 00-00-1949 to 00-00-1949

4. TITLE AND SUBTITLE Spillway for Benbrook Dam, Clear Fork of the Trinity River, Texas:Model Investigation

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S) 5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) U.S. Army Corps of Engineers,Waterway Experiment Station,3903 HallsFerry Road,Vicksburg,MS,39180

8. PERFORMING ORGANIZATIONREPORT NUMBER

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S)

11. SPONSOR/MONITOR’S REPORT NUMBER(S)

12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release; distribution unlimited

13. SUPPLEMENTARY NOTES

14. ABSTRACT A model study of the spillway for Benbrook Dam, Clear ,Fork of the Trinity River, Texas, was conductedon a 1:60-scale model for the Galveston District Office, CE. The study was concerned primarily w~th theinvestigation of flow conditions resulting from the concentration of flow over the low weir section located inthe center of the spillway and the determination of the adequacy of training walls to confine all flow to theexit channel. In the model tests flow in the approach channel and over the spillway of original designappeared satisfactory. However during the course of the tests, flow conditions at the abutments of both thehigh and low weir sections were improved slightly by rounding the abutments to the same curvature as theupstreanl portion of the high weir. Head-discharge relations as determined on the model were in closeagreement ’with computations. Downstream from the spillway it was observed that the concentration offlow through the low weir during periods of high discharge created standing waves which diverged fromthe junction of flow over the two weir sections to a point about 575 ft downstream and impinged on the leftside of the exit channel and right training wall causing considerable water to splash over the wallo Nomeans of eliminating or reducing the height of these waves, that was considered economically justified, wasfound in the model tests. To protect the sides of the exit channel from the wave action, the tests indicatedthat the left training wall should be extended about 140 ft. The length of the right training wall appearedadequate. High velocities were measured i~ the exit channel for all conditions of discharge. For lowdischarges flows were confined to the center of the channel.

15. SUBJECT TERMS

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT Same as

Report (SAR)

18. NUMBEROF PAGES

50

19a. NAME OFRESPONSIBLE PERSON

a REPORT unclassified

b ABSTRACT unclassified

c THIS PAGE unclassified

Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18

CONTENTS

ABSTRACT

PART I: INTRODUCTION • .

PART II : THE MODEL •

PART III: DESCRIPTION OF TESTS

Types A and B (Original) Design Alternate Designs Spillway Abutments • • •

PART DT: DISCUSSION OF RESULTS •

PLATES 1 - 17

. . . .

. 1

. 8

12 19 22

27

ABSTRACT

A model study of the spillway for Benbrook Dam, Clear Fork of the

Trinity River, Texas, was conducted on a 1:60-scale model for the Gal­

veston District Office, CE. The study was concerned primarily with the

investigation of flow conditions resulting from the concentration of

flow over the low weir section located in the center of the spillway and

the determination of the adequacy of training walls to confine all flow

to the exit channel. In the model tests flow in the approach channel

and over the spillway of original design appeared satisfactory. However,

during the course of the tests, flow conditions at the abutments of both

the high and low weir sections were improved slightly by rounding the

abutments to the same curvature as the upstream portion of the high weir.

Head-discharge relations as determined on the model were in close agree­

ment with computations. Downstream from the spillway it was observed

that the concentration of flow through the low weir during periods of

high discharge created standing waves which diverged from the junction

of flow over the two weir sections to a point about 575 ft downstream

and impinged on the left side of the exit channel and right training wall

causing considerable water to splash over the wall. No means of elim­

inating or reducing the height of these waves, that was considered eco­

nomically justified, was found in the model tests. To protect the sides

of the exit channel from the wave action, the tests indicated that the

left training wall should be extended about 140 ft. The length of the

right training wall appeared adequate. High velocities were measured in

the exit channel for all conditions of discharge. For low discharges

flows were confined to the center of the channel.

SPILLWAY FOB BENBBOOK DAM

CLEAB FOBK OF THE TBINITY BIVEB, TEXAS

Model Investigation

PABT I: INTRODUCTION*

1 . A comprehensive plari for flood protection and wat er conscrva-

tion on tho Trinity Biver and tributaries calls for modifications of

existing floodways at Dallas and Fort Worth, Texas, modification of the

existing Garza Dam and Beser-

voir, and the construct ion of

five now res~rvoirs in the

upper basin t o be operated in

conjunction with six existing

r eservoirs . In the interest

of navigation, the plan also

provides for canalization of

Trinity Bivcr from the ~outh

to Fort Worth, with a channel

across Galveston Bay to the

Houston Ship Canal . Figure 1

I l

\

l L A H 0 M AI'

ARK

!

Figure 1. Vicinity map

sb,ows the location of the proposed r .esorvoirs and floodways involved in tho

over-all plan .

2. Benbrook Dam, tho spillway of which is tho subject of this

* Information on the prototype was obtained from maps and data f urnished by the District Engineer, Galveston District, Cor ps of Engineers.

2

report, is one unit of the comprehensive plan for flood control and water

conservation. The dam is proposed for construction on the Clear Fork of

the Trinity River, approximately 10 miles southwest of Fort Worth, Texas.

In addition to serving as one unit in the comprehensive plan for the

Trinity River basin, it will, when operated in conjunction with throe

existing reservoirs on the West Fork of the Trinity River, provide flood

protection for the city of Fort Worth. The reservoir created by thedam

will have a surface area of 10,303 acres at maximum pool stage and an area

of 3,769 acres at conservation pool level. A total of 322,000 acre-ft

of storage will be available for flood-control purposes. The dam itself

will be a rolled-fill earth embankment containing over six million cubic

yards of material and will have a length of 9,200 ft and a height above

stream bed of 130 ft.

3. Located in the right abutment of the dam will be an outlet

works consisting of an intake structure with two slide gates 6.5 ft wide

by 13 ft high, a circular conduit 13 ft in diameter and about 600 ft

long, and a stilling basin (figure 2). Capacity of the outlet vrorks at

a pool elevation of 710* will be 7,340 cfs; capacity at maximum pool

elevation of 741 will vary from about 8,000 cfs to 8,600 cfs, depending

on taihrater effect caused by spillway discharge. To provide passage of

low flows, two welded steel pipes, 30-in. outside diameter, with inlets

at the intake structure will bo placed parallel to the main conduit on

either sido and will empty into tho outlet works stilling basin. Dis­

charge through oach pipe will be controlled by slido gates located in the

* All elevations are in feet above mean sea level.

I

Figure 2 . Proposed project plan for Benbrook Dam

4

intake tower . Combined capacity of the two pipes will be about 184 cfs

at a pool elevation of 710 and about 196 cfs at a pool elevation of 741 .

4. The spillway, located on the left bank west of the main nam,

will consist of an approach channel 500 ft wide, an uncontrolled ogee

crest at elevation 724 having a low section in its center at elevation

710J and an exit channel (figures 2 and 3 and plate 1). The portion of

the ogee crest at the high elevation will be 400 ft in H'idth while the

portion of the crest at the low elevation will be 100 f t in width. The

low- level crest is designed to pass a flow equal to the downstream chan-

nel capacity and the high- level crest is designed to pass greater flows .

The two-level spillway design utilizes effectively the valuable storage

area existing between the high and lmv spillway levels . The spillway

~~>,, / ) \ l~ f ..

·~~

SCALE IN fEET

s o m m

SECTION ALONG CENT ERLIN E SCALE IN r££T'

2() e r n op

Figure 3~ Spil lway pl an and pr ofil e

5

design flood, when routed. through tho reservoir with tho pool at elevation

710 at tho beginning of tho flood and the outlet works closed, produced

maximum heads of 31 ft on the 100-ft-wide low weir and 17 ft on tho high

weir with a combined discharge of 172,000 cfs. With the pool at spillway

crest stage (olovation 724), tho low weir section will have a capacity of

18,000 cfs.

5. Tho profile of tho high weir, designed for a head of 17 ft,

will follow the curve of Y = o.o44987Xl.85 downstream from the crest

line, while upstream from the crost the profile will bo shaped to arcs

of circles with radii of 8.5 ft and 3.4 ft. Tho profile of the low

portion of tho spillway, designed for a head of 31 ft, will follow tho

curvo of Y = 0. 0270X 1. 85 downstream from tho crost, while upstream from

tho crost tho profile will be shaped to arcs of circles >·ri th radii of

15.5 ft and 6.2 ft. Tho horizontal entrance curve of the low weir will

bo identical to the compound curvo of the upstream vertical section.

Height of tho low and high weirs above tho approach channel will bo 10

ft and 24 ft, respectively.

6. Tho spillway will discharge onto a paved apron approximately

159ft long, joining the exit channel at Sta 12+10. Tho paved section

will act as a transition from tho rectangular cross section at tho too

of tho spillway to tho trapezoidal cross section of tho exit channel.

Concreto training walls 11 ft in height will confine the flow in tho exit

channel for a distance of about 500 ft downstream from the spillway. The

left training wall will terminate at Sta 15+60, whilo tho right training

wall will be continued to Sta 22+60. The longer w·all on tho right was

desired to insure the prevention of right bank erosion 1.rhich 1.rould parmi t

6

spillway flow to attack the toe of the dam. A pilot channel 100 ft in

width will connect the exit channel with the old river channel about

7,000 ft below the dam. It is expected that the pilot channel will ulti-

mately scour to the full width of the exit channel and to the surface of

the underlying rock. No forn1 of energy dissipation is proposed, although

a hydraulic jump is expected in the exit channel at about Sta 43+50.

7. The following data pertain to the structural and hydraulic

features of the spillway:

a. Structural

Hidth Elevation of crest Maximum height

b, Hydraulic

Maximum head Maximlinl pool elevation Capacity (pool elev 724) Combined capacity

High Weir

1+00 ft 724 24 ft

17 ft 741

172,000

Low Weir

100 ft 710

10 ft

31 ft 741

18,000 cfs cfs

8. In order to check certain phases of the design not readily sus-

ceptible of analysis by theoretical means, a model study was deemed neces-

sary, Specifically the purpose of the model study was to (a) verify

spillway discharge coefficients, (b) investigate any objectionable flow

characteristics that might result from the concentration of flow through

the low portion of the spillwayJ and (c) determine the top grade of the

right training wall to prevent overtopping. Authority to undertake the

model study was granted by the Chief of Engineers in the second indorse-

mentJ dated 9 April 1947, to the basic request, dated 17 March 1947, of

the District Engineer.

9. During the course of the model study, Mr. J. H. Doun1a of the

Office, Chief of Engineers, t~. H. W. Feldt of the Southwestern Division,

and Messrs. W. E. Wood, A. Martelli, K. V. Speeg, and G. W. Demeritt, of

the Galveston District, visited the Waterways Experiment Station at inter­

vals in an advisory capacity. The model study was conducted in the Hy­

draulics Division of the Waterways Experiment Station during the period

May to November 1947 by Messrs. R. G. Cox and S. B. Burns, under the

general supervision of Messrs, F. R. Brown and T. E. Murphy.

8

P .ART II : THE MODEL

10. The model of the spillvray for Benbrook Dam 1vas constructed in

a brick flume to a linear- scale ratio of 1 :60 and reproduced about 900 ft

of the reservoir and approach areas, the entire spillway width, and about

2, 000 ft of the exit channel (figures 4 and 5) . The entire surface of

the model was mol ded in cement mortar to sheet-metal template . Care vras

exercised to obtain the proper shape of all surfaces, and those ar eas

which were to be paved in the prototype were continuously dusted with

cement and troweled until the practical limit of smootlmess was attained .

11. Water used in the operation of the model was supplied by a

circulating system made up of a large supply sump, numerous pumps, a con­

stant pressure header line, and a gravity fl0\1 return line. Two venturi

Figure 4. 1:60-scale model of Benbrook Dam spillway

b;j ..... ~

~ ~ <D

i \)1

:s: 0 p. <D ~

~ ~ ~ l!! :;; .. 0 i1 .

~ ~ a "

10 ~ g

10

meters located between the model and header line -vrere used to measure the

quantity of flow. From the supply lines flow spilled into a headbay where

it was stilled by baffles prior to its entrance into the model. After

passing through the model, the water flowed through the return line back

to tho sump. Steel rails, set to grade along either side of the model,

provided a datum plane for the use of measuring devices. \-later- surface

elevations were measured both by means of portable point gages (mounted

on an aluminum beam supported by the steel rails) and by means of piezom-

eters. Velocities were measured by means of a pitot tube. Surface cur-

rent clirqctions 1mre traced with confetti.

12. The accepted equations of hydraulic similitude, based upon

the Froudian relationships, were used to express the mathematical rela-

tionships between the dimensions and hydraulic quantities of the model

and the prototype. The general relationships existing for the Benbrook

model are presented in the following table:

Dimension Scale Ratio

Length Lr = 1:60

Area Ar = L 2 r = 1:3600

Velocity vr 1/2 = 1:7.746 = Lr

Discharge Qr = T 5/2 .ur = 1:28,886

Roughness nr = L 1/6 r = 1:1.989

13. Measurements of discharge, water-surface elevations, current

directions·, pressures, and velocities can be transferred quantitatively

from model to prototype by means of the above scale relationships. The

inability of the small-scale model to reproduce assumed prototype rough-

ness values might result in a slight decrease in spillway efficiency at

11

low heads. Slight variation from the theoretical model roughness should

have little or no effect on spillway efficiency at high or intermediate

heads or on general hydraulic performance of the spillway.

12

PART III: NARRATIVE OF TESTS

Types A and B (Original) Designs

14. As explained in paragraphs 4- 7 the spillway for Benbrook Dam

consisted of a 400- ft section with crest at elevation 724 and a 100-ft

section located in the center with crest at elevation 710 (figure 6).

Design heads for the two sections were 17 and 31 ft, respectively. In

order to align the vertical upstream faces of the two spillway sections,

the crest of the high weir was located at Sta 9+96.79, while that of the

low weir was a slight distance downstream at Sta 10+00 .74 (figure 3 on

page 4 and plate 1) . The abutments of both weirs were originally in­

stalled in the model with right angle corners (type A) and were

Figure 6. Upstream view of original two-level spillway design

13

subsequently rounded (type B) to conform to the same compound curve as

the upstream portion of the crest shape. The depressed center section

of the spillway extended for about 200 ft dovmstream until it joined the

exit channel. The exit channel was sloped on a 1.45 per cent grade in

the direction of flow, and in cross section consisted of a 100-ft-wide

level channel located in the center with sides extended on a 1-on-40

slope to an intersection with the side training walls. The slight de-

pression in the center of the exit channel was desired to confine low

spillway discharges to the center of the channel. Training walls on

either side of the spillway extended to Sta 15+60 on the left and to Sta

22+60 on the right.

15. Measurement of head-discharge relations of flow over the

spillway crest revealed that model and computed results were in close

agreement throughout the range in discharge (plate 2) . The design dis-

charge of 172,000 cfs was passed at a pool elevation of 741.2 as compared

to a computed elevation of 741.0. For the design discharge through the

low portion of the spillw-ay (18,000 cfs) there was no detectable differ-

enco between model and computed results. Computations were based on the

formula Q = CL 'n3/2, 1-rhere

Q = discharge in cfs

C = coefficient of discharge which varied from 3.09 at zero head to 3.90 at the design head for the high weir sec­tion and from 3.09 to 3.70 for the low weir section

L'= effective length= L- 0.04H

H = head on each particular weir section

16. Flow conditions in the approach channel upstream from the 1wir

were generally satisfactory for discharges up to about lQO,OOO cfs

a. Discharge 18,000 cfa; pool elevation 724.0 ft

b. Discharge 172,000 cfa; pool elevation 741 . 2 ft

Figure 7 . Flow patterns in the approach channel of original design for two discharges (confetti streaks show direction

and relative velocity of surface currents)

15

(figure ?a) . For the maximum discharge of 172,000 cfs velocities were as

high as 11 ft per sec upstream f rom the low weir and 9 ft per sec along

the rolled-fill earth embankment adjacent to tho spillway (figure ?b) .

Plates 3-6 present approach channel velocities for the selected dis­

charges .

17. With the exception of conditions at the abutments of both

weirs, flow over tho spillway was satisfactory. At tho abutments of each

of tho two weir sections it was observed that, unless tho corners wore

rounded, considerable turbulence resulted (figure 8) . Tho r ounding of

the corners to tho same curvature as tho upstream portion of the high

weir caused f low to follow mor e nearly the wall alignment, resulting in

smoother f l ow conditions. Pr essure measurements procured on tho side

walls of tho low weir ar c also indicative of tho improved performance ef­

fected by rounding the corners to provide a smoother transition o: flow

(plate 7) . With the originally-installed shar p corners (type A) and a

maximum discharge of 172,000 cfs, negative pressures on the side of tho

l ow weir section adjacent to the upstrerun face were as high as -14 ft of

water. The r ounding of the corners reduced tho magnitude of the negative

Sharp cornered abutment (type A) Rounded abutment (type B)

Figure 8 . Flow around right spillway abutment; discharge 172,000 cfs

16

pressure to -5 ft of water. No pressures wore measured on the side train­

ing walls of the high weir.

18. Flow conditions and water-surface profiles over the spillway

and through the exit channel for discharges of 172,000, 100,000, and

18,000 cfs are shown on figures 9-11 and plates 8-11. Concentration of

flow over the low weir section in the center of the spillway resulted in

two diverging waves in the exit channel for all discharges. Increased

discharge had little effect on the pattern of the waves, but did increase

the height. For discharges in excess of 100,000 cfs the energy of flow

was such as to cause the diverging waves to extend entirely across the

exit channel and attack the side banks at about Sta 16tOO. At the right

bank where the wave encountered the training wall some overtopping of

the wall resulted. On the left bank the wave extended downstream beyond

the end of the training wall at the maximum discharge and impinged di­

rectly on the 1-on-2 side slope of the channel. In general, the stand­

ing waves began at about Sta 10+06 at the downstream edge of the low weir

and terminated at about Sta 16tOO on the sides of the channel. The

water-surface profiles presented indicate the conformation of the waves

as they continued downstream from the spillway. Although the profiles

show sufficient freeboard on the training walls for the maximQm discharge

of 172,000 cfs, wave action caused some overtopping of the right wall at

about Sta 16tOO. Overtopping of the right wall creates an undesirable

condition in that drainage from the area. at the right wall is directly

toward the toe of the dam.

19. As mentioned previously in the report, no form of energy

dissipation was planned below the spillway. As a result, flow from the

a. Discharge 172,000 cfs; pool elevation 741 .2 ft

b. Discharge 18,000 cfs; pool olovation 724.0 ft

Figure 9. Flow conditions over original design spillway for two discharges

Discharge 172,000 cfs

Discharge 100,000 cfs

Discharge 18,000 cfs

Figure 10. Flow conditions in original design exit channel for three discharges

19

1 h ton

Discharge 172,000 cfs

Discharge 100,000 cfs

Figure 11. Wave action against right exit channel wall of original design for two discharges

spillway continued unchecked through the exit channel at sub- critical

depths. Velocities recorded ranged from 25 to 50 f t per sec at the maxi-

mum discharge to 19 to 45 ft per sec at a discharge of 18,000 cfs (plates

3-5) . Goodland limestone material will form the floor of tho exit chan-

nol from Sta 12+10 to 50+00. It is believed by tho District Office that

the erosional resistance of this material is such that paving of tho exit

channel will not be required.

Alternate Designs

20 . In an attempt to eliminate or reduce the standing waves

20

resulting from concentration of flow over the low weir during high dis­

charges, numerous alterations to the spillway and area immediately down­

stream w·ere investigated. Although it was believed that a smooth tran­

sition of flow could be effected from the spillway to the exit channel

by a deeper channel through the center of the exit area, the District

Office held that such a channel would not be economically justified.

Consequently, efforts to improve conditions were confined for the most

part to the spillway proper. Initial efforts were directed toward se­

curing more uniform flow distribution in the exit area by increasing

water-surface elevations below the high weir to parallel flow issuing

from the low weir section (types C and D designs, plates 12 and 13).

A similar attempt was made to lower water-surface elevations over the

low weir to agree with those over the high weir by moving the crest line

of the low weir upstream (type G design, plate 14).

21. Test results revealed similar flow conditions for all alter­

ations investigated. The diverging vrave below the low weir appeared to

be unchanged in height, although the point of intersection of the wave

and the training wall moved slightly upstream or downstream from its

original position. An increase in the level of the downstream portion

of the high weir raised water-surface elevations as anticipated but in

so doing also raised water-surface elevations belowthe low weir an

equal amount (plate 15). Relocation of the low weir 3.95 ft upstream

from its original position lowered water-surface elevations slightly,

but inasmuch as the distribution of flow· across the spillway was un­

changed the diverging waves were still present as in the original design

(figure l2). Alterations to the warped concrete apron immediately below

Figure 12 . Discharge 172,000 cfs; low weir moved 3.95 ft upstream, type G

Figure 13. Discharge 172,000 cfs; low weir with 1-on-l side slopes, type I

22

the spillway or variations- in the side slopes of the low weir (type I)

were all unsuccessful (figure 13).

22. In one series of tests the width of the low weir was varied

from 100 ft to a minimum of 22 ft. It was found th~t a:s the width of

the low weir was reduced the height of the wave in the exit channel de­

creased. For example, at Sta 12+00 the wave height was decreased from

6 ft for a width of 100 ft to 3.5 ft for a width of 22 ft. However,

sufficient improvement was not obtained to -vrarrant the decrease in

spillway capacity.

23. Since no solution to the problem appeared evident with a

single low weir section located in the center of the spillway, efforts

were directed toward improvements by relocation of the low weir. For

one test the entire low weir was moved adjacent to the right abutment

(type H) while in another test a low weir 50 ft in width was located

50 ft to the left of the right abutment (type J). If it appeared feasi­

ble to relocate the low weir, it was thought that perhaps two 50-ft

sections located near the abutments might be satisfactory. However, in

each case, resulting flow conditions were unsatisfactory (figures 14

and 15). The wave from the right side of the low weir overtopped the

right training wall, while the wave from the left side of the low weir

was similar to that observed with other designs installed.

Spillway Abutments

24. As explained previously the spillway was connected to the

rolled-fill earthen embankment on each side by concrete nonoverflow sec­

tions approximately 125 ft in length. It was the opinion of the District

Figure 14. Exit channel flow with 100-ft low weir at right abutment; discharge 172,000 cfs, type H

Figure 15. Exit channel flow with 50-ft low weir fifty feet north of right abutment; discharge 172,000 cfs, type J

24

Offico that some monetary saving could be effected by extending the oarth

embankment sections to the spillway, thus eliminating the concreto sec-

tiona . This procedure would require training walls at each abutment t o

protect the embankment sections from erosion resulting from spillway

discharges. To expedite testing a separate type of training wall was

installed at each abutment, thus permitting the investigation of two

type walls simultaneously. Unless considerable improvement in hydraulic

conditions could be effected, the maximum length of training wall that

could be economicall y justified was about 30 ft .

25. At the right abutment the use of a vertical training wall

~~th a top elevation of 747 and a length of 120 ft was investigated.

The length of wall was successively reduced from 120 ft to 60, 30, and 15

ft. In addition, one test was made with the 6o- ft wall at an angle of

45 degrees to the axis of the dam. For all tests the maximum discharge

was used. No actual change vas made in the location of the earth em-

bankmont in the model . Test results revealed that, although the use of

a training wall reduced currents in the area to the rear of the wall,

considerable turbul ence existed on the spillway side of the wall

120-ft wall 15- ft wall

Figure 16. Flow conditions at spillway with vertical training wall at right abutment; discharge 172,000 cfa

25

(figure 16). As the length of the wall was decreased, conditions on the

spillway side improved but velocities behind the wall increased. Loca-

tion of the wall at an angle of 45 degrees to the axis of the dam result-

ed in less favorable conditions than those existing with walls paralleling

the approach channel.

26. At the left abutment a sloped training wall paralleling the

channel was investigated. The wal l extended upstream for a distance of

about 130 ft and sloped from elevation 747 at the spillway to the eleva-

tion of the approach channel. The earthen embankment was continued to

the rear of the i-IB.ll. Tests indicated considerable turbulence at the

junction of the flow along the earthen embankment and the high-velocity

flow approaching the spillway . Velocities recorded over the embankment

section adjacent t o the wall ranged from 6 to 9ft per sec {plate 16).

In an attempt t o r educe velocities over the embankment section the wall

was extended above the elevation of the eribankment in successive tests

by 3, 5, and 7 ft. It v~s observed that, although a slight reduction in

velocity was effected by use of a 3-ft parapet wall, succeeding increases

in height had little effect (plates 16 and 17). However, each increase

Wall with no parapet Wall with 3- ft parapet

Figure 17 . Fl ow conditions at spi llway with trai ning wall and with and without parapet installed at left abutment; discharge 172,000 cfs

26

Hall with 5-ft parapet Wall with 7-ft parapet

Figure 18. Flow conditions at spillway with training walls and tvro heights of parapet installed at left abutment ; discharge 172,000 cfs

in height of wall increased the area of turbulence adjacent to the left

training wall (figures 17 and 18). Comparison of flow conditions at the

spillway abutments, with the earthen embankment extended, with flm-1 cor:-

ditions observed with the original design in place (figures 8 on page 15,

and 17 and 18) reveals that the use of the rounded abutment of original

design resulted in improved flow conditions over the spillway.

27

PART IV: DISCUSSION OF RESULTS

27. The test results described in the preceding paragraphs sug­

gested the retention of the original spillway design with a few revisions

to the training walls. While the original design is not ideal hydrauli­

cally, the benefits to be obtained, both hydrologically and economically,

are judged by the Galveston District to be sufficient to justify the sac­

rifice of hydraulic perfection. Although improved conditions could have

been secured by the use of a constant-level spillway or the excavation

of a deeper exit channel immediately below the low weir section, these

revisions were not considered necessary. The two-level spillway was de­

sired because the low weir permitted flood releases without exceeding

the capacity of the Fort Worth floodway downstream while the high weir

provided maximum storage! for flood control. Excavation of the exit chan­

nel immediately below the low weir was not permitted, since extra rock

excavation would be involved resulting in a more costly structure. Al­

though not investigated, it is believed that more favorable exit channel

conditions could have been obtained by a deeper central channel -vrhich

would have permitted an easier transition of flow from the low weir to

the full width of the exit channel. Consideration also -vras given to the

use of a considerable number of small openings, but this idea was aban­

doned because of extra construction costs and the possibility of the

small openings collecting debris.

28. Tests of the Benbrook spillway indicated that bottom veloci­

ties in the approach cham1el immediately upstream from the low weir sec­

tion and over the embankment along the upstream face of the nonoverflow

28

sections were in the range of ll to 9 ft per sec, respectively. These

velocities are sufficient to warrant the use of riprap to insure protec­

tion from erosion. Velocities in the approach channel upstream from the

high-level weir were about 6 ft per sec. Attempts to effect some monetary

saving by extending the earth embankment sections to the spillway, thus

eliminating the concrete nonoverflow sections, were unsuccessful. The

training walls necessary at each abutment to protect the earth embankment

section would be more costly than the originally-proposed concrete sec­

tions.

29. The close agreement between the spillway rating curve computed

by the Galveston District and that developed during the model study indi­

cates the reliability of assumed discharge coefficients. The rounding

of the abutments of the high, and low weir sections provided improved

flow conditions over those observed with square corners. A considerable

reduction in the negative pressures on the side walls of the low weir

section also was effected by use of the rounded walls.

30. Discharges over the low weir section at pool elevations below

724 were satisfactory. Flow was confined for the most part to the cen­

ter of the exit channel and where discharges were sufficient to spread

to the full channel width highest velocities were recorded in the center

of the channel. Velocities in the exit area for a capacity discharge of

l8JOOO cfs ranged from 19 to 45 ft per sec. However, at the maximum

design discharge of 172J000 cfs over the entire spillway width flow con­

ditions were not good. The concentration of flow in the center of the

spillway resulted in the formation of high standing waves which diverged

from tho side walls of the low weir section to the full width of the

29

exit channel about 525 ft downstream. Where the wave struck the vertical

wall on the right side of the channel, cons.iderable flow splashed over

the top of the wall; at the left side no overtopping of the bank was noted.

To insure protection of the banks from the wave action noted above, the

left training wall should be extended about 140 ft to Sta 17+00 and riprap

placed behind each of the walls at about Sta 16-t-OO. For discharges of

100,000 cfs or below no overtopping of the training walls by wave action

was observed. Bottom velocities in the exit area ranged from 25 to 50 ft

per sec for the maximum discharge of 172,000 cfs and from 20 to 50 ft per

sec for a discharge of 100,000 cfo. Velocities in the center of the chan­

nel were from 5 to 10 ft per sec more than along the side walls at the

maximum discharge, while velocities were about 10 to 15 ft per sec more

in the center for a discharge of 100,000 cfs.

PLATES

1)

r ~ JTI

8 ! ! .

LEH ~ : ~ ABUTMENT .

~ ,_.,1 .. ~

I'CC CUA8 EL£V 74.U» OT

£DCC OF II!! APPPOACH CHAHHCL ~Ill$ i "'1: ...

.... a ~! . I ::~

~I a:•,.

l I i;1 v:~

IIU

•. ~ / H IGH W EI R.

~§ )' ...

"' 0

~s .

<!"' Q: . ~ ~ ~ • X

< 1.~

I ~~

lll 'f~ liR . ~ V::t ~ · :;:8

-'--· II'•;; -

L-- CENTERLINE OF" SPILLWAY

HALF PLAN SCALE IN F'EET

0 •

~ s;

~

~

=

SECTION ALONG CENTERLINE SCALE IN FEET

r

NOTE : ORIGINAL DESIGN DESIGNATED TYPE A AND TYPE B

WHERE: I. TYPE A CONSISTED OF ORICIN.t.L OESICN WIT H ABUT!AENTS AND LOW WEIR H.t.VINC 90-DECREE CORNERS.

2. TYPE B CONSISTED OF ORIGINAL DESIGN WITH ABUTMENTS AND LOW WEIR AOUN0£0 AS SHOWN ABOVE.

SPILLWAY CREST DETAILS ORIGINAL DESIGN -TYP E 8

742 742

..::::. v / fo"""

..,...,. 736 736 v v

CIIRI £--. ~ v

MODE

734 734

f/ ~

/ ~

NOTE: RATING CURVE I S fOR HIGH AND LOW WEI RS.

COMPUTED DISCHARGE CURVE IS BASED ON r-- 730 730 v ''CG COM PI TED URVE f ORM ULA; Q •CL' Hf AND U • L-0.04 H _, _, U) U) ~LVES ON D ISTRIC ) WHERE L• LENGTH Of HIGH OR LOW WEIR ::E ::E ~ C• WEIR . COEI'F"ICIENT t-- 1-1- v H • HEAD ON WEIR ... ... ... ... "" "" .L 726~ ~ 726

I z z 0 0

t= t= _,.. HIGH WEIR CRES -ELE 724.0 ~ ~

I ... ... .J .J ... ...

722 . 722 v I COMl !NED I-LOW

I I

71& 718'

1 Lf LOW WEIR

_L I

I 714 714 160 180 200 220

~ OW~ IR q_~T-E EV l 0.0 710 SPILLWAY RATING CURVES 7100

20 40 60 eo 100 120 140 DISCHARGE IN 1000 CF'S

...

1--~«;.. .... ~

,~ STANDING WAVE

-;:"/ ..za.- .,I.U.- ~ .»Z---

~1--'~--. -_ -! --.-. ---~ ~-----k_ . 1-_---L_l====---~~----1 CENTERLINE OF SPILLWAY_./

TEST CONDITIONS

DISCHARGE POOL ELEVATION ORIGINI\L. DESIGN

172,000 CFS 741 .20 FT

NOTE: VELOCITIE~ ARE IN PROTOTYF>E FEET PER SECOND AND ARE ONE FOOT OFF THE BOTTOM •

" \ " \

CHANNEL VELOCITIES AT HIGH DISCHARGE

SCALE I N FEET

·;. . .: t

1J r ~ rn ~

TEST CONDITIONS

DISCHARGE POOL ELEVATION. ORIGINAL DESIGN

100,000 CfS 735.30 f'T

NOTE: VELOCITIES ARE IN PROTOTYPE fEET PER SECOND AND ARE ONE rooT orr THE BOTTOM.

't. \

CHANNEL VELOCITIES AT MEDIUM DISCHARGE

SCALE IN fEET ... ,.,.

DAM~ -

/, .. ~ TEST CONDITIONS

POOL ELEVATION ORIG INAL DESIGN

18,000 CF'S 724.00 F'T

NOTES: VE:LOCITIES ARE IN PROTOTYPE fEET PER SECOND 'AciD ARE ONE f'OOT Of'F THE BOTTOM.

ZERO VELOCITY INDICATES SLIGHT MOVEMENT IN DIRECTION SHOWN.

~

\ \

_, I I

CHANNEL VELOCITIES AT LOW DISCHARGE

SCALE I~ FEET

200 -

... .., <: <: oLL-. of.i....._ ~

=t I.J £L£V 700

----o!L_.

=t I.J

CLEV 71)0

~ 0 ~ p.---Q;

<1. <1. "(

~·

T EST CONDITIONS

DISCHARGE POOL ELEVATION

172.000 C FS 741.20 FT

NOTE: VELOCITIES ARE IN PROTOTYPE FEET PER SECOND ANO ARE ONE FOOT OFF THE BOTTOM.

PLATE 6

....

\ SPILLWAY FLOW PATTERN

ORIGINAL DESIGN SCALE IN FEET

AXIS OF DAM £LEY fZI.O

l ~

~ ® <D EL£¥ 11~.~ HIGH WEIR

I @ £LEV 111.0

J ® ® ® EL Ctl 11~. (1

I EL£V 1/0.D

I"" .. 0 18 ~ ~ 0

+ + + • : 2 ~ 2: L OW WEIR

c ~ ~ ~ • .. .. .. .. .... ..

i PIEZOMETER LOCATIONS

PRESSURES * DISCHARGE 172,000 CFS 1!>0,000 CFS 100, 000 CFS 80,000 CFS !>0,000 CFS

POOL · ELEV 741.2 FT MSL 7~~.4 fT MSL 7~~.~ fT MSL 733.2 FT MS\. 72~.e fT MSL. PIEZOMET ER T YPE TYPE TYPE T YPE T YPE TYPE TYPE TYPE TYPE TYPE

NO. A 8 A B A 8 A e A B I -14 .0 -~.0 -12 .0 -3.0 -8.~ - 1.0 • -7 .~ -1.0 -1.0

2 8.!1 4 .!> 8 .5 4 .5 7.!1 4 .0 6.!1 3 .!> -1.0

3 8. ~ 6.f> 8 .0 8 .0 6.5 !>.0 !>.!> 4 .!> O.f> 1.0

4 - 6.0 -6.0 - 4 .5 -4.0 - 2 .0 - O.f> -1.0 O.f> 1.0 1.0

!> 2.0 -3.0 2 .5 - 1.0 4 .5 2.0 5.0 3 .5 4 .0 4.5

6 4 .0 l.f> 4 .5 2 .0 6.5 4.f> 6,5 5 .0 4.5 5 .5

7 3.0 6.0 4 .0· 6.5 4.f> 7.0 5.0 7.5 7,0 6 .f>

8 7.0 6.0 7.5 6 .5 8.0 6.5 8.0 8.0 6 .5 5 .0

..

NOTES: ORIGINAL DESIGN DESIGNATED T YPE A AND TYPE B· WHERE: L T YPE A CONSISTED OF ORIGINA L DESIGN

WITH ABUTMENTS AND LOW WEIR HAVING 90-DEGREE CORNER~

2 . TYPE B CONSISTED OF ORIGINAL DESIGN WITH ABUTMENTS AND LOW WEIR CORNE:RS ROUNDED.

* PRESSURES AR£ IN PROTOTYPE FEET OF WATER.

SIDE PRESSURES AT LOW WEIR

PLAT E 7

750r-----.------,------,------,------r-----.------,------,------,------r-----,------,------.------,------r-----,750

h

74of=====9=====~==--==~~~~~----~~~~-~l--i------+------t-----~----~~----j_ ____ _J ______ L_ ____ ~----~740 172,~00 CFS--;;;!?---~ t'--1'-. \ LEGEND

CENTERLINE l -- N \ 100,000 CFS-_?Y ---- \ ---- 160FT RIGHT OF CENTERLINE

uo~----+-----~-----+------~----+---~~~~~~~~~----~------+-----~-----,-----,------,-----.------+----~730

1\\\

670 ~----~----~9~+700~---L------L------L----~I0~+~070----~-----L------L---~I~I+~0~0----~----~------~----12~+~0~0-----L----~S70

STATIONS ALONG CENTERLINE OF SPILLWAY TEST CONDITIONS

DISCHARGE POOL ELEVATION DISCHARGE POOL ELEVATION

DISCHARGE POOL ELEVATION

172,000 CFS 741.20 FT

100,000 CFS 735.30 FT

18,000 CFS 724.00 FT

WATER-SURFACE PROFILES ORIGINAL DE.SIGN

"'0 r ~ fll

<0

?Z ~r---r---~--~--,---,----r---r---r---r---.---,----r---r---~--,---,---,----r---r---r---.---,----r---r---~--,---,---~72~

I STATION 10 +40 I 5~L__l __ ~~j_~l__j __ ~~JL--~--L_~~_L-=~==~==~~==~=-j_--~_jl_~~JL~~~l__jl__l __ ~ __ j___j ~5

350L 300L ZSOL 200L 150L IOOL SOL 0 ~OR IOO R 150R 200R ZSOR 300R 350R DI STANCE IN FEET FROM CENTERL INE

70S 3SOL 300L 250L 200L I SOL

I STATION 10 + 00 1 L__l __ _l~j_~l__j __ ~~JL~~--L_~~_L-=~==~~c=~==~=-j_~~_jl__l~JL--l_ __ l__j __ _! __ _L __ j__j70$

IOOL SOL 0 SOR IOOR 150R ZOOR 250R 300R 350R

TEST CONDITIONS

OIS01Af\GE 172000 F'OOL £1.£VATION 74120

DISCHARGE 100000 POOL £LCVATION ns.3o DISCHARGE 18000 POOL ELeVATION 724.00 ORIQNAL OCSICN - TYPE e

Cf"S f"T CfS f"T CFS f'T

DISTANCE IN f"EET FROM CENTERLINE

WATER- SURFACE CROSS SECT ION AT SPILLWAY CREST

0

702

~ ~-- ~

~" 698

--~---- t----..,

694

.J (I)

~

:;; 690 .., . ... ; z 0

~ 686 > "' ..J w

682

6 78

I'-

674 11+00

200'

r--. ---......

12+00

1-t--v

-...... ~.0 '\ N-"

/y--1(

~-'\ f-.- -......

-~, ~ ~ -"i ---i~ .?48' RIGHT Or;-- --......_ .... r--.. CENTEHLINE

' ............... /

-~ /

~.~ ~% ........ +-- --- ~ --- ' ~.._, ....__ --......_ , ...... _-= 248' L~Fr'oF---......... --......_ 1--- - ~ t---.?o..,- --- CENTERLINE ---~ __/'-I'- ' ~~--~( CENTEHUNE PROFILE:

~ --- - --- '

I-- i -....

r--........ '· ' I-- --......_ '-.... ~c-vr. I'-.

~-vc I -- --......_ N,c- N% . c-r/r ~ .. --~ ~( ~ :::......... ....__

!---.... ---!---.... ~% ;-..... s~o ·r---~~

r-.. --......_ I--

--......_ r-...

13+00 14+00 15+00 16+00 ·17+00 18+00 19+00 .. 20+00 21+00

200' STATIONS ALONG CENTERLINE OF SP I LLWAY

.~ W IT H TRAINING WALLS W ITHOUT TRAIN ING WALLS

.., ~ .... ~ it ~

---- --......_ ~

702

698

694

.J (I)

:::;;

690 :;;

"' ...

' ' ~ .... ,

~ 'o~ w <' "' sc:o..oe-

~ z 0 .

686 ;:: < > w .J w

" --.__,

I--

. 22+00

~ ', -

--......_ t::---.-

23 ... 00

~ 682

678

;-.....

674 24+00

TEST CON DITIONS DISCHARGE POOL ELEVATION

17 2.000 CF'S 741.20 FT

TYPICAL CHANNEL SECTIONS NOT TO SCAL E

NOTE: TRAI N ING WALL ENDS AT STATION 15 +60 ON LEFT

WATER-SURFACE PROFILES EXIT CHANNELS (ORIGINAL DESIGN)

SIDE AND AT STATION 22 i-60 ON RIGH T SIDE.

~ 690

~

.... "' "' ... ! 680

z 0

~ > ... ..J 670

~ ~ 1/lj.

!~ .,~

-- - -

-- ---~ .. -.

... 3!10L 300L 250L 200L

TEST CONDITIONS

DISCHARGE POOL EU:VATION DISCHARGE POOL ELEVATION DISCHARGE POOL ELEVATION ORIGINAL DESIGN - TYPE B

172000 C FS 741.20 F"T

100000 CFS 735.30 F"T 18000 CFS· 724.00 FT

-

--- --I

I ~OL

( 0~ SPIL:LWAI' N r -toaooo cFs .l_.

_t.?,OO~ CFS I / I - . 1--·-!. .

A ~ ~·

~ t:; ... ...

-1=_-- ---- --- l----_ _l ___ 'c'l:_~o_o_~:!_ --- ---~- --- -- --=

680 ~

z 0

~ I

! I I :;;..---::.·

- > w ~ <r-: - ·. ';?', ~

3~o'}l0 d IOOL !IOL 0 !!>OR IOOR I !!>OR 200R 2!>0R 300R D ISTANCE IN FEET F ROM CENTERL.INE OF SPILL WAY

STATION 23+00

STATION 16 + 00

STAT ION 13 + 00

WATER-SURFACE CROSS SECTIONS IN EXIT CHANNEL

ELEV 747.00

£LEV 724.00

EL£V 7&1.86

<J/0

GRAD£= 145~--

SECT ION ALONG CENTERLINE OF SPILLWAY

SPH.:.LWAY REVISION-TYPE C SCALE IN FEET .• 20 2$

-u r )> ~ rn

•1.45~~

SECTION ALONG CENTERLINE OF SP IL LWAY

SPILLWAY REVISION -TYPE D SCAL E IN FEET

6 k) 20 2)

"U r )> -l JTI

EUV 747.00

£LEV 100.00

g +

;I· ~ ~ 0> • -c::li t- <( "'o

~10 &LEV ?:1000

EU:V Z06,00

SECTION ALONG CENTERLINE OF SPILLWAY

NOTE: TYPE G ALIGNMENT SAME AS ORIGINAL. EXCEPT

GRAOE • 1.4$"" --

ORIGINAL WAAPEO SURFACE

.· .. · ,.. ·, · .•.:'

LOW WEIR MOVED 3.95FT UPST REAM. SPILLWAY REVISION-TYPE G SCALE IN fEET

J

"' ::;

1-w w ... !': z Q 1-

~ w ..J w

68JzLO-L--~~2~40-L--~---16~0-L--~--8-0~L---L--~O--~L-~80~R~~--~1~60~R~-L--2~4~0~R~~~37.20~80

DISTANCE IN FEET FROM CEN TERLINE OF SPI LLWAY

STATION 12+00

7 10 710

700 700

690 690

680 i 680 320L 240L 160 L 80 L 0 80R 160R 240R 320R

DI STANCE IN FEET FROM CE NTE:R LINE OF SPI LLWAY

STATION II+ 5 0

680L---~--~--~--~--~----L---L---~---L--~~~--~~--L-~~--~--~ 680 320L 240L 160L SO L 0 80R 160L 240L 320L

LEGE N D

T YP E 8 (ORIGI NAL ) TYPE C TYPE D

DI STANCE IN FE:ET FROM CENTE:R L IN E OF SPI LLWAY

STATION II +OO

TEST CON DITIONS

DISCHARGE POOL ELEVATION

172,000 CfS 741,20 FT

- .. - T YPE G

..J </)

::;

1-w w ... !': z Q 1-<( > w ..J w

NOTES: FILL SHOWN NOT INSTALLED DURING TE STING OF T YPES B AND G.

ADDIT IONAL F'ILL IN PLACE A80VE STATION I I +00 FOR TYPED ON LY.

COMPARATIVE WATER-SURFACE CROSS SECTIONS

PLATE 15

, .. ~ ------- \ -111

APPROACH CHANNEL

\ -"' \

EU:V 700

I ~

OF DAM

EXIT CHANNEL

1ST ALTERN ATE-NO PARAPET

, .. ~ ~--- \ t

APPROA CH ~ "'< CHANNEL ' "'-..

CL~V 700

DAM

L srA 1o+oo

II I ~

EXIT CHANN EL

2ND ALTERNATE-3FT PA RAP ET

AI'PI?OACH CHANNEL

SECTI ON A-A (SCHEMATIC)

PLATE 16

ROUND LIN£

TOP or FIRM ROCK

TEST CONDITIONS

DISCHARGE POOL ELEVATION

172,000CFS 741.20 FT

NOTES: VELOCITIES A RE IN PROTOTYPE FEET PER SECOND AND ARE ONE FOOT OFF THE BOTTOM

ZERO VELOCITY INDICATES SLIGHT MOVEMENT IN DIRECTION SHOWN.

I ST AND 2ND ALTERNATES FOR LEFT ABUTMENT

SCALE IN FEET

. , .. ~

------- \ -

APPROACH CHAI'#>I£L

£LEV roo

I I

Extr CHANNEL

3RD ALTERNATE- 5 FT PARAPET

· , ,.~ --- \ ~ \

\

.4PPROACH ~ CHANNEL '

£ LEV TOO

I ~ '

OF DAM

Extr CHANNEL

4TH ALTERNATE -7 FT P A RAPET

SECTI ON A-A (SCHEMATIC)

TEST CONDI"TIONS

DISCHARGE POOL ELEVATION

172,000 CFS 741.20 FT

NOTE : VELOCITIES ARE IN PROTOTYPE FEET PER SECOND AND ARE ONE FOOT OFf THE BOTTOM.

3RD AND 4TH ALTERNATES FOR LEFT ABUTMENT

SCALE IN FEET

PLAT E 17