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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
Report Documentation Page Form ApprovedOMB No. 0704-0188
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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