Met

cube "e'' was effective when the best height, width, spacing, and posi-
tion on the apron were found. The front of the jump was almost vertical
and the water surface downstream was quite flat and smooth, much like
the water surface shown on Figure 20. Block "f", which is the same
shape used in the SAF basin, performed identically with the cubical block
"e"| The important feature as to shape appeared to be the vertical up-
stream face, The foregoing blocks were arranged in single and double
rows, The second row in each case was of little value, sketch "h", Fig-

ure 17,

Block "g" is the same as block "f" with the corners rounded,
It was found that rounding the corners greatly reduced the effectiveness
of the blocks. In fact, a double row of blocks with rounded corners did
not perform as well as a single row of blocks "b", "e", or "f". As block
"ft" is usually preferable from a construction standpoint, it was used
throughout the remaining tests to determine a general design with respect

to height, width, spacing, and position on the apron.

In addition to experimenting with the baffle blocks, variations
were tried with respect to the size and shape of the chute blocks and the
end sill, It was found that the chute blocks should be kept small, no
larger than Dy, if possible. The end sill had little or no effect on the
jump proper when baffle piers were placed as recommended, The basin
as finally developed is shown on Figure 18. This basin is principally an
impact dissipation device whereby the baffle blocks are called upon to do
most of the work. The chute blocks aid in stabilization of the jump, and
the solid-type end sill is for scour control.

Verification Tests

At the conclusion of the development work, a set of verification
tests was made to examine and record the performance of this basin,
which will be designated as Basin III, over the entire range of operating
conditions that may be met in practice. The conditions under which the
tests were run, the dimensions of the basin, and the results are recorded
on Table 4, The symbols can be identified from Figure 18.

Stilling Basin Performance and Design

Stilling basin action was quite stable for this design; in fact,
more so than for either Basins I or II. The front of the jump was steep,
and there was less wave action to contend with downstream than in either
of the former basins. In addition, Basin III has a large factor of safety
against jump sweepout and operates equally well for all values of the
Froude number above 4.5. The verification tests served to show that
Basin III was very satisfactory.

Basin III should not be used where baffle piers will be exposed
to velocities above the 50 to 60 feet per second range without the full
realization that cavitation and resulting damage may occur, For veloc-
ities above 50 feet per second, Basin II or hydraulic model studies are
recommended.

12


To determine the backwater effect of placing the underpass in
the channel, Figure 29B will prove helpful. Data from four different
underpasses were used to obtain the two curves shown. Although the
test points from which the curves were drawn showed minor inconsist-
encies, probably because factors other than those considered also af-
fected the depth of water upstream from the underpass, it is believed
that the submitted curves are sufficiently accurate for design purposes.
Figure 29B shows two curves of the discharge coefficient "'C' versus
average velocity beneath the underpass, one for underpass lengths of
1Dg to 2Dg and the other for lengths 3Dg to 4D2. Intermediate values
may be interpolated, although accuracy of this order is not usually re-
quired.

Example

To illustrate the use of the preceding data in designing an
underpass, a sample problem will be helpful.

Assume a rectangular channel 30 feet wide and 14 feet deep
flows 10 feet deep at maximum discharge, 2,400 cfs. It is estimated
that waves will be 5 feet high and of the ordinary variety having a period
less than 5 seconds. It is desired to reduce the height of the waves to
approximately 1 foot at maximum discharge by installing an underpass-
type wave suppressor without increasing the depth of water upstream
from the underpass more than 15 inches.

To obtain maximum wave reduction at maximum discharge,
the underpass should be submerged 33 percent. Therefore, the depth
beneath the underpass is 6.67 feet with a corresponding velocity of

12 ft/sec, 2, 400 To reduce the height of the waves f 5
= o reduce e hel; fe) ie waves trom
(V = 30% 6.67): 8

to 1 foot, an 80 percent reduction in wave heights is indicated, and,
from Table 7, requires an underpass approximately 2D in length.

From Figure 29B, C = 1.07 for 2Dy and a velocity of 12 ft/sec.

From the equation given on Figure 29B:

2 2

2, 400
Total head, h +h, = (——& = 2 ) = 1.95 feet
ve fie GOT x T-O7 x 200

h+ hy is the total head required to pass the flow and h repre-
sents the backwater effect or increase in depth of water upstream from
the underpass. The determination of values for h and hy is done by
trial and error. As a first determination, assume that h + hy repre-
sents the increase in head.

29



experimental results, Because of the wide use this curve has experi-
enced, a rather complete explanation is presented regarding the disa-
greement. The curve 1 experiments were performed in a flume 6
inches wide, with limited head. The maximum discharge was approxi-
mately 0.7 cfs, and the thickness of the jet entering the jump, Dj, was
0.25 foot for a Froude number of 1,94, The results up to a Froude
number of 2.5 are in agreement with the present experiments. The ex-
treme case involved a discharge of 0.14 cfs and a value of Dj of 0.032
foot, for Fj = 8.9, which is much smaller than any discharge or value
of Dj used in the present experiments. Thus, it is reasoned that in the
6-inch-wide flume, frictional resistance in the channel downstream in-
creased out of proportion to that which would have occurred ina larger
flume or a prototype structure. In laboratory language, this is known
as "scale effect" and is construed to mean that prototype action is not
faithfully reproduced, The experimenters were somewhat dubious con-
cerning the small scale experiments, and to confirm the above conclu-
sion, it was found that results from Flume F, which was 1 foot wide,
became erratic when the value of D, approached 0.10. Figure 7 shows
three points obtained with a value of Dy of approximately 0.085. The
three points are given the symbol [x] and fall short of the recommended
curve,

The two remaining curves, labeled 3 and 4, on Figure 7, por-
tray the same trend as the curve obtained from the current experiments.
The criterion used by each experimenter for judging the length of the
jump is undoubtedly responsible for the displacement,

As can be observed from Figure 7, the test results from Flumes
B, C, D, E, and F plot sufficiently well to establish a single curve, The
five points from Flume A, denoted by squares, appear somewhat erratic
and plot to the right of the general curve. Henceforth, reference to Fig-
ure 7 will concern only the recommended curve which is considered ap-
plicable for general use.

With the experimental information available, it is only a mat-
ter of computation to determine the energy absorbed inthe jump, The
total energy, E,, entering the jump at Section 1 for each test is simply
the depth of flow, D1, plus the velocity head computed at the point of
measurement. The energy leaving the jump is the depth of flow plus
the velocity head at Section 2, Et, the difference in the values, con-
stitutes the loss of energy, in feet of water, attributed to the conver-
sion. The ratioofenergy lost inthe jumpE],, tototal energy entering
jump, E1, is plotted in percent against the Froude number and is shown
as the curve tothe left on Figure 8, For aFroude number of 2.0, which
would correspond toa relatively thick jet entering the jump at low velocity,
the curve shows the energy absorbed inthe jump to be about 7 percent of
the total energy entering. Considering the other extreme, fora Froude
number of 19, which would be produced bya relatively thin jet entering the
jumpatvery high velocity, the absorption by the jump would amount to
85 percent of the energyentering. Thus, the hydraulic jumpcan perform


Water Surface and Pressure Profiles

Approximate water-surface profiles were obtained for Basin
Ill during the verification tests. The front of the jump was so steep,
Figure 20, that only two measurements were necessary - the tail-
water depth and the depth upstream from the baffle blocks. The tail-
water depth is shown in Column 6, and the upstream depth is recorded
in Column 29 of Table 4, The ratio of the upstream depth to conjugate
depth is shown in Column 30. As can be observed, the ratio is much
the same regardless of the value of the Froude number, The average
of the ratios in Column 30 is 0.52. Thus, it will be assumed that the
depth upstream from the baffle blocks is one-half the tail-water depth.

The profile represented by the crosshatched area, Figure 20,
is for conjugate tail-water depth. For a greater tail-water depth, D,
the upstream depth would be Dz. For a tail-water depth less than con-

2
jugate, Dy, the upstream depth would be approximately = There

appears to be no particular significance to the fact that this ratio is
one~half,

The information on Figure 20 applies only to Basin II, pro-
portioned according to the rules set forth. It can be assumed that for
all practical purposes the pressure and water-surface profiles are
the same, There will be a localized increase in pressure on the apron
immediately upstream from each baffle block, but this has been taken
into account, more or less, by extending the diagram to full tail-water
depth, peginning at the upstream face of the baffle blocks.

Recommendations

The following rules pertain to the design of the Type III basin,
Figure 18:

1. The stilling basin operates best at full conjugate tail-water
depth, D2. A reasonable factor of safety is involved at conjugate
depth for all values of the Froude number (Figure 11), but it is rec-
ommended that the designer not make a general practice of encroach-
ing on this margin of safety.

2. The length of pool, which is less than one-half the length of
the natural jump, can be obtained by consulting the curve for Basin
Ill on Figure 12,

3, Stilling Basin III may be effective for values of the Froude
number as low as 4.0, but this cannot be stated for certain.

4, Height, width, and spacing of chute blocks should equal
the average depth of flow entering the basin, or D1. Width of blocks
may be decreased, providing spacing is reduced a like amount.
Should Dj prove to be less than 8 inches, make the blocks 8 inches
high.
16


9. Stilling basin for diversion dams which can accommodate
both free and submerged flow.

10. Stilling basin for use on high head outlet works, utilizing
hollow-jet valves.

11. Slotted bucket for medium and low overfall dams.

12, Solid bucket for overfall dams where an excess of tail water
exists.

13. Flip bucket which discharges above the tail water.

The Operation and Maintenance Equipment Procedures Bul-
etin is published in the Commissioner's Office, Denver, Colorado.
It is circulated for the benefit of irrigation project operation and main-
tenance people. Its principal purpose is to serve as a medium of ex-
changing operating and maintenance information. Reference to a trade
name does not constitute an endorsement of a particular product, and
omission of any commercially available item does not imply discrim-
ination against any manufacturer. It is hoped that labor-saving de-
vices or less costly equipment developed by the resourceful water users
will be a step toward commercial development of equipment for use on
irrigation projects in continued effort to reduce costs and increase op-
erating efficiency.

iii



General Design Procedure

To design an underpass for a particular structure there are
three main considerations: First, how deeply should the roof be sub-
merged; second, how long an underpass should be constructed to ac-
complish the necessary wave reduction; and third, how much increase
in flow depth will occur upstream from the underpass. These con-
siderations are discussed in order.

Based on the two installations shown on Figures 27 and 28,
and on other experiments, it has been found that maximum wave re-
duction occurs when the roof is submerged about 33 percent, i.e.,
when the under side of the underpass is set 33 percent of the flow depth
below the water surface for maximum discharge, Figure 29C.. Sub-
mergences greater than 33 percent (for the cases tested) produced
undesirable turbulence at the underpass outlet, resulting in less over-
all wave reduction. With the usual tail-water curve, submergence and
the percent reduction in wave height will become less, in general, for
smaller than maximum discharges. This is illustrated by the upper
curve in Figure 29C. The lower curve shows a near constant value for
less submergence, but it is felt that this is a somewhat special case
since the wave heights for less than maximum discharge were smaller
and of shorter period than in the usual case.

It is known that the wave period greatly affects the perform-
ance of a given underpass, with the greatest wave reduction occuring
for short period waves. Since the designer usually does not know in
advance the wave periods to be expected, this factor should be elimi-
nated from design consideration as far as possible. Fortunately, wave
action below a stilling basin usually has no measurable period but con-
sists of a mixture of generated and reflected waves best described as
a choppy water surface. This fact makes it possible to provide a prac-
tical solution from limited data and to eliminate the wave period from
consideration, except in this general way: waves must be of the variety
ordinarily found downstream from hydraulic jumps or energy dissipa-
tors. These usually have a period of not more than about 5 seconds.
Longer period waves may require special treatment not covered in this
discussion, Fortunately, too, there generally is a tendency for the
wave period to become less with decreasing discharge. Since the sup-
pressor provides a greater percentage reduction on shorter period
waves, this tends to offset the characteristics of the device to give less
wave reduction for reduced submergence at lower discharges. It is,
therefore, advisable in the usual case to submerge the underpass about
33 percent for the maximum discharge. For less submergence, the
wave reduction can be estimated from Figure 29C.

The minimum length of underpass required depends on the
amount of wave reduction considered necessary. If it is sufficient to
obtain a nominal reduction to prevent overtopping of a canal lining at
near maximum discharge or to prevent waves from attacking channel
banks, a length 1D2 to 1.5D2 will provide from 60 to 75 percent wave
height reduction, provided the initial waves have periods up to about

27



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13



Entering Figure 12 with a Froude number of 11.42,

Lyy = 2.75, and the length of

Do

basin required Liy = 2,75 ¥ 17.80 = 48.95 feet.

(Notice that conjugate depth was used, not tail-water depth. )

The height, width, and spacing of chute blocks are equal
to D, or 1.130 feet (use 13 or 14 inches).

The height of the baffle blocks for a Froude number of
11,42 (Figure 19) is 2.5D, :

hg = 2.5 x 1.130 = 2.825 feet (use 34 inches).

The width and spacing of the baffle blocks are preferably
three-fourths of the height or

0.75 x 34 = 25.5 inches.

From Figure 18, the upstream face of the baffle blocks

should be 0, 8D, from the downstream face of the chute blocks,
or

0.8 x 17.80 = 14, 24 feet.

The height of the solid end sill (Figure 19) is 1.60D, , or
hy = 1.60 x 1.130 = 1.81 feet (use 22 inches).

The final dimensions of the basin are shown on Figure 21.

19



End Sill

The height of the solid end sill is also shown to vary with the
Froude number, although there is nothing critical about this dimension,
The heights of the sills used in the verification tests are shown in Col-
umns 27 and 28 of Table 4. The height of the end sill in terms of D,
is plotted with respect to the Froude number and shown as the lower
line on Figure 19, A slope of 2:1 was used throughout the tests.

Tail-Water Depth

The SAF rules suggest the use of a tail-water depth less than
full conjugate depth, Dg. Full conjugate depth, measured above the
apron, is recommended for Basin III. There are several reasons for
this statement: First, the best operation for this stilling basin occurs
at full conjugate tail-water depth; secondly, if lessthanthe conjugate depth
is used, the surface velocities leaving the pool are high, the jump ac-
tion is inpaired, and there is a greater chance for scour downstream;
and thirdly, if the baffle blocks erode with time, the additional tail-
water depth will serve to lengthen the interval between repairs. On
the other hand, there is no particular advantage to using greater than
the conjugate depth, as the action in the pool will show little or no im-
provement,

The margin of safety for Basin III varies from 15 to 18 per-
cent depending on the value of the Froude number, as can be observed
by the dotted line labeled, "Minimum Tail Water Depth--Basin III", on
Figure 11. The points, from which the line was drawn, were obtained
from the verification, tests, Columns 10 and 14, Table 4, Again, this
line does not represent complete sweepout, but the point at which the
front of the jump moves away from the chute blocks and the basin no
longer functions properly. In special cases, it may be necessary to
encroach on this wide margin of safety; however, it is not advisable
as a general rule, for the reasons stated above.

Length of Basin

The length of Basin III, which is related to the Froude number,
can be obtained by consulting the lower curve on Figure 12. The points,
indicated by circles, were obtained from Columns 10 and 12, Table 4,
and indicate the extent of the verification tests. The length is measured
from the downstream side of the chute blocks to the downstream edge
of the end sill, Figure 18, Although this curve was determined con-
servatively, it will be found that the length of Basin III is less than one-
half the length needed for a basin without appurtenances, Basin III may
be effective for values of the Froude number as low as 4.5; thus, the
length curve was terminated at this value.

15



5 seconds. The shorter the wave period the greater the reduction

for a given underpass. For long period waves, little wave reduction
may occur because of the possibility of the wave length being nearly
as long or longer than the underpass, with the wave passing untouched
beneath the underpass.

To obtain greater than 75 percent wave reductions, a longer

underpass is necessary. Under ideal conditions, an underpass 2D9

to 2.5Dg in length may provide up to 88 percent wave reduction for
wave periods up to about 5 seconds, Ideal conditions include a velocity
beneath the underpass of less than, say, 10 feet per second and a length
of channel 3 to 4 times the length of the underpass downstream from the
underpass which may be used as a quieting pool to still the small turbu-
lence waves created at the underpass exit.

Wave height reduction up to about 93 percent may be obtained
by using an underpass 3.5Dg to 4Dg long. Included in this length is a
4:1 sloping roof extending from the underpass roof elevation to the
tail-water surface. The sloping portion should not exceed about one-
quarter of the total length of underpass. Since slopes greater than 4:1
do not provide the desired draft-tube action, they should not be used.
Slopes flatter than 4:1 provide better draft-tube action and are, there-
fore, desirable.

Since the greatest wave reduction occurs in the first Do of
underpass length, it may appear advantageous to construct two short
underpasses rather than one long one. In the one case tested, two
underpasses each 1D2 long, with a length 5Dg between them, gave an
added 10 percent wave reduction advantage over one underpass 2D9
long. The extra cost of another headwall should be considered, how-
ever,

Table 7 summarizes the amount of wave reduction obtainable
for various underpass lengths.

Table 7
EFFECT OF UNDERPASS LENGTH ON WAVE REDUCTION

For Underpass Submergence 33 Percent and
Maximum Velocity Beneath 14 ft/sec

Underpass length Percent wave reduction*
1D2 to 1.5D2 60 to 75
2D2 to 2.5D2 80 to 88
3.5 to 4. 0Dg **90 to 93

*For wave periods up to 5 seconds,
**Upper limit only with draft-tube type outlet,

28


Length of basin necessary to confine the jump
L = 6,13 x 47.6 = 292 feet
Entering Figure 8 with the above value of the-Froude number,
it is found that the energy absorbed in the jump is 58 percent of the
energy entering.

By consulting Figure 9, it is apparent that a very satisfactory
jump can be expected,

10


many loopholes. Testing and analysis were synchronized to establish
valid curves in critical regimes, providing sufficient understanding
of the hydraulic jump in its many forms to establish workable design
criteria. Since all the test points were obtained by the same person-
nel, using standards established before testing began, and since re-
sults and conclusions were evaluated from the same datum, the data
presented are consistent and reliable.

This report, therefore, is the result of the first integrated
attempt to generalize the design of stilling basins, energy dissipators,
and associated appurtenances, General design rules are presented so
that the necessary dimensions for a particular structure may be easily
and quickly determined, and the selected values checked by other de-
signers without the need for exceptional judgment or extensive previous
experience,

The report emphasizes design procedures rather than the hy-
draulic aspects of the data. Certain design procedures recommended
in the past have been satisfactorily proved, while others have been
modified or discarded in favor of improved methods, Satisfactory ex-
planations are given for procedures, which in the past were considered
inconsistent; for example, it is now fully understood why certain hy-
draulic jumps require a stilling basin only 2.5 times the downstream
depth of flow while other jumps require basin lengths 4.5 times the
depth of flow.

In most instances design rules and procedures are clearly
stated in simple terms with limits fixed in a definite range. In other
cases, however, it is necessary to state procedures and limitations
in broader terms, making it necessary to carefully read the accom-
panying text,

Proper use of the material in report Hyd-399 will eliminate
the need for hydraulic model tests on many individual structures,
particularly the smaller ones. Structures obtained by following the
recommendations will be conservative in that they will contain a de-
sirable factor of safety. However, to further reduce structure sizes,
to account for unsymmetrical conditions of approach or getaway, or
tp evaluate other unusual conditions not covered in this discussion,
model studies will still prove beneficial.

‘€xperimental Equipment

Five test flumes were used at one time or another to obtain
the experimental data required in the present test program: Flumes
A and B, Figure 1; Flumes C and D, Figure 2; and FlumeF, Figure 3.
The arrangement shown as Flume E, Figure 3, actually occupied a portion



the position of the downstream raft is important. The second
raft should be positioned downstream at some fraction of the
wave length. Placing it at a full wave length could cause both
rafts to be ineffective. Thus, for narrow canals it may be ad-
visable to make the second raft portable. However, if it be-
comes necessary to make the rafts adjustable or portable, or
if a moderate increase in depth in the stilling basin can be tol-
erated, consideration should be given to the type of wave sup-
pressor discussed below.

Underpass Type Wave Suppressor

By far the most effective wave dissipator is the short-tube
type of underpass suppressor. The name "'short-tube" is used be-
cause the structure has many of the characteristics of the short-tube
discussed in hydraulics textbooks. This wave suppressor may be
added to an existing structure or included in the original construction.
In either case it provides a sightly structure, permanent in nature,
which is economical to construct and effective in operation.

The recommendations for this structure are based on three
separate model investigations, each having different flow conditions
and wave reduction requirements.

Essentially, the structure consists of a horizontal roof placed
in the flow chammel with a headwall sufficiently high to cause all flow to
pass beneath the roof. The height of the roof above the channel floor
may be set to effectively reduce wave heights for a considerable range
of flows or channel stages. The length of the roof, however, deter-
mines the amount of wave suppression obtained for any particular roof
setting.

Performance

The effectiveness of this wave suppressor is illustrated in
Figure 26. In this instance, it was desired to reduce wave heights
entering a lined canal to prevent overtopping of the canal lining at near
maximum discharges. Below 3,000 second-feet, waves were in evi-
dence but did not overtop the lining. For larger discharges, however,
the stilling basin produced moderate waves which were actually intensi-
fied by the short transition between the basin and the canal. These in-
tensified waves overtopped the lining at 4,000 second-feet and became
a real problem at 4,500 second-feet. Anxiety developed when it be-
came known that water demands would soon require 5, 000 second-feet,
the design capacity of the canal. Tests were made with a suppressor
21 feet long using discharges from 2,000 to 5, 000 second-feet. The
suppressor was located between the stilling basin and the canal.

Figure 27, Test 1, shows the results of tests to determine the
optimum opening between the roof and the channel floor, using the maxi-
mum discharge, 5,000 second-feet. With a 14-foot opening, waves
were reduced from about 8 feet to about 3 feet. Waves were reduced

25



shown on Figure 23 is a minimum requirement to accomplish

the purpose set forth. The width of the blocks is shown equal

to Dy and this is the maximum width recommended. From a
hydraulic standpoint, it is desirable that the blocks be constructed
narrower than indicated, preferably 0.75D1. The ratio of block
width to spacing should be maintained as 1:2.5. The extreme
tops of the blocks are 2D, above the floor of the stilling basin.
The blocks may appear to be rather high and, in some cases, ex-
tremely long, but this is essential as the jet must play at the
base of the roller to be effective. To accommodate the various
slopes of chutes and ogee shapes encountered, a rule has been
established that the horizontal length of the blocks should be at
least 2D}. The upper surface of each block is sloped at 5° ina
downstream direction, as it was found that this feature resulted
in better operation, especially at the lower discharges.

Tail-water depth. A tail-water depth 5 to 10 percent
greater than the conjugate depth is strongly recommended for
the above basin. Since the jump is very sensitive to tail-water
depth at these low values of the Froude number, a slight defi-
ciency in tail-water depth may allow the jump to sweep completely
out of the basin. Many of the difficulties that have been encountered
in small field structures in the past can be attributed to this aspect
of the jump for low numbers. In addition, the jump performs much
better and wave action is diminished if the tail-water depth is in-
creased to approximately 1. 1D9.

Basin length and end sill. The length of this basin, which
is relatively short, can be obtained from the upper curve on Fig-
ure 12. No additional blocks or appurtenances are needed in the
basin, as these will prove a greater detriment than aid, The addi-
tion of a small triangular sill placed at the end of the apron for
scour control is desirable.

Performance. If designed for the maximum discharge,
this stilling basin will perform satisfactorily for all flows. Waves
below the stilling basin will still be in evidence but will be of the
ordinary variety usually encountered with jumps of a higher Froude
number. This design is applicable to rectangular cross sections
only.

Alternate Stilling Basin Design - Small Drops

Performance

An alternate basin for reducing wave action at the source,
for values of the Froude number between 2.5 and 4.5, is applica-
ble to small drops in canals. The Froude number in this case
would be computed the same as though the drop were an overflow
crest. A series of steel rails, channel irons or timbers in the
form of a grizzly are installed at the drop, as shown on Figure 24,

22



STILLING BASIN I
NOTES

Jump occurs on flat floor with no
chute blocks, baffle piers or end
sill in basin. Usually not a practical
basin because of expensive length.

Elements and characteristics of
Jumps for complete range of Froude
numbers is determined to aid
designers in selecting more
practical basins I,1I,1v,V and V1.

HYDRAULIC JUMP
ON HORIZONTAL FLOOR

(FIGURE 4)

STILLING BASIN I
NOTES
Jump and basin length reduced
about 33 percent with chute
blocks and dentated end sill.
For use on high spillways, large
canal structures, etc. for Froude
numbers above 4.5.

(FIGURE 14)

TW. DEPTH

STILLING BASIN IL
NOTES

Jump and basin length reduced
about 60 percent with chute
blocks, baffle piers, and solid
end sill.

For use on small spillways, outlet
works, small canal structures
where 'V, does not exceed 50-
60 feet per second and Froude
number is above 4.5.

(FIGURE 17)

TW. DEPTH

* 8 12 16
FROUDE NUMBER

RATIO
OF TW
DEPTH
TO Dy
(Fie. 5)
a ae
FROUDE NUMBER
MINIMUM
TAILWATER
DEPTHS
(SWEEPOUT)
(Fig. 1)
ar a a a ed
FROUDE NUMBER
Ia
Re/
LL PA,
++ |
PAZ
HG PA
MINIMUM
TAILWATER
fs _| DEPTHS
‘minTwdepth] (SWEEPOUT)
~T Basin I
(Fi..11)

20

STILLING BASIN IZ
NOTES

For use with jumps of Froude
number 25 to 45 which usually
occur on canal structures and
diversion dams. This basin
reduces excessive waves
created in imperfect jumps.

May also use alternate design
and/or wave suppressors
shown to right, or Basin WI

/Slot width =

ALTERNATE DESIGN
(FIGURE 23)

STILLING BASIN &
NOTES

For use where structural economies
dictate desirability of sloping
apron, usually on high dam spill-
ways.

Needs greater tailwater depth than
horizontal apron.

0:

Fad

Figure 32) Z

( ) Z

—- F
TAILWATER

DEPTH RELATED TO
CONJUGATE DEPTH
(CASE D)

(FiGuRE,

31)



UNITED STATES
DEPARTMENT OF THE INTERIOR
BUREAU OF RECLAMATION

STILLING BASINS, ENERGY DISSIPATORS
AND ASSOCIATED APPURTENANCES
(From Hydraulic Laboratory Report No, Hyd-399)

OPERATION AND MAINTENANCE
EQUIPMENT AND PROCEDURES
RELEASE NO, 14

October, November and December 1955

Commissioner's Office
Denver, Colorado,
1955



SHORT STILLING BASIN FOR CANAL STRUCTURES,
SMALL OUTLET WORKS, AND SMALL SPILLWAYS
(Basin III of Frontispiece)

Basin II, as shown in the frontispiece, often is considered
too conservative and consequently overcostly for structures carrying
small discharges at moderate velocities. This can be especially true
in the case of canal chutes, drops, wasteways, and other structures
which are constructed by the dozen on canal systems. Any saving
that can be effected in decreasing the size of these structures can
amount to a sizable sum when multiplied by the number of structures
involved, In this section a generalized design is developed for a
class of smaller structures in which the velocity at the entrance to
the basin is moderate or low (5 to 60 feet per second, corresponding
to an over-all head of about 100 feet), Further economies in basin
length are accomplished with baffle piers,

Development

The most effective way to shorten a stilling basin is to modify
the jump by the addition of appurtenances in the basin, One restriction
imposed on these appurtenances, however, is that they must be self-
cleaning or nonclogging. This restriction thus limits the appurtenances
to blocks or sills which can be incorporated on the stilling-basin apron.

The Department of Agriculture developed a very short stilling
basin designated ''The.SAF Basin, " for use on drainage structures such
as the Soil Conservation Service constructs. The SAF basin, Figure 16,
fits the needs for which it was developed but does not provide the factor
of safety necessary for Bureau use. This was demonstrated by construct-
ing and testing several basins proportioned to SAF specifications. It was
discovered, however, that the arrangement of this basin had excellent
possibilities, and that by changing dimensions, such as the length, the
tail-water depth, the height and location of the baffle blocks, etc., the
desired degree of conservatism could be obtained.

In addition to the foregoing tests, numerous experiments were
performed using various types and arrangements of baffle blocks on
the apron in an effort to obtain the best possible solution. Some of the
baffle blocks tried are shown on Figure 17, The blocks were positioned
in both single and double rows with the second row staggered with re-
spect to the first. Arrangement "'a'' on Figure 17 consisted of a solid
bucket sill which was tried in several positions on the apron. This sill
required an excessive tail-water depth to be effective. The solid sill
was then replaced with blocks and spaces. For certain heights, widths,
and spacing, block "'b'" performed quite well, resulting in a water sur-
face similar to that shown on Figure 20. Block "'c" was ineffective for
any height. The velocity passed over the block at about 45° angle, thus
was not impeded, and the water surface downstream was very turbulent
with waves, The stepped block "d" was also ineffective both for asingle
row and a double row, The action was much the same as for "c"'. The

11



STILLING BASINS, ENERGY DISSIPATORS
AND ASSOCIATED APPURTENANCES*

GENERAL

Stilling basins are defined as structures in which all or part
of a hydraulic jump or other energy reducing action is confined. Other
structures, such as buckets or impact dissipators, are designated
energy dissipators.

Although the Bureau of Reclamation has designed and con-
structed hundreds of stilling basins and energy dissipation devices in
conjunction with spillways, outlet works, and canal structures, it is
still necessary in many cases to make model studies of individual struc-
tures to be certain that these will operate as anticipated. The reason
for these repetitive tests, in many cases, is that a factor of uncertainty
exists, which in retrospect is related to an incomplete understanding
of the over-all characteristics of the hydraulic jump and other types
of energy dissipators.

Previous laboratory studies made on individual structures
over a period of years, by different personnel, for different groups
of designers, with each structure having a different allowable design
limitation for downstream erosion, resulted in a collection of data
which on any plotting proved to be sketchy, inconsistent, and with
only vague connecting links. Extensive library research into the works
of others revealed the fact that these links were actually nonexistent.

To fill the need for up-to-date hydraulic design information
on stilling basins and energy dissipators, the laboratory initiated a
research program on this general subject. The program was begun
with a rather academic study of the hydraulic jump, observing all
phases as it occurs in open channel flow, With a broader understand-
ing of this phenomenon, it was then possible to proceed to the more
practical aspects of stilling basin design.

Existing knowledge, including laboratory and field tests col-
lected from Hydraulic Laboratory records and experience over a 23-
year period, was used to establish a direct approach to the practical
problems encountered in hydraulic design, Hundreds of tests were
also performed on both available and specially constructed equipment
to obtain a fuller understanding of the data at hand and to close the

*A condensation of Laboratory Report No. Hyd-399: Progress
Report II; Research Study on Stilling Basins, Energy Dissipators, and
Associated Appurtenances. Only the applicable Figures necessary for
illustration are included in this condensation. They will be found in
the Appendix and numbered as they appear in the original report toavoid
confusion,



5. The height of the baffle blocks varies with the Froude
number and is given on Figure 19, The blocks may be cubes or
they may be constructed as shown on Figure 18 so long as the
upstream face is vertical and in one plane. This feature is im-
portant. The width and spacing of baffle blocks are also shown
on Figure 18. In narrow structures where the specified width
and spacing of blocks do not appear practical, block width and
spacing may be reduced, providing they are reduced a like
amount, A half space is recommended adjacent to the walls.

6, The upstream face of the baffle blocks should be set at
a distance of 0.8D2 from the downstream face of the chute blocks
(Figure 18), This dimension is also important,

7. The height of the solid sill at the end of the basin can be
obtained from Figure 19. The slope is 2:1 upward in the direc-
tion of flow.

8. There is no need to round or streamline the edges of the
chute blocks, end sill or baffle piers. Streamlining of baffle piers
may result in loss of half of their effectiveness. Small chamfers
to prevent chipping of the edges is permissable.

9. Asa reminder, a condition of excess tail-water depth
does not justify shortening the basin length.

10. It is recommended that a radius of reasonable length
(R > 4D1) be used at the intersection of the chute and basin apron
for slopes of 45° or greater.

11. Asa general rule the slope of the chute has little effect
on the jump unless long flat slopes are involved. This phase will
be considered in Section 5 on sloping aprons.

As the Type III basin is short coupled, the above rules should
be followed closely for its proportioning. If the proportioning is to be
varied from that recommended, or if the limits given below are ex-
ceeded, a model study is advisable. Arbitrary limits for the Type III
basin are set at 200 cts per foot of basin width, or 100 feet of fall, un-
til experience demonstrates otherwise,

Example
Given the following computed values for a small overflow dam:
Q q V1 Di
ofs ets filsec ft
3,900 78.0 69 1,130
3, 090 61.8 66 0.936
2, 022 40.45 63 0. 642
662 13,25 51 0.260

17



LAST GOPY - BO NOT REMOVE

Department of the Interior

Bureau of Reclamation

OPERATION AND MAINTENANCE
EQUIPMENT AND PROCEDURES

RELEASE NO. 14
October, November and December 1955

LAST COPY - DO NOT REMOVE
LAST COPY - DO NOT REMOVE

CONTENTS

Stilling Basins, Energy Dissipators
and Associated Appurtenances


Definitions of the symbols used in connection with the hydrau-
lic jump on a horizontal floor are shown on Figure 4, The procedure
followed in each test of this series was to first establish a flow. The
tail water depth was then gradually increased until the front of the
jump moved upstream to Section 1, indicated on Figure 4, The tail-
water depth was then measured, the length of the jump recorded, and
the depth of flow entering the jump, Dy, was obtained.

All computations are based on discharge per foot width of
flume, or q.

The velocity entering the jump V1 was computed by dividing
q by Dy.

Froude Number

The Froude number is used throughout this presentation,
The Froude number is simply:

F,-—1 (1)

where F; is a dimensionless parameter, V, and D, are velocity and
depth of how, respectively, entering the jump, and g is the accelera-
tion of gravity. The law of similitude states that where gravitational
forces predominate, as they do in open-channel phenomenon, the
Froude number should be the same value in model and prototype, Al-
though energy conversions in a hydraulic jump bear some relation to
the Reynolds number, gravity forces predominate, and the Froude
number is very useful for plotting stilling basin characteristics, As
the acceleration of gravity is aconstant, the term g could be omitted,
Its inclusion makes the expression dimensionless, however, and the
form shown as (1) is preferred,

The theory of the hydraulic jump in horizontal channels has
been treated thoroughly by others and will not be repeated here. The
expression for the hydraulic jump, based on pressure-momentum, oc-
curs in many forms. The following form is most commonly used in

the Bureau.
2 2
D D. 2V,° D
Dy =- 21 1 1 1 2
2 z +) / tz (2)

This may also be written:

D, , /p,? av,” pb,”
Dy =--— +\/-L+
2078 4 gD;

4


~

of Flume D during one stage of the testing, but it will be designated
as a separate flume for ease of reference, Each flume served a use-
ful purpose either in verifying similarity or extending the range of
the experiments. Flumes A, B, C, D, and E contained overflow sec-
tions so that the jet entered the stilling basin at an angle to the hori-
zontal, The degree of the angle varied in each case. In Flume F,
the entering jet emerged from under a vertical slide gate so the ini-
tial velocity was horizontal.

The largest scale experiments were made ona glass-sided
laboratory flume 4 feet wide and 80 feet long, in which an overfall
crest with a slope of 0.8:1 was installed, Flume D, Figure 2. The
drop from headwater to tail water in this case was approximately
12 feet, and the maximum discharge was 28 cfs.

It is felt that the design information to be presented will be
found economical as well as effective, yet an effort was made to lean
toward the conservative side. In other words, a moderate factor of
safety has been included throughout. Thus, the information is con-
sidered suitable for general use with the following provision:

It should be made clear at the outset that the informa-
tion herein is based upon symmetrical and uniform action in the
stilling basin and buckets, Should entrance conditions or appur-
tenances near the head of any of these structures tend to produce
asymmetry of flow down the chute and in the stilling basin, these
generalized designs may not be adequate. In this case, it may
be advisable to make the basin in question of a more symmetrical
nature, more conservative, or it may be wise to invest in a model
study. Also, should greater economy be desired than these gen-
eralized designs indicate, a model study is recommended,

Hydraulic Jump on Horizontal Apron (Basin I)

A tremendous amount of experimental, as well as theoret-
ical, work has been performed in connection with the hydraulic jump
ona horizontal apron, There is probably no phase of hydraulics that
has received more attention; yet, from a practical viewpoint there is
still much to be learned,

As mentioned previously, the first phase of the present
study was academic in nature, consisting of correlating the results
of others and observing the hydraulic jump throughout its various
phases; the primary purpose being to become better acquainted with
the over-all jump phenomenon, The objectives in mind were: (1) to
determine the applicability of the hydraulic jump formula for the
entire range of conditions experienced in design; (2) as only a limited
amount of information exists on the length of jump, it was desired to
correlate existing data and extend the range of these determinations;
and (3) it was desired to observe the various forms of the jump and
to catalog and evaluate them,



splashing and created a backwater effect, resulting in submerged
flow at the gate for the larger flows. Submerged flow reduced the
effective head on the structure, and in turn, the capacity. Tests
on several suggested devices showed that rafts provided the best
answer to the wave problem when additional submergence could

not be tolerated. The general arrangement of the tested structure
is shown in Figure 25. The Froude number varied from 3 to 7,
depending on the head behind the gate and the gate opening. Veloc-
ities in the canal ranged from 5 to 10 feet per second. Waves were
1.5 feet high, measured from trough to crest.

During the course of the experiments, a number of rafts
were tested; thick rafts with longitudinal slots, thin rafts made
of perforated steel plate, and others, both floating and fixed. Rigid
and articulated rafts were tested in various arrangements.

The most effective raft arrangement consisted of two rigid
stationary rafts 20 feet long by 8 feet wide, made from 6- by 8-
inch timbers, placed in the canal downstream from the stilling
basin (Figure 25). A space was left between timbers, and lighter
cross pieces were placed on the rafts parallel to the flow, giving
the appearance of many rectangular holes. Several essential re-
quirements for the raft were apparent: (1) that the rafts be perfo-
rated in a regular pattern; (2) that there be some depth to these
holes; (3) that at least two rafts be used; and (4) that the rafts be
rigid and held stationary.

It was found that the ratio of hole area to total area of
raft could be from 1:6 to 1:8. The 8-foot width, W on Figure 25,
is a minimum dimension. The rafts must have sufficient thick-
ness so that the troughs of the waves do not break free from the
underside. The top surfaces of the rafts are set at the mean
water surface in a fixed position, so that they cannot move. Spac-
ing between rafts should be at least three times the raft dimension,
measured parallel to the flow. The first raft decreases the wave
height about 50 percent, while the second raft effects a further
reduction. Surges over the raft dissipate themselves by flow down-
ward through the holes. For this specific case, the waves were
reduced from 18 to 3 inches in height.

Under certain conditions wave action is of concern only
at the maximum discharge when freeboard is endangered, so the
rafts can be a permanent installation. Should it be desired to
suppress the waves at partial flows, the rafts may be made ad-
justable, or, in the case of trapezoidal channels, a second set of
rafts may be placed under the first set for partial flows. The
rafts should perform equally well in trapezoidal as well as rectan-
gular channels.

The recommended raft arrangement is algo applicable for

suppressing waves with a regular period such as wind waves, waves
produced by the starting and stopping of pumps, etc. In this case,

24



The overfalling jet is separated into a number of long, thin
sheets of water which fall nearly vertically into the canal be-
low. Energy dissipation is excellent and the usual wave prob-
lem is avoided. If the rails are tilted downward at an angle of
3° or more, the grid is self-cleaning.

Design

Two spacing arrangements were tested in the laboratory:
in the first, the spacing was equal to the width of the beams, and
in the second, the spacing was two-thirds of the beam width. The
latter was the more effective. In the first, the length of grizzly
required was about 2.9 times the depth of flow (y) in the canal up-
stream, while in the secend, it was necessary to increase the
length to approximately 3.6y. The following expression can be
used for computing the length of grizzly:

LG = ——2___

CWN V “gy (4)

Where Q is total discharge, C is an experimental coefficient, W
is the width of spacing in feet, N is the number of spaces, gis
the acceleration of gravity and y is the depth of flow in the canal
upstream (see Figure 24). The value of C for the two arrange-
ments tested was 0.245.

In this case, the grizzly makes it possible to avoid the
hydraulic jump. Should it be desired to maintain a certain level
in the canal upstream, the grid may be tilted upward to act as a
check; however, this arrangement may pose a cleaning problem.

Wave suppressors

The two stilling basins described above may be considered to
be wave suppressors, although the suppressor effect is obtained from
the necessary features of the stilling basin. If greater wave reduction
is required on a proposed structure, or if a wave suppressor is re-
quired to be added to an existing flow-way, the two types discussed
below may prove useful. Both of these types are applicable to most
open channel flow-ways having rectangular, trapezoidal, or other
cross-sectional shapes, The first, or raft type, may prove more eco-
nomical than the second, or underpass type, but rafts may not provide
the degree of wave reduction obtainable with the underpass type. Both
types may be used without regard to the Froude number.

Raft Type Wave Suppressor

In a structure of the type shown in Figure 25, there are
no means for eliminating waves at their source. Tests showed
that appurtenances in the stilling basin merely produced severe

23



STILLING BASIN

GHARACTERISTICS
%;

ws
g
&
Seo}
z /
5
&
5
Zz
i a a a rd
FROUDE NUMBER 2
So}
LENGTH OF JUMP oo 4 8 @ 2

FROUDE NUMBER

LOSS OF ENERGY IN JUMP
(FIGURE 8)

(FIGURE 70R 12)

5
F,+25-45, TRANSITION

¥
Wo apa
Lee

ee Se
F\=45-9.0, RANGE OF GOOD JUMPS

| ae

<5 an A

ee
Fi 9.0-UPWARD, EFFECTIVE BUT ROUGH
JUMP FORMS (Fis.9)

5 2
°
a
So
&
a
3
8
8
rr
FROUDE NUMBER A
LENGTH OF JUMP rr a

FROUDE NUMBER
(FIGURE 12)

Profile for greater thon
conjugate cepth~.
Pressure profile for
conjugate depth ---.,_
Ze

WATER SURFACE & PRESSURE PROFILES

(Ficure 13)

rs T a

Profile for
conjugate depth

€ 10 2
FROUDE NUMBER

LENGTH OF JUMP _€ 8 10 @ 4 6 1
FROUDE NUMBER
HEIGHT OF BAFFLE BLOCKS & END SILLS
(FIGURE 18)

(FIGURE 12)

WATER SURFACE AND
PRESSURE PROFILES

(FIGURE 19)

-Gate structure

Radial gate

r

RAFT TYPE

UNDERPASS TYPE

WAVE SUPPRESSORS

(FIGURE 24) (Figure 29)

SUMMARY SHEET
NOTES

This sheet summarizes the main
features discussed in this report,
and shows some of the important
charts. More charts are given
in the report. This sheet should
be used as a reference guide only;
the entire report should be read
before attempting to use any of
the material contained herein.

STILLING BASIN WZ
NOTES

For use on pipe or open channel
outlets, sizes and discharges
from table. \j should not exceed
30 feet per second. No failwater
required. Froude number usually
L5 to 7 but not important.

May substitute for Basin 1

Energy loss greater than in com-
parable jump, figure. 44.

(FIGURE 48)

FEET AND INCHES
WH La bleldig
5643/74 33 41 24/01 21|
24 314 38/69'53/90/311 51 240 2/26)

30 4.9159 80.63 108 4-7 61 3411-4 30|
[36207 85 937-3 i24/5-3. 71 310-7 36
429.62 115 10-6 8:0 14-0) 60180 45) 1-9 3]
[48 1257 151 1-99-0158. 69) Bul 4 20 45)
“1590, 191 13.0 9 100 5S 2-2 Hell
60 1963 |236/14°3)10-9 19-0 8-0,11-0 S11 25.54
T2_28.27/339 166 123 220 9-3)12-9 oll 2-9 62|

(TaBLe 11)



Chute Blocks

The recommended proportions for Basin III are shown on Fig-
ure 18, The height, width, and spacing of the chute blocks are equal
to D1. Larger heights were tried, as can be observed from Column 18,
Table 4, but are not recommended, The larger chute blocks tend to
throw a portion of the high-velocity jet over the baffle blocks. Some
cases will be encountered in design, however, where Dj is less than
8 inches. In such cases, the blocks may be made 8 inches high, which
is considered by some designers to be the minimum size possible from
a construction standpoint. The width and spacing are the same as the
height, but this may be varied so long as the aggregate width of spaces
approximately equals the total width of the blocks.

Baffle Blocks

The height of the baffle blocks increases with the Froude num-
ber, as can be observed from Columns 22 and 10, Table 4. The height,
in terms of Dj, can be obtained from the upper line on Figure 19. The
width and spacing can vary so long as the total spacing is equal to the
total width of blocks. The most satisfactory width and spacing were found
to be three-fourths of the height. It is not necessary to stagger the baffle
blocks with the chute blocks, as this is often difficult and there is little
to be gained from a hydraulic standpoint.

The baffle blocks are located 0.8D2 downstream from the chute
blocks, as shown in Figure 18, The actual positions used in the verifi-
cation tests are shown in Column 25, Table 4. The position, height and
spacing of the baffle blocks on the apron should be adhered to carefully,
as these dimensions are important, For example, if the blocks are set
appreciably upstream from the position shown, they will produce a cas-
cade with resulting wave action, Onthe contrary, if the blocks are set
farther downstream than shown, a longer basin will be required. Like-
wise, if the baffle blocks are too high, they can produce a cascade, while
if too low a rough water surface will result. It is not the intention to
give the impression that the position or height of the baffle blocks are
critical, Their position or height are not critical so long as the above
proportions are followed. There exists a reasonable amount of leeway
in all directions; however, one cannot place the baffle blocks on the pool
floor at randam and expect anything like the excellent action associated
with the Type III basin,

The baffle blocks may be in the form shown on Figure 18, or
they may be cubes; either shape is effective. The corners of the baffle
blocks are not rounded, as the sharp edges are effective in producing
eddies, which in turn aid in the dissipation of energy. It is advisable
to place reinforcing steel back at least 6 inches fromthe block surfaces
when possible, as there is some evidence that steel placed close to the
surface aids spalling.

14


may be considered as part of the over-all length. The sloping roof
will help reduce the maximum wave height and will also reduce the
frequency with which it occurs, providing in all respects a better
appearing water surface.

A close inspection of the submitted data will reveal that
slightly better results were obtained in the tests than are claimed
in the example. This was done to illustrate the degree of conserva-
tism required, since it should be understood that the problem of wave
reduction can be very complex if unusual conditions prevail.

The data and sample problem given here are for conditions
within the limits described. From these data it should be possible to
design a wave suppressor for general use with a good degree of accu-
racy. Care should be taken, however, that the data are not extended
beyond the limits given, When any doubt exists, a model study should
be made, particularly if the wave reduction must be accomplished be-
cause of a measuring device located immediately downstream from the
suppressor, Additional model tests will be required to be certain that
the limited amount of data, from which these conclusions were drawn,
represent typical problems encountered in the design of field structures.

31



STILLING BASIN DESIGN AND WAVE SUPPRESSORS

FOR CANAL STRUCTURES, OUTLET WORKS AND DIVERSION DAMS

(Basin IV in Frontispiece)

In this section the characteristics of the hydraulic jump and
the design of an adequate stilling basin for Froude numbers between
2.5 and 4.5 are discussed. This range is encountered principally
in the design of canal structures, but occasionally diversion dams
and outlet works fall in this category. In the 2.5 to 4.5 Froude num-
ber range, the jump is not fully developed and the previously dis-
cussed methods of design do not apply. The main problem concerns
the waves created in the hydraulic jump, making the design of a suit-
able wave suppressor a part of the stilling basin problem.

Four means of reducing wave heights are discussed. The
first is an integral part of the stilling basin design and should be
used only in the 2.5 to 4.5 Froude number range. The second may
be considered to be an alternate design and may be used over a
greater range of Froude numbers. These types are discussed as a
part of the stilling basin design. The third and fourth devices are
considered as appurtenances which may be included in an original
design or added to an existing structure. Also, they may be used in
any open channel flow-way without consideration of the Froude num-
ber. These latter devices are described under the heading Wave
Suppressors.

Jump Characteristics - Froude Numbers 2.5 to 4.5

For low values of the Froude number, 2.5 to 4.5, the en-
tering jet oscillates intermittently from bottom to surface, as indi-
cated in Figure 9B, with no particular period. Each oscillation
generates a wave which is difficult to dampen. In narrow structures,
such as canals, waves may persist to some degree for miles. As
they encounter obstructions in the canal, such as bridge piers, turn-
outs, checks, and transitions, reflected waves may be generated
which tend to dampen, modify, or intensify the original wave. Waves
are destructive to earth-lined canals and riprap and produce unde-
sirable surges at gaging stations and in measuring devices. Struc-
tures in this range of Froude numbers are the ones that require the
most maintenance. In fact, it has been necessary to replace or re-
build a number of existing structures in this category.

On wide structures, such as diversion dams, wave action is
not as pronounced since the waves can travel laterally as well as
parallel to the direction of flow. The combined action produces some
dampening effect but also results in a choppy water surface. These
waves may or may not be dissipated in a short distance. Where outlet
works, operating under heads of 50 feet or greater, fall within the

20


basin for use on small structures where tail water is nonexistent
or indefinite, "

The entire study as planned and given below has not been com-
pleted. Laboratory Report No, Hyd-399, designated ''Progress Report
Il, Research Study on Stilling Basins, Energy Dissipators and Associ-
ated Appurtenances, " is divided into six sections and covers the first
6 items of the planned program. The other items will be completed
as time and funds permit. Items 3,4, and 6 pertain to canal structures
in general and are the items included inthis issue of the Bulletin. For
the benefit of those desiring additional detail and the technical analyses
of the problems involved, report Hyd-399 can be obtained by writing the
Assistant Commissioner and Chief Engineer, Bureau of Reclamation,
Denver Federal Center, Denver, Colorado.

SCOPE

1. General investigation of the hydraulic jump on a horizontal
apron,

2,  Stilling basin with horizontal apron, utilizing chute blocks at
the upstream end anda dentated sillat downstream end such as are often
used on high dam and earth spillways. The appurtenances modify the
jump, causing it to form ina shorter than normal length.

3. Unusually short type of stilling basin suitable for canal struc-
tures, small outlet works, and small spillways where baffle blocks
are used to effect a further shortening of the jump.

4, Stilling basin, alternate basin, and two types of wave suppres-
sors, for use on canal structures, outlet works, and diversion dams.

5.  Stilling basin with sloping apron for large capacities and high
velocities, where appurtenances in basin are undesirable.

6, Extremely short impact-type stilling basin for use on outlets
where tail water is nonexistent or unknown.

7. Overchute type of dissipator where baffle blocks distributed
over the entire length and width of the chute dissipate the energy in
the water as it falls.

8, Stilling basin for diversion dams where temporary retrogres-
sion is expected.

ii


over a wide range of conditions. There are poor jumps and good
jumps, with the most satisfactory occurring over the center portion
of the curve.

Another method of expressing the energy absorption ina
jump is to express the loss, EL, in terms of Dj, The curve to the
right on Figure 8, shows the ratio ne plotted against the Froude

1
number. Losses in feet of head are obtained by this method. As
there are those who prefer this method of plotting, the latter curve
has been included.

The hydraulic jump may occur in at least four different dis-
tinct forms on a horizontal apron, as shown on Figure 9. Incidentally,
all of these forms are encountered indesign. The internal character-
istics of the jump and the energy absorption in the jump vary with each
form. Fortunately these forms, some of which are desirable and some
undesirable, can be catalogued conveniently with respect to the Froude
number.

The form shown in Figure 9A can be expected when the Froude
number ranges from 1.7 to 2.5, When the Froude number is unity, the
water would be flowing at critical depth; thus, a jump could not form.
This would correspond to Point 0 on the specific energy diagram of Fig-
ure 4, For the values of Froude number between 1.0 and 1.7, there
is only a slight difference in the conjugate depths Dj and D2. A slight
ruffle on the water surface is the only apparent feature that differenti-
ates this from flow with uniform velocity distribution. As the Froude
number approaches 1.7, a series of small rollers develop on the sur-
face as indicated in Figure 9A, and this action remains much the same
but with further intensification up to a value of about 2.5. Actually,
there is no particular stilling basin problem involved; the water sur-
face is quite smooth, the velocity throughout is fairly uniform, and the
energy loss is low.

Figure 9B indicates the type of jump that may be encountered
at values of the Froude number from 2.5 to 4.5, This is an oscillating
type of action, so common in canal structures, where the entering jet
oscillates from bottom to surface and back again with no regular per-
iod, Turbulence occurs near the bottom one instant and entirely on
the surface the next. Each oscillation produces a large wave of irreg-
ular period which, in the case of canals, can travel for miles doing
unlimited damage to earth banks and riprap. The case is of sufficient
importance that a separate section has been devoted to the practical
aspects of design.

A well-stabilized jump can be expected for the range of Froude
numbers between 4,5 and 9 (Figure 9C). In this range, the downstream
extremity of the surface roller and the point at which the high-velocity
jet tends to leave the floor practically occur in the same vertical plane.
The jump is well balanced, and the action is thus at its best. The energy



4, No particular difficulty is encountered in the form shown
on Figure 9C. Arrangements of baffles and sills will be found
valuable as a means of shortening the length of basin.

5. As the Froude number increases, the jump becomes more
sensitive to tail-water depth. For numbers as low as 8, a tail-
water depth greater than the conjugate depth is advisable, to be
certain that the jump will stay on the apron, This phase will be
discussed in more detail.

Conclusions

The foregoing experiments and discussion serve to associate
the Froude number with stilling-basin design where it offers many ad-
vantages, The ratio of conjugate depths, the length of jump, the type
of jump to be expected, and the losses involved have all been related
to this number. The principal advantage of this form of presentation
is that one may see the entire picture at a glance, The foregoing in-
formation is basic to the understanding of the hydraulic jump. The
following sections deal with the more practical aspects, such as modi-
fying the jump by baffles and sills to increase stability and shorten the
length,

An example follows which may help clarify the information so
far presented,

Water flowing under a sluice gate discharges intoa rectangular
stilling basin the same width as the gate. The average velocity and
the depth of flow after contraction of the jet is complete are V1 = 85 ft/sec
and Dj = 5.6 feet. Determine the conjugate tail-water depth, the length
of basin required to confine the jump, the effectiveness of the basin to
dissipate energy, and the type of jump to be expected.

Vv
F, - —_- 85 = 6,34

Vedi

Entering Figure 5 with this value

(32,2 x 5.6

De

= 8.5
Dr

The conjugate tail-water depth
D2 = 8.5 x 5.6 = 47.6 feet

Entering the recommended curve on Figure 7 with a Froude
number of 6,34

L. 6.13
Dy



INTRODUCTION

This is Release No. 14 of the Operation and Maintenance
Equipment and Procedures Bulletin and is being devoted to studies
made on stilling basins, energy dissipators and associated appurte-
nances by the Bureau's Hydraulic Laboratory, located at the Denver
Federal Center, Denver, Colorado, The full study made by Engi~
neers J. N. Bradley and A, J. Peterka was published as Hydraulic
Laboratory Report No, Hyd-399, The portion of the report condensed
and given in this issue of the Bulletin includes only that part of the
study devoted primarily to canal structures or necessary to under-
stand the material on canal structures,

A forward to the report, by Engineer H. M. Martin, Chief
of the Hydraulic Laboratory, explains the need for the study in the
design of irrigation works and is quoted in part below:

"* %* * Since 'Stilling Basins' did not lend themselves to
mathematical analysis, they became the center of discussion be~
tween laboratory and design personnel. It was realized that un-
explainable gaps existed in the available information resulting in
uncertainty, confusion, and sometimes apparent contradiction,
when stilling basin designs were attempted without individual hy-
draulic model tests. To resolve these differences, to close the
gaps, and to generalize the design of stilling basins, the labora-
tory's general investigation program for the past 2 years has in-
cluded a coordinated program of stilling basin research,

"As the study progressed and the outcome became increas-~
ingly promising, numerous requests for design criteria even in
draft form were received. To satisfy this immediate demand, a
tentative and limited edition of Hydraulic Laboratory Report No,
Hyd-380 was issued. Comments and criticism were invited.

"The immediate requirement having been temporarily satis-
fied, the tentative edition was next given a critical review and
certain parts were rewritten for the sake of clarity. More in-
formation, along with more definite design limits, was rewritten
into other parts. New material, gathered since the first publish-
ing, has been added to meet some of the deficiences of the tenta-
tive edition. The written material has been broken down into
more titled units to make the report more useful asareference
volume, To also aid in this respect, a pictorial summary of
the six sections of the report has also been added as a Front-
ispiece. Section 4 has been entirely rewritten to include the
most recent developments in wave suppressors for open channels.
Section 6 is entirely new and presents an economical stilling



Then channel approach velocity, V1 = 2 =

2,400

TO +T. 95730 = 6.7 ft/sec

_ 1)? (6.7)?
2g «64.4

= 0.70 foot

and h = 1.95 - 0.70 = 1,25 feet

To refine the calculation, the above computation is repeated
using the new head

2,400

Vi= TO FT, 25750 = 7,1 ft/sec

hy = 0.72 foot

and h = 1.17 feet
Further refinement is unnecessary.

Thus, the average water surface upstream from the under-
pass is 1.2 feet higher than the tail water which satisfies the assumed
design requirement of a maximum backwater of 15 inches. The length
of the underpass is 2D9 or 20 feet, and the waves are reduced 80 per-
cent to a maximum height of approximately 1 foot.

If it is desired to reduce the wave heights still further, a
longer underpass is required. Using Table 7 and Figure 29B as in the
above problem, an underpass 3.5 to 4.0D2 or 35 to 40 feet in length

-reduces the waves 90 to 93 percent, making the downstream waves
approximately 0.5 foot high and creating a backwater, h, of 1.61 feet.

In using the above heads, allowance should be made for waves
and surges which, in effect, are above the computed water surface,
One-half the wave height or more, measured from crest to trough,
should be allowed above the computed surface, Full wave height would
provide a more conservative design for the usual short period waves
encountered in flow channels.

The headwall of the underpass should be extended to this same
height and a seawall overhang placed at the top to turn wave spray back
into the basin. An alternate method would be to place a cover, say 2D5
long, upstream from the underpass headwall.

To insure obtaining the maximum wave reduction for a given
length of underpass, a 4:1 sloping roof should be provided at the down-
stream end of the underpass, as indicated on Figure 28. This slope

30


and the tail-water curve for the river, identified by the solid line
on Figure 21, proportion a Type III basin for the most adverse con-
dition, utilizing full conjugate tail-water depth. The flow is sym-
metrical and the width of the basin is 50 feet. (The purpose of this
example is to demonstrate the use of the jump height curve. )

The first step is to compute the jump height curve. As V1
and Dj are given, the Froude number is computed and tabulated in
Column 2, Table 5, below:

Table 5

Q a Dy Dy Jump height elevation

cfs Fi 1 Te Te Curve a Curve a!

qy =, wo 6 6) (6) 0)
3,900 11.42 15.75 1,130 17.80 617.5 615.0
3,090 12,02 16.60 0.936 15,54 615.2 612.7
2,022 13.85 19.20 0.642 12.33 612.0 609.5

662 17,62 24.50 0.260 6.37 606.1 603.6

Entering Figure 11 with these values of the Froude number, values of

TW are obtained for conjugate tail-water depth from the solid line.

Dy

These values are also ne and are shown listed in Column 3. The con-
1

jugate tail-water depths for the various discharges, Column 5, were

obtained by multiplying the values in Column 3 by those in Column 4,

If it were assumed that the most adverse operating condition
occurs at the maximum discharge of 3,900 cfs, the stilling basin apron
should be placed at elevation 617.5 - 17.8 or elevation 599.7,

With the apron at elevation 599,7 the tail-water required for
conjugate tail-water depth for each discharge would follow the eleva-
tions listed in Column 6, Plotting Columns 1 and 6 on Figure 21 re-
sults in Curve a, which shows that the tail-water depth is inadequate
for all but the maximum discharge.

The tail-water curve is unusual in that the most adverse tail-
water condition occurs at a discharge of approximately 2, 850 cfs rather
than maximum, As full conjugate tail-water depth is desired for the
most adverse tail-water condition, it is necessary to shift the jump
height curve downward to match the tail-water curve for a discharge of
2, 850 cfs (see Curve a', Figure 21). The coordinates for Curve a' are
given in Columns 1 and 7, Table 5. This will place the basin floor 2.5
feet lower, or elevation 597.2 feet, as shown in the sketch on Figure 21.

Although the position of the basin floor was set for a discharge

of 2,850 cfs, the remaining details are proportioned for the maximum
discharge 3, 900 cfs.

18


Cover Photograph

Performance of wave suppressor--Flatiron Afterbay Dam
Outlet Works--Colorado-Big Thompson Project, Colorado.


to less than 2 feet with an opening of 11 feet. Smaller openings pro-
duced less wave height reductions, due to the turbulence created at
the underpass exit. Thus, it may be seen that an opening of from 10
to 12 feet produced optimum results.

With the opening set at 11 feet, the suppressor effect was
then determined for other discharges. These results are shown on
Figure 27, Test 2. Wave height reduction was about 78 percent at
5, 000 second-feet, increasing to about 84 percent at 2, 000 second-
feet. The device became ineffective at about 1,500 second-feet, when
the depth of flow became less than the height of the roof.

To determine the effect of suppressor length on the wave re-
duction, other factors were held constant while the length was varied.
Tests were made on suppressors 10, 21, 30, and 40 feet long for dis-
charges of 2,3,4, and 5 thousand second-feet, Figure 27, Test 3.

Roof lengths in terms of the downstream depth Dg for 5, 000 second-feet
were 0.62D9, 1.31Dg9, and 2.5Dg9, respectively. In terms of a 20-foot-
long underpass, halving the roof length almost doubled the downstream
wave height while doubling the 20-foot length almost halved the result-
ing wave height.

The same type of wave suppressor was successfully used in
an installation where it was necessary to obtain optimum wave height
reductions, since flow from the underpass discharged directly into a
Parshall flume, in which it was desired to obtain accurate discharge
Measurements. The capacity of the structure was 625 cubic feet per
second, but it was necessary for the underpass to function for low flows
as well as for the maximum, With an underpass 3.5Dp2 long and set as
shown in Figure 28, the wave reductions were as shown in Table 6.

Table 6

WAVE HEIGHTS IN FEET-- PROTOTYPE
Maximum Head

Discharge 625 550 400 200 100
in cfs pstream*|/Downstream*/ U[D [U] D/[ U|D{U]D
Wave heights} **3.8 0.3 4,210.3)/4.5/0.4/3.6/0. 4/1. 7/0.3
in feet plus

*Upstream station is at end of stilling basin. Downstream
station is in Parshall flume.
**Recorder pen reached limit of travel in this test only.

Figure 28 shows some of the actual wave traces recorded by an
oscillograph. Here it may be seen that the maximum wave height, meas-
ured from minimum trough to maximum crest did not occur on succes-
sive waves. Thus, the water surface will appear smoother to the eye
than is indicated by the maximum wave heights recorded in Table 6.

26



range of Froude numbers between 2.5 and 4.5, a model study of the
stilling basin is imperative. A model study is the only means of
including preventive or corrective devices in the structure so that
proper performance can be assured.

Stilling Basin Design - Froude Numbers 2.5 to 4,5

Development Tests

The best way to combat a wave problem is to eliminate
the wave at its source; in other words, concentrate on altering
the condition which generates the wave. In the case of the still-
ing basin preceded by an overfall or chute, two schemes were
apparent for eliminating waves at their source. The first was
to break up or eliminate the entering jet, shown on Figure 9B,
by opposing it with directional jets deflected from baffle piers
or sills. The second was to bolster or intensify the roller, shown
in the upper portion of Figure 9B, by directional jets deflected
from large chute blocks.

The first method was unsuccessful in that the number and
size of appurtenances necessary to break up the roller occupied
so much volume that these in themselves posed an obstruction to
the flow. This conclusion was based on tests in which various
shaped baffle blocks and guide blocks were systematically placed
in a stilling basin in combination with numerous types of spreader
teeth and deflectors in the chute. The program involved dozens
of tests, and not until all conceivable ideas were tried was this
approach abandoned. A few of the basic ideas tested are shown
on Figure 22, a, b, c, f, g, andh.

Final Tests

Deflector blocks. The second approach, that of attempt-
ing to intensify the roller, yielded better results. In this case,
large blocks were placed well up on the chute, while nothing
was installed in the stilling basin proper. The object in this
case was to direct a jet at the base of the roller in an attempt
to strengthen it. After a number of trials, the roller was actu-
ally intensified, which did improve the stability of the jump.
Sketches d and e on Figure 22 indicate the only schemes that
showed promise, although many variations were tried. After
finding an arrangement that was effective, it was then attempted
to make the field construction as simple as possible. The di-
mensions and proportions of the deflector blocks as finally adopted
are shown on Figure 23.

The object in the latter scheme was to place as few ap-
purtenances as possible in the path of the flow, as volume occu-
pied by appurtenances helps to create a backwater problem, thus
requiring higher training walls. The number of deflector blocks

21



Sarrying D , over to the left side of the equation and substi-
tuting F 2 for V12,
gD,

Dg VV 2
=- + 4+2
1/2 1/ F

1

or

22 2 1/2(Vi+ ar? -1) (3)

1

5]

Expression (3) shows that the ratio of conjugate depths is
strictly a function of the Froude number. The ratio 72 is plotted with
1

respect to the Froude number on Figure 5, The line, which is virtu-
ally straight except for the lower end, represents the above expression
for the hydraulic jump; while the points are experimental. The agree-
ment is quite good for the entire range. There is an unsuspected char-
acteristic, however, which should be mentioned.

Although the tail-water depth was sufficient to bring the front
of the jump to Section 1 (Figure 4) in each test, the ability of the jump
to remain at Section 1 for a slight lowering of tail-water depth became
more difficult for the higher and lower values of the Froude number.
The jump was least sensitive to variation in tail-water depth in the mid-
dle range, or values of Fj from 4.5 to 9,

In cases where chutes or overfalls were used, the front of
the jump was held near the intersection of the chute and the horizontal
floor, as shownon Figure 4. The length of jump was measured from
this point to a point downstream where either the high-velocity jet be-
gan to leave the floor or to a point on the surface immediately down-
stream from the roller, whichever was the longer. In the case of Flume
F, where the flow discharged from a gate onto a horizontal floor, the
front of the jump was maintained just downstream from the completed
contraction of the entering jet. The point at which the high-velocity jet
begins to rise from the floor is not fixed, but tends to shift upstream
and downstream. This is also true of the roller onthe surface. The
ratio of length of jump to the conjugate tail-water depth Dg is plotted
with respect to the Froude number on Figure 7, This method of plotting
was used throughout the study. The points represent the experimental
values,

In addition to the curve established by the test points, curves
representing the results of three other experimenters are shown on
Figure 7. The greater portion of curve 1 is at variance with the present



absorption in the jump for Froude numbers from 4.5 to 9ranges from
45 to 70 percent (Figure 8).

As the Froude number increases above 9, the form of the
jump gradually changes to that shown in Figure 9D. This is the case
where Vj is very high, Dj is comparatively small, and the difference
in conjugate depths is large. The high-velocity jet no longer carries
through for the full length of the jump. In other words, the downstream
extremity of the surface roller now becomes the determining factor
in judging the length of the jump. Slugs of water rolling down the front
face of the jump intermittently fall into the high-velocity jet, gener-
ating additional waves downstream, A rough surface can prevail, Fig-
ure 8 shows that the energy dissipation for this case may reach 85 per-
cent,

The limits of the Froude number given above for the various
forms of jump are not definite values, but overlap somewhat, depend-
ing on local factors, Returning to Figure 7, it is found that the length
curve catalogs the various phases of the jump quite well. The flat por-
tion of the curve indicates the range of the best operation. The steep
portion of the curve to the left definitely indicates an internal change
in the form of the jump. In fact, two changes are manifest, the form
shown in Figure 9A and the form, which might better be called a tran-
sition stage, shown in Figure 9B, The right end of the curve on Fig-
ure 7 also indicates a change in form, but to less extent.

Practical Considerations

As stated previously, it was the intention to stress the aca-
demic rather than the practical viewpoint in this section. An exception
has been made, as this is the logical place to point out a few of the
practical aspects of stilling basin design using horizontal aprons. View-
ing the four forms of jump just discussed, the following is pertinent:

1. All forms are encountered in stilling-basin design.

2, The form in Figure 9A requires no baffles or special con-
sideration. The only requirement necessary is to provide the
proper length of pool, which is relatively short. This can be ob-
tained from Figure 7.

3. The form in Figure 9B is one of the most difficult to handle
and is frequently encountered in the design of canal structures, di-

version dams, and even outlet works. Baffle blocks or appurtenances

in the basin are of little value. Waves are the main source of dif-
ficulty, and methods for coping with them are discussed, The pres-
ent information may prove valuable in that it will help to restrict the
use of jumps in the 2,5 to 4.5 Froude number range. In many cases
its use cannot be avoided, but in other cases, altering of dimensions
may bring the jump into the desirable range,