Hydraulic
Design
f Stilling
Basin
Channel
Outlets
Research
Report
No. 24
l
A WATER RESOURCES
TECHNICAL
PUBLICATION
Hydraulic
Designof
StillingBasinfor Pipe
or Channel
Outlets
Basin VI in the
Bureauof Reclamation
designation
By G. L. Beichley
Division of Research
Denver, Colorado
UNITED
STATES
DEPARTMENT
BUREAU
OF THE
OF RECLAMATION
INTERIOR
As the Nation’s
principal
conservation
agency, the Department
of the Interior
has
responsibility
for most of our nationally
owned public lands and natural
resources.
This includes fostering
the wisest use of our land and water resources, protecting
our fish and wildlife,
preserving
the environmental
and cultural
values of our
national
parks and historical
places, and providing for the enjoyment
of life through
outdoor recreation.
The Department
assesses our energy and mineral resources and
works to assure that their development
is in the best interests
of all our people.
The Department
also has a major responsibility
for American
Indian
reservation
communities
and for people who live in Island
Territories
under
U.S. administration.
First Printing: 1971
Revised Reprint: 1976
Revised Reprint: 1978
UNITED
STATES
GOVERNMENT
DENVER:
PRINTING
1978
OFFICE
PREFACE
High-energy
forces in flowing or falling water must
be contained
or dissipated
to prevent
damaging
scour
or erosion of downstream
channels.
Various
means for energy dissipation
are employed
at hydraulic
installations.
Stilling
basins are among
the most common.
Ten types, I through
X, are used
by the Bureau of Reclamation.
(The Roman numeral
classifications
are internal
Bureau designations.)
The
variety of operating
conditions
necessitates
this wide
range of stilling
basin designs.
Criteria
for design of the 10 stilling basin types were
first summarized
in Engineering
Monograph
No. 25,
published
in 1958
and revised
in 1963.
The
monograph
was based on a series of earlier papers and
laboratory
reports.
This study of the type VI stilling basin, which is used
for pipe or open channel
outlets,
was made to
standardize
and modify
existing and previously
used
procedures
in the design of this impact stilling basin.
Development
of the type VI short impact-type
basin
originated
with a need for some 50 or more stilling
structures
on a single irrigation
project.
Relatively
small basins providing
of a tailwater
curve
required.
The information
resource centers,
industrial
irrigation
Included
energy dissipation
independent
or tailwater
of any kind were
in this report is intended
for water
government
agencies, municipal
and
water
systems
in this
operators,
designers.
publication
and
hydraulics
and
is an informative
abstract
with a list of descriptors,
or keywords,
and
identifiers.
The abstract
was prepared
as part of the
Bureau
of Reclamation’s
program
of indexing
and
retrieving
the literature
of water
resources
development.
The descriptors
were selected from the
Thesaurus
of Descriptors,
which
is the Bureau’s
standard
for listing of keywords.
CONTENTS
Page
...
Preface
.....................................................
Introduction
.................................................
..................................................
The Models
..............................................
The Investigation
111
1
3
5
Standardization
of the basin dimensions
in terms of basin width
...........................
Standardization
of the basin flow entrance
Standardization
of the basin size ..................................
.....................
Standardization
of the entrance
velocity
limitation
.......................
Standardization
of the discharge channel riprap
.....................................
Tailwater
recommendations
Performance
evaluation
........................................
Alternate
end sill design ........................................
....................................
Debris barrier and trashrack
..........................................
Self-cleaning
feature
............
.............................
Design Conclusions
and Recommendations
...................................................
References
. ..... ..... .... ...... .... ..... .... ...... ... ...... ...
Abstract
5
5
7
8
19
19
19
24
24
24
27
29
31
TABLE
Page
Table
1
Design specifications
for the outlet
Picacho North and South Dams
works structure
at
. . . . . . . . . . . . . . . . . . . . . . . . . . .
25
FIGURES
Page
Figure
1
2
3
4
5
6
7
8
9
10
General design of the type VI impact stilling basin
. . .
Test flows with uncontrolled
tailwater
. . . . . . . . . . . .
Test flows with controlled
tailwater
. . . . . . . . . . . . . .
Entrance
pipe flowing full with uncontrolled
tailwater
in
2.4-foot
(730 -mm) wide basin . . . . . . . . . . . . . . . . .
Erosion for uncontrolled
tailwater
with entrance
pipe
flowing
full in 2.4-foot
(730-mm)
wide basin . . . . . . .
Entrance
pipe flowing half full with uncontrolled
tailwater
in 2.4-foot
(730-mm)
wide basin
. . . . . . . . . . . . . . .
Erosion for uncontrolled
tailwater
with entrance
pipe
flowing half full in 2.4.foot
(730.mm)
wide basin . . . .
Design width of basin
. . . . . . . . . . . . . . . . . . . . . . .
Entrance
pipe flowing one-fourth
full with uncontrolled
tailwater
in 2.4-foot
(730-mm)
wide basin . . . . . . . . .
Entrance
pipe flowing three-fourths
full with uncontrolled
tailwater
in 1.6-foot
(490-mm)
wide basin . . . . . . . . .
V
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
4
6
7
. . . . . . . . . . . .
9
. . . . . . . . . . . .
10
. . . . . . . . . . . .
11
. . . . . . . . . . . .
. . . . . . . . . . . .
12
13
. . . . . . . . . . . .
14
. . . . . . . . . . . .
15
CONTENTS-Continued
Page
Figure
11
12
13
14
15
16
17
Entrance
pipe flowing half full with uncontrolled
tailwater
in 1.6-foot
(490-mm)
wide basin
. . . . . . . . . , . . . . .
Entrance
pipe flowing one-fourth
full with uncontrolled
tailwater
in 1.6-foot
(490-mm)
wide basin . . . . . . . . .
Prototype
operation
. . . . . . . . . . . . . . . . . . . . . . . .
End sill velocity,
water surface drop from end sill, and
energy loss through
basin . . . . . . . . . . . . . . . . . . . .
Recommended
riprap stone size . . . . . . . . . . . . . . . . .
Entrance
pipe flowing full with controlled
tailwater
in
2.4-foot
(730-mm)
wide basin . . . . . . . . . . . . . . . . .
Entrance
pipe flowing half full with controlled
tailwater
in 2.4-foot
(730-mm)
wide basin
. . . . . . . . . . . . . . .
vi
_.-.
__...
. . . . . . . . . . . .
16
. . . . . . . . . . . .
. . . . . . . . . . . .
17
18
. . . . . . . . . . . .
. . . . . . . . . . . .
20
21
. . . . . . . . . . . .
22
. . . . . . . . . . . .
23
INTRODUCTION
revision
of these design
standards.
Unforeseen
operating
conditions
in the field
and the
over-generalization
of the present
design rules have
This study was conducted
to standardize
and modify
existing procedures
used in the design of the impact,
type VI stilling basin.
Stilling Basin VI, as referred to in section 6 of report
and in Engineering
Monograph
No. Hyd-399
[ I]’
No.
25 [Z],
is an impact-type
energy
caused operating
structures.
Four
principal
dissipator,
refer
operating
at some of the prototype
problems
that
have
occurred
at various
installations
are: (1) the basin
tends to clog with debris upstream
of the hanging
baffle. Russian thistles and similar weeds are the main
contained
in a relatively
small boxlike structure
which
requires
no tailwater
for successful
performance.
Although
the emphasis in this discussion
is placed on
use with pipe outlets, the structure
may be used with
an open channel
chute.
It was originally
developed
for use as an energy
dissipator
at several locations
on the Franklin
Canal.
Many
of these basins are in use on other
Bureau
projects
and generally
have been designed
in
accordance
with the procedures
outlined
in these two
publications.
However,
operation
of the various
prototype
structures
has revealed
the need for
* Italicized
numbers
in brackets
cited at the end of this report.
problems
source of the debris, which is not generally
a problem
in cultivated
areas. (2) Excessive splash overtopping
the compartment
walls upstream
of the baffle, usually
resulting
from too small a basin for the quantity
and
velocity
of flow involved,
has eroded the fill outside
the basin walls. (3) The discharge
from the entrance
pipe passes under the baffle, resulting
in very little
energy dissipation
in the basin and excessive erosion
of the downstream
channel.
This has occurred
with
a sloping
entrance
pipe or with an oversized
basin
having a horizontal
entrance
pipe discharging
at less
than the design flow. (4) Channel
erosion at the end
of the basin where the size of riprap was not adequate.
to references
1
-___-
__
THE MODELS
Two model
millimeters)
basins,
wide,
1.6 and 2.4 feet
were constructed.
(490 and 730
The other
tests,
as well as flow from
an 8-inch
(200-mm)
rectangular
open channel.
Each of the two basins discharged
into a canal
section lined with 1.5~inch (38-mm)
gravel. Tailwater
dimensions
were related to the width of the basin, as
shown in figure 1.
An 8-inch (200-mm)
inside-diameter
pipe was used
at the entrance
to each of the basins. Deflectors
of
various sizes were installed
on the crown of this pipe
upstream
of the portal to vary the velocity
of flow
entering
the basins.
One-fourth,
one-half,
three-fourths,
and full pipe flows were used in the
elevations
were controlled
by stoplogs at the end of
the canal section. The bottoms
of the canal sections
were at the same elevation
of the basin end sill and
were as wide and as long as the basin width. The side
slopes were 1.5 to 1 for the 2.4-foot
(730-mm)
wide
basin and vertical
for the smaller basin.
3
r
4
SEC. A-A
4 :I
.-
I--
a
-L
PLAN
I
PLAN
-0 :.‘.‘.-6:
:..
..‘.
‘.i?‘.
:
:
‘p,’
La
Bedding/
SECTION
H= 3W/4
L=4W/3
a-W/2
b- 3w/0
ALTERNATE
END SILL AND
WING WALL
c= w/2
d-W/6
e- w/l2
t-w/@
suggested minimum
Riprap stone size diameter = W/Z!0
Figore L-General
SECTION
design of the type VI impact stilling basin.
4
THE INVESTIGATION
intermittently
The investigation
was primarily
concerned
with
relating
the basin size to the discharge
and velocity
and in relating
the downstream
channel
and riprap
requirements
to the basin size. It was also concerned
with special situations
involving
debris, silt, tailwater,
sloping entrance
pipe, and rectangular
open channel
entrances
not usually
encountered
design of the type VI basin.
Standardization
Terms
of Basin
of the
Width
Basin
and splashed
high
on the basin
moved a short distance away from the sidewalls.
Also,
the slope of the top of the basin sidewalls was reduced
to increase the height of the wall at the downstream
end of the basin. The modification
to the notches
reduced
the splashing
and the height
of the water
surface rise on the sidewalls.
Increasing
the height of
the sidewalls
provided
additional
freeboard
at the
downstream
end of the basin. These modifications
are
incorporated
into the standard
design dimensions
shown in figure 1.
in the standard
Dimeneions
surged
walls immediately
downstream
from the hanging
baffle
and overtopped
the sidewalls
at the
downstream
end of the basin (figs. 2 and 3). To
improve
these flow conditions,
the width
of the
notches
in the baffle was reduced
and the notches
in
A test basin was constructed
having
dimensions
related to the basin width in accordance
with those
developed
for the basin in the earlier study [Z].
To test the adequacy
of this 2.4,foot
(730~mm)
wide
model basin, tests were conducted
over a range of
flows that had been determined
in the earlier tests [Z]
to be the limits of exceptionally
mild operation
and
of safe maximum
operation
for a given basin width,
provided
the entrance
flow velocity
did not exceed 30
feet per second (9.1 m/s) (prototype).
These test discharges were related to the basin width
in accordance
with the equation:
Q = (W/C)=
where “Q” is the discharge
in cubic feet per second,
“W” is the inside width of basin in feet, and “C” is
a coefficient
that varies for the maximum,
minimum,
and intermediate
flows. The coefficient
in English
units is 1.46 for the maximum
permissible
flow, 1.80
for the minimum
mild
flow,
and 1.60 for the
intermediate
flow.’ Each test discharge
was run at
approximately
half full and full pipe to obtain highand low-entrance
velocity
conditions
and with
controlled
and uncontrolled
tailwater
depths (figs. 2
and 3).
The larger
flows
with
the higher
velocities
Standardization
of the
Basin
Flow
Entrance
The flow will usually enter the basin from a circular
pipe but may enter from a rectangular
open channel.
The pipe may flow full or partially
full. If it flows
partially
full and the upstream
entrance
to the pipe
is submerged,
the pipe should
be vented
to the
atmosphere.
The vent should
be located
near the
upstream
end of the pipe and have a diameter
of
about one-sixth
the pipe diameter.
Although
the entrance
pipe or channel
is usually
horizontal
or on a very slight downward
grade, some
installations
may require
an entrance
pipe on a
relatively
steep slope. The hydraulic
performance
of
the 2.4-foot
(730-mm)
wide
model
basin
was
determined
with the entrance
pipe sloped downward
about
12’.
Both high- and low-velocity
test flows
partially
impinged
on the hanging
baffle
and the
bottom
of the baffle was only partially
submerged,
resulting
in incomplete
energy dissipation.
The model tests showed that a horizontal
fillet on
the invert
of the pipe for a distance
of one pipe
diameter
upstream
from the portal caused greater jet
impingement
on the baffle,deeper
submergence
of the
bottom
of the baffle, and consequently
better energy
dissipation.
The same improvement
could be obtained
by placing
the entrance
pipe horizontally
for a
distance of one or more pipe diameters
upstream
from
the basin entrance.
Either of these two methods
may
’ 1 ft3/s = 28.3 L/s = 28.3 x 10-s m3/s. To obtain
the discharge
in cubic meters per second (ms/s),
the
width must be in meters and the discharge coefficient
must be multiplied
by 1.27. To obtain the discharge
in liters per second (L/s),
either multiply
the cubic
meters per second value by 1000, or calculate
with
the width in millimeters
and the discharge coefficient
multiplied
by 80.
5
be used for entrance pipe slopes up to 15 0. Entrance
pipes having a downward grade exceeding 15 ° should
performance of the basin. However, flow from the
basin backed up into the open channel, making it
necessary to raise the channel waIls to the same height
as the basin waIls. To further contain the flow, the
invert of the channel should be horizontal for a
distance equivalent to at least two channel widths
upstream from the basin entrance.
be horizontal for at least two diameters upstream
from the basin entrance.
Replacing the sloping entrance pipe in the model
with an 8.inch {200-mm) wide rectangular channel on
a similar slope did not change the hydraulic
v
=
9.9 ft/s
(3.0 m/s)
v
c
Q
v
=
7.89 ft/s
=
=
5.84 ft/s
3.47
ft31s
c
1.60
(98
21.58
(6.6
m/s)
=
17.15
ft/s
(5.2
m/s)
2.76 ft31s (78 Lis)
(1.8 m/s)
v = 12.69 ft/s (3.9 m/s)
C = 1.80
Q = 2.04 ft3/s (58 Lis)
Q = (W IC)2.S where W =basin width of 2.4 feet (730 mm)
V = velocity of flow at entrance
Tailwater
ft/s
Lis)
v
=
=
1.46
(2.4 m/s)
Q =
v
=
elevation in tailbox is below basin end sill
Figure 2.-Test flows with uncontrolled
6
tailwater.
v
=
9.9
v
=
7.89
ft/8
(3.0
ft/s
m/8)
(2.4
v
m/s)
c
Q =
v
=
5.84
Ct/s
(1.8
v
c = 1.46
Q = 3.47(131s(98 Lis)
2.76
=
=
=
21.58
17.15
ft/s
ft/s
(6.6
(5.2
m/s}
m/s)
1.60
fl31s (78 Lis)
v
m/s)
=
12.69
ft/s
(3.9
m/s)
c = 1.80
Q = 2.04 ft31s (58 Lis)
Q = (WIC)2oSwhere W = basin width of 2.4 feet (730 mm)
V = velocity of flow at entrance
Tailwater elevation in the tailbox is at d + bl2 (see fig. 1)
Figure 3.-Test flows with controlled tailwater.
Standardization
of the Basin
Size
by the basin width,
the quantity
and velocity
of flow
After standardizing
the basin dimensions in relation
to the basin width, the next step was to standardize
by the Froude number of the incoming jet.
It was believed that the shape of the incoming jet
was relatively
unimportant
in evaluating
the
the size in relation
adequacy
flow entering
to the quantity
and velocity
of the
the basin. The basin size is represented
standardize
of a type
the method
VI
basin.
Therefore,
of computing
to
the Froude
number
of the incoming
flow,
it was assumed
that
than
the
number
cross sectional
area of the jet in the circular
pipe or
rectangular
channel had the shape of a square; thus,
the depth of the incoming
flow “D”
was considered
to he the square root of its cross sectional
area.
a riprapped
channel
simulated
end of the basin.
Water
at the
surface
In figure 8, the straight
points with the highest
roughness and erosion, together
with the ability of the
basin to contain
the flow, were used as guidelines
in
evaluating
the hydraulic
performance
test flows
corresponds
to a flow
having
a Froude
of 1.1.
Additional
tests were run in this smaller model (figs.
11 and 12) to confirm
the findings found in the larger
model basin. The results of these tests are plotted
in
figure 8 at W/D ratios of 3.8 and 6.1. The two models
showed very good agreement
in what was considered
satisfactory
and unsatisfactory
performance,
as seen
by comparing
figures 4 and 6 with 11 and 12.
The test flows (figs. 2 and 3) used in verifying
the
standard
dimensions
of the basin in reference
II were
repeated
in the 2.4.foot
(730-mm)
wide model basin,
but with
downstream
3, which
line drawn through
the data
Froude
numbers
for which
satisfactory
operation
existed indicates
the minimum
width of basin that can be used for a given Froude
number.
Data points above the line indicate
that it
should be permissible
to increase the size of the basin
approximately
25 percent;
however,
this should not
be done as these points represent
the condition
when
the basin
is operating
at less than
the design
discharge.
If the basin is too large, the incoming
jet
will pass under the baffle as has occurred
at some
installations
and effective
energy dissipation
will not
occur. For best results, the basin should be designed
for the minimum
width indicated
in figure 8.
(figs.
4 through
7). Each of the test flows was judged to be
satisfactory
or unsatisfactory
and plotted
in
dimensionless
terms (Froude number
of the incoming
flow “F”
versus
the ratio of basin width
to the
incoming
depth of the flow ‘*W/D”)
in figure 8.
To increase
the range of data to be evaluated
for
figure 8, the cross sectional
area of the incoming
flow
was reduced
to one-fourth
the area of the 8-inch
(200-mm)
pipe , and the velocity
of the flow entering
the 2.4-foot
(730-mm)
wide model basin was increased
(fig. 9). Thus,
both the Froude
number
and the
width/depth
ratio increased.
The width/depth
ratio
for these tests was 8.15, at which the Froude number
of a theoretical
square jet at the entrance
was 6.70
for the minimum
satisfactory
operation.
Because the
size of the jet was becoming
very small in relation
to
Standardization
Limitation
of the
Entrance
Velocity
In previous
studies [Z], the design criteria
for this
type of structure
were based on discharge
alone. The
maximum
incoming
velocity
was arbitrarily
limited to
30 feet per second
(9.1 m/s).
However,
some
prototype
structures
have been designed
and
operated
at velocities
exceeding
this limit. The type
VI stilling
basins for the outlet
works of Picacho
South and North
Dams were designed
for velocities
up to 39 and 48 feet per second (11.9 and 14.6 m/s),
respectively,
for flows of 165 and 275 cubic feet per
second (4.7 and 7.8 m3/s), respectively.
They have
operated
satisfactorily
at 80 percent
capacity
at
velocities
of 32 and 37 feet per second (9.8 and 11.3
m/s) (fig. 13).
To prevent
the possibility
of cavitation
or impact
damage to the basin, the maximum
entrance
velocity
should be limited
to about
50 feet per second (15
m/s). At this velocity
the maximum
Froude number,
9.00, for which the basin is recommended
will occur
at a design flow of 46 cubic feet per second (1.3 m3/s).
For Froude
numbers
less than about
4, this basin
would not be feasible at this velocity
because of the
enormous
size of the structure
involved.
the width of the basin, the design curve in figure 8
was not extended
beyond
a width to depth ratio of
10, which
corresponded
to flow having
a Froude
number
of about 9.
To increase the range of data in the other direction,
the cross sectional
area of the incoming
flow was
increased
in relation
to the basin width by switching
to the 1.6-foot
(490-mm)
wide
model
while
maintaining
the 8-inch (200-mm)
entrance
pipe (fig.
10). The tests were evaluated
and plotted
in figure 8
at a W/D
ratio
of 3.08.
The side slopes
of the
downstream
discharge
channel
were vertical
and the
same distance
apart as the basin sidewalls.
Although
this was not typical of the usual prototype
installation
and is not recommended,
it was not considered
to be
critical
in evaluating
the performance
of the basin.
For these tests, the Froude number was in the vicinity
of 1.0 and the height of the incoming
flow was near
the top of the baffle.
Therefore,
it did not appear
practical
to design this basin for W/D ratios smaller
8
F = 1.34
WID = 4.06
Satisfactory
F = 1.81
WID = 4.06
Satisfactory
F = 2.27
WID = ~.O6
Unsatisfactory
Note: For erosion results, see figure .5; for plot of these operating conditions, see figure 8.
Figure
4.-Entrance
pipe
flowing
full
with
uncontrolled
9
tailwater
in 2.4-foot
(730-mm)
wide
basin.
F
=
1.34
WID
No
=
4.06
erosion
Satisfactory
F
=
WID
1.81
=
4.06
Erosion
Satisfactory
F
=
WID
2.27
=
Excessive
4.06
erosion
Unsatisfactory
Note: For plot of these operating conditions, see fig lIre 8.
Figure
5.-Erosion
for uncontrolled
tail water
with
entrance
10
pipe
nowing
full
in 2.4.foot
(730.mm)
wide
basin
F = 3.53
WID = 5.98
Satisfactory
F = 4.77
WID = 5.98
Unsatisfactory
F = 6.01
WID = 5.98
Unsatisfactory
Note: For erosion results, see figure 7; for plot of these operating conditions, see figure 8.
Figure
6.-Entrance
pipe flowing
half
full
with
uncontrolled
"ailwater
in 2.4-foot
(730-mm)
wide
basin.
F
=
WID
3.53
=
Minor
5.98
erosion
Satisfactory
F
=
WID
4.77
=
5.98
Excessive
erosion
U nsatisfactory
F
=
WID
6.01
=
5.98
Excessive
erosion
U nsatisfactory
Note: For plot of these operating conditions. see figure 8.
Figure 7.-Erosion for uncontrolled tailwater with entrance pipe nowing half full in 2.4.foot (730-mm) wide basin.
12
IC
9
-
8
-
7
-
6
-
5
-
4
-
n
2
-
3
-
‘(0
at W/D = 3.08,
F = 0.91)
-
2
-
0 - Satisfactory
Hydraulic Performance
X - Unsatisfactory
2
Hydraulic Performance
3
4
FROUDE NUMBER
5
6
7
8
V/AjI7
is the inside width of the basin.
represents the depth of flow entering the basin and is the
square root of the flaw area.
“v” is the velocity of the incoming flow.
The tailwater depth is uncontrolled.
“w”
I’D”
Figure E.-Design width of basin.
9
IO
F
=
WID
No
5.87
=
8.15
erosion
Satisfactory
F
=
WID
No
6.67
=
8.15
erosion
Satisfactory
F
=
WID
Minor
7.59
=
8.15
erosion
Unsatisfactory
Note: For plot of these operating conditions, see figure 8.
Figure
9.-Entrance
pipe
nowing
one-fourth
full
with
14
uncontrolled
tailwater
in 2.4.foot
(730-mm)
wide
basin.
F
=
WID
No
0.91
=
3.08
erosion
Satisfactory
F
=
WID
No
1.07
=
3.08
erosion
Satisfactory
F
=
WID
Minor
1.27
=
3.08
erosion
Unsatisfactory
Note: For plot of these operating conditions, see figure 8.
Figure
IO.-I':ntrance
pipe flowing
three.fourths
full with
uncontrolled
15
tailwater
in 1.6.foot
(490-mm)
wide basin.
F
=
1.01
WID
No
=
3.81
erosion
Satisfactory
F
=
1.56
W/D=3.81
No
erosion
Satisfactory
F
=
WID
1.83
=
Minor
3.81
erosion
Unsatisfactory
F
=
WID
2.17
=
3.81
Excessive
erosion
Unsatisfactory
Note: For plot of these operating conditions, see figure 8.
Figure
11.-i'~ntrance
pipe nowing
half
full
with
uncontn,lIl'd
16
tailwatl'r
in 1.6-foot
(190-mm)
wide
basin.
F = 3.72
W/D=6.14
No erosion
Satisfactory
F
=
5.11
WID
=
6.14
Excessive
erosion
Unsatisfactory
F
=
WID
6.28
=
6.14
Excessive
erosion
Unsatisfactory
Note: For plot of these operating conditions, see figure 8.
Figure
12.-Entrance
pipe
nowing
one.fourth
full
with
uncontrolled
17
tailwater
in 1.6.foot
(490.mm)
wide
basin.
Picacho South Dam outlet works
structure discharging 130 ft3/s
(3.7 m3/ s) (80 percent of maximum capacity).
Picacho North Dam outlet works
structure discharging 210 ft3/s
(5.9 m3/s) (80 percent of maximum capacity).
Scour below Picacho North Dam
outlet works following flood of
August 20, 1954. Evidence points
to undersized riprap.
Note: At full capacity the basins are approximately
13 percent IIndersized, based on present design standards.
Figure 13.-Prototype
18
operation.
Standardization
of the
Discharge
Channel
plotted
in figure
Riprap
flow
Channel
bed erosion tests were not conducted
to
prove
the required
size of stones
in the riprap.
Instead, a reasonable
riprap size was chosen to fit the
size of the basin. Having predetermined
the basin size
and relative size of stones in the riprap, the discharge
Tailwater
capacity
and entrance
velocity
limitations
determined
as already described.
A model riprap
was chosen that approximated
basin width-to-stone
diameter
size appeared
to be reasonable
from
14 is not
the center
as high
as the velocity
of
of the sill.
Recommendations
The effect of tailwater
on the basin efficiency
was
determined
by repeating
the above tests using a
maximum
tailwater
controlled
to a depth of d+ b/2
above the basin floor. [Z] (See fig. 1 for definitions.)
A comparison
of these flow conditions
(figs. 16 and
were
a
ratio of 20 to 1. This
and satisfactory,
as was
confirmed
by the tests described
in the preceding
section on standardization
of basin size. These tests
showed
that slight erosion
of the riprap
began at
about
the same time as excessive
water
surface
roughness
appeared
within
and downstream
of the
basin. The model
stones were rounded,
although
angular ones would be preferred
in the prototype.
The gravel was placed on the channel bottom
at end
still elevation
and on the l-1/2
to 1 side slopes to a
normal depth equal to the height of the end sill (fig. 1)
and for a distance
downstream
equal to the basin
width.
This arrangement
was satisfactory
in the
model
tests and is, therefore,
recommended
for
prototype
construction.
In some instances,
the discharge
channel
bed may
be several
inches or a few feet below the end sill
elevation.
This will considerably
increase the riprap
stone size requirement.
To determine
the increased
riprap
stone size requirement,
the average
flow
velocity
at the end sill was determined.
It was then
related to the average entrance
velocity
and plotted
versus Froude number in figure 14, and plotted
versus
the stone size requirement
in figure 15.
The additional
head as provided
by the lower
channel bed should be added to the velocity
head at
the sill to determine
the velocity
of flow entering
the
channel.
Having
determined
the increased
velocity,
figure 15 can be entered to determine
the riprap stone
size requirement.
The stone size requirement
for end sill velocities
is
compared
in figure 15 with the stone size requirement
for bottom
velocities
in channels
downstream
of
stilling basin [Z]. Th e comparison
indicates
that the
stone size recommendation
here is conservative;
however,
the flow from the sill is in a downward
direction
as there is a drop in water surface from end
sill to channel
(fig. 14). Also, the average
velocity
19
17) with the uncontrolled
(figs. 4 and 6) shows that
tailwater
flow conditions
the water surface roughness
and bed erosion are reduced
by the higher tailwater
but not sufficiently
to allow a reduction
in the basin
size. The riprap stone size could be reduced
slightly
as determined
by,the reduced velocity
using figure 15.
Performance
Evaluation
Energy
dissipation
is initiated
by flow striking
the
vertical
hanging baffle and being turned upstream
by
the horizontal
portion
of the baffle and by the floor,
in vertical
eddies. Its effectiveness
is best illustrated
by plotting
the percent
of energy loss between
the
entrance
portal
and the end sill for a range
of
operating
conditions
as represented
by the Froude
number
(fig. 14). Comparing
the energy loss with the
losses in a hydraulic
jump shows the impact basin to
be more efficient.
Prototype
structures
[Z] that meet these design
standards
have operated
successfully.
The outlet
basins at Picacho South and North
Dams, discharing
at 80-percent
capacity,
are examples
(fig. 13). The
design requirements
for the 80-percent
capacity
and
for the loo-percent
design capacity
are given in
table 1.
For operation
of these structures
at 80-percent
capacity,
the table shows the width
of basin and,
therefore,
the size of basins to be adequate
to meet
design requirements.
However,
for loo-percent
design
capacity,
the table shows the basins to be about 13
percent
undersized
based on the design standards
presented
herein
(fig. 8). The actual
performance
proved this to be true (fig. 13).
The prototype
structures
at Picacho
South
and
North
Dams
can also be used
to verify
the
recommended
size of riprap.
According
to
construction
specifications
for both dams, the riprap
below the outlets was to “* * * consist of durable rock
velocity
velocity
at
to 1,he basin.
entrance
\
over
the
\
i i i I
I i i i i i i i i 7--H4J
elevation
.06
from
end sill
nnel with
the
to
the
.05
.03
.--ns!
7
L” is the energy
loss in the
flow from
basin
entrance
to the end sill.
is the flow
energy
at the
.6
.5
/I
I
0
I
2
I
FROUOE
(Where
area
Figure
IL-End
“D”
of
sill velocity,
I
3
I
I
4
surface
I
5
NUMBER
is the sqllare
the entrance
water
I
root
flow
drop
front
6
I
7
I
I
8
I
I
9
= v/m
of the cross-sectional
area.)
end sill, aud energy
20
___
loss through
basin.
VELOCITY
0
I
IN
METERS
2
PER
3
SECOND
4
5
IO00
900
CnJO
W
I
:
500
f5
+
I”
4
400
0
WN
300 uY
:
200
100
0
0
0
2
4
VELOCITY
6
6
IO
IN
FEET
I2
14
PER
16
I6
SECOND
Note: The riprap
should
he composed
of a well-graded
-mixture but most of the stones should bc of the size indicated
by the curve.
End sill velocity
in riprap.
in type
VI Basin
.___ Bottom
velocity
in a channel
riprap. (See figure 165 in reference
Fire
15.-Recommended
21
vs stone size required
vs stone
2)
riprap
size required
stone
size.
in
E
u)
F
=
1.34
WID
=
No
4.06
erosion
Satisfactory
F
=
1.81
WID
No
=
4.06
erosion
Satisfactory
F
=
WID
2.27
=
4.06
Excessive
erosion
Unsatisfactory
Note: Tailwater
Figure
16.-Entrance
pipe
flowing
full
with
= d + b/2; see figure 1 for definitions.
controlled
22
tailwater
in 2.4-foot
(730-mm)
wide
basin.
F
=
3.53
WID
No
=
5.98
erosion
Satisfactory
F
=
4.77
WID
=
Minor
5.98
erosion
Unsatisfactory
F
=
WID
6.01
=
Excessive
5.98
erosion
Unsatisfactory
Note: Tailwater
Figure 17.-Entrance
= d + b/2; see figure 1 for definitions.
pipe flowing half full with controlled
23
tailwater in 2.4.foot (730.mm) wide basin.
fragments
reasonably
graded
in size * * *” from
3.4
channel
wave
cubic feet (95 dms) to 0.1 cubic foot (3 dm3). The
individual
rocks would
range from about
18- to
5.5.inch
(460- to 140-mm)
cubes and have a mass of
500 to 15 pounds
(225 to 7 kg). Although
it is
impossible
Dam (fig.
Debris
from the photograph
of the outlet at North
13) to determine
the size of stones in the
Sill
reduce
erosion
tendencies
and
heights.
Barrier
At some
and
prototype
Traehrack
installations,
have
portal
weeds
and debris
been trapped
in the
and the baffle.
This
debris has compacted
to the extent of blocking
the
portal,
thus reducing
the capacity
of the structure.
The compacted
weeds will not wash out and are very
difficult
to remove.
The only satisfactory
field
method
of removing
the debris has been to destroy
at the beginning
of the run; but, here again, the bank
riprap
and the waterfall
effect of the flow over the
end sill in figure 13 indicate
that there is a drop from
the end sill to the channel,
as shown in the table.
Therefore,
the majority
of the stones in the riprap
should
be 28 inches
(710 mm) in diameter
as
recommended
here. Since the specified
stones were
smaller than this size, the riprap
would be expected
to fail and did.
At South
Dam,
the photographs
of the outlet
discharging
do not show a waterfall
effect from the
end sill. Therefore,
the riprap was probably
nearer to
end sill elevation
than specified
in the table. This
would
reduce
the required
stone
diameter
to
something
less than 18 inches (460 mm), but greater
than 8 inches (200 mm). Since this range is within that
specified,
the riprap would be expected
to remain in
place and did.
End
thereby,
such as Russian
thistles
basin between
the pipe
channel riprap at the start of the run, the bank riprap
indicates
that there were very few pieces of the
500-pound
(225-kg)
size. The few remaining
pieces
hear the man at the right seem to be in the upper
range of sizes and are not washed
out.
It is also
difficult
to determine
the elevation
of the channel bed
Alternate
and,
portions
of the baffle.
This condition
was tested in the two models using
Russian
thistle
branches.
The model demonstrated
that
the thistles
would
not wash out and no
satisfactory
method
of making the basin self-cleaning
of weeds and debris was developed.
At structures
where thistles or other debris are likely
to be a problem,
it is suggested that screening be used
to cover the upstream
portion
of the basin and that
a screen or trashrack
device be used where the flow
enters the pipe to the basin.
Self-Cleaning
Feature
Sediment
may accumulate
in the basin below the
hanging baffle during periods of nonuse. The notches
were installed
in the baffles
to provide
an opening
through
which a jet would discharge to begin erosion
and removal
of the sediment
from the basin.
The 2.4-foot
(730-mm)
wide basin was operated
with the portion of the basin below the hanging baffle
blocked
to simulate
a sediment-filled
basin. It was
determined
from this test that the design discharges
could be passed over the top of the baffle with very
little splashing
outside the basin and, in general, only
minor
erosion
in the riprapped
area. This type of
operation
could be tolerated
for a limited
time while
sediment
is being washed
from the basin.
If it is
anticipated
that the basin beneath
the baffle
will
remain relatively
free of sediment,
the notches
may
Design
The alternate
end sill design (fig. 1) having
45’
wingwalls
was not tested in this study. Examination
of the data and photographic
results of the earlier
studies [ 21, however,
indicated
that height of boil and
drop in water
surface
elevation
to the channel
(fig. 14) will be reduced by using the 45’ wingwalls
and a longer end sill. The use of this sill would allow
the flow to spread
more uniformly
over a wider
be omitted.
24
Table 1.-Design specifications for the outlet works structure
at Picacho North and South Dams
Estimated maximum
flood ( record
North Dam
South Dam
Discharge (Q)
130 ft3/s (3.7 m3/s)
Estimated entrance
velocity (V)
3 1.8 ft/s (9.7 m/s)
Cross-sectional
area of flow (A)
4.09 ft2 (0.38 m2)
Depth (D)
2.02 ft (0.616 m)
Froude number (F)
3.94
Width to depth ratio
(W/D), figure 8
6.21
Width recommended (W) 12.54 ft (3.82 m)
Width actually used
12.50 ft (3.81 m)
Percent undersized
