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> Table A.3 Velocity Estimation and Riprap Size
Table A.2 Uniform Flow Computations
REVETMENT TOP AND END PROTECTION
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Appendix A: Design Procedure for Riprap Armor
Table
A.3
Velocity
Estimation
and
Riprap
Size
V
SS1
Computed
D
302
D
30
(min)
of
Trial
3
Trial
D
100
(max)
(in.)
(fps)
(ft)
(ft)
0.48
0.59
9.9
12
0.73
0.53
9.5
18
0.97
0.48
9.2
24
1
From
Figure
A.2
using
trapezoidal
channel.
2
From
Equation
(A.3).
3
From
gradation
information
given
in
Table
A.2.
This
example
demonstrates
that
the
increasing
rock
size
for
the
three
trial
gradations
results
in increasing
depth
and
decreasing
velocity.
The
minimum
acceptable
routinely
produced
gradation
is
the
18-in.
D
100
(max).
(e)
In
braided
streams
and
some
meandering
streams,
flow
is
often
directed
into
the
bank
line
at
sharp
angles
(angled
flow
impingement).
Precise
guidance
is
lacking
on
determining
the
imposed
force
for
this
condition.
Until
better
guidance
can
be developed, a
local
velocity
of
1.6
times
the
average
channel
velocity
at
the
impingement
point
is recommended
for
use
in
riprap
design.
The
discharge
used
for
design
conditions
should
correspond to a
stage
at or
just
above
the
tops of
the
mid-channel
bars.
A
velocity
distribution
coefficient
C
V
of 1.25 is
typical
of
low
R/W
bends
and
should
be
used
for
impingement
points
in
Equation
(A.3).
(f)
Transitions
in
channel
size
or
shape
may
also
require
riprap
protection.
The
procedures
presented
here
are
applicable
to
gradual
transitions
where
flow
remains
tranquil.
In
areas
where
flow
changes
from
tranquil
to
rapid
and
then
back
to
tranquil,
riprap
sizing
methods applicable to
hydraulic
structures
should
be
used
(Hydraulic
Design
Criteria,
Chart
712).
In
converging
transitions,
the
procedures
based
on
Equation
(A.3)
can be
used
unaltered.
In
expanding
transitions,
flow
can
concentrate on
one
side
of
the
expansion
and
design
velocities
should
be
increased.
For
installations
immediately
downstream
of concrete
channels,
a
vertical
velocity
distribution
coefficient
of 1.25
should
be
used
due
to
the
difference
in
velocity
profile
over
the
two
surfaces.
A.5
FILTER
REQUIREMENTS
Guidance
for
filter
requirements
for
riprap
and
other
armors is
given
by
USACE
(1986),
and
Pilarczyk
(1984).
A-26
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