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Coastal and Hydraulics Laboratory Fact Sheets
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> River Profiles and Bed Material
Dominant Discharge in Alluvial Rivers
General River Response to Change
4_010491
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5.4.8
River
Profiles
and
Bed
Material
The
slope
of a
river
channel
or a
river
system
is
usually
steepest
in
the
headwater
regions.
The
river
profile
is
concave
upward
and
the
slope
of
the
river
profile
can
be
represented
by
the
equation:
S
x
= S
o
e
- α x
(5.12)
where:
S
x
=
Slope
at
any
station
a
distance
x
downstream
of
the
reference
station
S
o
=
Slope
at
the
reference
station
α
=
Coefficient
Similarly,
the
bed
sediment
size
is
coarser
in
the
upper
reaches
where
the
channel
slopes
are
steep
and
the
bed
sediment
size
becomes
finer
with
distance
downstream.
Generally,
the
size
of
the
bed
material
reduces
with
distance
according
to
the
relationship
D
50
x
= D
50
o
e
- β x
(5.13)
where:
D
50
x
=
Median
size
of
bed
material
at
distance
x
downstream
of
reference
station
D
50
o
=
Median
size
of
bed
material
at
the
reference
station
=
Coefficient
β
The
hydraulic
geometry
relations
are
applicable
to
continuous
channel
behavior.
In
some
cases,
this
behavior
(in
this
case
the
slope)
may
become
discontinuous
as
the
channel
pattern
changes
from
meandering
to
braided
by
the
formation
of
cutoffs.
Application
of
Equation
5.13
is
illustrated
with
field
data
in
Section
5.9
(Problem
7).
5.5
QUALITATIVE
RESPONSE
OF
RIVER
SYSTEMS
Many
rivers
have
achieved
a
state
of
practical
equilibrium
throughout
long
reaches.
For
practical
engineering
purposes,
these
stable
reaches
can
be
also
called
"graded"
streams
by
geologists
and
"poised"
streams
by
engineers.
However,
this
does
not
preclude
significant
changes
over
a
short
period
of
time
or
over
a
period
of
years.
Conversely,
many
streams
contain
long
reaches
that
are
actively
aggrading
or
degrading.
These
aggrading
and
degrading
channels
pose
a
definite
hazard
to
any
highway
crossing
or
encroachment,
as
compared
to
poised
streams.
Regardless
of
the
degree
of
channel
stability,
local
human
activities
may
produce
major
changes
in
river
characteristics
locally
and
throughout
the
entire
reach.
All
too
frequently
the
net
result
of a
river
improvement
is a
greater
departure
from
equilibrium
than
that
which
originally
prevailed.
Good
engineering
design
must
invariably
seek
to
enhance
the
natural
tendency
of
the
stream
toward
stable
conditions.
To do
so,
an
understanding
of
the
direction
and
magnitude
of
change
in
channel
characteristics
caused
by
human
activity
and
natural
processes
is
required.
This
understanding
can
be
obtained
by:
(1)
studying
the
river
in a
natural
condition;
(2)
having
knowledge
of
the
sediment
and
water
discharge;
(3)
being
able
to
5.28
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