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> Table 5.8. Channel Response to Changes in Watershed and River Condition
Resulting Problems at Highway Crossings
Bank Stability - 4_0104910347
4_010491
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Table
5.8.
Channel
Response
to
Changes
in
Watershed
and
River
Condition
(after
Keefer
et
al.
1980).
Observed
Channel
Response
Condition
Stable
Unstable
Degrading
Aggrading
Alluvial
Fan
Upstream
X
X
Downstream
X
X
Dam
and
Reservoir
Upstream
X
X
Downstream
X
X
River
Form
Meandering
X
X
Unknown
Unknown
Straight
X
Unknown
Unknown
Braided
X
Unknown
Unknown
Bank
Erosion
Unknown
Unknown
Vegetated
Banks
X
Unknown
Unknown
Headcuts
X
X
Diversion
Clear
Water
Diversion
X
X
Overloaded
with
Sediment
X
Channel
Straightened
X
X
Deforested
Watershed
X
X
Drought
Period
X
X
Wet
Period
X
X
Bed
Material
Size
Increase
X
X
Decrease
X
Unknown
X
5.8
STREAM
STABILITY
PROBLEMS
AT
HIGHWAY
CROSSINGS
In
the
United
States,
the
annual
damage
related
to
hydraulic
problems
at
bridges
and
highways
has
been
estimated
as
high
as
million
during
years
of
extreme
floods.
Damages
by
streams
can
be
reduced
by
considering
channel
stability
in
site
selection,
bridge
design,
and
countermeasure
placement.
Ideally,
a
stable
channel
is
one
that
does
not
change
in
size,
form,
or
position
through
time.
However,
all
alluvial
channels
change
to
some
degree
and
therefore
have
some
degree
of
instability.
For
engineering
purposes,
an
unstable
channel
is
one
whose
rate
or
magnitude
of
change
is
great
enough
to be a
significant
factor
in
the
5.52
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