Damage
to
culverts
is caused
primarily
by
floodwaters
eroding
culvert
entrances
or outlets
and road
embankments,
and
usually
results in
a full or
partial
washout or
misalignment
of the
culvert.
These
damages
may be due
to
insufficient
design
capacity
or end
treatments,
inadequate
slope
protection,
or
inadequate
protection
from
floating
debris.
Careful
determination
of the
cause for
the damage
is
necessary,
as
different
causes
require
different
mitigation.
Assessing
the Causes
of Culvert
Damage
Selection
of
appropriate
mitigation
depends
upon the
culvert
flow
conditions
at the
time of
damage.
Surveys of
high water
marks
located
upstream
and
downstream
from the
culvert,
and
surveys of
the inlet
and outlet
elevations
are
necessary
to
identify
the flow
conditions.
Flow
through a
culvert
may be
controlled
by its
entrance
or outlet
conditions,
or by
downstream
channel
features.
A culvert
will flow
full if
its outlet
is
submerged
or if the
depth of
water
above the
top of its
entrance
is greater
than 1.5
times its
diameter.
Damage to
a culvert
flowing
full
usually
occurs
when the
road
embankment
is
overtopped
and is
fully or
partially
washed
out.
Appropriate
mitigation
measures
include:
- Increasing
culvert
size;
- Increasing
efficiency
of
the
entrance;
and/or
- Raising
the
culvert.
If
raising
the
culvert
causes it
to flow
partially
full,
adjust the
slope to
return it
to full
flow
conditions.
A
culvert
may flow
partially
full when
the water
depth is
above the
top of the
culvert's
entrance
and below
the top of
the
outlet, or
when the
water
surface is
below the
top of the
culvert at
both
entrance
and
outlet.
Damage to
a culvert
flowing
partially
full
usually
occurs
when
embankment
erosion
has
occurred.
Appropriate
mitigation
for these
conditions
include:
- Increasing
culvert
entrance
efficiency,
and/or
- Decreasing
the
slope
of
the
culvert.
If the
culvert
was
flowing
partially
full and
the flow
at the
outlet was
in a
subcritical
or
tranquil
state, the
damage
will
likely be
confined
to its
entrance.
In this
case,
appropriate
mitigation
includes:
- Increasing
the
efficiency
of
the
entrance,
and
- Armoring
the
entrance
embankment.
If the
culvert
was
flowing
partially
full and
the flow
at the
outlet was
in a
supercritical
or
turbulent
state,
damage may
have
occurred
to either
the
entrance
or outlet
or both.
Appropriate
mitigation
would then
include:
- Increasing
efficiency
of
entrance
and
outlet
conditions,
and
- Armoring
the
entrance
and
outlet
embankments.
If, as
a result
of these
conditions,
erosion of
the
streambed
with
subsequent
head
cutting
and
embankment
erosion
occur,
appropriate
mitigation
would be:
- Installation
of
a
stilling
basin,
and/or
- Armoring
of
the
stream
channel
and
road
embankment.
To
identify
the flow
conditions
that
caused the
damage,
determine:
- Water
surface
elevations
upstream
and
downstream
from
the
culvert
at
the
time
of
damage;
- Elevations
of
the
culvert
entrances
and
outlets;
- Whether
downstream
channel
erosion
and
head
cutting
occurred;
and
- Evidence
of
road
embankment
erosion.
Floodwaters
frequently
carry
debris
both as
the flows
rise or
recede.
Debris
carried by
rising
flood
flows may
become
caught or
wedged in
culverts,
plugging
the flow.
Culverts
can then
be washed
out or
damaged
due to
increased
surface
flow
elevations.
Mitigation
measures
should be
designed
to protect
against
debris
impact and
accumulation,
and to
assist in
passing
debris
through
the
structure
openings.
Debris
carried by
receding
flood
flows will
generally
be
deposited
on the
stream
overbanks
and draped
over
culvert
entrances.
Damage to
culverts
will most
likely
occur due
to factors
other than
floating
debris,
and
mitigation
should be
developed
based upon
the most
probable
cause of
damage.
