Hi, I'm Will deRosset
hydraulic engineer and consultant
to the Federal Highway Administration.
I have over 20 years experience
in river engineering and
transportation hydraulics.
I also teach courses for the
National Highway Institute.
In this video we are going to provide
examples and information on performing
a channel assessment.
We will focus on field
activities, but before we go,
there are a few items to
review in preparation.
An office data review is a critical key
to a productive site visit.
It will inform and focus
your field activities
and give you an opportunity to gather
needed field equipment
and documentation forms.
This video is part of a series,
and companion videos "Data Mining"
and "Preparation for
Project Field Scoping"
and "Bridge Hydraulic
Design - Field Scoping"
cover these activities in detail.
In addition, a detailed review
of aerial photography
and channel survey data
will help identify
channel stability trends
you need to investigate further.
Obtain and annotate a current
aerial image of your site,
identifying features and resources
you need to document in the field,
including the extents of the
reach you need to review,
the presence of
infrastructure and utilities,
private property and
potential access issues,
upstream and downstream
hydraulic controls,
critical habitat and
wetlands, vegetative changes,
and the presence of
hydraulic countermeasures.
Past changes to the site and
indicators of channel responses
can also be identified using
historical aerial imagery.
Obtain, overlay, and
review these historical
aerial images of the site.
They will provide insight into changes
in land use and developments in the reach,
including changes in
agricultural practices,
development, gravel mining, flood,
drought, fire, or other impacts.
You can also identify
infrastructure changes through time.
You can also review the channel alignment
and locations of river features,
such as point bars and cut banks.
It is particularly useful
to outline the channel banks
and significant features,
and overlay the traces to evaluate
past channel instability trends.
It can guide your field
evaluation of channel features
such as this mid-channel bar's evolution.
Any available historical
channel survey data
can also provide insight into
vertical stability trends.
Resources that may provide
vertical stability information
include bridge inspection
reports for nearby structures,
USGS gage records, reservoir
sedimentation surveys,
and Federal Emergency Management Agency
flood insurance study
model cross sections.
During your desktop reconnaissance
you will have compiled a
long list of observations,
which should have generated
an equally long list of questions.
You should have identified areas
that may pose a design
or analysis challenge,
and recorded your
assumptions about the site.
Make a list of these assumptions,
questions, and areas of concern.
You will want to take this list,
as well as annotated aerials
and field reconnaissance forms
to the field with you.
Now it's time to grab our field equipment,
our personal protective
equipment, and head to the field.
Today we are here to evaluate
the stream stability of this reach.
We are going to be primarily looking for
field indicators of processes
that we have identified as part of
our office evaluation,
and we are going to be
looking at our river reach
upstream, at our stream crossing,
and downstream for field indicators,
and also documenting any
processes that we see
happening here.
So, right now, before we get started,
I would like to point out that
you should be wearing
proper protective equipment,
personal protective equipment,
and also that you are evaluating
your location and situation for safety.
With that, let's move into
evaluating this stream a little bit.
I'm standing here on river left.
You are looking downstream
on the river left,
so we measure river left and river right
relative to floating downstream
as if we were in a canoe.
So, here we are on river left,
and that is river right.
This particular stream is a
single-thread meandering stream.
We are standing at the outside
of one of the meander bends.
Now,
stepping back a little bit,
why do we want to perform a
stream stability analysis?
Well, first off, we are
interested in how the stream
is going to interact
with its sediment supply,
its flow regime, and the
existing infrastructure
and environment around it.
We want to be able to
evaluate any potential impacts
to our infrastructure,
and also be able to successfully
design that infrastructure
to work in concert with this
naturally dynamic system.
So, we are out here today
to review the findings of
our office study
for this review,
also to document what we see
and identify field indicators
of potential stream instability
for further evaluation
once we get back into the office.
So, here, we're well
upstream of our bridge.
Our bridge is downstream
and around the bend here,
and we are going to be taking a look
at the sediment
characteristics of the stream.
We want to look at the lateral
stability of the stream
and identify features that may
indicate lateral instability.
We also are going to
evaluate vertical instability
where the stream tends to
adjust its slope over time
due to changes in its
sediment supply and water supply,
or changes in the downstream
base level control.
So, here, we are going to take a look
at this upstream meander bend.
We're on the outside of a bend,
and a couple of things to note here
are that the bank is relatively steep.
This has also been reconstructed.
This reach was subject
to a major flood in 2013,
and this entire left overbank
where we are standing
was reconstructed to
protect the infrastructure
to the river left.
You will notice that this
is a relatively steep,
uniform slope bank,
and that the toe is protected
with coarse, angular rock riprap.
Also notice that in this reach
this channel is developing
a pool-riffle sequence
where we have a relatively
steeper, coarser reach,
followed by a deeper,
slower pool region.
Finally, we notice that
on the inside of the bend,
we have an actively developing point bar,
so that is that region of
gravel and cobble material
on river right that we can observe.
Note that there's no vegetation on it,
on the active portion of that,
but there is another section further back
that is river right
that has been vegetated
and actually was historically dissected.
So, there is a secondary backflow
channel river right there.
All of this we identified as
part of our office review.
Okay, so we are now looking upstream
still on the river left,
looking upstream at our site,
and what we want to look
for are field indicators
of stream instability,
either lateral instability,
or vertical instability.
For lateral instability,
what we are primarily interested in
are finding areas of
over-steepened or sloughing banks,
riprap revetment or other bank protection
that has failed,
the presence and expansion
of un-vegetated point bars,
and then, also, when you look at
a vegetated portion of the reach,
such as you see on river right
just upstream of the point bar,
you can see that we have
exposed vegetation roots,
leaning trees, and if you
look closely further upstream,
you'll notice that there are
some large, woody material
that are present in the bed
that will get entrained by higher flows.
So, all of these are indicators
of lateral instability
in this system.
Also, we note that seven years ago,
this system experienced
a major flood event,
and as part of that, many
banks significantly eroded
as this stream eroded
the outside of the bend
and built a point bar,
including this site.
So, you can see down in the bed,
you can see the remnants of prior
failed embankment protection,
and also, when they rebuilt this slope,
they placed a toe riprap revetment
on the outside of the bend
to hold this bank in place
to protect the infrastructure
here on the river left side.
So, while you are at this approximate
upstream of the bridge,
upstream of the region where
the flow tends to contract,
you also want to find a spot
that will be representative
and capture a
bed sediment sample.
Now, it would be really convenient
if we could just go out and take a look
at the material that's
present on that point bar,
but that has been subject
to lower flow events
that tend to entrain smaller materials,
so that is actually coarser
than the actual sediment supply
coming in from upstream.
Consequently, we're going to have to dig,
and we'll do that at our approach section
just upstream of the bridge.
All right, so we've moved downstream
just upstream of the bridge
to the head of a riffle
immediately upstream.
So, couple of other lateral
stability notes here.
Notice that on the outside of the bend
we have a number of different generations
of rock riprap that has been
placed in order to stabilize
the outside of this bend.
This river is eroding its outside bank,
and they are preventing that.
They are protecting the
infrastructure at the top there.
Looks like there's a road.
There's also a house.
There may be septic systems.
One other indicator of lateral instability
that we do not see in this location
is exposed utilities.
It's very common in the
vicinity of roadway embankments
for utilities, water,
sewer, gas, electric lines
to be placed along those nice corridors,
and so it's one of the
things we want to check for,
both for vertical stability,
which we'll talk about in a minute,
and for lateral stability,
is whether or not those
features are exposed.
We don't see any here.
Now,
again, we are on an un-vegetated
point bar river left
at the head of a riffle.
This is actually an excellent place
for us to obtain
representative bed material.
So, when we look at the bed,
we can see that there is
this coarsened surface layer
that is formed at the top.
This is not representative
of the sediment supply.
We want to
capture the material
that is representative.
We need this material
for a couple of reasons.
One is if we perform a bridge scour
or culvert scour evaluation,
we are going to need to evaluate
the contraction scour potential
at that location.
Two,
if the bed
contains significant quantities
of coarse bed sediments,
it may form what we call an armor layer,
which is material that is too large.
This material may be too
large for the channel
to mobilize during high flow events.
That in turn may limit the degree to which
vertical channel instability
is exhibited in the reach.
So, we want to capture two things here.
One is the surface layer gradation,
and the other is a representative sample
of the bed sediment supply.
So, to that end,
we get to dig through that
coarsened surface layer.
Typically, you will dig through...
Take a look at your cobbles.
Find kind of the coarser materials
that you are seeing present,
and you want to be about
three times that depth deep
before you start taking a sample
of the underlying bed material.
Once you have that,
you will see that the material underneath
is significantly finer
than this relatively coarse
cobble boulder material.
The other thing that you want to do
is grab a large enough sample.
There are references,
and your geotechnical colleagues
can help you establish
how large a bulk sample
you will need to capture
in order to capture the full range
of the underlying bed sediment.
Because this coarse surface
material is very coarse,
we don't want to bring
home a dump truck full of
coarse cobble material to capture.
So, we are going to use
an alternate analysis
known as a Wolman Count
in order to capture the gradation
of the coarse surface layer.
We need both of those information
in order to evaluate in the office
whether or not a coarse
armor layer will form
for this system.
And we have moved downstream
of our crossing of interest
just upstream of a pedestrian bridge,
and we just have a few more
things to take a look at.
We've seen extensive evidence
of lateral instability here.
We also need to evaluate as
part of this field effort
the vertical stability of this system.
Everywhere where we have
observed the bank stability,
wherever there is not bank protection
we see that there are over-steepened banks
that are losing their toes,
have leaning vegetation and exposed roots,
and that results in large, woody material
falling into the river
and being entrained.
These are all evidence
of additional lateral instability
in this reach.
Also, we need to consider
our vertical stability.
When we consider vertical stability,
field indicators of that
are going to include
upstream and downstream
base level controls,
so things such as dams,
exposed bedrock layers,
irrigation diversion structures,
and/or very, very coarse
lag deposit materials.
In this particular case, there is a
diversion structure just
downstream that is holding grade,
and what we mean by that
is if you have a channel
and something has changed
in its watershed, the
discharge has increased,
or the sediment supply has been reduced,
that channel is going to tend to rotate
about that base level control to seek
a new equilibrium slope as
that channel gets flatter.
Consequently, that base level control,
maintaining that base level
control's a critical key
to limiting degradation.
If you lose that base level control,
you're going to tend to initiate
another cycle of degradation,
and that will tend to propagate
from downstream to upstream over time.
Another thing in this particular site
that may limit that vertical degradation,
that degradation over time,
is the presence of very
coarse material in the bed.
We would need to evaluate
this coarse surface layer
to determine whether
or not it is competent
to form an armor layer,
whether or not it is thick
enough and coarse enough
that the ordinary range of flows
do not mobilize that armor layer
and expose the finer materials underneath.
Now, this particular site,
we also notice a mid-channel bar.
That mid-channel bar is coarse material.
You'll notice that the upper
edge of it is un-vegetated,
and that indicates that it
may be growing through time.
That would be another
thing we'd want to evaluate
as part of our office review
with aerial photography.
It's also very commonly
observed upstream of
diversion structures.
So, with that, this system
has relatively limited
vertical instability potential
due to the presence of that
base level control downstream
and this coarse material here.
Consequently, we're going to move
for the remainder of our discussion
to a site that has significant
vertical stability issues.
We have moved to another
site in order to evaluate
a site that does have significant
vertical stability issues.
Here we are standing downstream
of our crossing
on river right,
and you all are looking
upstream towards the crossing.
Now, this is a site that is subject to
significant historical
long-term degradation.
We can observe that by taking
a look at the floodplain here.
The level of the floodplain historically
was pretty much where we're standing,
and as we look here,
we notice that there's a
small inset floodplain,
a large, vertical scarp,
and unstable bed and banks.
Now, the reason we call
this historical degradation
is that this system,
couple of things to note,
there's a sign that it
experienced some degradation
subsequent to the
construction of this pipe,
because the pipe is perched
above the base level,
and also, you will notice
that it is no longer connected
to its floodplain.
We call that
a degraded or an incised channel.
Now, there's pretty good evidence
that this was a historical thing
and not actively degrading,
and one of the things we can look at
is the fact that it has
formed an inset floodplain
or terrace, or is forming one
in this relatively fine
and cohesive material.
As we look at the bank,
we notice that the system has developed
near vertical
scarps
on both sides of the bank.
That is actively slumping over time,
so under spring runoff conditions,
this tends to slump,
and the system is migrating
into this vertical scarp.
Now, that material,
because it is fine-grained
and relatively cohesive,
that's going to tend
to slow the time scale
of this system's response to
this historical degradation.
There's one other thing
we really need to consider
with this stream, given
its history of degradation
and lateral instability,
and that's the presence
of our structure itself.
This structure provides
a base level control
for vertical stability
of this stream upstream
of the crossing itself.
So, if we were designing
a replacement structure,
or doing a rehabilitation here,
we would want to consider the impacts,
the stability impacts of the
replacement of that structure,
because this thing is
currently holding grade.
With that, we want to organize
and document our findings.
Federal Highway's HEC-20 manual includes
forms and other analysis tools
that will assist you with that task.
I'd like to thank you
all for participating
in our channel assessment video,
and with that, like to wrap it up.
Thanks a lot.
