Hi, my name is John Hunt.
I'm a consultant to the
Federal Highway Administration
on transportation hydraulic engineering.
I've got over 30 years
experience in the field,
and I'm also an instructor
in the National Highway
Institute's training courses
on transportation hydraulics.
We put together this video
to give you some ideas
and provide an example
of field scoping for
a bridge hydraulics study or design.
The focus of this video is
going to be on the field portion
but there's a lot that
you can do in the office,
think of it as office reconnaissance
prior to going out in the field
that will really help
you to be more efficient
and get more insightful information
when you're in the field.
So let's take a look at that.
You can learn a lot
about the crossing site
and the river reach
using some really
helpful online resources.
Here we're using an
aerial imagery application
and we're going to use it to
get more familiar with the site.
So let's zoom in on the project area.
And let's say we want to do
a hydraulic study and design
for the replacement of this crossing.
You've got the road here and the bridge.
We can see the river channel
and the adjacent land
use and ground cover.
We can look as far upstream
or as far downstream as we want to.
And by the way, in this example
the flow is from northwest to southeast.
We're going to start by looking
at the channel alignment.
So imagine that you're in a raft
and you're floating down river.
Starting here.
You go through this bend to the right
and then immediately
into a bend to the left.
And in that bend to the left
you've got the bridge site.
And what's happening downstream?
Well the channel splits in two
to get around this mid-channel bar.
And downstream of that,
what is this?
It's a diversion structure.
It's essentially a low
dam across the channel
and it causes weir flow
conditions for this low flow.
But also, even for flood conditions
we can expect that this will become
a critical depth control section.
And that means that it makes
it a really good candidate
for the downstream end
of our hydraulic model.
Whether it be a 1D or 2D model.
Well also, we can get
a look at the terrain.
So let's get an oblique view.
Now we're looking downstream
with an oblique view.
The bridge site is right there,
and this hill side to the right,
that's essentially the south edge
or right edge of the floodplain.
So it's obvious that in
flood flow conditions
this terrain feature
is going to have an
effect on the hydraulics.
Now a really great thing you can do here,
the Federal Emergency
Management Agency, FEMA
has created something called
the National Flood Hazard Layer.
You can go to their web page
and download that layer
and when you do
you can plot
the flood inundation
here on this image.
And that's what we've got here.
These shaded zones that
represents the floodplain
according to FEMA for
this reach of the river.
And really any reach of any river
that's in a FEMA regulated floodplain
you should be able to get this data for.
Well, we've just barely
scratched the surface
of the things you can do,
but as you can see you
can accomplish a lot
of pre-field reconnaissance
using an application
like this one.
Before you go to the field
you want to anticipate
some of the major design considerations.
Most bridge projects have
regulatory constraints
or permit requirements
that are going to need
some hydraulics input.
For instance, you might need
a floodplain development permit.
And you'll need to know
what agency at the state
or local level has the
authority to issue that permit.
The example we showed you before
that was a crossing
over a FEMA floodplain,
and in that case the county
would issue the permit.
You should engage the environmental
experts in your agency
to find out if there's
a way on this project
to provide some environmental enhancement.
Maybe some in-channel or
riparian habitat improvements
that will complement the project.
Other design considerations
you will look into
will have to do with the
hydraulic capacity of the crossing
and the floodplain,
how you want to model it,
and stability issues.
And you can use aerial imagery
like we did with the application
we were looking at earlier
to develop these considerations.
So for instance, hydraulic controls.
You want to find out are
there any that are visible
whether they're structures
across the channel
or structures in the floodplain.
What about if we want to calibrate,
which we should try to,
we want to know if there are
recent past floods,
and did they leave
behind high water marks.
Is the crossing skewed?
Meaning not perpendicular to the flow.
What about ice and debris?
Are those causing issues for maintenance?
And is there any data that suggests
that there are stability
problems with the channel
either laterally or vertically?
Examining these things
before you go into the field
will inform you as to what things
you need to look at and look for
when you're in the field.
Another pre-field activity
is gathering as much
information as you can
about the existing bridge.
And of course that
includes as-built plans.
Take a copy of these
with you into the field
and verify the span lengths.
Do the piers look like in the field
what they look like on the as-built plans?
If the as-built plan show countermeasures
can you still see those in the field?
Have things been added since
the original construction
that don't show up on the as-built?
Also, the as-builts show you
things that are under ground
that you can't see in the field
like these spread footings
and the elevations of the spread footings.
There's great information for you
in the bridge inspection reports.
Now these item numbers and the code values
are subject to change,
but as of right now a code of three
under item 113 tells us that
this bridge is Scour Critical.
Item 61 and 71 in the current coding guide
are also important because
item 61 for instance
that code six tells us
that the inspectors found
a problem with the channel bank protection
or there's some minor stream
bed movement that's evident.
This code of nine under Waterway Adequacy
that tells us that in
the inspector's opinion
there is the chance of roadway
or bridge deck overtopping
in a flood, is remote.
Also in the bridge inspection reports
look for this.
It's the measurement of
the stream bed elevation
at the bridge faces ideally
at different points in time.
And you can see if there's a
trend towards channel lowering
which is degradation
or the channel increasing
in elevation over time,
and that's aggradation.
Well once you've got your
pre-field reconnaissance done
and you're ready to head into the field
make sure you've got all
the equipment that you need.
That includes safety equipment of course,
and also that you've got appropriate forms
that are going to help you to
be as efficient as possible
in the field.
And to help you with this the FHWA
has produced another video in this series
with the title "Data
Mining and Preparation
for Project Field Scoping".
Please refer to that video.
And now, let's get outside.
In this portion of the video
we're going over what you're
going to want to look at
and document when you're
doing the field reconnaissance
for a typical bridge project.
Now this could be a bridge replacement,
it could be a new bridge where
there is no existing bridge,
or it could be for the evaluation of scour
at an existing bridge.
The observations that we
take from this field visit
are going to help us
make an appropriate and
accurate hydraulic model
and also provide information and insight
that are going to be needed
in the design of the bridge.
Here we're standing
a pretty good distance
upstream of the bridge.
I'm looking down at where the bridge is.
So, you're looking upstream
and I'm standing at the
left bank of the river.
When we say left and right
we're talking about as if
you were looking downstream.
So I'm at the left bank of the river
and we can see that this portion
of the river is in a bend.
This left bank is at
the outside of the bend.
Across the channel you see this point bar,
and that is basically sediment building up
that builds up at the inside of a bend.
It's a natural process,
and that same process
that's building the bar
on the inside of the bend
is working to erode this
left bank or trying to
which is at the outside of the bend.
And you can see
that this bank on the
left side is pretty steep.
It's been undergoing erosion,
but I can also see,
looking down at the
lower portion of the bank
that there is some bank protection
in the form of rock riprap
that is trying to hold that erosion.
Now if we're doing a one-dimensional model
of this stretch of the river
then we're going to be
talking about cross sections.
What would a cross section
of the flow look like?
Well as we look across the channel here
number one, we can see that
this channel is pretty deep.
The channel will carry quite a bit of flow
before the flow starts going
out into the floodplain.
Over on the right floodplain
that looks pretty high.
It's hard to tell how
much flow would be carried
in the right portion of the cross section
because it is pretty high
compared to the left side.
Over here we have the
left overbank portion
of the floodplain,
and it's pretty wide
and it's also pretty clear.
It's going to be carrying a lot of flow
in the left overbank of say a large flood
like the 100-year flood at this point.
But also, at this point,
this is where the flow would
have to start contracting
to get through the bridge
because not far downstream from here
we have the road embankment,
and that road embankment,
unless the flow we're interested in
would overtop the road,
but assuming it doesn't overtop the road
that flow has to contract to
get through the bridge opening.
And so, downstream of this point,
the two overbanks, the left and right
will begin to become
less and less effective
in carrying flow downstream.
That concept is called ineffective flow,
and we're going to talk more about that
a little later on in this video.
Now this field reconnaissance
is really your best opportunity
to get a good handle on what
the roughness value should be,
your Manning n values for
your model cross sections
or if it's a two d model
then the n values you're going
to assign to the elements.
And so let's just see what we
can see about roughness here.
I'm going to start with the
bottom of the main channel,
and as I look downstream from here,
the bottom of the main
channel is pretty smooth.
There's smooth cobbles
and gravel on the bottom.
Not a lot of obstructions or undulations,
and so I would call that
an n value of about 0.025,
maybe as high as 0.03.
But behind me which is further upstream
we see these boulders sticking up,
and they're going to add
a little more roughness,
and that portion
the bottom of the main channel
might be more like 0.04.
So that's the bottom of the main channel
and of course that's unvegetated.
Now we're going to start looking at areas
that will have varying
degrees of vegetation.
That point bar over there
which we've talked about before
it doesn't have any vegetation on it
because it's fairly new,
that again is going to have
those same low n values.
The 0.025 to 0.03.
But, you can see a vegetated area behind
or to the right of the point bar,
and we're here in January
so all the leaves are off,
and you can't really see
much of the undergrowth.
But if we were here in the summer
which is really our flood season here
you'd see a lot of undergrowth there
and it's really pretty dense vegetation.
So in that area that's
covered by that vegetation
I'm going to want to assign
an n value that's maybe 0.08
to as high as 0.12.
Looking over here on the left bank,
well it's a different kind
of vegetation isn't it?
There's grass and weeds,
but there's not really
anything that's substantial
that would stand up and resist the flow.
So this left bank is pretty smooth.
I'd say 0.04,
0.05 at the max.
And then if we're looking
at the left overbank area.
Well, we do have scattered trees.
In fact, the overbank does
go as far as those big trees
you see over there to the
right of the bathroom facility.
But those are just isolated.
In general, the roughness
of this left overbank
is pretty low because of the
type of vegetation we see.
Again, in the summer it
would look different.
But this is basically
grasses and weeds and sedges.
I'm going to say this is 0.05,
maybe as high as 0.06
in the left overbank.
So that's basically how
you capture information
about the roughness.
And of course, you want to
take a lot of photographs
at all of these vantage points.
So here at this point you
want to take a photograph
of what your overbanks
look like on both sides,
photograph of the main channel
so you're capturing the
roughness characteristics there,
and capturing that sense of
deepness to the main channel.
Well now we're standing
just a little ways upstream of the bridge,
and I'm standing in the
river bottom really.
If this were during spring runoff,
if I were standing here
I'd have to be in waders.
But we're here to look at
the bridge opening itself,
and talk about the effects
that the bridge will have
on the hydraulics of this river reach
So we can start at the far right end,
the right end of the bridge,
we call that the right abutment.
Now, this abutment is a vertical wall,
a really tall one,
with a wing wall,
with wing walls on both sides really,
and the vertical wall
comes up from the bottom
of the channel bank slope.
Well what that means is,
that flow in the main channel,
if we had a flood that was large enough
to fill this main channel,
well on the right side there
there'd be a triangular
wedge of obstruction
formed by that vertical abutment wall
that's coming up out of the bottom
of the channel bank slope.
Next we can look at the pier.
This is a two span bridge
so we just have the
one pier in the center.
It's what we call a wall pier
in that it's continuous
from the upstream end
to the downstream end,
and it's got a rounded
nose which is much better
than if it were a squared off nose.
Now for flow that's strictly
contained in the channel
for what I should say is channel flows
the pier looks to be pretty
well aligned with the flow.
It's not so easy to tell here,
but this is actually within a bend.
Now we're at the inside of the
bend here on the left bank,
and the right bank is at
the outside of the bend,
and when you look at aerial imagery
as to how the floodplain
is configured here
it's easy to imagine that
during a flood flow condition
the flood flows would
probably attack that pier
at more of a skew rather than head on,
there would be a bit of a
skew angle for the pier,
and that's going to effect
the hydraulics as well.
Well we can only see a portion
of the left abutment from here,
but if you could see the upstream side
you'd see that it's built the
same way as the right abutment
in terms of being a vertical wall
coming up from the bottom
of the channel bank
forming again, a wedge of obstruction
to the flow that's along the left bank.
I also want to look at
the bridge superstructure.
The bridge deck itself.
Now the bottom part,
the bottom line of the superstructure
we call that the low cord.
And on this bridge that's pretty high up.
I can imagine it would take a lot of flow,
a very large flood to get
to where that low cord
was submerged by the flow.
If it was that would be
called pressure flow.
Pressure flow is a condition
that really causes a big impact
on the hydraulics.
I am guessing that even a 100-year flood
we would not reach pressure
flow at this bridge.
Now our hydraulic model
will either confirm or
refute that statement,
but my guess at this point
is that it's high enough to
be out of the 100-year flood.
So, another thing we
would want to think about,
if we thought the bridge
deck was going to overtop
we would need to look at the rail
and ask ourselves
"Well, is the top of the rail
really the control on overtopping flow
or is it the top of the
concrete bridge deck?"
In this case that rail is pretty open.
So my guess is that the
way I would model it
would be to have overtopping begin
not at the top of the rail,
but at the top of the concrete deck.
One of the things that
you need to think about
when you're doing field reconnaissance
for a bridge project
you're going to be doing
scour calculations,
and for contraction scour
that can either be a live bed
or a clear water
contraction scour equation.
Well how do you know
which one of those it is?
The answer to the question is based on
here at a point upstream of the bridge,
a significant distance
upstream of the bridge.
The main channel flows during a flood.
Now again this is a
very low flow condition.
The flow would be where I'm standing
and a lot wider than this,
and it'd be a lot deeper
than what we see now.
The question is in a big flood like that
would the velocity of flow be fast enough
to pick up this bed material
and move it downstream
into the bridge opening?
The hydraulic modeling
is going to be needed
to really answer that question.
At this point we're just making
a qualitative assessment of it.
Well this is very coarse bed material.
It's gravel.
You can see cobbles.
Three to six inch, even
some eight inch cobbles.
It's going to take a high
velocity to pick this up.
Now one thing I'll mention
is that this particular stream reach
this is an armored bed.
If I took a shovel and dug
down a foot to two feet,
well even less than that,
I would get into a much finer material.
Underneath these cobbles we'd
be more in a gravel situation.
So really an assessment you have to make
at a site like this is
would the flows be high enough
to move this armor layer out of the way,
and get down to
the
finer bed material that's underneath?
And if the answer to that is yes
then this would likely be a
live bed contraction scour situation.
And if the model tells us no
that velocity is not
going to be high enough
to move this armor layer
then we'll really have
a clear water contraction scour situation
inside the bridge.
Where I'm standing now is right upstream
of the road embankment.
The bridge is that direction from me,
but I'm to the left of the bridge.
I'm in the left floodplain,
and I want to talk about
visualizing the flow conditions
in the floodplain
in the vicinity of a bridge.
So off in that direction
that's the left overbank,
the left portion of the floodplain.
And further upstream
it's going to be carrying a lot of flow.
And even right here where I'm standing
there is going to be water here.
It could be pretty deep.
It could be up to my shoulders.
A pretty deep stand of water.
However, in this location
it's not actively flowing downstream,
and you know why?
Well it's because that road embankment
is preventing that from happening.
Because of the road embankment,
unless the flow is able
to overtop the road,
which in this location the
FEMA floodplain mapping
says it wouldn't.
So unless it's overtopping the road
then the flow here,
if it's moving at all,
it has to be moving in this
direction towards the bridge.
So that means it's not being
effectively carried downstream.
So you have to visualize that,
and incorporate that
if you're doing a one-dimensional hydraulic model.
You incorporate that idea as
what we call ineffective flow.
Because there's a large
portion of this left overbank
that's not effective for
carrying flow downstream
because the flow here has to
be moving towards the bridge
which is lateral and not downstream.
And by the way, this
is a good time to plug
two-dimensional modeling
because if you use 
two dimensional modeling
you don't have to imagine
the ineffective flow,
and make guesses as to what
that flow pattern looks like
because a two-dimensional model
will actually directly compute it for you.
When you're here at the bridge
during your field reconnaissance
you have a chance to
look for signs of scour.
Well at this pier here
you can see a little dip in the ground
against the side of the pier
at the upstream nose of the pier.
It's pretty clear to me
that that is a remnant
of a pier scour hole.
It's important to realize though
when you're in the field
looking for signs of scour,
we did have a big flood
here six years ago,
it would be a mistake to think that
that is how deep the scour
got during that flood.
It probably got a lot
deeper at that location,
but what happens in live bed conditions
if indeed this was live bed,
is that after the peak
of the flood has gone by
and you're now in the
receding part of the flood,
towards the end of the flood the material
infills the scour hole.
So what we're seeing here is
what's left of a scour hole
that at one point was a lot deeper
and then it got infilled
by the lower flows
at the end of the flood.
But this does show you
that scour can be generated
at the upstream nose of that pier.
Now we can look here at the left abutment.
Now as I've been talking about the flows
in the left overbank being
prevented from going across
the road by the embankment,
meaning that the flows have
to come through the bridge,
well that concentrates a high velocity
and very turbulent flow
here at the abutment.
You can see the riprap that they've put
at the upstream corner of the abutment.
Now, it's interesting that
it stops where it does,
but this is indicating to me that
the bridge owner has
experienced scour problems
at that abutment,
and has decided to prevent further
or later formation of scour
by putting that riprap there.
Now normally at a bridge abutment
we would expect to see that protection
carrying all the way along the
length of the abutment wall
and wrapping around the downstream corner
in a similar fashion to what it does here.
It's unusual to see it stop here,
and at first that could
cause me some concern
because there's not riprap
where I expect to see it,
however there's something to point out.
There was a big flood here six years ago
and that tree was no doubt
standing there six years ago,
and there wasn't enough
scour to affect that tree
or to rip it out of the ground.
So that is a clue that the hydraulic,
the scour conditions must not
be that bad at this abutment
in a flood condition.
Not only that there's another smaller tree
that's probably over six years old
at the downstream corner of the abutment.
So, that's an interesting sign
that we probably don't have
really bad scour conditions
at this abutment.
And now we've moved to a vantage point
downstream of the bridge
because there's a lot of
information we need to gather
about the conditions
on the downstream side.
So first, we can look up at the bridge,
and now this could get confusing
because we're looking upstream,
and I'm going to talk
abut the right abutment
which we can see,
but it's to our left.
So that's the right abutment.
We can see how that looks
from the downstream side.
Now we're seeing almost
the full length of the pier
that the downstream nose of it.
Something's that worth
point out here is that
I mentioned before that
the bridge was in a bend
and that the left side is
the inside of the bend.
And what I didn't mention before
is that there's a point bar
coming off from the left bank
because it's at the inside of a bend,
and that point bar extends
throughout the whole length of
the left span of the bridge.
It extends all the way out
to the pier in the middle.
And so that of course
slightly effects the capacity
of that span.
We can see that we've
got a lot of vegetation
on both the right and left bank.
It's heavy vegetation on the bank slopes.
Again, if we were here during the summer
which is really the flood
season for this area
we'd see a lot of undergrowth
and that this is pretty thick vegetation.
So again, I'm going to say n
values on the order of 0.08
to 0.12 here.
Now,
that same things applies to the left bank.
We've got the same kind
of vegetation over there.
Because the right bank is
at the outside of a bend
we see that there's a lot
of places where the bank
is nearly vertical
and in some cases undercut.
We see places where bank protection
has been placed along there.
Some of it formal,
but most of it kind of
a informal placement
of bank protection.
We don't know how well that would stand up
to a really large flood,
although again we can say we know
that this flood six years
ago was pretty heavy,
and this bank protection
seemed to sustain that.
Something really important to
point out I think here is that
we are located near the upstream end
of a mid channel bar.
We can see the mid channel bar
extending further upstream
from where we're at.
Again this is downstream of the bridge.
Now a mid channel bar that's
an aggradational feature.
That means that there's been deposition
that has occurred in this reach.
Whether it's still
ongoing deposition or not,
or if it's more historic,
we don't know until we
do further investigation
from various data sources,
but we know that
it has aggraded here,
and we can look at the
n values in the channel,
these channel n values again
are going to be fairly low on
the order of 0.025 to 0.03.
We notice that we have some woody debris
that's been at some point deposited
onto this mid channel bar.
That's not surprising.
Woody debris tends to,
during a flood,
tends to flow along the
center of the channel,
and if we get to an area
where velocity is lower
or the depths are shallower
we could expect to see debris
hang up there
on a bar like this.
And so that's not very surprising,
but it's something to note.
Now, when we were upstream of the bridge
I didn't notice any debris
laying in the channel upstream,
and that's a good sign
because you don't want to have
a lot of potential for debris
to hang up on the bridge pier.
Being downstream we
don't expect this debris
to cause any significant
problem for the bridge.
Now we're in the same location,
but our focus is downstream
instead of upstream
towards the bridge.
So, why is it important to
be downstream of the bridge
and looking downstream?
Well it's because in hydraulic modeling
you need to have a good handle
on downstream conditions
because of the effect that they can have
on the water surface profile
through the bridge reach
and even to an extent
the stability through the bridge reach.
Now, I want to focus on
this mid channel bar.
We mentioned it before.
Here we're seeing the
downstream extent of it.
It's big and from the vegetation
we can see that it's been
here for quite a long time.
And again, this is an
aggradational feature,
but something that happens
with the formation of a mid channel bar,
it puts added stress on the
left and right river banks.
So if we look over there at the left bank
we see undercutting and vertical banks.
You see exposed tree roots.
And that just shows us that
this bank has been eroding.
There's also been informal
attempts at holding the bank
with stone and rubble
that don't seem to be working very well.
So, these are things we need to notice
when we're downstream of the bridge.
This mid channel bar with
all of its vegetation
puts a lot of roughness in
the main channel portion
of your cross section.
So this is going to be a 0.08 to 0.12
or higher n value region right here
with the two parts of your main channel
with their much lower n
values off to the side.
Now by the way, looking
back at that debris
or the exposed tree
roots on the left bank,
something I should mention is
if we saw that upstream of the bridge
we would have to be concerned that
this was going to be a source
of debris in future floods
that these trees could
get undermined completely
fall into the stream,
and then get hung up on the bridge.
But thankfully these are
downstream of the bridge
and not upstream.
Now remember that you need
to take a lot of photographs
during this field reconnaissance.
You're going to be glad for all
the photographs that you took
when you get back to the office.
So for instance here,
from this location
downstream of the bridge
you'll take a photo upstream
towards the downstream face of the bridge,
and downstream towards
what'll be the downstream
end of your model.
Now we've mentioned stream
stability issues a lot
during this video,
but only briefly.
And so there's an FHWA
video on channel assessment
that also features the same stream reach
and a lot of these stability issues
will be discussed in more
detail in that video.
Well thank you for taking
the time to watch this video
on field reconnaissance
for hydraulic analysis
and design of bridges.
