Hello, my name is Jim Schall
and I am an instructor for Federal Highway Administration Training
delivered under the National Highway Institute.
The National Highway Institute, or NHI,
is the training branch of the Federal Highway Administration.
The videos you're about to watch are based
on some of the training courses
developed by the National Highway Institute,
in particular,
the Introduction to Highway Hydraulics course,
NHI course 135065
and the Culvert Design course,
NHI course 135056.
I've ben an instructor for National Highway Institute training courses
for 25 years.
I completed my undergraduate engineering degree at Purdue University,
did my Masters and PhD
at Colorado State.
The videos are based on demonstrations and experiments
that we do primarily in these two training courses
built around a portable flume.
The flume is six inches wide
and approximately five feet long
and recirculates water from a 30 gallon sump.
The videos, there are six of them to watch.
The recommended order in which to watch those videos,
first, open channel flow concepts,
then grate inlets,
culvert hydraulic concepts,
hydraulic effects of culvert liners,
aquatic organism passage design concepts,
and energy dissipaters for culverts.
Thank you for your interest in Federal Highway Administration Training.
I hope you find this video series
helpful in completing highway drainage design projects.
This video demonstrates grate inlets
commonly used in storm drain design.
The grate inlets we're going to use are based
on Federal Highway Administration standards
presented in Hydraulic Engineering Circular Number 22,
Urban Drainage Design.
These are standard designs, typically
a 2 by 2 foot grate.
I have them placed in a street section
that simulates a 2 percent cross slope
and on a model scale, a 6 inch high curb.
We want to look at the amount of flow
intercepted,
the distance to complete interception,
whether or not we have any splash- over velocity occurring,
water going beyond the end of the grate,
and an important concept in grate design
is the frontal flow versus the side flow.
We'll see that most of the flow approaching the grate is intercepted,
the flow going by the side
tends to continue on down the street.
The first grate we're going to look at is the parallel bar grate.
The parallel bar grate is a common inlet design
that's been used for many years.
It is hydraulically very efficient,
intercepting most all the water
very close to the beginning of the grate.
As you can see, we're getting complete interception
of the frontal flow,
very close to the beginning of the grate.
We see the side flow continuing on down the street section.
This is common of any grate inlet
designed and installed on grade.
The frontal flow is a very important design factor.
A grate that's wider
is going to perform better hydraulically than a grate
that is longer,
because of the frontal flow conditions.
One disadvantage of any grate
is it must be installed in the correct orientation.
So if the grate is turned 90 degrees,
we see that the hydraulic performance is diminished.
Now we're seeing water close to halfway down the grate.
We still don't have any splash-
over so this grate's performing okay at this low condition.
We still have the side flow.
Another complication
when the grate is installed incorrectly is, in this orientation,
it's going to trap more debris coming down the street section
and that will further diminish the hydraulic performance.
So the parallel bar grate,
hydraulically very efficient
when it's installed in the right direction.
But a disadvantage of a parallel bar grate
is that it's not very bicycle safe.
The bar spacing is wide enough that a bike tire
could fall through the bars,
creating a hazard for a bike rider.
That led to research by the Federal Highway Administration
in the 1970s
to develop both hydraulically efficient
and bicycle safe grate designs.
One of the first options was taking that same parallel bar grate
and installing transverse rods
or cross rods
to make the grate hydraulically efficient and bicycle safe.
As we see, the installation of the transverse rods
really didn't diminish the hydraulic performance.
We're still getting complete interception
very near the front edge of the grate.
Of course, we see the same bypass flow
going down the street section
but the grate is hydraulically very efficient.
It does suffer, again, like most grates
if it's not installed in the right direction
from its hydraulic performance.
Now we see water about halfway down the grate,
still no splash-over of any significance
but certainly more debris trapping
when the bars are not parallel to the flow.
So the parallel bar grate with the transverse rods
is hydraulically very efficient and also bicycle safe.
A variation of that design concept was the narrow parallel bar grate,
very similar in design,
it's just the bars are closer together.
This grate, when installed,
performs very similar to the parallel bar.
The narrow parallel bar captures
nearly all the flow very near the front edge
of the grate.
We see the same flow bypassing out on the street section,
no splash-over occurring and like most grates,
though,
when installed 90 degrees incorrectly,
now we see we're down to a half maybe two-thirds
before we get complete interception.
And certainly on a narrow parallel bar
with those bars being closer together,
installed incorrectly like this, we're going to,
have significantly more debris issues.
So the narrow parallel bar,
hydraulically efficient and bicycle safe.
Another design concept is called the reticuline grate;
It's more of a honey comb type structure
on the grate design.
This grate performs reasonably well
and perhaps not as good as the parallel bars,
you see we're down around a third
to maybe a half for total interception
with the grate installed in the correct orientation.
We're still seeing the flow along the side,
this is called side flow continuing down the street section.
When we turn this grate 90 degrees,
the performance doesn't suffer too much,
mostly because the grate
is not as hydraulically efficient to begin with.
A disadvantage, and many DOTs don't like this grate,
because it is more trouble with debris
with that tight of a honeycomb structure,
a lot of debris is captured on the grate
even when it's installed in the correct orientation.
Another design concept
that would maybe first appear to work quite well is the diagonal bar.
I was surprised the first time we put this grate in the flume
and saw that it really isn't as hydraulically efficient
as you might think.
Even with the grate oriented
so the bars would tend to push the flow
towards the curb section,
we find that its hydraulic performance is not as you might expect.
As you can see, we're now getting some splash-over,
we're getting some water going beyond the end of the grate.
The diagonal bars are not really directing the flow
to the curb section as you might expect.
Of course, we still got the side flow
going on down the street section
and this grate, as well, if it's put in
in a different orientation suffers.
It's not as dramatic
as a more hydraulically efficient grate
and I have to hold this one in place because of the grate design.
As you can see, we get quite a bit
of bypass, splash-over flow,
as we call it,
as the flow continues beyond the end of the grate.
We're not getting 100 percent interception
like we do on the more hydraulically efficient grate designs.
There are many variations of a diagonal bar
in use throughout the country,
depending on the manufacturer different bar spacing.
My conclusion is none of them will probably perform
as well as you think.
Certainly, the best and the most hydraulically efficient design
is the parallel bar.
However, there are two other options
that are both very similar in design concept.
One's called a tilt bar,
the other's called a curved vane.
The words describe pretty much
the direction and the orientation of the vanes on these grates.
The tilt bar is what it sounds like.
It has bars that are tilted
to intercept more flow.
The bars are straight, they are tapered
but they are straight as opposed to a curved vane.
So I'll insert the tilt bar first
and we'll see hydraulic performance
very similar to the parallel bar grate,
if not, even a little bit better.
The same concern
with the flow going down the street section,
the so-called side flow, compared to the frontal flow.
One big issue with this style vane
is if it gets put in the wrong direction,
the hydraulic performance is surprisingly poor.
In fact, now we're seeing very little interception of water.
We have a lot of splash-over,
we have quite a bit of water going beyond the end of the grate.
This is a common characteristic of this grate design and one thing
that manufacturers do to try to prevent that
is to stamp an arrow in the casting
pointing to the curb
or to put some kind of a key on the grate
to align with the frame.
And yet it is still possible, at times,
for this grate to be installed incorrectly.
We had one DOT that had just installed these types of grates
on a large highway drainage project,
around 100 catch basins.
The contractor wasn't sure which way to put the grates
in, and so they went every other grate
with one installed correctly, the next one incorrectly.
The same performance is true of the curved vane.
Again, very similar in design concept
but the vane is actually curved
as opposed to being straight.
Hydraulically, very efficient and very bicycle safe,
capturing 100 percent of the flow,
very near the leading edge of the grate.
Still seeing the usual amount of side flow
but 100 percent interception
of what we call the frontal flow,
until we turn the grate the wrong direction
and as with the tilt bar,
we see quite a bit of flow going beyond the end of the grate.
A lot of splash-over,
very little interception even of the frontal flow.
So the tilt bar and the curved vein,
hydraulically
very efficient and bicycle safe.
The demonstrations we've done are all
for a street section on grade
But of course these same grates are used in a sump condition.
In a sump a grate
is typically operating at weir flow
under low discharge
and orifice flow under high discharge.
So at low flow depths,
water coming into the sump
sees the grate opening as a three-sided weir
and we use the weir equation.
At higher flows, when water starts to pond
over the top of the grate,
it begins to act more like an orifice.
All of the design equations for the grates
that we've reviewed
are presented in Hydraulic Engineering Circular Number 22.
For grates on grade there are equations describing the frontal flow interception
versus the side flow.
For grates in a sump, there are equations
for when it's operating in weir versus
when it's operating in orifice flow condition,
or the transition in between.
If your grates do not match the standards
that are in Federal Highway Administration Hydraulic
Engineering Circular Number 22,
you will not have the design equations
in which to evaluate the interception capability
of a given grate design.
You would need to have model study tests
done on a different design concept
to really know exactly how a grate's going to operate
when it's installed out in the field.
Those equations and those relationships
are also implemented
in the Federal Highway Administration software program,
the Hydraulic Toolbox, which is a collection
of utility programs useful for doing highway drainage design.
Thank you for your interest in this Federal Highway
Administration video.
I hope you found the information on grate inlet design
concepts useful and helpful
in your highway drainage design projects.