Hi, this is Chris Morris
and I'm the surface water specialist
at the USGS in Las Vegas, Nevada.
I will be talking about surveying
the geometry of pipe and pipe-arch culverts.
In previous videos, I discussed surveying
the approach high water marks,
the tailwater high water marks,
an approach cross-section,
and the general attributes of any culvert.
The information presented in this single-component video
can also be found in TWRI 3-A3,
Measurement of Peak Discharge at Culverts
by Indirect Methods.
As mentioned in previous videos,
good clear notes are needed.
With complex culverts or when combined with other methods,
say a road overflow,
it is easy to forget to measure the beveling
or note if you measured the top
or bottom of the culverts.
There is nothing worse than finding
that a critical element was missed a few months ago,
and an additional site visit or survey
is needed to finish the computation.
Even for someone who has surveyed many culverts,
I find having a checklist is useful
to get everything you need.
I typically wait and do the following steps
after I completely finished with the survey.
Although all of these could be done during the survey,
I find you are less likely to mismeasure
or not note an important element
if you aren't doing two things at one time.
Additionally, some of these are easier
to measure with two people.
First, let's determine the area.
For a circular pipe, the diameter should be measured.
Remember geometry class,
we can compute area from the diameter.
I will typically measure at least a couple diameters,
both vertically and horizontally using a measuring tape.
Sometimes corrugated metal pipe culverts
can be squeezed out of circular
due to traffic on the road,
becoming more like ovals.
Such pipe and pipe arches that are made that way
should have both a rise and span measured.
The key is to try and do our best
to compute the area of the pipe.
For concrete pipes, I will measure the inside diameter
and the thickness at the top
if I end up surveying the top rather than the bottom.
For corrugated pipes, the inside diameter
should be measured.
This is the distance from the most inside portion
of the corrugation on one side to the other side,
i.e., the minimum inside diameter.
For the next few steps,
we will most likely be measuring
and noting the conditions of various
small elements of the culvert.
These are the ones most likely to be missed.
These various elements will be used to determine
adjustment factors for various coefficients
during the actual computation.
The material used to make the pipe
should be noted and photographed.
Typically, it will be concrete or corrugated metal.
For corrugated metal pipes,
you also need to measure the size of the corrugations.
First, I measure the width using a folding ruler
to measure the distance between two adjacent high points.
Next, I measure the depth by putting the folding ruler
into a low point and using a small hand level
or a straight edge to line up with the two high points.
For a standard corrugated metal pipe,
these are two and 2/3-inch width, and 1/2-inch depth,
however, other sizes may be encountered.
Next, let's focus on the entrance.
Depending on the culvert, it may or may not project
or stick out from the embankment.
If the culvert does not project,
it should be noted and photographed.
Sometimes pipes are mitered,
or cut at an angle to match the slope of the embankment.
If such a pipe does not project,
it should be noted and photographed.
If the culvert is thick walled, such as concrete,
even if it does project,
it should be noted and photographed,
but no additional measurements are needed.
However, if the culvert is thin walled,
such as a corrugated metal pipe,
the projection needs to be measured.
Typically, if the culvert is projecting,
the embankment it's projecting from is uneven.
I will measure the projection on the top,
the bottom, and both sides.
I will later average these values to determine
the projection used for the adjustment factor.
The final factor to examine are the culvert walls
at the entrance.
They can be squared, rounded, or beveled.
For rounding, the culvert walls will be rounded
on the side at the entrance.
I measure this by using a small hand level
or another straight edge to line up flat on one side
and a folding ruler flat on the other side.
In between, there will be a gap
from the 90-degree corner created by the two tools
and the rounded corner of the culvert.
I then measure the distance on the folding ruler
from the hand level to where the culvert wall
is again parallel with the folding ruler.
This is where the gap ends.
Repeat this step by switching the positions
of the straight edge and the folding ruler.
The average of the two values is the radius
of the entrance rounding.
With a culvert entrance that is beveled,
the culvert wall entrance has corners
not at 90-degree angles.
To measure the bevel, you need to measure
the angle as well as the thickness of the wall.
I measure the angle by using a protractor
facing away from the bevel.
I then line up the folding ruler with the bevel
and read the angle between the ruler and the wall.
This should be done at all bevels
since they won't necessarily be the same on all sides.
Sometimes only the top of the culvert
is rounded or beveled.
If the flow being computed did not reach this height,
it is not necessary to measure,
although it could be used for future computations.
If you need help in the field,
call your supervisor, surface-water specialist,
or indirect measurement specialist.