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 a culvert.
In another video, I discussed surveying the approach
high water marks, the tailwater high water marks,
and an approach cross-section to a culvert.
It would be useful to watch that video first.
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.
So you have surveyed the high-water marks
and the approach cross-section of the culvert.
You still need to survey the culvert itself.
At first glance it may seem easy.
It's just a concrete box or a metal pipe.
However, once you start looking closely, you can see
rounding or bevels, projections and wingwalls,
natural channel bottoms or fill in the culvert.
Many of these features will be covered in other videos.
In this video, I will discuss some of the more general
culvert features that need to be surveyed or measured
that can occur in any type of culvert
you may encounter in the field.
What if I'm really busy and I have an indirect
from 10 years ago at this site?
They already surveyed all the attributes.
I don't need to do this, right?
Wrong, don't assume that an old indirect, survey sheets,
blueprints or even station descriptions
have the correct culvert attributes.
Culverts can be replaced, modified, or even mis-measured.
Additionally, depending on the surveys being done,
they may not share the same gage datum.
For example, a 10-year-old total station survey
may have set an RM at 100, 100 while the modern-day
GPS survey may be using real northing and easting locations.
Although this data could be made to work together,
it is often faster just to resurvey.
A critical element to surveying a culvert
is taking good, clear notes about everything 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.
In addition to good notes, take many good photographs.
I typically take photographs of the rod person
holding the rod horizontally at the height
of the water surface in the approach from both banks,
and at the culvert entrance and exit.
For complex culverts, I would also take pictures
of someone pointing out key features of the culvert itself.
Okay, let's start surveying the general culvert features.
The most obvious feature you would want to measure
is the size or area of the culvert.
For box culverts, the width and height are needed.
For circular pipes, the diameter should be measured.
For pipe arches, the rise and span should be measured.
The elevation of the entrance and exit floor of the culvert,
also known as the inverts, needs to be surveyed.
Sometimes the inverts cannot be directly surveyed
because of obstructions such as trees or the culvert barrel
itself sticking out too far,
not allowing you to hold a rod level.
With GPS surveys, often you have poor satellite coverage
due to the embankment blocking a portion of the sky.
If this is the case, the top of the culverts
can be surveyed, and the culvert height subtracted
to determine the invert elevation.
However, the additional thickness of the culvert top
or material needs to be taken into account.
This should also be clearly noted in the survey notes.
If you are surveying at a site far away from the gage
and it is in an arbitrary datum, it could be very unclear
if the top or bottom of the culvert was shot
when you are trying to work up the data
a month later in the office.
The length of the culvert is also needed.
When I am surveying, I'm typically using a GPS
or a total station and measuring points
in northings and eastings.
I can use the distance formula along with the entrance
and exit invert shots to compute the length,
but a measuring tape could also be used as a check.
The material the culvert is constructed from
should be noted.
Corrugated metal pipes and concrete are most common,
but clay, cast iron, welded steel, and even wood
can sometimes be encountered.
After a flood, material can be found in the culvert itself.
Often here in Nevada I find sand fill
in the first few feet of the culvert.
When this material is loose, it is typically assumed
that at the peak itself, high velocities keep
the culvert clean, and the material is deposited
on the recession of the peak.
The next peak again removes it on the rise.
However, at some sites the material is well compacted
or naturally cemented and doesn't appear to change
as a result of the peak.
The same can be true for debris at the entrance.
If it is thought the material was present at the peak,
the obstructed area needs to be measured.
For example, if I found gravel well-compacted
and cemented in a box culvert, I would need to survey
a cross-section in the culvert.
If that was not possible, I could dig out part
of the culvert clear and measure the depth of the fill
to compute an accurate area.
Take time in the field to assess if the culvert
was clear at the peak or was impacted by debris or fill.
We aren't done yet.
Continue to other videos for more detailed survey notes
about the culvert you have.
If you need help in the field,
call your supervisor, surface-water specialist,
or indirect measurement specialist.