Hi, this is Chris Morris
and I'm the surface water specialist
at the USGS in Las Vegas, Nevada.
I will be talking about culvert indirects.
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.
To use a culvert to compute a peak flow,
we need to survey attributes of the channel and the culvert.
In this video we will be discussing surveying
the high-water marks and the approach cross-section.
Surveying the culvert itself is discussed
in the other videos.
Culvert indirect measurements have several assumptions,
including that sub-critical flow occurs
in the approach to the culvert.
In the culvert barrel itself, flow may be sub-critical,
critical or super-critical.
To determine if there was sub-critical flow in the approach,
you will survey the high-water marks upstream
of the culvert entrance to verify
the water surface profile was mostly flat
and uniform without large changes in slope,
except associated with the culvert.
The proper techniques for identifying
and flagging high water marks and techniques
for surveying high water marks and cross-sections
can be found in other videos.
First, where do we want to survey these high-water marks?
During the flood, depending on the height of the water,
there may be been drawdown or a drop in the water surface
at the culvert entrance.
You are going to start surveying the high-water marks
at least one to three culvert widths upstream
from the entrance to ensure you are not only
in this zone of drawdown.
If there are multiple culverts, the sum of the width
of each individual culvert will be the distance you want
to be upstream, three five-foot culverts.
You want to be at least 15 feet upstream.
If there are wingwalls associated with the culvert,
measure the distance between the upstream
end of the wingwalls.
For example, if that distance is 30 feet,
the survey should start at least 30 feet upstream
of the upstream end of those wingwalls.
High water marks should be surveyed over a distance
of at least one times the width of the culvert.
Depending on the site, there may be a gage
located in this area.
Surveying a crest stage gage
and then using the computed peak can be a simple way
to get a good high-water mark.
This can be especially useful if most
of the high-water marks available are poor or degraded.
High water marks can be surveyed in any direction,
although I find it is less confusing in the office
to start from the upstream end on both sides.
Once the high-water marks have been surveyed,
take a break and plot the high-water marks in the field.
It can be tempting to skip this step,
but if the high-water marks indicate, for example,
that there was a drop associated with a riffle upstream
of the culvert, the computation won't be valid
and doing the entire survey will be useless.
You can use software like Excel, SAC-GUI,
or IRIC to create a high-water mark plot.
The high-water marks will likely not plot
in a perfectly flat line.
Especially if there are many poor marks,
there may be considerable spread, but they should indicate
a mostly flat and uniform water surface
without large changes in slope,
except associated with the culvert.
At this point, while it is still fresh in my mind,
I like to start my site sketch
and make any important notes about the high-water marks.
This topic is discussed in more detail in another video.
Now you are ready to survey a cross-section in the approach.
Like the high-water marks, the cross-section
should be one to three culvert widths upstream
to ensure we are not in the zone of drawdown.
If possible, place your cross-section in between a few
of the surveyed high-water marks since you
will be using the high-water marks to determine
the water surface at the approach.
The cross-section should be perpendicular to the flood flow.
This may be at an angle to the culvert.
For a simple cross-section, you may only need five
to 10 points, however, for a complex one,
you may need many more.
You want to characterize the changes in the approach,
so you can compute an accurate cross-sectional area.
Make sure your first and last cross-section points
were above the water surface during the peak.
If you have really good marks, this may be obvious,
but when marks are poor, it is always better
to go a little higher just to make sure you end up
being above the final water surface.
Finally, don't forget to take pictures of the approach.
The rod person holding the rod horizontally
at the height of the water surface
from both sides is quite useful.
At this point, while it is still fresh in my mind,
I like to continue my site sketch and make
any important notes about the approach cross-section.
I will also estimate a roughness value for the approach.
This topic is discussed in more detail in another video.
Using a measuring tape, determine the distance
from the approach cross-section,
pick the middle of the approach, to the culvert entrance.
This step is slightly different for mitered culverts,
or those culverts whose entrance are
at an angle to match the slope of the embankment.
Measuring of this distance for mitered culverts
is covered in another video.
You have the approach cross-section
and high-water marks surveyed, so let's go to the exit
of the culvert to document our tailwater conditions.
High water marks should be close to the outlet.
Continue using the number sequence from upstream.
If the last high-water mark on the left side
was named LH-12, name the first one
on the downstream side LH-13.
Again, a clear numbering system is very helpful
back in the office.
Depending on whether you are surveying to gage datum
by starting on a RM or are using a temporary RP to
set to an arbitrary datum, the downstream high-water marks
need to be surveyed in the same datum,
both horizontal and vertical,
as the upstream high-water marks.
Depending on the instrument, this may require turning points
to come out of the channel, over the road,
and back down in the channel,
or even shooting through the culvert itself.
By planning ahead, it is often possible to be able
to shoot everything from one setup.
What about the culvert?
Don't worry, this will be covered in another video.
If you need help in the field, call your supervisor,
surface-water specialist or indirect measurement specialist.