Hi, this is Megan Poff
and I'm the Field Office Chief
at the USGS in Las Vegas, Nevada.
During the other slope-area video,
we discussed the theory of the slope-area method.
In this video, I'll be discussing the survey requirements
for the slope-area indirect measurement method.
You can find more information on this topic
in TWRI 3-A2: Measurement of peak discharge
by the slope-area method.
As we learned in the last video,
the field data required for a slope-area measurement
involves surveying the channel cross-section geometry,
high-water mark profiles along each bank,
and then estimating roughness coefficients.
First things first though,
we have to flag our high-water marks.
Finding a good reach for a slope-area measurement
is very similar to finding a good reach for a streamgage.
You want a relatively uniform and straight reach
with minimal expansion and contraction,
as close as we can find for a natural stream, anyway!
However, for the slope-area method,
you also need a distance of at least three times
and preferably five or more times
the width of the flood channel,
and there has to be at least half a foot
of fall in the reach.
Flag the high-water marks on both banks.
If you need information on flagging high-water marks,
see the high-water marks video.
You will want to flag enough marks
to define what the water-surface profile
looked like at the time of the peak.
That's going to vary for every channel
depending on slope, the channel roughness,
and the types of high-water marks you encounter.
For the conditions I encounter in the southern Nevada area,
I generally think, "Okay, less than 10 high-water marks
"per bank isn't enough.
"20 per bank might be enough,
"and more than that might be even better!"
If you aren't sure how many marks to flag,
make sure you take an experienced person
with you to help you out.
Also see the guidelines provided on page 36 of TM 3-A24
on Identifying and Preserving High-Water Mark Data.
Okay, we've flagged our high-water marks,
and now it's time to survey!
You can complete the slope-area survey
using a conventional level, Total Station,
or with RTK or RTN GNSS if you can achieve acceptable
vertical precision with these instruments.
For this video, I will demonstrate
how to do a simple survey with a Total Station.
Before you leave for the field,
you will need to gather the following items:
tripod, prism pole or fiberglass rod, prism,
Total Station, field notebook, site sketch forms,
wooden stakes, nails, data collector if you have one
along with any required cables,
a cable to download the data, a field laptop,
site-specific information such as a Station Description,
and extra pencils.
Remember to tape, that is measure, your rod with the prism
on it and check the collimation on your instrument
and enter these data in your notes.
Extendable fiberglass rods are extremely helpful
for total station surveys, you can measure the extension
lengths with prism attached and record them
for quick reference during the survey.
Here we are in the field.
The first thing I'm going to do is walk the reach
and check out its extent in characteristics.
This is a pretty simple reach to survey
so we're going to set the instrument up
in a place where we can see the entire reach
and we'll do the entire survey from a single set up.
Find or make a stable and repeatable point
you can come back to just in case you need to.
For this survey we have a stable rock
in the channel that we marked with a permanent marker
but you could also install a stake and set up
the instrument over that.
Look in the scope on the side of the Total Station
to set up the instrument exactly
over your point of reference.
Tape up from that point to the line or marking
that's even with the instrument eye piece
to determine the height of the instrument
over the point and record
the tape up distance in your notes.
We will name this point hub-1 and will give
it arbitrary coordinates of 1000 feet northing,
1000 feet easting, 100 feet elevation.
If you're serving a large site,
you could also use coordinates of 5000, 5000.
If you're serving near the gage you can also
reference to gage datum.
If you do decide to reference to gage datum,
tie in two or more gage RMs and also survey
any gage high water marks that where flagged.
Just like with any other level circuit,
you will want to start your survey by taking a shot
on a reference mark,
two or more if you're starting from a streamgage,
or some other stable and repeatable point.
However, remember that Total Stations also measure
both horizontal and vertical angles along with distances.
We have to tell the instrument where zero degrees,
zero minutes is located so our angles
and distances will be correct.
What's another name for zero degrees, zero minutes?
That's north!
However, we have options.
It doesn't truly matter which direction
you set to zero degrees, zero minutes,
because we are surveying in an arbitrary datum.
You have three main options:
one, set the gun's zero to true north using a compass,
two, set the gun's zero to upstream in the channel,
or three, set the gun's zero to the direction
of the backsight shot.
I actually prefer option three because I can check
my angle at the end of the survey very easily,
but all options are valid.
Focus the instrument on the prism and take the backsight.
A note on accurately shooting the prism,
you want to focus the Total Station eyepiece
so the stadia lines on the gun align
with the lines on the prism back-plate.
You can improve the precision of your backsight shots
by shooting the prism direct,
then rotating the instrument and inverting the telescope
to shoot the prism indirect,
average the two vertical readings.
We've taken our backsight,
so now it's time to survey the high-water marks.
Instruct your rod person to tell you if they change
the height of the rod and, this is important,
write down every rod height change in your notes,
just in case you forget to enter
the change in your gun or data collector.
Survey all the high-water marks on both banks
to the hundredth of a foot,
making sure to record any rod height changes
and the composition and quality of each mark
if the flaggers didn't already do that for you,
the rod person or other party member
can help with this documentation.
Your data collector will record the point names,
northings, eastings, and elevations for you,
but if you don't have a data collector,
you will need to record these items as well.
At this point, stop and take a break.
We are required to plot our high-water marks
in the field to identify significant breaks
in the water surface profile.
Remember that whole discussion about steady, uniform flow
and gradually-varied flow concept
I covered in the theory video?
Putting our cross sections at major breaks
in the water-surface profile will allow us
to approximate steady, uniform flow
through the entire surveyed reach;
additional cross sections can be placed
in more uniform parts of the reach as appropriate
so we have an adequate number of cross sections
in the study reach.
We will need at least three cross sections
for a slope-area measurement.
Placing your cross-sections correctly
will result in a more reliable measurement
using the slope-area method.
Plus, it's nice to get out of the sun for a few minutes,
have some water and a snack,
and collect your thoughts before continuing.
You can also see if you have any glaring gaps
in your water-surface profile and make notes
to see if you can find more high-water marks
to fill those gaps.
I also take this time to make sure
we have that required half a foot of fall in our reach.
Download the data from your data collector
and use a program like the SAC-GUI, iRIC-SAC or Excel
to create a quick high-water mark profile plot.
Check out the video on data input
into SAC-GUI for more information.
This is what a high-water mark profile plot
might look like for an ideal site.
However, more often than not,
our high-water mark profile plot
is going to be a messy affair.
Regardless, this is what our field site gave us,
so we're going to have to work with it.
Let's find the breaks in slope in our profile!
I usually use the edge of a piece of paper
to help me with this task,
because most high-water mark profiles
can be a little messy, you usually want to give
more weight to the higher, better-quality marks.
Let's put a cross section close
to the beginning of the reach.
I'm going to disregard this area of left bank marks
that appear to be going uphill,
and let's focus on the rest of the profile.
I'm going to place the first cross section at RH-2,
then, using my piece of paper on my screen
as a straight edge, let's see where the first change is.
Looks like that occurs right around RH-13.
Our next change occurs around RH-5,
so I'll place a cross section there.
Three cross sections are our absolute minimum
for a slope-area survey, so I will try to add in a fourth
if my reach length is long enough.
Looks like we can fit a fourth cross section
around RH-9 before the next potential slope change occurs.
If you plot your high-water marks in the field
and can't detect a trend or breaks in slope,
and therefore have no idea where to start,
you can try displaying the left and right
high-water marks separately.
Sometimes that will help give you a starting point
to figuring out where to place the cross sections.
Great!
We've now figured out where to place the cross sections.
Let's get back to our survey.
We'll survey our cross sections perpendicular
to the flood channel flow to an accuracy
of a tenth of a foot.
I like to use the stake-out feature
in my Total Station controller
to help find high-water mark locations.
This function tells me which direction to point the gun
so I can quickly find the high-water mark
where I want to start my cross-section surveys.
There's no need to survey the high-water marks
again because we already shot them,
although you may want to search again for marks
at or near the cross sections you've selected,
especially at sections where none of the marks
flagged initially were close.
How many points should you choose
for your cross-section survey?
Well, it depends.
Keep in mind that your entire measurement
will be based on the area that you define
from this single cross-section survey,
so it will be in your best interests
to make your survey relatively detailed.
Start and end each cross section at points
well above the high-water mark elevation,
you don't want to have to extrapolate
your cross-section geometry later in the office.
Make sure you define the major changes in the shape
of the channel as your rod person
moves across the cross section;
ideally left and right edge of water for each cross section
and document any characteristics of interest.
Remember to record any rod height changes.
You can find more information on surveying
both high-water marks and cross sections
in the high-water marks
and cross sections video in this series.
At the end of the survey, break the HI to turn
and close the circuit by taking a final shot
on your starting point.
Take pictures, draw your plan-view
and cross-section site sketches,
and estimate n-values for each cross section.
You're done!
You can find more information on the type of notes
to record and what to put on your site sketches
in the field notes video.
If you will have a more complex survey
that requires you to move the gun,
see the Total Station surveying video.
If you need help in the field, call your supervisor,
surface-water specialist,
or indirect measurement specialist.