Hi, my name is Nick Stasulis and I am a hydrologic technician with the Maine office of the New
England Water Science Center.
In this screencast I will discuss the basics of reviewing extrapolation methods for midsection
measurement data collected in RiverSurveyor Stationary Live.
It’s important to note that the USGS extrap program cannot be used to evaluate extrapolation
methods in midsection software, it is a manual process, as we’ll discuss.
-Before reviewing extrapolation in RSSL, be sure the screening distance is set appropriately
in the software, as this will cause data near the surface to be removed, and might change
your evaluation of the methods used.
As a reminder, the screening distance is set to 0.52 plus the transducer depth for open
water measurements.
For ice measurements, the screening distance is still based on the water surface, but must
include ice information.
So, you would add the 0.52 to the transducer depth below water surface, which would vary
for each station if ice information was entered into the software.
-When evaluating extrapolation methods for any ADCP data, it is useful to have some knowledge
of the site conditions to aid in your understanding of an appropriate extrapolation method.
For example, it’s useful to know about the bottom substrate type and size in relation
to the channel depth, the channel shape and hydraulic features upstream and downstream
of the cross section.
Certain channel shapes can tell us something about what the expected velocity profile might
be, as we see in this diagram from Chow.
It shows that some site conditions might cause us to expect a bend back at the surface, indicated
by the blue arrows, while some might cause to expect an increase in velocity towards
the surface, indicated by the red arrows.
Also, the roughness of the bottom could tell us something about the expected power exponent.
If the average depth is 4 ft and there were 1 ft boulders on the channel bottom, you would
expect the profile to trend towards zero higher in the water column than if the channel were
a sand bottom.
The rougher the bottom, the higher you would expect the extrapolation power fit exponent
to be.
-Start your evaluation of the extrapolation method by scrolling through each vertical
and looking at the velocity profile plot.
This is easily done using the left and right arrow keys.
Remember that the blue measured data is always used, and we want to evaluate how the green
top fit and orange bottom fit line up with the measured data and extend beyond that data
into the unmeasured areas.
-If you start noticing a consistent trend of measured data and the extrapolated areas
not agreeing, it’s likely a change is needed from the power/power default.
If a change in slope to the measured data will help align the extrapolation, it may
be as simple as increasing or decreasing the exponent, RSSL software labels the exponent
as coefficient.
A lower coefficient will cause a more vertical profile (due to a smoother bed), while a higher
coefficient will flatten the profile (due to a rough bed).
If the coefficient alone doesn’t allow a proper fit, you might need to change the method,
as well.
Note that when changing the exponent for a power fit, the coefficient value needs to
be changed for both the top and bottom method.
-Constant/no slip will take the upper most bin and carry that velocity to the surface
and fit a power curve through the lower 20% of the data.
The cells to use for the constant method is typically not changed, left at 1, and the
% of the cells used for the no slip method should typically not be changed, either.
It is important to note that you can, and should, change the coefficient for the no
slip method if the standard exponent doesn’t follow the trend of the measured data.
-Let’s make a few other points on the available methods.
No slip should only be used on the top for ice measurements, never for open water data.
Also, constant should never be used on the bottom, for any measurement, as we know the
velocity eventually goes to zero.
3-point will fit a line through the top three bins, and is useful in conditions where strong
winds cause a severe bend towards zero on the surface.
Use of this method should be confirmed with detailed field notes.
-While many measurements will have the same extrapolation method used for the entire measurement,
some cross sections may require different methods for each station.
If the portions of your cross section vary significantly in bed roughness, bed type,
and/or flow direction the velocity profile for some sections may also vary.
In RiverSurveyor Stationary Live you can specify extrapolation methods, including the power-fit
exponent, for individual stations when the shape of the velocity profile varies from
station to station.
-After your evaluation is complete, and you’ve selected the method that you feel is appropriate,
there are a couple steps left.
First, apply the newly determined extrapolation settings to each station, or the entire measurement,
as appropriate.
Take a look at the change that resulted to the final discharge, as this will help determine
the uncertainty associated with your new extrapolation method.
If you are confident in the selected method and the change in final discharge is quite
small, it’s likely the change in extrapolation method will have little influence in how you
rate the measurement.
If the change is quite large or uncertain, you should likely down-rate your measurement
due to the increased uncertainty with the extrapolation method.
Also, any time you apply a change to the extrapolation methods, make sure that the change is documented
in field notes.