Evaluating Extrapolation in TRDI SxS Pro

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Detailed Description

This video will discuss the basics of reviewing extrapolation methods for midsection measurement data collected in TRDI SxS Pro software. Note: Use of trade names is for descriptive purposes only, and does not imply endorsement by the USGS.


Date Taken:

Length: 00:05:58

Location Taken: Augusta, ME, US


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 SxS Pro.

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.

-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
us 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.

At this site, there are large boulders on
the river bottom in the manned cableway section,

and we would expect this to cause a bend towards
zero on the bottom higher in the water column

than the standard power curve.

Also, this increased friction is expected
to cause a higher exponent as the rougher

the bottom, the higher you would expect the
power exponent to be.

-In this data set, we can actually see in
the contour plot that velocities decrease

near the bed in nearly every station due to
those large boulders on the streambed.

-Your evaluation should begin by scrolling
through each vertical using the playback controls

or by hitting the control left or control
right arrow keys.

While scrolling, look at the velocity profile
plot and evaluate how the green top fit and

orange bottom fit line up with the measured
data shown in red and extend beyond that data

into the unmeasured areas.

The blue line represents the power fit, but
remember that the software will always use

the measured data, in red, where it exists.

-In the plots, look for consistent trends
of measured data and the extrapolated areas

not agreeing, and if there are consistent
differences, it’s likely a change is needed

from the power/power default.

In this data set, we do notice the data on
the bottom going towards zero higher in the

water column than the standard power fit,
causing a large difference between the measured

data and the bottom extrapolation.

If a change in slope to the measured data
will help align the extrapolation, it may

be as simple as increasing or decreasing the

A lower exponent will cause a vertical profile
(due to a smoother bed), while a higher exponent

will flatten the profile (due to a rougher

The power exponent is changed for the entire
measurement in the processing dialog.

If the exponent alone doesn’t allow a proper
fit, you might need to change the method,

as well.

The method can be changed for each individual
vertical when appropriate.

-The wind driven method uses constant/no slip
and 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.

It is important to note that you can, and
should, change the exponent for the no slip

method if the standard exponent doesn’t
follow the trend of the measured data.

Remember that the exponent can only be changed
for the entire measurement in the processing


In this example, we see that constant/no slip
with the standard exponent doesn’t define

the shape of the measured data very well,
but an exponent closer to 0.39 shows a much

more reasonable shape (as would be expected
in a cross section with rougher than typical


-The bidirectional method uses a 3 point on
the top, fitting a line through the top 3

bins, and a no slip on the bottom.

Three point 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.

Note that in version 1.14 the bidirectional
method does not work properly and should not

be used.

-Use of the ice method utilizes a no slip
on both the top and bottom, and this method

should only be used for ice conditions, where
velocities go to zero on the surface.

Be aware that changes to the power exponent
apply to the ice method as it is using a power

fit of 20% of the data on the top and the

-There is some trial and error associated
with selecting the most appropriate method

for the measurement, as some verticals, particularly
on the edges and behind bridge piers, may

show profile shapes much different than the
main channel where the majority of the flow


-After your evaluation is complete, and you’ve
selected the method that you feel is appropriate,

there are a couple steps left.

First, check the change in final discharge
from the default power/power to the method

you’ve selected.

If the change is very minimal, say less than
1%, it is typically not worth additional consideration.

If the change is quite large and uncertain,
you might need additional examination and

down-rating your measurement based on the
amount of estimated uncertainty from the extrapolation

method would be appropriate.

Also, if applying a change, make sure that
change is documented in the field notes.