Issues during Review of SonTek RiverSurveyor Stat. Live Data

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 some of
the common issues identified during field

review of midsection measurement data collected
in RiverSurveyor Stationary Live.

-One of the most common issues in midsection
ADCP measurements is a lack of angle correction

or angles that are measured incorrectly.

First, ensure angles were corrected for using
either the XYZ method, which requires the

unit to be held in a specific orientation,
or the ENU method, which uses the compass

and a tagline azimuth to correct for angles
automatically.

Indicated angles are easily viewed in the
Transect Tab and the Samples Tab.

-With the XYZ method, errors are usually caused
by the user not holding the unit at the correct

orientation, causing angles that do not make
sense hydraulically, as we see here.

-With the ENU method, errors are usually related
to the azimuth entered in the software being

measured incorrectly, the compass not being
calibrated properly, or compass errors during

the measurement, either due to a poor calibration
or interference from a local source.

-Good documentation is always important to
confirm angles that are indicated by the software

or noted by the user.

In this case, the angles may be very reasonable,
or due to a compass error or misalignment,

and without good field notes, we wouldn’t
have any way of confirming them.

-Ideally, errors with either method would
be identified during data collection and addressed.

With XYZ, there is not much that can be done
to correct erroneous angles during review.

With ENU, only the tagline azimuth can be
edited, if it was known to be entered in error.

-Another common issue when reviewing RSSL
data is inconsistencies in the extrapolation

methods selected by the user and the selected
extrapolation methods not representing the

measured data.

First, ensure the selected methods agree with
the site conditions.

For example, if making an ice measurement,
ensure the ice water surface type is selected

and that the no-slip/no-slip extrapolation
method is used.

Also, evaluate the velocity profile plot to
ensure the appropriate method and exponent

are used and applied consistently to all stations.

Here is an example of where the measured data
and the extrapolated data don’t seem to

make much hydraulic sense.

A separate screencast in this series discusses
extrapolation in more detail.

-OSW Memo 2014.02 requires a screening distance
to be entered in the manufacturer software,

which includes RSSL.

This screening distance is equal to 0.52 plus
the transducer depth.

In open water measurements, this is easy to
compute, but in ice conditions, this screening

distance is referenced from the water surface,
so you must add the 0.52 to the transducer

depth below the water surface, which would
include ice thicknesses.

If you don’t see blacked out data on the
contour plot, review the screening distance

settings.

-OSW Memo 2010.07 requires an independent
temperature comparison for each ADCP measurement,

and this includes midsection data.

A common issue observed during midsection
review is a change in temperature due to the

instrument not being equilibrated.

Ideally, this would be prevented by the independent
temperature check obtained before the measurement.

Changes in temperature are easily seen in
the Time Series Tab or the viewing the tabular

data for each station on the left of the screen.

If large differences in temperature are noted,
consider entering the independent reading

on the System Tab.

Understand that this fixes the temperature
for the entire measurement.

-Separation in SNR is sometimes observed in
the profile plot on the Transect Tab in fast

velocity conditions where the ADCP is not
deep enough in the water.

This separation is likely due to air entrainment
under the transducer.

Again, if observed during data collection
the ADCP can be lowered into the water, but

during playback, there is nothing that can
be done to remove these anomalies and it’s

difficult to determine how they might affect
the final discharge.

-When using the midsection method with ADCPs
it’s important to select the appropriate

references, and a fairly common mistake is
the improper use of these settings.

In the System Tab, ensure the track reference
is set to System.

It should not be set to Bottom-track as this
could introduce errors in moving bed conditions

or places where bottom tracking is difficult.

If there are errors in the bottom track or
vertical depths they cannot be screened out,

and the best option is to change to the other
reference, assuming it doesn’t have the

same errors.

Also, ensure the discharge method is set to
mid-section, not mean-section.

-Another common error in the settings is the
coordinate system, which tells the software

how you would like to compute angles.

XYZ is used to correct for angles in cases
where you hold the unit at a known orientation

to the flow.

If this is not done, you cannot use XYZ.

ENU is used in cases where a tagline azimuth
is obtained and the compass is used for angle

correction.

Be aware that these settings can be made for
each station individually, and you would want

to ensure they are consistent for the entire
measurement using the apply to all button.

More information on the use of these references
can be found by visiting the OSW hydroacoustics

webpage and viewing the podcasts on open water
and ice midsection ADCP use.

-Possibly the largest source of error and
uncertainty with all midsection measurements

is the use of a poor cross section, improper
stationing and use of an instrument not appropriate

for the channel conditions.

When reviewing the data, consider how a different
cross section or instrument might improve

data collection.

During data collection, ensure your stationing
defines the river conditions and flow, and

insert additional stations, if needed.

When possible, use an instrument and deployment
that is appropriate for the depth and velocity

conditions you’ll be measuring in.