[Acoustics] Flowtracker in shallow depths with high velocities (around 8 ft/sec)

[Timothy J Reed]

Hello,

Has anyone measured high velocity with the Flowtracker (at least 8 feet per
second). I made a measurement in a shallow stream (under one foot deep)
with velocities around 8 to 9 feet per second. At the edges, the velocity
seemed reasonable (what was being displayed/measured by the Flowtracker),
but near the center of the stream, the velocity at times registered as
negative velocities when they should have been positive (downstream)
velocities. I would retry the measurements when they were negative until I
would get a positive velocity. The pileup on the wading rod and the rubber
handle/stem of the Flowtracker was substantial and there was a pocket of
air in the stream were the water was parting at the wading rod and
Flowtracker handle/stem.

After finishing this measurement, I went downstream to a section where it
was deeper and the velocity was 5 feet per second. The two measurements
made at this section matched each other. The stream is small and flashy.
The first measurement (with the negative velocities) was made on the rise
of the peak and the next two (no negative velocities) were made at the
peak.

Measurements at established gages have compared well with the rated
discharge and measurements made with comparison to the current meters have
compared well at a range of velocities below 8 feet per second.........

Dave Velasco  from Sontek looked at the output and gave the attached
response:


Hi Tim

What you are observing are commonly called ambiguity jumps or wraps.  All
ADVs use the pulse-coherent technique to calculate velocity and the data is
subject wrapping if the measured velocity goes above the maximum measurable
velocity for that velocity range.  Wrapping is when the velocity goes above
a certain threshold, beyond which they become negative, then positive
again,
etc. because we can only measure between pi and -pi.  Think of a carousel
with just one horse on it.  If you look at it once, close your eyes for a
few seconds, then look at it again, you have no way of knowing how many
times it has gone around, since after one complete revolution, the horse is
back where you first saw it.  Once it's past that original point, it has
"wrapped" around.  See the attached picture; should help.

The max velocity of 15 ft/s we spec for the FT assumes that we have a
purely
X velocity.  In your case, the velocity is actually a bit off pure X -
typically by about 15 degrees.  Though this does not seem like much, it
actually makes a big difference in our beam velocity data and hence pushes
us over the ambiguity level.  A quick example:
 - For a pure X velocity of 8 ft/s, if we assume our bistatic angles are
exactly 15 degrees, we get beam velocities of +2.1 ft/s and -2.1 ft/s (both
are 8*sin(15))
 - If we have an 8 ft/s flow that is 15 degrees off of the X axis, our beam
velocities are 4 ft/s and 0 ft/s (8*sin(15+15) and 8*sin(15-15)).
 - Our bistatic velocity limit for the FT is about 1.15 m/s (3.8 ft/s).
With the flow directions you were seeing, we violate this value (we can
confirm this by manually rotating your velocity from XY to bistatic/beam).

Unfortunately there's not much that can be done to the data.  Using an
estimated velocity for that station and calculating flow by hand is the
best
solution.  Next time you encounter this in the field, try to place the
probe
as aligned with the flow as possible.  The higher the angle, the lower the
maximum measurable velocity.

Let me know if you have any questions.

Cheers,

-Dave 
+-------------------------------------
David W. Velasco
Technical Support Department
SonTek/YSI, Inc.
6837 Nancy Ridge Drive, Suite A
San Diego, CA, USA  92121
(t) 858.546.8327
(f) 858.546.8150
dvelasco at sontek.com
www.sontek.com



-----Original Message-----
From: Timothy J Reed [mailto:treed at usgs.gov]
Sent: Tuesday, October 05, 2004 9:15 AM
To: David Velasco
Cc: Timothy J Reed
Subject: Reverse flows at high velocities?

Hello Dave,

I recently made 3 discharge measurements at a site here in New Jersey on
September 8th, just after a heavy rainfall. The first measurement was on a
rising stage. The second measurement was near the peak, and the third
measurement was on the recession. The channel is lined with gabion gaging
(streambed and some of the sloping bank) upstream of the culvert were I
made the first measurement. The velocities were fast and the stage was
changing rapidly, so I set the Flowtracker to a 20 second measuring time
instead of 40 seconds. My concern is that on some of the measurements, the
Flowtracker was measuring up to and over NEGATIVE 8 feet per second when
the flow was moving downstream (not negative). I would not accept the
negative velocities at the prompt and would remeasure until I got a
positive velocity (except for station #5, here I am crossing out the -0.834
ft/sec and substituting an estimated 6 ft/sec).

The next two measurements later that day seemed fine (54.1 cfs and 50.5
cfs). They were made downstream of the culvert were the depths were over 2
feet and the velocities were slower.

I am attaching the files for the first measurement that day were you can
see the negative velocities. Can you please contact me for further
discussions on this? I will fill you in on more details about the stream
conditions.

(See attached file: 01387485.908.ctl)(See attached file: 01387485.908.dat)
(See attached file: 01387485.908.dis)(See attached file: 01387485.908.sum)
(See attached file: 01387485.908.wad)

You probably only needed the .wad file, but I sent the others, just in
case.

Hope all is well out there on the West Coast. Good day to
you............... -Tim

~~~~~~~~~~~~~~~~~~~~~~~
Timothy Reed
United States Geological Survey
810 Bear Tavern Road, Suite 206
West Trenton, NJ  08628
phone 609-771-3967
fax       609-771-3915
treed at usgs.gov
~~~~~~~~~~~~~~~~~~~~~~~

(See attached file: Ambiguity Jump.bmp)
~~~~~~~~~~~~~~~~~~~~~~~
Timothy Reed
United States Geological Survey
810 Bear Tavern Road, Suite 206
West Trenton, NJ  08628
phone 609-771-3967
fax       609-771-3915
treed at usgs.gov
~~~~~~~~~~~~~~~~~~~~~~~
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