[Acoustics] Question on Sontek SL beam vel's

[Keith Ging]

I wanted to run this across everyone to see if my line of thought was near correct.  I've recently found that our rating has changed at one of our index-velocity sites based on 4 discharge measurements this year.  We cannot find any visual clues to suggest the channel or vegetation has changed.

After digging into the data through ViewArgonaut, I noticed two things that stood out.

1.    The multi-cell data shows the velocities of the last two cells are fluctuating more than usual at very slow vel's (<0.20 ft/sec) as compared to the previous 2 years (2004-2005).  However, the diagnostic data does not show anything in the path of the beams based on signal strength and SNR's.  Visual observations have found nothing in the channel to be different.

2.    I then looked at the beam velocity data and found the vel's of beam 1 & 2 to roughly equal one another fairly consistently in 2004 (the magnitude is near the same but one is negative and one is positive).  This changed in 2005 and 2006.  Beam2 is about 0.10 ft/sec slower than Beam1 at the slower vel's.  Many times Beam2 is half of Beam1 vel's.  Could this indicate the SL has moved slightly?  The SL was replaced during the off-season between 2004 and 2005.  We do have it marked to prevent this from happening.  We also recently shot levels to make sure the SL had not changed in elevation which it had not.  I can only guess it may be turned slightly when it was replaced and we will be checking this soon.

Please let me know your thoughts.  I'm interested in how others have used the beam vel's to diagnose problems or any other parameter logged by the SL.

Thanks,

Keith Ging, P.G.,  Senior Hydrologist
River Operations
Lower Colorado River Authority
3505 Montopolis Dr,  Austin, TX  78744
Phone:  512-356-6009
Toll Free:  1-800-776-5272 ext. 6009
Fax:  512-356-6059
Email:  kging at lcra.org