[Acoustics] Questions:extrapolation methods and 3 beam solutions

[Michael R Simpson]

I'll try to answer the first one, and let someone else answer the three 
beam question.

As you cross a typical channel,  the depth will increase significantly at 
some point and many times, the highest water velocity is found near the 
deepest portion of the channel (but not always, and especially not on 
curved reaches).  In tidal channels, profiles near the edges may have a 
non-standard shape.  In channels having a salinity gradient, the velocity 
profile shape may be significantly different than a standard logrithmic 
profile (the bottom may be moving in a different direction than the top) 
because of fluid density differences. Bottom roughness also plays a part 
in the shape of the velocity profile. Wind may be affecting the surface 
velocity on one side of the channel, but not the other.  Averaging the 
whole cross section would simply not provide an accurate picture of the 
"average" velocity profile.

It would be really easy to simply say that a short average, taken in the 
deepest, fastest part of the channel, where the depth does not vary too 
much, would be an ideal sample for use in examining the velocity profile 
shape.  Maybe that would work in 75% of the cases, but what about the 
other cases where the edge profiles are non-standard, or there exists 
density gradients in only certain portions of the cross-section.

The answer is:  There is no right answer that would cover all situations. 
Bottom velocities most often roll off to zero, so that is not a bad 
general assumption, but velocities near the surface are unpredictable. 
Each case must be examined on its own merits and the proper profile chosen 
based on common sense and the examination of all the collected data 
(separated into different zones if need be.)

................................................................................
Michael R. Simpson
USGS
Placer Hall, 6000 J. Street
Sacramento, CA 95819-6129
(916) 278-3130  cel (916) 997-8048
................................................................................
" The wonderful thing about standards is that there are so many of them to 
choose from" - Grace Murray Hopper
...............................................................................





"Thomas P Suro" <tsuro at usgs.gov>
Sent by: acoustics-admin at simon.er.usgs.gov
07/08/2003 12:14 PM

 
        To:     acoustics at simon.er.usgs.gov
        cc: 
        Subject:        [Acoustics] Questions:extrapolation methods and 3 beam solutions



   I have two questions about how others are applying the new 
extrapolation
      methods and the use of 3 beam solution.

   1. As the new OSW memo states, and I agree, we should not be using
      constant on the bottom. OK, but my question is about choosing 
between
      constant and 3 pt. slope on the top and power and no slip on the
      bottom. I usually average the entire transect and look at the
      profile. Then I average it into three or four pieces and look at the
      areas with the most flow (velocity). I some cases I will use the
      method that looks best in the major flow areas instead of using the
      entire transect average to pick the extrapolation method.
    I know everyone has an opinion, but does anyone have any specific
      reasoning to always average one way or another when picking an
      extrapolation method?

 
   Tom


   ====================
      Thomas P. Suro
      Hydrologist & Engineer
      U.S. Geological Survey
      425 Jordan Road
      Troy, New York 12180
      Tel:  (518) 285-5642
      Fax: (518) 285-5601
      tsuro at usgs.gov
   ====================


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