[Acoustics] mode use for slow vel

[rwellman]

Important measuring conditions in your cross section have not been 
mentioned, such as the bed material. Is it silt? mud? how soft is it? 
gravel? cobble? armored or unarmored? what's the size distribution of 
the cobble? uniform size or variable? round or broken with flat sides? 
any boulders? what do they look like? is the shape of the bed uniform 
with gradual changes in elevation?

Bed conditions relate to tubulence in the lower parts of the water 
column, even at the velocities you mention. Tubulence will cause data 
correlation problems and lost/bad bins/ensembles in modes 5 and 11, 
leading to larger areas of estimation aka. uncertainty in your final 
discharge. Consistancy of the software computed total discharge doesn't 
make me feel good if the ratio of estimated
Q  to total Q is high.

I've seen no mention of making a stationary acoustic measurement. The 
technique removes (or lowers) the issues surrounding boat speed and 
other operator errors. If the water velocities are extremely low, you 
can collect for longer than 40 seconds. Whatever it takes to get your 
standard deviations to an acceptable level. If you don't have the 
'newest and coolest' equipment, your 1200 could do it but there would be 
more post-processing. Consider all your options.

Roy Wellman
USGS, Portland, Or

========================================
David M Hernandez wrote:

>
> To all, The conditions are: Channel width 100 to 200 ft. Depth 3 to 7 
> ft. velocity < .1ft/sec. Used Rio 1200khz water mode 12sb bottom mode 
> 5. Not sure it was the best combination for those conditions, does 
> anyone have similar conditions? Which mode combinations would work 
> best for collecting repeatable transects?
> Dave
>
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