Upstream flow greater than downstream flow - --- is this possible?

[login67]

I have a task of measuring low flows in a stream using a pygmy meter. Although for the most part the result shows flow generally increases as I go downstream, there are some places where an upstream flow is greater than the next one downstream. I am very skeptical about the later finding and would like to know under what circumstance will upstream flow be greater than downstream flow. You help is highly appreciated.

Thanks.

 

[psmart]

Do you mean flow (cfs) or velocity? (fps)

Unless there are losses or diversions, the total flow (velocity x cross-section) will generally increase as you go downstream. The velocity, however, will depend on the exact stream characteristics at any given point, rather than the upstream/downstream location.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[login67]

I mean flow. The pygmy meter computes the average velocity at a cross section and then multiplies the average velocity with the measured cross-sectional area to come up with the flow. Flows for some of the upstream cross sections are higher than those downstream and I am trying to figure out the reason behind this odd trend.

Thanks.

 

[psmart]

Forgive my ignorance, but how does the meter determine the cross-section? I thought it just measured velocity at a specific point in the channel, and you had to do the final calculation of flow rate based on a surveyed/measured cross section.

http://www.rickly.com/sgi/pygmy.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[beej67]

I'm also curious how the pygmy meter determines average velocity, since the velocity profile is going to change throughout the water column. See my response in the Storm/Flood forum for a half dozen possible sources of error.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[login67]

The device we are using is called AquaCalc 5000 and it has a USGS pygmy current meter attached to it. For more information about this device and how it works, please go to website below:

http://www.advmnc.com/JBS/AQ.html

Thanks.

 

[psmart]

Would have helped to know that in the first place...

Anyway, the device still requires manual input for the position and channel width, correct? There's a lot going on in that (black) box, and it seems that you need to take a closer look at exactly what it's doing. Basically, you need to be able to replicate and verify all the calculations it's doing.

BTW: Please avoid posting the same question in two forums. It just gets confusing and wastes other people's time.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[login67]

Yes the device still requires manual input for position and depth along a transect.

You are right I should not have posted the same question in two forums. What happeded was that I first posted it in the modeling forum and later made the decision to repost it in the storm/flood engineering forum since the question is more in place with this forum. My appologies. Anyway I can take of them off the forum?

 

[psmart]

I sent a request for the two threads to be merged.

Peter Smart
HydroCAD Software

http://www.hydrocad.net