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Pipe flow, Manning's n, low flow

Pipe flow, Manning's n, low flow

Pipe flow, Manning's n, low flow

(OP)
My problem is following: I am trying to generate a elevation-disharge curve for a pipe with Manning's equation. I know the material (concrete), diameter (20 inches) and the slope (3%) of the pipe.

There are some discharge measurements, but unfortunately these measurements are from low flow period. The water depth is 0.4-6.8 inches.

When I used Manning's equation, I got almost perfect correlation to these measured flows by using the slope of 3% and Manning's n of 0.06. However, in the litearture Manning's n for concrete pipe is around 0.01-0.015 and using these values doesn't look good against the measured values.

I think Manning's equation is giving too small discharges with higher water depths with the fitted Manning's n.

Do you have any links/references to studies that deals with low flow Manning's roughness coefficients?

Any suggestions to solve this problem are very welcome too!

Thank you,

Johann.


RE: Pipe flow, Manning's n, low flow

Manning's is applicable for finding the normal flow depth in the pipe.  

It is not clear from your post where the measurments were taken.  Unless they were "mid-pipe", Manning's will not be entirely accurate.

To find water depth at the pipe inlet, you need to account for entrance head losses.  The simplest way to do this is with a culvert nomograph, which you can find in a local drainage manual, hydraulics text book, or FHWA documents such as HEC-22.  

Accounting for entrance losses will yield higher water surfaces, which will more closely match results you got using the higher n-values and Manning's.

RE: Pipe flow, Manning's n, low flow

Manning's n varies between 1.0 and 1.3 of its value, with depth of flow.  It is only equal to 1.0 times its full flow value at full flow and 0 flow.  

It is about 1.2 times full at about 0.08 and 0.65 full depth, 1.3 times full from 0.30 to about 0.35.

If you draw a chart and plot d/D on X and n/nfull on Y, and put a smooth curve through these pts, you will have the relationship given on page A-39 of Lindbergs CERM, which is widely used for PE exam prep in the USA.

Engineering is the practice of the art of science - Steve

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