Pipe flow, Manning's n, low flow

[Ravn]

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.

 

[bltseattle]

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.

 

[LHA]

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