Maximum flow capacity of a water diversion tunnel

[m1e9r5c6]

I have been tasked with researching the capacity of a very old water diversion tunnel. This tunnel was constructed in 1939 and very little design information has survived. In the absence of historic data, how can I calculate the maximum flow of this tunnel?

Particulars of construction are as follows: 12ft high, 16ft wide, 1050ft long. Change of elevation from inlet to outlet is 2.5ft. Maximum expected head is 15.2ft, minimum head is 7ft. The tunnel was blasted through solid bedrock and is not lined.

Any help you guys can give would be greatly appreciated.

 

[msquared48]

Pipe gravirty flow, probably 80% full, depending on the wetted perimeter. You will need the elevation of the inlet and outlet too for the slope. Be conservative on your inlet loss, and the tailwater elevation.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dicksewerrat]

Is this a rectangle or some type of arch?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[m1e9r5c6]

Hi, thanks for the responses. The slope was stated in my previous, i.e.: 2.5ft in 1050ft. And yes this is a rectangular tunnel.

I haven't even looked at my hydroligy books in 12 yrs so I am a bit foggy on the subject. Thnaks a lot.

 

[froude]

You could use Mannings Equation. You could also use the FHWA nomographs for a box culvert (depending on whether it's inlet or outlet controlled) and get a reasonable estimate, even though it's meant for short spans of concrete box. But a better way may be to use software similar to StormCAD to determine the capacity.


Good luck!

 

[beej67]

Presuming there's no obstructions in your tunnel, fish through tables for a manning's roughness coefficient that best describes your tunnel's walls, and document your assumption, with photos if possible. Then use Manning's equation.

If it were me, I wouldn't assume a pressure head in it at all. I'd determine the open channel flow capacity at a depth equal to the height of the tunnel and report that as the "maximum flow capacity." Technically it has infinite capacity under infinite head.



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

http://www.campbellcivil.com

 

[cvg]

first, determine inlet or outlet control. if inlet control, which I suspect is the case, then beej's method will over estimate the flow. If outlet control, then flow is related to the downstream hydraulic control section, not to the tunnel