Pipe Flow from Resevoir 1

[Knute]

Hello All,
I am working on a zoo exhibit with a large pond. We will be circulating 250 gallons per minute thru the pond. Water will be drained from the pond at 1/4" per foot back to a holding tank were the pump will be located. The pump will then pump water to a man made creek that will flow back down to the pond. My problem is that I am having a problem sizing the pipe from the pond to the holding tank. The pipe will discharge from the pond 15" below the water surface. Can anyone help me size the pipe from the pond to the holding tank.
Thanks

 

[byrdj]

Gravity drain flow can be calculated with the "Manning equation". Here is a pipe site with the equation and "look up" graph. Your slope 1/4" per foot would be 2ft/100ft, with smooth plastic pipe, 4" would only give 0.4CFS, where 6" should give 1.1 CFS. for your desired flow of 250 gpm (equivlant to 0.6CFS), thus 6" should work.

Howevere I would verify with pipe supplied

http://www.cpp-pipe.com/techpapr.htm#references