Design of storm pipe flow

[chicopee]

This question was asked before,however it was not answered. My question is when sizing a storm drain pipe, is it sized for full flow or for partial flow as in an open channel?

 

[gbam]

I would say that it depends on the governing Entity that will ultimately approve the design. For instance our local city gov. policy states and i paraphrase (that storm drains may be designed for either open channel or pressure flow. However prior written approval is required from city engineer for open channel flow design.)

So I would first check with your governing entity to see what they require then proceed in that manner. If left up to the designer then I would design for pressure flow. A spread-sheet can be used to optimize sizes etc... Pipe slopes will not dictate pipe velocity in a pressure system as in a free surface system. It also depends on the topography of your design site. Just some things to think about.

 

[gibfrog]

gbam is right on track. 1st, You must meet the criteria of the governing regulator. 2nd, You must meet the existing commonly used local standards, usually your state DOT design criteria. 3rd, How much safery factor or excess flow capacity can you afford. In southwestern florida, with very flat grades and a high water table, I usually design based upon the pipe flowing full, but not pressure flow, based upon the FDOT storm sewer tabulation chart. (Remember, all drainage systems will fail if a large enough storm hits).
Best of Luck..

Clifford H Laubstein
FL PE 58662

 

[chicopee]

Thank you for your replies