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Riprap Design for Multiple Pipes

Riprap Design for Multiple Pipes

Riprap Design for Multiple Pipes

(OP)
I'm designing the riprap apron (and rock diameter size) for (4) four force main pipes that are discharging into a channel.  The four pipes are spaced 1 ft. apart.  Each individual pipe is 6 ft. in diameter with a flow (Q) of approximately 210 cfs.  I'm using the HEC 14 publication, specifically equation D.5 from Appendix D (Fletcher & Grace).  The invert of all four pipes is 13 ft. below the water surface (TW = 13 ft.)  The equation is as follows:

D(50) = 0.02D(Q/D^2.5)^4/3 x (D/TW)

Where D(50) = riprap size, ft.
Q = flow, cfs
D = diameter, ft.
TW = tailwater, ft.

My only issue is that the equation (and all of the others that I run across) only account for the diameter or area of a single pipe/culvert.  Is there an equation to obtain the equivalent diameter for multiple pipes so that I can use the above mentioned equation?  Or is there riprap equation out there that considers both tailwater and multiple pipes?

RE: Riprap Design for Multiple Pipes

(OP)
Would designing the riprap for each individual pipe be the solution?  The discharge pipes are 12' apart from center line to center line which would put a gap of roughly 6' from outside of pipe to outside of pipe.  Is this enough of a gap to handle each pipe independent from another or would the combined flow (Q) from all four pipes govern?

RE: Riprap Design for Multiple Pipes

I am not sure un your situation, I have never designed an outlet like this one; but, I have designed using HEC14 outlet scour countermeasures for multiple pipes.  I typically design for a single pipe and widen the structure accordingly.  This will work if all pipes are hydraulically equal.   

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