Riprap Design for Multiple Pipes

[Trackfiend]

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?

 

[Trackfiend]

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?

 

[gbam]

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.

 

[TerryScan]

There is coverage of outlet protection with multiple culverts in USDA SCS Minnesota Technical Release 3.

http://www.mn.nrcs.usda.gov/technical/eng/pdf/tr3web.pdf