Sizing Rip-Rap apron at Blow-off

[Hock29]

I need to size a rip-rap apron for a 10" blow-off for an 18 water transmission line. The flow at the blow-off is 3.7 cfs, which calculates to around 8 fps, but the line has 120 psi of static pressure. I can't use a gravity flow calculation because of the static line pressure.

 

[mflam]

How much space do you have?
That's a lot of water.
I don't know of any calculation that would support this but, I suggest you use the biggest rip rap available. In the northeast, it is commonly called "crusher run."
Also, you'd better be aware of where the water is headed, even after you have dissipated the velocity.

 

[Hock29]

Thanks. We do have plenty of room to work with, but it looks like it is going to need grouted too.

 

[beej67]

Crusher Run isn't rip rap, it's gravel. At least anywhere I've worked. You don't want gravel for your energy dissipation.

Rip Rap sizing is based on velocity, and based on the characteristics of the receiving body of water. (tailwater condition / etc) It will vary by location. In Georgia, we use this:

http://www.gaepd.org/Documents/esc_manual.html

Page 6-181.

If you want to get fancy, use this:

http://www.fhwa.dot.gov/engineering/hydraulics/pubs/06086/hec14.pdf

You know your velocity, work from there.


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