Rip-Rap Sizing
Rip-Rap Sizing
(OP)
For a dam project I have an 8" siphon pipe flowing at 15 cfs and roughly 40 cfs. I can't use the standard nomograph to size the rip-rap, because 8" isn't shown, and the graph says clearly "do not interpolate". Our original design is to have the pipe at a concrete headwall (and plenty of concrete thrust blocking), and would prefer to keep this design and just size the rip-rap appropriately instead of switching to a commonly used "bent" siphon where the siphon jet shoots up into the air and back down. Either way, I don't see how anyone can really know the right "answer", but thought I'd give it a try.





RE: Rip-Rap Sizing
RE: Rip-Rap Sizing
You may find more response if your post includes a few question marks to let the reader understand where you are seeking advice.
Not trying to bust your chops, just an observation.
f-d
¡papá gordo ain’t no madre flaca!
RE: Rip-Rap Sizing
RE: Rip-Rap Sizing
When you mention thrust blocking I am led to believe you are going to construct a concrete energy disipator/stilling basin. (what is probably called for here).
It is for the channel downstream of the stilling basin:
"Loose Riprap Protection" USDA TR3
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RE: Rip-Rap Sizing
This is all I can say, if anyone can answer please respond if not don't.
thanks
RE: Rip-Rap Sizing
Try USBR EM-25 - Hydraulic Design of Stilling Basins and Energy Dissipators
RE: Rip-Rap Sizing
One thing I couldn't find in the garble of government crap is the gradation guidlines? Or is specifying D50 of 0.8 FT good enough? In the real world, however, rock is actually specified by weight, so I was thinking 100lbs, but wasn't sure about whether needing gradation with different rock sizes.
Any problems from more experience guys are welcomed.
RE: Rip-Rap Sizing
RE: Rip-Rap Sizing
Regarding Rip-rap sizes, our Department of Transportation has common gradations and specs that suppliers have readily available, whatever state your in (or city your near) would almost certainly have them as well, no need to reinvent the wheel. Call a aggregate supplier in the area and they should be able to help you spec something that isn't odd or custom(and unnecessarily expensive). I'll say something like: "Rip-Rap DOT Type C" for example.
RE: Rip-Rap Sizing
Using the most basic formula of Q=VA, Velocity is 42.97fps. I do not see how one could expect 10fps.
For perspective, consider that a 1000gpm is not an unusual minimum fire flow in a water supply (can be as low as 500gpm). With a 5"id hose (the larger of fire hose sizes), that's 16fps velocity out of the hose end.
I would certainly be concerned about using 5ft of riprap to protect a channel from a flow with the characteristics described.
I think some of why responses here have seemed difficult, is that it is unusual to attempt to handle this flow using only riprap. The guides for riprap design do not describe this scenario, because they all imagine much different settings for the application of riprap. Riprap design guides cover flows in "normal" roadway culverts, not dam discharges.
RE: Rip-Rap Sizing
Civilatl, is this the drawdown pipe? How often would you expect flow of 40fps to be developed? If its infrequent, I am still inclined to believe rip-rap apron would work.