Rip-Rap Sizing 4

[civilatl]

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.

 

[civilatl]

40 cfs = 40 ft/sec

 

[fattdad]

Not sure what the question is. . . .

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!

 

[civilatl]

What reference do I need to size this rip-rap based on the given info. In more technical terms energy dissipation for an abnormally high velocity and small pipe size using rip-rap as the means.

 

[TerryScan]

As fattdad mentioned, the scenario is not entirely clear where this riprap is called for.

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

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

 

[civilatl]

The rip-rap is called for at the outlet headwall of the 8" siphon pipe. The channel is kinda narrow at this point, so excavating a pool seems stupid when first we want positive drainage, and second there is already a pool of water not too far downstream caused by a road culvert.

This is all I can say, if anyone can answer please respond if not don't.

thanks

 

[cvg]

note that riprap does not by itself readily dissipate energy for high froude number flows. this is normally done by some sort of baffles, blocks, high tailwater / or transition the flow to subcritical by causing a hydraulic jump. To force the jump, you may well need to excavate a pool. Placing large riprap in the channel may not do the trick.

Try USBR EM-25 - Hydraulic Design of Stilling Basins and Energy Dissipators

 

[civilatl]

Thanks for the responses, sorry for my impatience its my first post and this is a new world of communication for me and most engineers (no plans). In FHWA HEC 14 I found an equation for circular conduits which calculates a d50 and I only needed the dia, velocity, and T.W. depth (which I assume worst case is equal to pipe dia). With D50 of 0.8', I'll use 10" for D50 and 3D50 for thickness. And I'll use geotextile underneath and rap the rock up the slope about a foot above the TW depth.

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.

 

[cvg]

real world, rock is specified by both size and weight depending upon local custom. Unless you want to take a scale out to the job site to check the rock weight, I suggest using a tape measure instead. Also, make sure you specify the minimum specific gravity of the rock as the size must increase if the contractor brings in light weight rock. Check the attached file for gradation recommendations.

 

[655321]

Civilatl, your right, any method is just a guess, the real right answer is one that doesn't erode the channel in a time span that the people who maintain it find acceptable. I would feel very comfortable having an apron of 10' long for an 8-inch pipe with 15cfs of outflow, even 5' would probably be plenty. You should check your velocity calcs though, 40 ft/s seems really high for a 8" pipe carrying 15cfs, i would expect it to be under 10 ft/s.

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.

 

[TerryScan]

655321,

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.

 

[655321]

Your right TerryScan, around 40 fps is correct for the situation described, I had flip-flopped the pipe size and flow rate in my head when I was reading this, no additional perspective needed. With that in mind, yes, 5' isn't enough rip-rap.

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.