Lots of rain this winter here in MO. I happened to arrive at my pond this weekend after an overnight rain when the primary spillway pipe was still just barely running in siphon mode. With the water only over the plate about an inch and some waves coming in from the wind, the pipe was sucking in gulps of air and had a nasty vibration/water hammer going on. I could feel it through the ground. I am concerned this vibration is not good for either the pipe or may cause seepage around the outside of the pipe. About an hour later it quit and went into typical water flow down the pipe. Anyone have any experience with this?

I have a typical angled down through the dam pipe with anti-vortex plate over the opening. 8" PVC pipe with 12" wide plate. The top plate is the recommended 1.5 times the width of the pipe - I can certainly put a larger plate on, but there is still going to be some transition point when the discharge goes from siphon mode to standard flow.

Well, I was hoping to see some responses over night because you have a curious phenomenon going on. I do not believe it is water hammer which is good as that type of vibration has some serious forces involved.

Is your drain pipe corrugated on the inside or does it have smooth walls? How long is it and are there any joints underground.

My 2 cents suggests that it is the gulps of air that are rattling along the corrugations. If that is the case, I would be concerned about any underground joints you might have, but without joints, the vibrations cold be helping to settle any loose fill and actually packing the pipe. IDK, just looking on the bright side.

Hopefully adjusting your anti-vortex plate will prove to be an improvement.

Q-A:
Its 8" schedule 40 PVC with five 20' sections (bell-end) . Smooth wall of course. I don't have any dam leaks now so don't see any upside to the vibration. I plan on getting an 18" or wider top plate

Would you mind showing a pic of the top plate? I’m trying to picture it, but my imagination isn’t working this morning. I’ll google it, but if you have a pic, I’d appreciate it.
Thx
Jeff

Here it is just after installation. Its a 12" wide plate over an 8" diameter pipe.



NRCS calls it a hood inlet: https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_023318.pdf

I see on the NRCS document they increase the width of the plate for higher slope of pipe. I will look for a larger aluminum plate.

Thanks! I didn’t imagine it looking like that. Sometimes I wish I wouldn’t have even bothered with a overflow pipe. It could be a source for a leak. (Draining down around the outside of the pipe.)
I’m sure I have water going out my pipe now, but not a whole lot. I put in a 10” pipe, and I’ve never seen more than about a 2” deep stream flowing out the bottom. Water has never reached my emergency spillways. My guess is our next drought will be starting soon, and I’ll be hoping for water before too long.

Is it possible that the plate was flexing downward with the suction until the vacuum broke (by sucking air)and then popped back up which could case the plate to spring up and down? This could cause a vibration that resonated down the tube. Water carries vibrations really well. Consider putting a stiffener (or two) lengthwise from the u-bolts out to the end of the plate (in line with the pipe). A couple pieces of angle iron (1-1/2 to 2" by 1/4" thick) drilled to match the u-bolts and then drilled and bolted to the end of the plate would take any flex out of the plate itself.

Curious, thats steel pipe showing, where and how is your transition to PVC pipe?

Q-A - there was some movement of the top plate, but not much. Its either 3/16" or 1/4" thick aluminum plate. Still, good idea to look at. I did place a tool against it to try to get it to suck in enough air to drop out of syphon mode to no avail, and the noise did not stop when I had the bar against it. Should have put my foot on it. Its worth stiffening the new larger plate with a couple strips of aluminum angle. Maybe place some rubber between the plate and the pipe.

Jake - its PVC pipe that has been painted brown so its UV protected and also not a white eyesore. The plate has been painted brown also since the photo was taken.