Had some rain last night and this morning. Water is flowing pretty well out the 12" overflow pipe. (First time it has reached the inlet to the pipe)
Now it's half way up the pipe. I'm looking at the two grass emergency spillways, at each end of the dam, and they are close to coming into use.
Question, aren't the emergency spillways supposed to see use, "after" the overflow pipe is completely submerged? I do not see how it could siphon, if it didn't go under?

Pipe:


Spillway:

You are correct.

Thanks,
I'll have to get the USDA guy back out. Get his take.

Just for a reference, on my pond (3 acres) the overflow pipe was an 8" and my emergency was designed at 1' above full pool. So as designed the top of my pipe would have been under 4" of water when water level was at the emergency overflow level.

Some things that might make a difference on different ponds. One would be the size of the watershed in relation to the pond size and the potential size of rainfall events. The other thing that might come into play would be the amount of freeboard above full pool.

My thinking is that a pond with a large watershed area in an area of large rainfall events might have a design with a higher freeboard (to take the surge of water). The ultimate purpose of the emergency overflow is to keep water from topping the dam, so its width, height above full pool, rainfall events, watershed area, and freeboard all would part of the design consideration, at least the way my non-expert mind would envision it.

I think the key, for a pipe that goes downhill, is to have enough freeboard so that the pipe pulls a siphon without pulling air. That can dramatically increase the volume of water that goes through the tube during heavy rain.

Thanks guys. I appreciate the feedback. I was concerned with the siphoning effect, as I knew it would dramatically increase flow.
I am trying to eyeball the height of the emergency spillways vs the height of the 12" pipe. It's difficult for me, and past experiences are telling me I'm not particularly good at it. (Guessing and comparing elevations) the USDA guy has a laser measuring device. Hopefully he can come by.
Technically, since I built the pond in the woods, and not in an erodible field, he can't offer me much help.
I needed to confirm the spillway process, and you have helped.
Thanks again
This pic shows how much free board I have, at 4" above full pool. 1 AC pond, with 8 AC of drainage. It also shows spillway on right side of dam. There's an identical one on the left side.

I also cannot judge elevations very well. Lets hope that the laser gives good results!

If you have a straight 2x4 or similar straight edge and a carpenters level, you can come pretty close.

Lay one end of the 2x4 or straight edge at the lowest point of the spillway where water would flow out first. Point the other end of the 2x4 towards the water. Lay the carpenters level on the 2x4 and bring the end nearest the water up till the level reads level. Measure from the bottom of the 2x4 to the water. That is how much your water is below the emergency overflow. If the water is at less than full pool, measure the distance the water is low of full pool at your overflow pipe and subtract that from the previous measurement for the distance the emergency spillway is above full pool level.

I've got a construction grade laser level that we use on the farm when we do dirt work. But sometimes simple tools will get the job done simpler and quicker and close enough for government work.

Thanks. My dam is pretty wide by each spillway. I'd guess they are close to 25' long, and not perfectly flat. I'll try to determine the highest point, but I may have to use string with a level to measure different elevations.
Thanks again.

String level would work.............. Why didn't I think of that!