Scott, are you thinking the angle of the pipe would help water increase it's velocity once it started following along the pipe versus a standpipe situation? I would think that could make a big difference once a leak started.
No just the opposite. Well, sort of. I think a standpipe helps pack the water into the horizontal pipe by the large vertical fall before going horizontal. A standpipe can also have a sort of collar on it so it takes water from the bottom of the pond..... The collar is above the top of the highest water level, and extends down to near the bottom of the pond where the horizontal pipe goes thru the dam. The area of the gap between the collar and the pipe is equal to or larger than the drain pipe. The inner stand pipe sets the desired water level of the pond. If the water in the pond is higher than that, then the water that goes into the overflow pipe comes from near the bottom of the pond, or at least the level of where the pipe goes thru the dam.
With the way the OP had the pipe thru the dam, the pipe had to be completely covered with water before it would act as a siphon. Without an anti-vortex device, it most likely wasn't as effecient as it could have been.
Ok. I see what you are getting at. The two of us are actually talking about different things. You are talking about the pressure inside the pipe, and getting the maximum flow thru the pipe. My reference was to a leak following the outside surface of the pipe, and what would cause a leak in that area and why it might deteriorate so quickly once it started.
I don't think the problem in this case was from the pipe size being to small or not having maximum flow thru the pipe. The water level never rose enough to get close to his spillway to indicate this. Perhaps a vortex around the pipe might have washed away the dirt within it's vicinity before it could rise to his spillway level. Possibly a vortex was caused by not having a standpipe or by the pallet stuck at the pipe's opening, can't rule that out. BUT, would that vortex have kept eating away at that soil around the pipe 5, 10, or 15 feet down the length of the pipe? Once you get outside the vortex's range, the dirt should have been sufficiently packed around the pipe to prevent a leak.
edit: I will also add a simple farmer's common sense approach and question to this. If the pallet got stuck at the pipe, blocking it's flow capacity and creating a vortex, wouldn't that vortex be smaller than when/if the pipe would be fully open and flowing???
