jsec, I really like your rock island/dock. That's cool.
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1812,
Just a hypothetical thought, but, I wonder how well it would work if you were to install a concrete overflow pipe through the top portion of the dam, at water level, and eliminate the riser altogether.
If you install it horizontally level you'll have an erosion issue on the backside of the dam, which would require a concrete apron to prevent the erosion. But, if you install it on a diagonal slope you would eliminate the erosion issue.
Also, a poured concrete anti-seep collar around the first joint would hold this joint tight, anchoring the pipe, allowing it to extend outwards from the face of the dam, thus preventing it from snapping off.
If nothing else, this may be a good option for a secondary overflow, thereby eliminating the need for a primary spillway, which would allow you to build a road on top of the dam.
Also, it may be better to place these pipes on the ends of the dam, where you can excavate into the undisturbed soil of the ravine's banks, giving the pipe a solid base to rest on, reducing the possibility of settling.
One more thing. It seems to me, that water draining into the diagonally sloped pipe would have an increased velocity, much like a siphon overflow. Especially when the intake side is completely submerged.
Like I said, it's just a thought. I'm sure there are some issues I'm not seeing.
