I would use the red pipe diagram and extend the pipe further into the pond, close to the bottom, if you can.
We got the pipe finished today. We wound up using 10" PVC pipe. The contractor was a lot more careful than I would have been. He does a lot of sewer installations, and I guess getting the slope just right is important in those jobs. Installing pipe has come a long ways since the last time I looked into it. He had a neat laser light that he setup to create a beam that went through the pipe all the way from one end to the other (270 feet) as he dug the trench. Here he is setting up the slope at the outlet point.
This shows the red laser beam shinning through the pipe at the black mark on the shovel. He aligned each section as he installed it, so the slope is very even.
Here is the view looking from the catch basin back towards the pond. I'll install an elbow at the inlet of the green pipe, so I can rotate it to control the flow and block possible backwards flow from the pond. .
Finally, here is the outlet. We'll put in some rip rap to break the flow. With a maximum 3 feet of head, the flow will probably be substantial. I am debating whether to add another section or two to carry it farther towards the bottom.
I would install an AgriDrain in the ditch.
That will be the next step. I had thought about some sort of slide valve, but the flap valve looks like a better solution to me. I can hardly wait for the next gullywasher.
