I currently have a 24" outlet pipe, two 20 footers with a collar that appears to be leaking. The water goes over my emergency 2 or 3 times a year. I am thinking about digging up the pipe, lining the area with clay then installing a 45 mil pond liner, covering the liner with sheet aluminum then oversize septic rock. It will be 18' wide x 3'deep and 35' long spillway. Am I missing anything? Any Ideas on a who to purchase the liner from?

With that volume of water, I don't expect the rock to stay on the liner.....

If you are set on the liner idea, I'd carry it a ways into the pond and anchor it down there, well below the full pool level.

Or make a concrete dam on the pond side of the liner, and sink a 4"x4"x 5/16" or 3/8" thick angle into the concrete so the surface is flush with the top of the concrete and pointed back towards the overflow area. Bolt the liner to it using a heavy piece of flat stock.

But before doing that I'd research siphon drains. Sounds like a perfect time to put one, two or three in.

Thanks esshup....I was wanting to cover the liner for uv protection, I could put larger rock on top of the oversized septic rock {3-5 inch round} but....I will look into the siphon system.

Siphons are really amazing withdrawl systems.....just be sure to put on a good screen/intake setup to prevent clogging.

Siphons are also cheap by comparison to install. For the price of liner, aluminum, rock and labor, you could install a good concrete overflow, yet still run a much bigger risk of leaks going under the spillway area.