dhall,

Any impermeable structure you place over your standpipe, that ALSO extends above the surface of the water will only allow water to be drawn from the lower edge of that structure.

Imagine placing an 18" culvert over a 12" standpipe. Utilizing a 10' long culvert, that stuck up in the air by 2', the water draw would then be 8' below the surface of the pond.

If a rain event raised the water elevation of the pond by 1', then the water inside the 18" culvert would also go up by 1'. As it spilled into the standpipe, additional water would enter the 18" culvert from the bottom edge.

I hope that helps with the conceptual part.

However, that type of arrangement would restrict the water flow out of the standpipe. You definitely want your standpipe to flow at the designed capacity to avoid water reaching your emergency spillway whenever possible.

Maybe add a bottom water drain ahead of the main standpipe, or as a separate system?

Another option would be to add a bottom siphon system. Then you could easily control your water draws, and you would not be adding another pipe through the lower portion of your dam.

I have attached a link to an automatic siphon system that you can control from the surface.

Siphon Diagram

Good luck on your new pond/lake! Welcome to Pond Boss.