I recently installed a debris shield and anti-vortex plate on my 15" vertical standpipe (corrugated ABS). This lasted about a week before it got clogged with leaves and small sticks, restricting water flow and causing my pond to rise a few inches.
The pond is about 1 acre and there are tons of trees around the property so I expect this to be a continuous problem. Any thoughts on a solution?
It will be continuous. There is nothing that you can do to stop that. There are band-aids but the end result will be the same.
You can put a larger screen around that intake screen, say 4' square made from cyclone fencing and smaller wire, and extend that 12" below the standpipe and 24"-36" above it. That way the stand pipe screen will stay relatively clean but you will have to clean the bigger screen, just not as frequently.
The only way to keep the screen clean is to NOT have debris floating on the pond, and that is impossible.
Another option is to put a larger UNPERFORATED barrier around your existing pipe.
A short piece of large diameter culvert would be perfect. Secure it to some posts driven into the pond bottom. The top of the pipe should be higher than your high water mark during flood stage when flow is also going out of your emergency spillway. (You don't want flood debris going over the top and plugging your standpipe.)
The lower edge of the large barrier pipe should be several feet below the water level at normal pool. (Depending on your water depth at the standpipe.)
That set up will allow water that is 2' BELOW the floating debris to enter beneath the lower edge of your barrier pipe. The floating debris should still be drawn to that area of the pond, but it should not clog your outlet. (You could still skim the area when convenient, just to keep that floating debris from eventually becoming organic sludge on the bottom of your pond.)
I don't know how "pretty" the solution needs to be at your pond. Large diameter culvert is typically very expensive. However, you could make do with an overturned plastic trashcan or 55 gallon drum, or even use old chain-link fence lined with heavy plastic. If using a "closed" system like a drum, then I think you need an open air hole on the top so you don't pull a vacuum on the large barrier pipe.
I do like your existing shield and plate. Hopefully, the solution above will keep your existing system running at 100% efficiency with debris-free water from below the surface.
The flip-side is that all of the floating debris in your pond is now going to STAY in your pond. That will cost you pond depth over the years in your tree-lined location. I suspect that some type of pond cleaning in the fall will greatly extend the life of your pond.
I would not recommend running the overflow without the trash rack in place. If the floating junk plugs the overflow pipe, it will be extremely dangerous to clear the jam, if the OP can. People have been killed doing so. They don't realize how much suction the pipe has, were cleaning out the pipe from the pond side, the plug broke free and they were sucked up against the pipe.
If you ever have to clean out an overflow pipe with the water level over the top of the pipe you must clean it out from the discharge end of the overflow pipe, not the intake side.
I was proposing an additional large diameter pipe that would be installed leaving all of the existing standpipe protection in place (the plate and guard).
Further, any such pipe should be significantly larger than the standpipe equipment that it is engulfing. If it is only slightly larger, then surface debris could be sucked down below that pipe during periods of high water flow into the standpipe.
