Hope you have good luck with your project. Your inflow pipe looks like a well thought plan. You can use gunfire to discourage the GBH but they will come back. Could you get some old fencing and place around the top of the water to deter them. I wish I had a spring like yours, I would put a spring house on the top and have the overflow going into the pond. Good to see someone with enough drive to do something of substance.
I'm also impressed seeing he is only a teenager. Sounds like a future aquaculturist or engineer to me! And not many teenagers have something like this to work with.
