My brother in law had a fairly large pond built according to the Arkansas County Extension requirements which I believe are 160 feet square and must be 12 feet deep below grade not counting the levee. They wouldnt let him put in an overflow pipe but rather require that a trench be cut from the highest elevation side (built on a slope as most ponds are) then the trench follows the outside of the dam to control the runoff. The trench is lined with river rock to prevent washing. They said the reason is that you can never size an overflow pipe to control the run off as you dont know how much rain is going to fall and how fast. The trench is set to start diverting the water about 3 feet from the top of the dam. He had to conform to the state requirements if he wanted to get his partial reimbursement cost. Anyway if works well.
I built a small ponda couple years ago in Sept and was going to put in a concrete spill way in center of the lower dam, but didnt get it done (procrastinating too long waiting for the temps to fall then back to work overseas so it sat for over 5 months. Finally we got about 4 days of rain in Feb.which didnt fill the pond but about half full. BUT then the wet weather springs started flowing and filled it up to overflowing overnight. I ended up with a uphill side overflow cut into the dam to keep it from washing the lower levee away. This all happened when I was out of the country and luckily my Brother In Law saw what was happening and took his FEL and cut the upper dam side. When I came home, I filled it in a bit and left it to drain when the pond is 3" from top of lower dam. It works pretty good, but do have the constant draining around the side of the pond all winter and spring. The springs usually stop running in June when the weather normally turns dry. This year they stopped about April due to the drought.
I know you dont need rain for your pond, but if it keeps it up, rather than loose your lower dam, cut the upper side with your backhoe and divert the water outside of the pond sides downhill. You can always fill it back in later.