Mike, I've been following your progress, super fantastic pond project, especially that outflow waterfall. I don't think you'll have any problems with discharge, my pond has 2 12" discharge pipes, and with a large drainage area into the pond with heavy spring rains onto 3ft of snow they easily handle the runoff. Keep the photo's coming.
P.S. According to my laymans calculations, one 8" pipe should be able to handle at least 900 GPM.
