We are planning on building a 1.5 acre pond using an existing creek below a 5 acre pond. The creeks maximum discharge/flow is 15 cfs but the pond above this area could have an emergency spillway of 600 cfs. We need some advice on how to handle this?

What size drop inlet principal spillway would manage the normal flow plus some of the emergency?

What size emergency spillway would be needed given the drop inlet principal spillway installed?