once again, I'm not a professional pond builder. I recommend you contact a professional , that can correctly calculate your watershed, and truly calculate if a 500 year storm hits your pond. if it's even a 100 acre watershed , and a 12" down pour hit's your spillway , I'm afraid your dam will be topped. I would think expansion done correctly will help reduce your risk , how much, I don't know, and neither do you. Get a professional involved. Without a solid number on a maximum potential rain, and actual size of watershed, no one , even a licensed Engineer , can give you good numbers as to pond size, spillway design , Siphon design , etc. What little I know , I would feel like I was playing Russian Roulette , in this situation . Sorry not to be of help , you need a professional , IMHO.
