Based on your description, you have a watershed pond, rather than a ground-water pond. Installing the recommended drains will likely be the cheapest alternative long-term. Most critical is a well designed emergency spillway that is wide enough to handle heavy rain events that might occur every 2-5 years. The slope of the emergency spillway should be as shallow as possible, and along with the the right width, should keep the water velocity low enough to prevent the soil from washing out. It should be planted in grass ASAP after construction, and should be constructed in virgin soil, not fill. The primary drain should handle most rain events and should be designed based on your watershed area and local rainfall history (like the emergency spillway). Both the size of the pipe, its length, and drop effect its capacity to carry water, along with the free-board of the dam before the emergency spillway kicks in. This all can be complicated, but your local NRCS or DNR may be able to help. For me, I still go bigger to be safe. Good luck!
