I think you need to make some mathematical calc's about your drainage. Meadowlark has great background in ho-made, DIY ponds...he and I both refer alot to Pub 590 for the basics. For openers, I will quote from text: "The principal spillway normally is sized to control the runoff from a storm ranging from a 1-year to a 10-year frequency event. This depends on the size of the drainage are. For pond sites where the drainage area is small (less than 20 acres) and the condition of the vegetated spillway is good, no principal spillway is required except where the pond is spring fed or there are other sources of steady baseflow. In this cse, a trickle tube shall be installed."
I think you are well beyond the described conditions. Personally, I think you will be making a mistake to presume that a vegetated principal spillway will reguarly handle the potential outflow that you may/will experience. The answers lie in the hard mathematics. What is your projected containment size? What is the drainage area size, soil hydrology, and how are these soils used (timber, crops, structures, etc.). This, combined with rainfall histories, can be plugged into basic formulae that will render a snapshot of what to do to handle run-off excess.
Regarding the drain pipe, this is where Meadowlark and I respectfully disagree. He maintains a valid posture that I understand and respect. I believe, tho, that it can be a positive device when installed correctly and I intend to do so. The experts that I have conferred with for my project (approx 35 - 40 ac/ft) lead me to a 6" pvc sched 40 pipe. I have purchased a victaulic 6" butterfly valve. I intend to use 2 anti-seep collars, both 48" sq. I am still researching the actual collar product construction. I am leaning away from the basic neoprene rubber and leaning toward the sold PVC model....tbd. I have no intention of using anything more than elbow grease, hand packed clay around both. Pub 590 also has formulas for frequency and location of application.
