I’m building a pond that’s almost exactly the same size. Min is spring fed by a year round spring. Spring may flow 10 gallons per minute. Trying to come up with best pipe, and size for my overflow that will go through the dam. My core is 12’ deep and 15’ wide and is red clay soil that has been layered and compacted with a vibratory roller. I’m thinking of installing mine somewhere in the 3 or 4’ depth from future water surface.