I looked online, and cannot find a flapper valve larger than 4 inches. They are listed as check valves. I cannot imagine why an engineer would spec a flapper. Most, if not all small lakes that have valve towers have gate valves.
The OP may have some seepage around the valve. Such valves are rated for gravity flow at low head. As I said before, 480 gallons per day is not much for a 12 inch valve to leak. You could gravity flow that much through a garden hose in two or three hours with much less head.
Last edited by John Fitzgerald; 11/13/17 05:36 PM.
