An important practical consideration is the operating design of the check valve. Some require positive differential to open. For every 1 psi required, there must be 2.3 feet of differential in order for the check to open. You want a low pressure opening requirement if one is implemented. The designed elevation of the discharge will need to incorporate the pressure opening requirement. Also unless the check fully opens, the flow will be a bottleneck to the flow rate. The type of check that gehajake recommended is ideal

Jscummy, you have received a lot of great advice and none of it was bad. Everyone is trying to help. The flexible drain pipe idea, I know, doesn't address your present challenges but I offered it as an alternative to you and as first choice for others who need quickly drain their ponds to a level and maintain that level for a short time. Not as durable as Sched 40 PVC but it is affordable and durable enough to last a few years. One can drain and coil for storage until he needs it again.

I will also mention that it doesn't have to be as complicated as I pictured it. It can be much more temporary. A jug can serve as a raft and you don't need to stand the pipe up. For example, one could lay the pipe over and down at the edge of the dam (or spillway) and form a loop back up the valley to the desired elevation and then drive a stake to secure it. One could lay some sheet plastic und the exit if erosion was a concern. You do need a belly in the siphon to keep it primed and maintain a level and so something like this could work temporarily without a lot of expense.

Last edited by jpsdad; 08/24/22 07:35 AM.
It isn't what we don't know that gives us trouble, it's what we know that ain't so - Will Rogers