Glad to hear you do have a working siphon outlet!

They are a pretty ingenious design, and do not require that you "breach" your dam at a low level for installation.

However, they move MUCH more water when they actually siphon. Can you go down and observe the outlet of your siphon the next time your pond has water going through? Water should almost be exploding from the outlet if it achieves full siphon. The usual cause of poor performance is no upturned elbow at the outlet. If air can run back up the outlet, that can sometimes keep your siphon from hitting full flow through capacity.


Your bigger question was designing your additional outlet size. As you said, the tables can make your head spin. The reason for that is that there are so many variables that control how much water will be entering your pond during a big rain event. You could have zombie Albert Einstein perform the actual math solutions, but I still don't think he would be that accurate, because so many assumptions are required for the calculations. We can't just say a 6" siphon for a 1.5 acre pond should work, we need to know so much more about how much water can reach the pond.

However, we do know that your outlet design is too small, because water went over the dam - which is a worst case scenario. (Unless the siphon inlet got clogged during a big rain event? Does the pond have beavers, lots of leaves, etc. that might have plugged your siphon?)

Let's assume for now that your outlet is too small, but it does function correctly. In that case, you clearly need some type of supplemental "emergency" spillway. Lots of different ways to do that.

Do you have a decent route AROUND the dam? That is usually your safest option. You can just build a wide, open spillway that is either hardscaped or well-maintained grass. If that fails, it usually only cuts down a few feet and does not drain your whole pond. It is also MUCH easier to repair than a dam breach.

Your idea for a culvert would also work - especially if you could route that water around your dam. Installing a culvert always creates a potential pathway for water to trickle along just outside of the culvert walls. When that happens during a big rain, the trickle usually starts eroding more material and eventually washes out your culvert. Be sure to read up on "anti-seep" collars if you go the culvert route.

Do you live at your pond location? Another option is to deliberately close your siphon "air break" inlet prior to a big rain and start your siphon running. (You may need to pump water into a valve at the highest point to do that.) If you see a big rain coming in the forecast, you could always start running the siphon on your pond. If you created 1-2' of additional freeboard on your dam by lowering the pond level, you would probably prevent some of the dam overtopping events.

Just throwing out some more ideas for you to consider!