Hello Neo, welcome to the forum. Myself and others wish that we would have found this forum before building our ponds, as I wish that you would have found this forum before constructing your pond.
Hate to be a downer and I hope that I am wrong, but from looking at the pics I suspect it’s just a matter of time before the runoff from a heavy rain event completely overwhelms that shallow buried 15” drain pipe. The resulting flow of water over the dam could completely unearth the pipe as it carves a deep trench thru the dam, draining the pond
Normally, the top of the primary drain pipe should be about 1ft below the level of the emergency spillway. ( the spillway is a wide flat path going around the end of the dam, over undisturbed soil, often covered with grass, rip rap, or concrete) The elevation of the emergency spillway should be 2 or more feet lower than the top of the dam. The difference in elevation from the top of dam to the spillway is known as the free board. (from the pictures it appears you only have less than 1ft of freeboard above the primary drain)
When determining the size of the primary and secondary drains it is important to calculate the area of watershed which funnels into the pond. From the pictures it appears to me that your pond could receive a massive influx of water during a heavy rain event. (then again, I don’t know the total size of the watershed, or how much of the runoff may be diverted around the pond)
Personally, I would either lower the primary drain, or increase the height of the dam, thereby increasing the free board. And I would definitely use a tractor or other equipment to carve an emergency spillway around the dam.
If you do add to the existing drain pipe down the backside of the dam you should probably dig a trench and bury the pipe, as a way of anchoring it. A 15” lightweight pipe pulling a siphon needs to be securely anchored.
Oh yeah, I forgot to mention that your pond is actually pretty cool looking, and I hope you're successful with it.
Ok.... well the predicted did happen... so now, I am planning on digging an emergency spillway in the area indicated on the pics... I believe this is an old logging road cut into the hillside probably 15-20 years ago so it should be solid. I was planning on just digging it out with my Kioti backhoe and FEL. I know I don’t have much freeboard and I’m planning on building the dam up some over the current pipe with the excess dirt I take out for the spillway. Although some of the dirt will also be used to fix the dam on the overflow side where it is washed out. I would like some ideas as to if this is the best way to do it without spending a fortune. I am going to create a concrete and rock tailrace under the overflow pipe as well to keep it from eating away the dam on the overflow side.
The way I see it right now I have possibly 3 options, as listed below:
1) Dig out an emergency spillway on the area marked in the pic. It is probably about 75' to the ravine from the pond that I would need to dig out.
2) Remove the existing 18" overflow and create a concrete spillway wider that goes over the dam where the pipe was and forget the emergency spillway.
3) Remove the 18" overflow pipe and replace it with a larger 24" - 30" pipe.
