Hi,

Looking to construct a 1 acre pond with a 9 1/2 acre watershed that is primarily ground water fed. The pond will be mostly excavated, with a low wide damn needed on the lower end of the slightly sloping yard. Dam will be approximately 4 foot above grade and 20 feet wide with a 2 foot free board.

Thinking of utilizing 6 inch schedule 40 PVC that transitions into smooth wall corrugated beyond the dam area. Drain pipe will enter the water about 2 feet below normal surface level. see the attached picture please. Gate valve would be installed to service and adjust the drain, male/female screw union for future adjustments of height of drain, and clean out plug if I ever needed to lower the level by 2 feet or so Or for direct access to remove debris.

Comments welcome on this design, also tables seem to indicate that 6 inches would be fine for drain but opinions valued here as well. I will be installing an emergency overflow. Thanks

I used SDR26 Heavy Wall Gasket Joint Sewer Pipe which is less likely to sheer off. With only 2' of freeboard, at what elevation will your emergency spillway be, and how wide?

Hmmm, doesn’t leave much room does it. Perhaps 3 ft of freeboard with a one foot drop - 10 foot wide emergency spillway.

It's hard to believe what a flooding rain can do. I try to overengineer things myself. I have a wetland that the Indiana Department of Transportation had built to mitigate one that they destroyed. I am glad it is a mitigation because the water floods over the berm regularly and will wash out. There is no emergency spillway and it only has a 6" pipe drain. My upstream pond has a 15" drain, and even with that, the pond's emergency spillway gets used occasionally. The downstream wetland gets even more water from its watershed. The emergency spillway should really be constructed in virgin dirt. They will have to fix it when the berm washes out. Don't really think that they knew how to engineer it, but they sure did a great job with the plantings and it is wonderful for wildlife.

Thinking about it, I’d probably install the gate valve horizontally to keep it below the ice.

And RAH, you’re making me think 8 in might be a safer choice.

Its been so long since I ran calculations so I'm rusty but find out what 100 year flood would be for your area. With that small of a watershed the water will only be on the ground for a few hours before it will be downstream of your pond. If the max rain event over a 4 hour period is 4" that will tell you how much water will be on the watershed. If the max flow is say 50 CFS find out what your pipe will handle and then you can figure out how big the spillway needs to go. A 15' wide spillway that is 1' deep will handle a bunch of water.

Go with as big of a pipe as your budget will allow. You wont regret it. Also keep in mind if you will be driving over the spillway often. If the pond is constantly going over the spillway it will stay went and you will rut it up.

May want to put shut off gate on pond side of drain. Then you could extend pipe deeper in your pond and drop it lower. Would not need a cap in the water.

Adding to nehunter thoughts. I ended up using an economy pvc slide gate in the water side at the bottom of the pond. The slide gate is connected to a 1 1/4” pvc that allows me to open and close the gate to lower the pond at any time. The 1 1/4” piece of pvc is enclosed in a 4” pvc pipe about 6 feet long with a cap on top to prevent anyone from “accidentally” draining the pond. I have 2 telephone poles about 5 feet apart with 5 2x4s that act like a vertical ladder to secure the 4” pipe with pipe strapping. One thing that I recommend is to attach a female 1 1/4” thread at the top of the 1 1/4” pipe that is connected to the slide gate at the bottom of the pond. You can piece 1 1/4” pvc to make a T handle. I use the 1 1/4” male thread at the bottom of the T handle. To use my slide gate I pop off the 4” top cover that has the 1 1/4” pipe inside it, screw the 1 1/4” T handle and pull up to open the gate or push down to close it.

Economy slide gate link

https://sepipe.net/portfolio-item/plastic-slide-gates-and-valve/
These aren’t cheap but I found a 10” to 8” at a local plumbing supply company for a steal.
I hope this helps!
Edit: The one that I have is the middle picture.
Heppy