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#550986 07/31/22 08:21 PM
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Hello everyone,

First off, I'm not only new to the site, I'm completely green when it comes to ponds. I am in the middle of having an old over grown beaver pond dredged and an overflow added since there wasn't one; don't know why the beavers didn't put one in. Anyway, my contractor originally requested an 50' x 18" thick wall steel pipe system; 40' stretching under the dam, and 8' standpipe/riser for the pond overflow. After discussing the $13k cost for just the materials and fabrication, we went back and forth and agreed on 12" thick wall steel pipe under the dam (which I picked up new on Facebook for over $11k less), with whatever size I chose for the standpipe/riser. Now, his original idea was to install the 18" pipe with an earthen emergency spillway on one end of the dam. What we are doing instead is a smaller standpipe/riser, with a 12" plastic culvert slightly higher than the standpipe/riser, and an emergency earthen spillway off on one end of the dam. I think this will work, the question I now have is 8" or 10" standpipe/riser? The 8" is readily available and very cheap. The 10" has been hard to find; retailers want nearly what I paid for the 40' of 12" for just 10'. However, I've found some used 10", just not very convenient to get to and transfer. So, would the 8" be enough?

Some background here, the pond is approx 1/3 acre, with the deepest area just over 8'. And I'm not sure about rainfall/heavy rain periods, but there has always been a spring feeding into the pond with a constant flowing trickle, and a foundation drain around the house, also dumping a trickle of ground water. Before the dredging, a notch in the dam was created to lower the water level to rid of the beavers. That was done 20 years ago and it worked; water level dropped approx 3' and beavers were gone. So, for 20 years, the only spillway for water was an 18" wide dug notch into the dam itself; no pipe, rock, nothing. Nothing ever eroded, and water never has reached anywhere near the top. The contractor suggested 18" not because of the volume of water, but because he knows it's overkill and will work, plus that's just what he's always done. I think the 8" standpipe/riser with a 12" culvert should take care of it all, with the earthen spillway likely never being used, but I would like to know the thoughts of those experienced. Will 8" plug up far easier than 10" or any other problems with the smaller pipe I'm oblivious to? Are my three methods of discharging overflow a great idea that will work, or a flop that will lead to several headaches?

And lastly, what about escaping fish. From what I've read, it's best to not screen overflows with anything to prevent fish from escaping during heavy rain periods. Sounds like losing some fish is normal from time to time, most important thing is to have the water flow; fish are far more cheaper and easier to replace than an eroded or blown out dam. Thoughts?

Last edited by Johnny821; 07/31/22 08:22 PM.
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It's a combination of the size of the pond, the type of vegetation on the watershed, the type of soil in the area and how much rain can fall in a 50 to 100 year rain event. Earthen spillways tend to erode and wash out, that is why they are only used for emergencies. Go to your counties NRCS office and talk to their pond guru. They can pull up the watershed around the pond and properly size the overflow pipe. If the 18" pipe was spec'd out and you dropped the size to 12", I'd be concerned that it is too small and would put in two pipes both 12" dia.

https://www.nrcs.usda.gov/wps/portal/nrcs/main/national/contact/local/


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I have a six acre pond and have two 24” black plastic double wall pipes for my overflow. So far no problem with it handling the overflow ( when it is full). I have a retention pond just on the other side that does catch small fish that I usually trap and return (sometimes) to the main pond. This year we are down about 2’ so far with no rain in sight for months to come. La Ninya is to thank for that. The overflow has worked good for us. They wanted to put in a 24” syphon system out of carbon steel pipe….. too much money and chance of it rotting out., so we went with the flow through of 2 24” pipe

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Johnny, not sure I’m reading this right. But, I’m not too wild about an under the dam overflow system. If anything goes wrong, the water would all have to be drained and all rebuilt. Most of us go through the dam at whatever top water level we want(or can get).


It's not about the fish. It's about the pond. Take care of the pond and the fish will be fine. PB subscriber since before it was in color.

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Thanks guys for the replies, much appreciated. Esshup, 18" pipe wasn't spec'd, it's just what the contractor always installs; being overkill, he doesn't need to think/calculate anything. But at a nearly 250% increase in the cost of pipe, overkill isn't reasonable. I've researched sizing pond overflows and found three sites with tables/calculations based on my square acreage (1/3). One site said 3"-4", another site 6", and the third site said 10"; hence why I came here. For comparison, my neighbor across the street has a slightly larger pond in square acreage and depth, and has only a 10" culvert pipe with an emergency earthen spillway, and in 30 years, never had a problem.

Pat, I'm going to upsize the black plastic culvert pipe to 15". I would assume with an 8" standpipe/riser, in conjunction with the 15" horizontal culvert pipe, it would be more than adequate given a total orifice of 23" to discharge water. I also forgot to add that I will be adding a bell mouth to the standpipe/riser opening at the top, to reduce the chance of the 8" standpipe/riser clogging. I'll also have a small retention pond below the back side of the dam, so I guess I'll likely be doing the same as you regarding fish.

Dave, I'm likely explaining incorrectly. I thought the idea didn't make sense at first either, but after researching it more, I found most ponds and lakes utilized this type of drain instead of or with a horizontal culvert pipe.

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Epphus, just researched the USGS and FEMA flood maps, and appears I am no where near an area of concern. The entire town is not shaded, except a small area around a lake well south of town in a low lying area. I've seen complete monsoons, and a little 18" dug hole in the dirt handle it, while I watched a raging river from my french drain spilling down towards it.

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I am no expert but what your talking about does not set right with me. The pond size is not what you look at for your emergency drain, it is the drain field that drains into it. Pluss the height of the dam above the main spill way pipe which determines the amount of rain your pond will hold before it goes over the top and washes out.
I have heard of using a smaller vertical pipe than the horizontal. To help with clogging. But in my uneducated mind your choking it down way to much. If you have a 8 inch riser and then bell that yet more on top you probably need only an inch bigger horizontal pipe. There is only so much water that can go threw an 8 inch pipe and an 18 inch pipe can take 2 to 3 times that amount.


61 acre water shed lake. bass, channel cat, black crappie, wiper, walleye, redear sunfish, blue catfish and bluegill. To many bullhead and common carp
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OK just reread your 1st post you are sizing the main spillway not the emergency drain. But I would still not change size of the vertical pipe other than the bell on top. You will need to make it so when the beavers come back ( and they will ), they can not block drain very easily.
Brother has a 2 acre pond that 40 acres drains into and has a 12 inch pipe and has worked great for the past 10 years. We even went threw a 13 inch rain once.


61 acre water shed lake. bass, channel cat, black crappie, wiper, walleye, redear sunfish, blue catfish and bluegill. To many bullhead and common carp
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Johnny,

I think you should download USDA Handbook No. 590

Ponds - Planning, Design, and Construction

It does not directly show all of the civil engineering for constructing a large pond. But their recommendations are based on that engineering.

There is a big section on surface run-off into the pond and designing the correct spillway to match that expected water flow.

There is also a section on drop-inlet pipe spillways:

"The diameter of the riser must be somewhat larger than the diameter of the barrel if the tube is to flow full. Recommended combinations of barrel and riser diameters are shown in the tables. In these tables the total head is the vertical distance between a point 1 foot above the riser crest and the centerline of the
barrel at its outlet end. Because pipes of small diameter are easily clogged by trash and rodents, no pipe smaller than 6 inches in diameter should be used for
the barrel."


I hope this helps. Good luck on your pond project!

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Johnny821, how many acres of watershed will flow into your pond? Like was said above, it's not the size of the pond it is how many acres of watershed "feed" it.

If you are determined to figure it out yourself vs. asking people that are trained how to do the calcs, then I would strongly recommend downloading the USDA booklet and going through it. It's a lot cheaper to do it right the first time than to have to re-do it later.


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A Bottom Withdrawal Siphon Spillway is much less expensive and can move a lot of water much faster than a standard spillway during a rain event. My pond has about an 11 acre watershed and I put the system in a couple of years ago when I rebuilt the dam. I used 4" schedule 40. I wish I would have used 6".

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My watershed is supposed to be 240acres but I doubt that is right.. all woods and sand. Anyway I put in 2/24” culverts for the spillway. It’s only been used 2-3 times in almost 10 years. We don’t get the rain here for some reason. A 2” rain won’t add much to pond due to no runoff even with a large watershed

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Originally Posted by Pat Williamson
My watershed is supposed to be 240acres but I doubt that is right.. all woods and sand. Anyway I put in 2/24” culverts for the spillway. It’s only been used 2-3 times in almost 10 years. We don’t get the rain here for some reason. A 2” rain won’t add much to pond due to no runoff even with a large watershed


Better off to have it and never need it.

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A customer's 1 acre forage pond was drained down when we seined it last Monday. He drained it further when we finished to almost zero and applied hydrated lime to kill any remaining fish. That evening he had a rainstorm and the pond filled and overflowed Tuesday. Yesterday he had a big enough rain event that the forage pond and the main pond became one......


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