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newbie to Experts...pond and dam questions, building one now.

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I’ve never built a dam. I need help, lol. I bought a track loader with bucket and forks. I have 20 bags of bentnite and 20 bags of concrete. I have to make a waterfowl and serenity pond for the wife and kids. More info…

So the dam that I’m building will cross a natural drainage area. We don’t get much water in it usually…maybe 8” when the irrigation water is flowing top rate. There are irrigation shares about 5 months a year during summer, and then it returns to a semi-boggy swamp. We rarely get hard rains for some reason over my mountainous area and most runoff from the mountains goes down our irrigation shares.

The span of the dam will be about 65 feet from one side to the next (measured on the very top. It will have two-each 12” drain pipes placed nearer to one side, about 4 apart from each other, slightly pitched towards the outgoing end, but not much (almost level). I was going to lay the pipes about 9’ up from the bottom of the mush and put the bottom edge at 9’. Put a lot of bentonite down the pipe body and concrete the ends in large plastic forms. Pack them in like crazy.

My max depth of the pond can be 10’ at deepest, thus drain pipes at 9’.

The pond water that will occur could potentially back up about 300 feet to another small dam that holds a small pond that also gets fed from the same drainage/irrigation shares through summer. At the 9’ elevation point, the dam is about 65 ft long. The drainage area consistently narrows as it goes upstream towards termination to about 15-20 ft across. So it’s shaped sort of like a squiggly cone, with the big end on the dam side.

There is a natural deep channel that snugs up against the far side. I’ve left it open for now so water can pass through. I’ve put the boulders all the way across thus far, touching each other. My plan is to put the drain pipes in and then fill that in with extra fortified dirt (400 lbs of bentonite) and hope it holds.

I have no ability to properly excavate the spot where the dam will go because it is about 12” of muddy mush at bottom and is constantly wet. I cannot put a clay or concrete core in it. All I can do is pack boulders at the base, bentonite in layers, and pack it over and over with the track loader (9,500 lbs).

I plan on layering the bottom with a layer of large boulders (about 2,500 lbs each, 3-4ft in diameter). Then I’ll pack it with dirt from the adjacent field. And layer bentonite over it every 3-4 feet of build up.

Question, how wide might I want to make the base of the dam to be sure the water’s load at a depth of 9’ will be held back? The top needs to be driven across by pickup trucks, etc, nothing too heavy. The top’s flat service will be at least 10 – 12’ wide. So how wide should I make the base, with the above dimensions? Are there equations to figure with water and earth quantity inputs?

Dimensions again: 9’-10’ target water depth at dam. Pond will back up and gradually be perhaps 1-2’ deep at little/far end. Dam span at 9’ height is 65’ across. Its thickness will be about 25’ wide at the 9’ height. Top needs to be 10-12 ft wide. Materials used: track loader, 30 large 2,500lb boulders, 20 bags bentonite, 10 bags concrete, 2 each 12” drain pipes placed at 9’ depth, level across dam.
Posts: 863
Location: North Georgia / Appalachian mountains , Zone 7A
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This site has some really good info on engineering a small dam properly for water retention and safety:

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