I'm not a soil engineer but my guess is that's not going to work long term. I assume the exact same thing would happen as happened before. There will be a void between the concrete and soil where the curved part goes into the soil because they expand and contract differently. Same thing happened with the culvert. But maybe it would last another 20 to 30 years, I sure don't know.

I've never seen an earth dam with a culvert for over flow. The overflows I've seen were vertical pipes out in the pond set to the max desired height and then run underground, under the earth dam and far enough away to not be an erosion problem.

The other system I've seen is used by fish farmers to drain the pond completely. A large concrete drain at the bottom of the pond and a large pipe running underground and under the dam. In this case there's a concrete buttress where the water comes out. They open that and the shrimp or fish come out with the water.

So in my limited experience I've only seen continuous earth for dams. And I've always thought any kind of overflow over the top was a no no.

I'd call around to some gov agencies like Fish & Game, EPA, local building department and your local water district. It's completely free to talk with them and no harm getting info specific to your property. Plus they might give you free advice on proper building for your area. Plus you want to attract waterfowl and that's something the gov or Ducks Unlimited would probably be happy to help you do. They might even kick in some grant money.

The downside of talking to the gov is if they see the breached dam you might get stuck with having to reduce erosion or something. When you brought the property you also bought the previous owner's problems which sucks. In general have I've worked with gov agencies they've been helpful and really try to not ruin your day. But I'd not mention the previous pond and the breach to start with and just get info on building a pond, because that's really what you're doing. Just because there was a pond there for 20-30 years doesn't convey any grandfather type stuff I don't think.

Keep in mind that if Fish & Game says it's OK that doesn't mean it's OK with EPA, the state or local building departments. In most western states the landowner doesn't own surface water automatically. I think in Colorado certificates of water ownership are issued, like a deed, but for the water. I think you need a permit or something from your water court just for the storage of water. How you get the water to store is another matter. And that's right, Colorado has a court system devoted solely to water issues. People in the west take water ownership real serious.

As for just the block structure...I'm also not a structural engineer and only have experience with a max of 4' above ground walls holding water. Horizontal rebar does most of the work. I like to use all bond beam blocks in stead of regular block. That allows for horizontal rebar and a contiguous horizontal beam of poured concrete.

The curve and the buttresses could be a problem, I don't know. If a structural engineer told me to build that way I would. But if this is just what looks stronger to you that should be a red flag. Sometimes structures that look stronger are actually weaker.

A curve imo couldn't use block, it would have to be formed. Just my guess.

The buttresses I think have to be placed with care. For example the curve you have is an basically a single arch on its side, but you have 3 buttresses. I think that middle buttresses might cause a weakness. If you wanted 3 buttresses I think you need 2 arches. Buttress on each arch end and the buttress in the middle holding the other end of both arches. The buttresses on the ends are pointing the wrong way imo. The idea of the arch is to transfer stress to the buttresses. The way the side buttresses are positioned the stress wouldn't be transferred, the wall would want to side past or pull away.

The one thing I am certain of is a 9.5' wall would need a serious footer.

I think calling around will get you way better info than asking in a forum. There are pond forums more specific to your type of pond, but even then I'd be careful where I was getting info. Even spending a couple of hundred bucks to talk to a local engineer could actually save you some money in the near term and a lot of money in the long term. A soil engineer might be able to tell you how to install an overflow pipe run under the dam (you already have most of the trench) and then can tell you exactly the kind of soil/material to fill in the void and how to properly compact it as you go. That would be a lot cheaper and I suspect last longer.

Thanks for the help. Some clarification: the wall would not be very curved, just a little. I think gradual enough where CMU could be used, but the wall could probably be straight too. The front of the wall would be no more than 6' tall with a portion in the middle missing 3 bricks to allow the water to spillover there, giving a waterfall like action, concentrating the water flow in the middle of the existing channel which is rock and clay at the bottom. Same on back wall, except it is about 3 to 4 feet tall, with 2 bricks missing in the middle. This would allow a pool of water in the channel that is about 3' deep, where I plan to put trout for the summer months when my irrigation water flows real good.

At the ends of the wall, where it overlaps the dirt from the old earth dam, I could secure heavy plastic, to further prevent erosion at those points. There would be packed concrete behind those junction points, with bentonite on the soil to help seal it further.

How did you lay rebar horizontal? My design would have rebar every 12 to 18", vertically from top to bottom, between the CMU holes, then I'd pour concrete down the CMU voids.

How did you lay rebar horizontal? My design would have rebar every 12 to 18", vertically from top to bottom, between the CMU holes, then I'd pour concrete down the CMU voids.
Bond beam blocks instead of regular block. CMU come in many configurations to accomplish different tasks.

Vertical rebar has to be anchored in a footer or it doesn't really do much.

At the ends of the wall, where it overlaps the dirt from the old earth dam, I could secure heavy plastic, to further prevent erosion at those points. There would be packed concrete behind those junction points, with bentonite on the soil to help seal it further.
I know this all sounds like it should work but I'm fairly sure if you tell this to a soil engineer they might be horrified. The concept of "seal" is the problem. A large chunk of earth moves a lot as temp and moisture levels change. An earth dam moves as a unit, with all the tiny grains of material interlocking and moving together.

Plus the sheer weight of the structure which is compromised now by the trench.

When you try to connect another material to the soil there is a big problem. The plastic and concrete isn't going to move with the soil. When the soil shrinks it pulls away from plastic, concrete, steel, etc. That creates a gap.

The earlier culvert spent most of its life I assume above the water line. Your structure is going to be in the water so gaps I assume would be eroded very fast.

In N CA I had a property where a neighbor decided to dam a stream. It was a massive structure to create a tiny little pool. Probably 10 yards of poured concrete. It lasted less than 1 day I kid you not. Water just eroded the edges where the concrete met the soil and also under the dam. When I first saw this mess Fish & Game was there trying to figure out how to remove this monolith. I don't know what the total bill was to the dude who built this, had to be more than $5k just for the concrete and removal.

This type of thing happens a lot.

Earlier this year Andy Johnson was in the news about a pond he built in Wyoming. I wouldn't want to be Andy Johnson.

Thanks. I know its not ideal but I can't afford the $3,500 equip cost (plus labor and materials still required in addition, about $4500 and a week of time). This will cost me some elbow grease and about $600 of total materials, all of which I can lift by hand, me and one other dude. I'm going to have a footer row of 2 CMU at bottom, then a layer of bond for horz rebar, drill holes for vertical rebar into the footers, and again a bond row at the top, with the vert and horz tying into each other in the bond layer.

I planned to drill holes into the corners of the dam and pound rebar about 6' into the existing dam dirt, every 18" up and down the walls (only at the 4 corners) and then pour concrete into the voids behind, between, and thru the CMU wall and buttresses.

I think I'll lower the front to 5' at top, so the culvert area will only be at the 4' level. Back culvert will be at 3'. Depth of flow area will be about 2' in my canal. I'd encase the culvert in reinforced concrete to keep water flow off the wall directly (on the backsides where water exits).

Might not last, or maybe so. Won't know until I install, but I need something under $1,000 total cost.

Any other ideas that undercut mine and don't require heavy equip and machine drivers?

And skip the permit process too. Sounds like a plan. Hopefully you can post pictures.