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Re: 70 acre dam build NADam

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FishinRod

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Originally Posted by NADam

What do you mean by the "nice flat area" that I must create? That raging river in your video must be tamed for your dam to survive. Generally there are two ways to do it.

1.) You can build an earthen dam, but you must have a concrete chute to pass the entire amount of floodwater at maximum flood stage. If the flood in your video is 50m wide and 2m deep, then you need a chute of 100 square meters plus a safety margin. That is a big concrete project!

2.) You can build an earthen dam, but use an emergency spillway to direct floodwaters AROUND the dam. Imagine a "nice flat area" about 10.5m above the river bed and beside your dam. If you had an area 500m wide, then the same floodwater flow would only be running 0.2m deep. That is much less force than what is shown in your video which literally can rip trees from the ground.

For a normal lake water level 10m above the river bed, you would probably need a dam that is 12-13m tall. The extra 2-3m is called the freeboard (as gehajake mentions above). As the floodwaters rise, the water level will move up the freeboard of your dam and the water will continually get deeper flowing through your emergency spillway. You still must prevent the water from rising over the top of your dam at the peak of the flood. That is the purpose of the freeboard.

You also will need a concrete spillway with a large pipe through the dam to safely pass "normal" river flows through your dam, such that the emergency spillway does NOT get regular use.

A big flood will probably do some damage to your emergency spillway. However, it should be eroding across a wide plain rather than just cutting through a narrow dam. Usually there is lots of stone placed against the wing of the dam that is adjacent to the emergency spillway, so that any erosion does not reach the dam. You can then repair the damage in the emergency spillway, make it as flat as possible again, and replant your vegetation.

I personally, am still worried about the size of the project required to dam a river that wild.

One option would be to build some pond basins on higher ground and pump water from the river to fill them.

If you have some dry waterways (low ground) running down to the river banks, another option would be to build some dams adjacent to the river. You could run pipes at the elevation of your desired pond surface water level into the river. That way the river waters would flow into your ponds at flood stage. If you widen the banks of the main river there, that will calm the flow and you can put rock on your dam faces to prevent erosion.

Do you have any sandy ground adjacent to the river? I have some "groundwater" ponds on my property that are in very sandy areas adjacent to a small creek. Even when the creek goes "dry", that sand is still full of water. I have dug out a pond that is BELOW the creek bed, and it has water in it even though we are in a 3-year drought.

If your sandy layer is extensive and contiguous, you do not need to dig out a pond immediately adjacent to the river. You might be able to excavate a pond beyond a bank that is not over-topped when your river is at flood stage. You can test this with a large backhoe or even a small excavator by digging a test pit and observing how fast it fills with water. The overnight water level should very closely match the water level in the river if the sand readily allows for subsurface water flow.

Good luck and keep learning on this project! The more you know, then the better ideas and questions you can have for the engineer during the design phase.

I'm sure the experts will jump in here shortly. I'll let you know my results. We found a lot of rock in the soil where we built our pond. The dozen operator stopped taking dirt from a couple of areas because it was just too rocky. One of the three guys working on the pond told me if it was his pond, he would stop construction right then, because we were in so much rock. I was already three quarters of the way through, so we went for it.
Indeed, I do have a small leak. (I lose between a 1/4" and 1/3" per day. Doesn't sound too bad until you go through a six month drought, and the pond is 4' below full.
We took every other precaution that we could, including bringing in a sheeps-foot roller to compact the soil as much as possible. We fluctuate a bit on water depth, but it's 15' deep at full, so I don't worry too much about the fish.
Do I wish it didn't leak? Yep... If I had a do over, would I build it anyway? Absolutely! We love it. Swim in it, and are catching lots of fish with the grandkids. I just have to block out the 2' of exposed bank right now due to the low rainfall totals and leak. A buddy just put in a swimming pool. He spent a lot more than our 1-acre pond cost us. I wouldn't trade for anything. Try to get as much help from experts as possible. Dig test holes, etc. Good luck! I hope it works for you.

9 yr old pond, 1 ac, 15' deep.
RES, YP, GS, FHM (no longer), HBG (going away), SMB, and HSB (only one seen in 5 yrs) Restocked HSB () Have seen one of these.
I think that's about all I should put in my little pond.
Otter attack in

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