Pond Boss Forums POND CONSTRUCTION Building a dam Seeking Help on Dam Against a Rock Outcrop

I am looking at a pond site in southeast Kansas in the "Flint Hills". I could impound 9+ surface acres along an ephemeral stream with a dam length of only 460'. Maximum water depth would be 15 feet.

The Flint Hills are alternating layers of limestone and shale. Each bed is 5-30' thick. Some of the limestone layers are very resistant and act as caprocks. In an arid climate the hills would be nice mesas, but here they are tall grass prairie with some 10' "cliffs" of exposed rock where a limestone layer is eroding. Overall, some of the hills can be 200-300 feet tall from the top of the crest to the bottom of the stream.

I have about 260 acres of watershed above the dam site which should keep the pond mostly full without requiring a massive emergency spillway. (NRCS recommendation for this area is 25:1 drainage acres to impounded surface acres.) The ephemeral stream also receives water from some springs. The springs are currently running even with the dry winter we had in the area.

I have two questions for the PB lunkers.

1.) One end of the dam will tie into an exposed cliff of slightly fractured cherty limestone. What are my odds of success in getting the core trench to seal against the rock? I don't think the rock fractures are extensive, but is it possible/likely to get a good dam seal and still leak all of my water back into the rock formation where it acts for some distance as a shoreline?

[The big state reservoirs in the area usually have a dam that ties into the "cliffs" in the narrow spot between two ridges. The dam obstructs a large stream or small river. I don't know if these dams are watertight, or if they leak but the water inflow rate to the lake exceeds the leakage rate.]

2.) I am worried about impounding water where the pond surface would be 6-10' above the current level of the spring effluent. I believe I would push the springs back into the surrounding hills. However, the top of the Flint Hills within one mile is 100' higher than the projected pond surface. I think the potentiometric gradient would still feed into my pond.

I would welcome any expert (or even non-expert) advice on either of these questions.

Thanks,
Rod

Rod, our resident pond builder expert is Mike Otto.
Here is his contact info.
Otto's Dirt Service
Otto's Dirt ServiceSpecializing in fish pond construction and design, Mike Otto can help you with the answers to these and any other questions. Engineering service available.
Toll Free:(800) 882-3478Email:mikeotto@ottosdirtservice.com


Also It might help if you would post pics of your pond site. What kind of soil material is in the basin? Any chance you could extend the dam down the side were the rock outcropping is and seal it off in front with the materials from the basin? Are there any more ponds or lakes close by that hold water?
Good luck
Rocky

Rocky,

Thanks for the help. I got Mike on the phone this morning. As usual he was both cheerful and knowledgeable. He said he has never seen a well-built dam have a structural problem where it meets a rock ledge.

He did say there is a chance my rock wall "shoreline" could leak. His solution was exactly the same as yours. He said you need a minimum of 1' of clay sealing off the wall (2' would be even better). You build it up just like a dam with 6 inch lifts and about a 3:1 slope.

My drainage area is too large so I will have "excess" water. He agreed it would probably be best to build the dam right and see if the lake holds water properly. If not, I can drain it to perform the required modifications and try again.

There are lots of good ponds and watershed lakes in the area that hold water. However, this project is a lot of money out of the family budget. I wish I knew if the good lakes were 19 out of every 20 attempts or only 10 good lakes out of 20 attempts.

zhkent is a PB guy in the Flint Hills north of me and has built ponds and watershed lakes in similar terrain. He also called me and dropped some more knowledge.

He said to utilize the biggest excavator you can get and tear into the rock outcrop where the dam will connect. (He has a Cat 330.) He said you should be able to pull rock away until you get up to about car-sized chunks. When you can't tear any more rock away then you know you are through the fractured zone due to weathering and exposure. He said he has not observed any leaks on the dams he built in this way.

The only complication he recalled was that one time he excavated back to a small spring in the rock outcrop. It was on an engineered watershed lake, so they were able to install a toe drain to protect the core trench.

I would still appreciate any more advice from people willing to speculate on my odds of success. I don't own this land - it is coming up for auction next week. If I get the land I will definitely post pictures and more information.

If you have both Otto and Kent, you've talked to 2 of the best.