What should I do about building this pond dam back?

ShowroomShine · Mar 30, 2017

This is a buddy of mines pond, maybe 2 ac in size and the deepest part is 15ft (the part where the dam busted), the rest is maybe 7ft or so. It blew out during hurricane Matthew last year, and he's trying to find a cost effective way of rebuilding it.

It was built incorrectly 20 years ago, but survived through Fran and Floyd hurricanes. It was built in eastern NC where sandy soil is prevalent, and they didnt use a clay core. The spillway was an 8" pipe and a bucket right at the top of the dam.

He's been told the "correct" way to build it back would be to bring in about 15-20 truckloads of clay, pack it as they go, and install a gate/valve setup to meter the water level during a storm. Sounds great, but comes with a $15,000-20,000 price tag. The valve alone is $3000-4000.

There has to be a more economical way to build it back. I know the clay core is key, where I think the money savings can be is in the valve/overflow. What if instead of a valve he did 2 or 3 15" pipes at the top of the dam, just above normal level. That way when a storm came it would have significant enough flow out to keep the water level. Possibly install some sort of erosion device around them so the water wont just cut out around the pipes.

Thoughts? The spot in the picture is where the dam blew out, about 15' deep and 10' wide at the top and maybe 20' at the bottom.

breadtrk · Mar 30, 2017

Hire a pro or let it dry up and plant it.

Egon · Mar 30, 2017

Consider a syphon tube for the overflow line. A smaller valve right at the top can break the the syphon action. It's probably not what you are looking for as it would be a strictly manual operation.

EddieWalker · Mar 30, 2017

Avoid pipes, they are only capable of handling so much water, and in an extreme storm, they fail. Pipes also give water a path to flow along the outside of the pipe, slowly, steadily, eroding the dirt packed around the pipe. There are collars and sleeves that can be added to the pipe to stop this, but they only work in ideal conditions. Just avoid using a pipe for overflow and design an overflow spillway instead. On my pond, it's virgin soil that goes around my dam. Kind of off to the side, where nothing was disturbed, the grass is thick, and no amount of water flowing across that area will affect the dam.

If that wasn't an option, then a wide, flat concrete spillway across the top of the dam and down to virgin soil, then lots of rock to protect the soil from the water coming off of the spillway.

I'm not sure what you mean by being built incorrectly other then the pipe not being sized properly. If the pond held water, didn't leak, and survived several hurricanes, it was probably built just fine with soil that was fine. I'm curious why you need to bring in clay if the existing soil worked for so long? 20 truckloads of clay is not a lot of material, so this is either a fairly small dam, or somebody is just trying to make money selling clay.

Dams are very simple to build, slope is easy to do, getting rid of the excess water under extreme conditions is what your buddy needs to account for!!!

Larry Caldwell · Mar 30, 2017

Another vote for a spillway on one side. A spillway at the deepest part is stupid. It weakens the structure where it needs to be the strongest. He can beef up a 7' dam a lot cheaper than a 15' dam, and if a side spillway fails you only lose the top 7' of water and repairs are a lot cheaper.

davesl708 · Mar 30, 2017

As Eddie and Larry stated, the spillway is the answer to your friends problem. Lower cost and can handle much more water. DIY is possible on that small of a dam. The spillway should have a gradual slope on the down stream side to minimize erosion. Good deep root grass and no brush or trees on the dam.