Planning on a small 1/2 acre pond down in a holler behind me. The builder man has built several in this area and is used by the USDA too. He doesn't think I need a spillway. He wants to build a massive wide dam so that the water will slow down over it as a sheet. He built his the same way and has had no issues.

The sides of the holler are kind of steep so putting in a spillway would involve cutting into the sides of the hills pretty far. I do want to buy and have him install a syphon system too.

I'm a bit nervous about the entire thing so have scheduled a meeting with the local conservation official at my place to check it all out. One more concern for stability is the neighbors pond which is about 30 feet down past my pond, same holler. A dam blowout from me would probably blow his pond out too.

I'll try and post some pictures tomorrow. Comments please. Thanks.

Siphons are amazing and generally take care of all or any problems. However, a spillway is for the proverbial 100 year flood that, sooner or later, is going to come. If your dam ever blows out it takes all of the soils downstream. If that happens you have to rebuild the whole thing. Patches rarely work well.

I doubt that the dam will stay level enough to allow the water to go over as a sheet. It's going to settle even if well packed. An overflow generally starts at one place and erodes there.

I look at it as relatively cheap insurance. Knowing that I am mortal, I buy life insurance. And, I buy health insurance before I get sick.

Dave, I couldn't have said it better. My initial thought is there's no way that he can get dirt to be completely level especially after settling.

Thanks. good info.

Even if you can't cut into the hill sides, a nice concrete emergency spillway can save your butt...

Went down and looked at it again. I wasn't thinking new water level when looking for spillway location but it appears it should work fine. Length is around 320, width 70. Elevation change now around 20'.


Looking downslope toward where dam will be.



Looking upslope from where dam will be.



Looking downslope from dam toward neighbors pond.


Spillway will probably end up being in slope on right side of dam spot (picture looking toward neighbors pond). The area is nice and "roundish" in the upslope area and then narrows down like a funnel toward the dam.

Mike, what area are you in. Im looking to do the exact thing in future and will be following. Keep us updated.

So much depends on what kind of rain events and the amount of runoff you have. For example, a very small pond with a very large runoff area had better have very good water control leaving the dam. A small pond with relatively small runoff area, not so much so.

A half acre pond is not so big. If it is fairly close to the top of a hill and has just a few acres runoff, then your builder might not be expecting there ever to be huge amounts of rain flow across the top. I do agree with others though, a dam will rarely stay level and the water is likely to find a low spot and start washing there.

If you have 40 acres running into that half acre pond and sometimes get 12 inches of rain in two days like we can get where I live, better have a very good emergency spillway.

I'm in no way an expert on these matters. Just a farmer with quite a few years of experience with terraces and waterways handling water flow. How much flow there will ultimately be makes all the difference in the world.

I am in NE TN. The drainage area is almost 10 acres. After talking with the gubment and reading more, I have decided that I will have an 8" siphon drain even though I probably only need a 6" and I will have an emergency spill way. This will require my contractor to be very careful in measuring as the siphon spillway and dam are all separated by only a foot each. Not a lot of slop to play with. I'm sure he is up to the task though.

Dirt settling is a good point. I want to go with a sipon and spillway. When should I put in that siphon? With that device being what sets my water level, I could end up with a mess if I have too much settling.

If compacted properly settling will likely be minimal. You may find you have more erosion loss than settling if you don't get grass established quickly.

Contractors today don't have much excuse for not getting the heights correct on drains and overflow areas. Electronic self-leveling laser levels are not that expensive any more (we have one for farm use) and they save so much time and labor (compared to a manual operated sight level that take two people to operate) I would question any contractor that is not smart enough to own one and that still uses old technology. I had a manual adjust laser level (lower accuracy cheaper model) to do about 90% of our 3 acre pond project and by using repeated manual leveling and bench marks set up with a more expensive unit, we got within about an inch of everything being right. Unfortunately or fortunately, depending on how a person looks at it, one of my operators came up over a berm and forgot where he put the laser transmitter (important point, try not to station the laser in the work area ) and mashed it. Bit the bullet and bought a good low end commercial contractor grade unit. What a pleasure to use. Almost no setup time, very accurate, useful for everything from building ponds to laying out building sites and foundations, terraces, waterways. A good contractor should have the tools to make sure the drains and overflow levels are correct. If not, look for a better contractor.

Rainman makes a good point. Even if you end up needing the spillway to run down the back side of the dam because of surrounding hills, a person can always use concrete or rip-rap for the water to run over to prevent erosion.

My old pond has only a spillway ( I may put a pipe in next summer and raise the water level a foot so this may change) and it can have a pretty good flow of water over it during large rain events. Most cattle watering ponds in our area have simple spillways that run out into pasture grass. It is very common to see that over many years a ditch erodes and lowers the pond level. I used rock and shaped the overflow so all the water runs over the rock. So far at least, no erosion. And if there appears that some will start at the margins, I'll add more rock. If I end up putting an overflow pipe in, will rework this area down to clay and cover it up to be part of the dam and a new emergency overflow will be in another area.

Point is, with rock or concrete water can be controlled so it does not erode the dam when leaving the pond. Costs a little more, but if you need it you need it. If your emergency overflow runs out over a fairly flat area not prone to severe erosion, then likely a grassed area would be fine.

Remember, the emergency overflow is only used in the most severe of rain events where the main overflow can not handle it. In my case and our 3 acre pond, we had 12" rain in 48 hours and water came within 1" (witnessed by the debris line) of utilizing the emergency overflow, but best I can tell no water ran over it (I was gone and only seen results later).