Pond Boss Forums POND CONSTRUCTION Building a dam Another overflow pipe question

1 acre pond
1-2 acre watershed
NE Texas Lamar County

The existing pond on this property appears to never had an overflow installed until it overflowed and took out the fence line that ran across the dam/levee. Then it appears the owners ran a 6" schedule 40 pvc horizontally through the dam as an afterthought.

I am rebuilding the dam area and would like some suggestions on overflow pipe size. The local USDA NRCS office does not want to run calculations for an existing pond. Their engineer claims they only do it for new construction. I downloaded the USDA Ag Handbook 590 but the tables make my head spin.

I am considering a 12" double-wall HDPE culvert pipe as I have seen on multiple YouTube vids.

Any thoughts on the pipe sizing/type or pipe finishing on the pond side i.e. angle cuts, standpipe, screening etc?

1-2 acres of watershed for a 1 acre pond in Texas sounds awfully small.

Does the pond get very low in late July and August? Does the pond get very, very low in drought years?

If not, is there some type of spring flowing into the pond?

With the 6" outlet functioning normally, does the pond have a typical "normal" water level?

The 590 tables and the subsequent calculations are very complicated because it is not an easy task to determine how much water actually hits a given pond after a big rain due to all of the variables.

I believe the numbers are usually used to get a decent estimate, and then the outlet is designed a little larger than that as a safety factor. [How has the pond done with the 6" outlet for the rains that you have observed? Was the dam almost overtopped by a 2" rain? If so, then a 4" rain will be more than 2x as much water.]

Is there any seepage around the existing 6" pipe? If not, and you are not working on dam repairs in that area, then I would leave it installed. Even if you install a new outlet, having a back-up is "free insurance" if the new main gets plugged off during a storm event.

Another factor is the amount of freeboard margin in the existing dam. If you have several feet of freeboard above the normal pool water elevation, then your pond can handle a fairly large rain event without water overtopping the dam while the 6" pipe outlet passes water.

It is not clear how deeply you are planning to cut the dam during your repair operations. Another option for you is to install a pond siphon system for your outlet. That may work well for an existing dam.

Here is a link to a drawing of a typical siphon system.

Automatic Siphon Diagram

I am just throwing out some ideas. Hopefully, that will help you make some choices that fit the circumstances at your pond.

Good luck on your pond improvement project!

Thank you for posting this.

I have an automatic siphon setup but could not figure out how it works. Water flows out. I couldn't figure what the pipe on top was for and now I know it is to break the siphon at a given point.

Of course, can't attach image cause it is too big. It's a 6-inch pipe for a 1.5 acre pond.

Glad to hear you do have a working siphon outlet!

They are a pretty ingenious design, and do not require that you "breach" your dam at a low level for installation.

However, they move MUCH more water when they actually siphon. Can you go down and observe the outlet of your siphon the next time your pond has water going through? Water should almost be exploding from the outlet if it achieves full siphon. The usual cause of poor performance is no upturned elbow at the outlet. If air can run back up the outlet, that can sometimes keep your siphon from hitting full flow through capacity.


Your bigger question was designing your additional outlet size. As you said, the tables can make your head spin. The reason for that is that there are so many variables that control how much water will be entering your pond during a big rain event. You could have zombie Albert Einstein perform the actual math solutions, but I still don't think he would be that accurate, because so many assumptions are required for the calculations. We can't just say a 6" siphon for a 1.5 acre pond should work, we need to know so much more about how much water can reach the pond.

However, we do know that your outlet design is too small, because water went over the dam - which is a worst case scenario. (Unless the siphon inlet got clogged during a big rain event? Does the pond have beavers, lots of leaves, etc. that might have plugged your siphon?)

Let's assume for now that your outlet is too small, but it does function correctly. In that case, you clearly need some type of supplemental "emergency" spillway. Lots of different ways to do that.

Do you have a decent route AROUND the dam? That is usually your safest option. You can just build a wide, open spillway that is either hardscaped or well-maintained grass. If that fails, it usually only cuts down a few feet and does not drain your whole pond. It is also MUCH easier to repair than a dam breach.

Your idea for a culvert would also work - especially if you could route that water around your dam. Installing a culvert always creates a potential pathway for water to trickle along just outside of the culvert walls. When that happens during a big rain, the trickle usually starts eroding more material and eventually washes out your culvert. Be sure to read up on "anti-seep" collars if you go the culvert route.

Do you live at your pond location? Another option is to deliberately close your siphon "air break" inlet prior to a big rain and start your siphon running. (You may need to pump water into a valve at the highest point to do that.) If you see a big rain coming in the forecast, you could always start running the siphon on your pond. If you created 1-2' of additional freeboard on your dam by lowering the pond level, you would probably prevent some of the dam overtopping events.

Just throwing out some more ideas for you to consider!

The pond builder who built my lake a year ago said he uses concrete spillways rather than pipe because there can be so many problems with piping. Now I am glad I don't have pipe because I see a lot of sticks, limbs, grass, etc flow thru my lake that could clog up pipe.

I envy you guys. My pond is about 35 to 40 years old and the overflow pipe is still a virgin.

Worker: "What size pipe should we use for Dave's overflow outlet?"

Builder: "Just run 3/4" PVC to save him some money since it will never see any water anyway."

Ouch!

Garden hose maybe