Pond Boss Forums POND CONSTRUCTION Selecting a site Pipe flow calculator

Good morning, gents. After 'thinking' of rebuilding a 5 acre pond on the property that stood for 23 years, I'm finally at phase one, which is clearing the property of trees and identifying the rerouting of the creek that feeds it. On a flow calculation of that 6in deep x 6ft across creek, we get an estimated year round flow of 750 gallons/min. My local (and experienced) pipe and culvert supplier was initially recommending a tandem 10in siphon system, but I would like to educate myself on what the actual pipe sizes should hold.

The previous lake used only a gate for draining, used a large earthen emergency spillway year round, and had no riser pipe.

Thanks for any input you have- this has been a dream since 1972 of my Grandfather, Father, and now me. In 2025, we're going to get it done!

It's not as simple as that. You have to calculate the amount of watershed that is feeding the pond, and determine how much rainfall will run to the pond in 100 and 500 year rain events. That amount depends on the type of vegetation or lack of vegetation in that watershed. The easiest way to do that is go to your counties NRCS office and talk to them.

Not a simple calculation as figuring out how much water your creek runs, its a matter of how much it runs on that biggest rain event day, and be able to handle it without washing out.

Its a matter of calculating how many acres drain into it and a bunch of other variables, such as what vegetation, if any, is on that acreage and how steep is it, which will relegate how fast its gonna all get to your overflow, and how much freeboard you have if your overflow system will not handle all of it immediately, hopefully you have an emergency spillway for such an event.

Not a bad idea to consult an engineer to put some numbers together for you, could save you money and or headaches in the future.

Good Luck! sounds like a fun project that everyone is looking forward to.

1.) Diverting a flowing creek into a pond is perhaps a violation of some federal laws and probably a violation of state law. Some states designate waterways as small as a dashed blue line on a USGS topo map as a stream. Obstructions and diversions in streams are usually legislated pretty heavily.

You probably need to internet search the SC rules. They may start with a simple publication like, "Rules to Build a Pond on Your Land". It will only have the basics, but that may lead you to the more relevant specific rules.

Note - After you do the clearing, I would take good photos and try to document the prior pond. Frequently, the rules may be much different if you are just modifying a pre-existing pond.

2.) How high did the creek run when you got hit by Helene (or a different hurricane)? From the comments above, you can see that if you divert the entire creek to your pond, the outlet of your lake would have to survive your biggest possible flood event. The engineering for that is possible, but gets VERY expensive very quickly.

3.) Does your creek run year round or most of the year? If so, one plan may be to just divert a portion of the creek flow to your pond site. Let the floods keep going down the creek.

750 gpm flow is over 1,000,000 gallons per day. That would add at least 3' to a one acre pond every day.

Many new ponds require many months, or even more than a year for their initial fill up. You can see that you don't need to divert much of the flow to have a nice pond.

Good luck on your new pond project!

Thanks for the replies. I'll review the points above and keep those in mind.

A local 9 acre family pond that was established in the 50's is still standing strong (the galvanized piping was replaced in the 90's) and we may copy their design. The watershed is no joke and worthy of respect as we receive the runoff of the NC mountains. Helene did a fair amount of damage, but that is why the emergency spillway still stands- 20ft wide, 6ft deep, and drops down a total of 20ft, but in two different 10ft sections.

As of right now, I'm still in the planning process of deciding to go with a conventional pipe system, or single vs. double siphon system.

If you go with a conventional system, remember that the galvanized pipe lasts about 40 years like you found out. Did you replace the pipe in the '90's with galvanized or something else?

Well, great question!

No, they did not replace with galvanized. Actually, the plan goes against a lot of what engineers say "won't work", but has been doing just fine since 92.

This 9 acre lake receives a year round stream flow of about 350 gallons/min, and any and all runoff from the surrounding acres of over 100. This is regulated by an emergency earthen spillway (Helene), but for the most part, has (1) 10in PVC riser pipe (no trash rack) that lies on the edge of the lake. This riser drops approximately 8ft, then moves horizontal underneath a red clay/ gravel road, and then empties into another 2 acre pond that simply empties into the woods. It's made its own path of course.

The dam is approx 75 yards long, and appears to be a true 3:1 slope on both sides.

Images of the riser pipe inlet and the exit into another smaller pond. The other white PVC system at the dam appears to be a type of siphon system, but flow out of the downstream side of the dam is just more than a trickle...

I am still failing to fully grasp the exact function you are designing an outlet system for.

Did your previous 5-acre lake receive all of the flow of the incoming creek? With no outlet pipe? Are you saying that under "normal" conditions there was a continuous amount of roughly 750 GPM flowing over the earthen emergency spillway? Or was the gate used for "draining" the water inflows, as opposed to draining down the lake?

Did the remnant of your 5-acre pond survive Helene without damage? I am impressed that the nearby 9-acre pond survived so well. That drop outlet shown in your pics almost certainly did NOT handle all of the flow, therefore the emergency spillways did a great job. I would definitely copy the design and materials of those emergency spillways.

As to some rough flow calculations: I just ran a 6" pipe to start under GRAVITY flow. For a 20' pipe, with 5' of drop, that system should flow approximately 2,200 GPM. A siphon of the same size would obviously flow at an even higher rate in "full siphon" mode.

A 6" siphon would easily take care of your normal water flows. It would be completely inadequate if you are designing for heavy rain events.


P.S. Notes about the 9-acre lake outlet. A pipe like that always develops a vortex, unless the water level is WAY above the lip of the pipe. The vortex actually reduces the amount of water that will flow through the pipe outlet. Further, a pipe outlet like that shown is at great risk of plugging if the storm was big enough to break off some branches, or even wash full trees into the creek.

If you copy that design, I would definitely add some anti-vortex baffles and a trash guard.

Here is a link to a Pond Boss thread about the baffles. I am sure that better discussions exist, but this is one that I could find easily.

Anti-Vortex Baffles


P.P.S. In your pond rebuild, are you going to rebuild the dam, or is it mostly intact? I (a non-expert) like siphon systems if adding drainage to an existing dam that does not leak. If you are re-building the dam and packing a core trench, then running a traditional drop outlet of a large size is probably your best solution to handle large rain events and keep water off of your emergency spillways.

"Did your previous 5-acre lake receive all of the flow of the incoming creek? With no outlet pipe? Are you saying that under "normal" conditions there was a continuous amount of roughly 750 GPM flowing over the earthen emergency spillway? Or was the gate used for "draining" the water inflows, as opposed to draining down the lake?"

Yes, no outlet pipe for flow, only for draining the entire contents. Keep in mind that I was minus 2 years old when they decided to 'controlled bust' the concrete dam that had stood for about 23 years, so what is truth vs. folklore is in play, lol. One dam side remains locked into the red clay bank, whereas the opposing end was placed against sedimentary rock. My Dad (now 71) says that he remembers that the dam was not a true 3:1, and had small leaks throughout. Built around 1949, he also doesn't believe a solid core trench was dug.

My initial thoughts were to bust up the remaining concrete, and place that in the emergency spillway. But yes, the entire dam will be made from red clay, core trenched, packed in lifts, etc.

My local culvert supplier does not like/sell galvanized piping, and the max height of his PVC riser is 14ft. Our lake was closer to 20' at the dam when built.

FishinRod, Helene did not harm us much, as you can see that the lakebed is actually in one large canyon, and the spillway just reroutes water around an island and dumps it back into the same stream. The spillway is roughly pictured here, and also downstream of the old dam. you can see the first 'shelf' of the spillway that created a pool, then waterfalled onto the second large boulder, then fed the stream. when looking at the downstream pic of the dam, the spillway is about 20ft to the viewer's left.

Beautiful little creek. Some of my favorite memories as a kid were fishing in rocky creeks like that where you had to sneak up to every pool. However, the pools would frequently hold a surprising number of aggressive fish!

You might do some searches on Pond Boss about outlet designs or even just read some of the threads under the "Building a Dam" section. When I read sections of Pond Boss, I frequently find an answer to a question that I didn't even know to ask.

I believe (non-expert) that the current best material for a large diameter outlet is to use heavy-wall steel pipe and weld the connections from the drop outlet to the horizontal run. That stuff should last over 50 years (?) in many situations.

Another option that may be available for you, but that is not common for most ponds, is to design to frequently use your emergency spillway. Most spillways take some damage due to fast-moving water during a flood event. However, if you have rock outcrops available on one side of the dam, then you could probably create a very robust emergency spillway that was almost erosion proof. That way you might get by with a PVC siphon pipe for 90% of your rain events.

Definitely be advised that there is literally nothing worse that can happen at your pond than allowing moving water to go over the top of your dam. It WILL start cutting down, and can cut 20' in an hour. Consequently, much of the advice you will receive will be to make sure you avoid that catastrophe.

I am not surprised that the old dam leaked a little bit. It is difficult to seal a clay/earthen dam against a rock outcrop. Further, even if you achieve a perfect seal, there is a chance that the rock formation itself will leak once you impound water. The rocks can either have small pores or a few fractures or joints. Either system can pass water.

I think the "safest" way to design your pond is with a very large outlet pipe. However, I don't believe the neighbor's pond gets by with the small outlet pipe you showed without frequently having water go through his emergency spillway. (Unless he has a very small watershed above his pond.) If your spillway design could easily match his successful design, then that might be a possible alternative to save some money.

Building a large dam always takes LOTS of money. Saving a little money on your "insurance policy" (your flood control structures) is not a great idea, unless you are very confident in your less expensive solution.

Hope that gives you a few more ideas to compare to what you actually see with your eyeballs on the site.

Thank you for your time and reply, FishinRod. I recently talked with my excavator guy, and he has previously placed, and recommends, a tandem 10in siphon system. Any waters that get past that can be handled by the emergency spillway. The priority for now is to ensure that the 'new' dam site is rock free and suitable- fingers crossed!

Thanks, again!