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Kris B Offline OP
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Despite the category title, I thought this would be the best place for this question, please correct me if I'm wrong.

How can I provide an adequate overflow system for a pond that was dug out, and not built with a dam (and therefore has no low ground to which a siphon/spillway system can drain excess water).

---

I am in the middle of a pond renovation project, in which the final state of the pond should be about 3/4 acre surface area and 8ft average depth (ranging to 15 or more feet in spots, depending on soil suitability testing, still pending). Probably +/-2M gallons capacity.

In the past the water level of the pond varied considerably (2-3ft or so, probably). It is my understanding that the pond, when overfilled, naturally overflowed into the front or side street bordering my property and from there into the storm drains/culverts. As far as I know, this only happened once, in August 2016 during major rains and flooding throughout S Louisiana (I didn't own the house at the time). Photo of possible drainage path below - not sure whether it went to the culvert or storm drain.

In the future I plan to keep the pond full using well water. However, if I am keeping the pond full all the time, rain events are going to much more quickly affect me. I'd rather not soak and wash the pond out through my yard every time it rains, so I need a dedicated drainage/overflow set. Since my pond is dug out and does not have a dam, I'm not sure that the standard siphon/spillway system work for me, though I'd certainly like it to.

I know at this point that I need to measure the depth of the storm drains and culverts on each side of the property to see if tapping into the bases of those would be a viable option. How much deeper than the "normal" pond level would these drains need to be to get a siphon system to work? Alternatively, do I just need to make a dedicated/cemented channel for water to flow in these events? (or perhaps constantly, if I go with a solar water well pump?)

(One other data point - when it is raining HARD, my 3in Honda trash pump rated at 18,000gph at times cannot keep up with the rainfall and drainage from surrounding yards.)

Looking forward to hearing some thoughts and ideas on this problem. Thanks!

Thanks to Redonthehead for posting this explanation of the pond siphon system on this thread.


Last edited by Kris B; 08/28/19 01:16 PM. Reason: typo - gpm is actually gph

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On further review, is this statement (from the linked pdf explaining siphons) correct? I don't think it is:

Quote:
The low point of joint C should be at least 4’ below the low point of point E.


It seems that the low point of joint C should be perhaps some minimum distance below point A, but drawing from the bottom of the pond shouldn't affect the hydraulic situation, right?


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Kris - dug out ponds are difficult to spill via a pipe as you have stated. A siphon would likely never go into full siphon (vacuum) due to the lack of elevation change. Now, if it is the type of dug out that has a bit of water backed up against the spoil pile, then it may work in that situation. For your case, I may just install a straight pipe, give it some fall from inlet to outlet (as much as you can) and let her rip. It probably won’t do much except slowly return the water back to normal pool elevations after the rain...but that would get you ready for the next rain...so that may work. You’d have to stay on top of the maintenance and keep the pipe trash free, but that goes for all pipes.

Just some thoughts...

Last edited by Hogfan; 04/29/19 10:57 PM.
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As far as your question about the 4 foot elevation difference, correct, the elevation difference between the inlet and outlet can be 0’ and it will still siphon. Shouldn’t matter. What matters is the outlet to pond waterline elevation difference.

Not totally sure why the vent tube has to go 2’ into the air either. May be just a personal preference thing or there might be something more technical to it. We usually just come up a few inches with the vent tube and add angled fittings until the vent is running parallel with the slope of the inlet pipe. Then we install a perforatied sewage pipe to function as a vent tube trash guard and use big stainless hose clamps to clamp the vent tube to the 6 or 8 inch inlet pipe.

Also, the “normal pool” will be the elevation of the vent tube inlet. Once the thing goes into siphon after a rain event, it will stop flowing at the vent opening elevation. I would call that normal pool. Again personal preference. If it flows but doesn’t go into siphon, then the pool elevation would be at the point A on the drawing.

Having said that, that’s a good drawing, and if you build it just like it depicts, it should function correctly.

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Also, I don’t know the specifics of the storm drains you are referencing, but I would avoid adding any additional water to them that wouldn’t already be going into them. Those are designed for a specific flow rate and a specific rain event. Just a thought...

Have a good one!

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Kris B Offline OP
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Thanks for the reply! That makes sense, I'll just have to give it a bit of fall and let it do the best it can do. I'm realizing that pond dams with siphon systems are more critical when ponds are very large and very high flow rates must be generated in order to manage water levels.

In my case, since it's going to be a 3/4 acre pond draining a total 2-2.5 acre area, I think something like an 8in pipe with normal flow ought to suffice even in heavy rains based on the reading I've done so far. Thanks again.


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Originally Posted By: Hogfan
Also, I don’t know the specifics of the storm drains you are referencing, but I would avoid adding any additional water to them that wouldn’t already be going into them. Those are designed for a specific flow rate and a specific rain event. Just a thought...

Have a good one!


I think in my case it would be fine (of course I'd have to get permission from the city to tap it somehow). If I filled in the pond those storm drains would have to manage the rainfall, so I would think I'll be OK.


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Here is what the landscape architect came up with. I'm thinking about backing off from 12in to 8in pipe (it's only a 3/4 acre pond, after all) and going to PVC or other plastic pipe rather than corrugated metal. It also just seems like overkill for the type of project I'm doing. The elevation of the bottom of the storm drain 110ft away is 25.83ft and the water level will be at 28ft so I can't probably do much more than 1% grade on the outfall.

I need something, for sure, but the design as it stands now is adding about $10k to my pond construction cost.

Attached Images
Control.PNG (81.82 KB, 761 downloads)

Kris

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I see a hole or two in your flow logic. You say that the 18,000 gpm pump cannot keep up at the extremes. Generically speaking, with little to no head pressure at the pipe inlet, an 8" pipe can carry only 950 gpm...a 12" pipe might be upwards of 4700 gpm with decent head pressure.

Either you pump is not pushing 18,000 gpm or your pipe sizes are too small for the extreme rain events.

I don't pretend to be an expert in fluid dynamics, but I suggest you look deeper into this.

I can say that pump specs are typically advertised at maximum capacity, meaning that the pump has NO lengths of hoses attached but somehow magically receives the water. This spec is to sell pumps not to be very practical, but should lend an idea of what it can do. Even at half spec, 9,000 gpm is more than a fairly level and short run of 12" pipe can carry.

How much water shed does your pond have?

Point of reference...My 1/4 acre pond with about 15 acres of watershed can overtake my 15" drain pipe. It has not gone over the dam yet, but it will eventually. Before renovation, it had a 18 inch pipe and overflowed a time or two. I, now, have extra freeboard to help during those extreme rains, but that only buys me time.

Hopefully I am missing something, but a pond drain expert may be a good idea.


Fish on!,
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Kris B Offline OP
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That's a fair point, QA, I'm probably getting sub-optimal performance from my pump. I've got 45ft suction hose with probably 10-15ft head lift plus probably 100-125ft of discharge hose.

HOWEVER... the biggest issue with what we're discussing is that my pump is not rated to 18,000gpm!! That would be impressive indeed. I'm afraid it's only rated to 18,000gph (technically 317gpm). A fairly critical typo in my original post! (now corrected) Sorry about that.

I've got probably about 2.2acres watershed, but in the worst possible case, about 4.2 acres - in a developed residential area, google earth calcs attached.

Attached Images
Est. Actual Watershed.PNG (1.62 MB, 302 downloads)
Max. Possible Watershed.PNG (1.67 MB, 204 downloads)

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I'm glad to hear it was a typo! I had a strange feeling the I was either cross-wired upstairs or the pump spec was off. I was thinking it was an extra zero maybe.

I like the way it's sounding now and I'm glad my wires are in good order...thanks!


Fish on!,
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Kris B Offline OP
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Ok, attached are four principle ways I can think of to drain my pond. I think the siphon system is out due to shallow grade attainable for the outlet pipe.

A CMP standpipe system is what the landscape architect designed, but materials & labor for this system is $8-10k, which seems a little crazy for a backyard pond with a 100-120ft run of buried pipe (some of the cost I'm stuck with, including the pipe run and tapping into the nearby storm drain). Of course the shrouded standpipe option - or better yet a deeper version of it - have some distinct advantages.

Any thoughts on these options?

Attached Images
Straight Pipe.PNG (12.57 KB, 322 downloads)
Deep Draw Straight Pipe.PNG (16.88 KB, 444 downloads)
Deep Draw Siphon Straight Pipe.PNG (17.24 KB, 821 downloads)
Control.PNG (81.82 KB, 892 downloads)

Kris

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Warning: Redneck logic being applied.
Flow is flow, I would suggest the siphon system and size the piping to work without going to siphon mode. I would just make a small u-shaped vent at the proper level and just count on it working as a regular straight pipe system. Much less cost and I think there is a definite benefit to having a deep water draw. In the rare case you you need to draw down the water level, you could always pump it for the money saved on install. To me, simpler is usually the best solution. added bonus is the lack of drainage system visibility in your situation, it would make a nice clean install.


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