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Hello all.... I have been skimming the posts in search of something similar to the situation I have, but I deemed it may be better if I provide my own post with pictures.
Background... Just had this pond built on new property about 6 months ago, and now I am seeing some erosion where the water exits the 15" corrugated exit pipe. I had stacked a large number of rocks around the exit and down the dam, but it appears the water when running with force has sort of sidestepped them. Not sure if I need to fill the washed out area back in with dirt/clay and the put some quickcrete around it, or what the best possible solution may be.... Also, on the entrance of the pipe, do I need to pour some concrete there as well around the pipe? Sorry for the stupid questions but I really do not know much about this. I have attached pics for more detail. Thanks!

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that pipe is a straight shot through the dam? so any erosion it causes is eroding the dam correct? If so you're going to need a tailrace to keep the dam from being damaged by the outflow for sure


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Perhaps either extend the pipe down the backside of the dam to the ditch below, or build a thick concrete tailrace for the water to flow on down the backside. Otherwise you will be battling erosion forever.


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Thanks for the replies.... yes, it is a straight shot they the dam. I thought about extending the pipe down the dam wall, but do they make a 15 flexible corrugated pipe? Or would I just do like a 45 degree coupling to turn it down and straight sections from there? The tailrace sounds like a good idea as well. Also, would I need to put anything around the pipe at the entrance into the dam on the pond side? To keep it from possibly washing out around it from maybe suction? Thanks!!

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Is this the only outlet? I don't see an emergency spillway.

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I can tell you what I did on one of my pond and it seemed to work. It has a 12" PVC pipe and it angles down dramatically which increases velocity and capacity during flooding. I dug a pit below the outlet side of the pipe and placed a 6" drain to another downstream pond. I capped up-end of the smaller drain with a wire trash rack and filled the hole with rip rap. During heavy flow the structure spreads out the water flowing over the grass (that the 6" drain will not handle). During the rest of the time, the small drain gives the above-ground area time to grow tall grass. Tall grass lays down during flooding protecting the soil beneath. I used the same strategy on my newly finished pond (but on a smaller scale), but it has not yet been tested. I have emergency spillways on all 3 of my "fish" ponds and wish that I had them on 2 constructed wetland that do not. They were constructed by contractors and have a very level berm which has so far handled overflow. Good luck!

Last edited by RAH; 01/30/18 07:31 AM.
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This is the only outlet...

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That sounds like it would work, but for me to place something like that below the outlet I would be digging right into the dam itself, not sure I would want to do that. I am thinking I will look at placing more large stones where it has eroded and then using concrete between any of the gaps of the rocks and build it up on the sides to essentially make a tailrace.

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Think that if it were me, I'd extend the pipe:

https://www.menards.com/main/plumbing/ro...44451515603.htm

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Originally Posted By: RAH
Think that if it were me, I'd extend the pipe:

https://www.menards.com/main/plumbing/ro...44451515603.htm



Hmmmm... I didn't know they made 22.5 degree elbows.. that I think will work. Thx!

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Post some pictures of the project for us to see and let us know how it works out.

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Originally Posted By: RAH
Post some pictures of the project for us to see and let us know how it works out.


Will do...

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Just curious...If this outlet is extended with more corrugated pipe, could you place a T or two inline to serve as clear outs?? Would capping the T help prevent the vacuum created?


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Originally Posted By: Matzilla
Just curious...If this outlet is extended with more corrugated pipe, could you place a T or two inline to serve as clear outs?? Would capping the T help prevent the vacuum created?


Well, if they make a Y then that may be better than T as a clear out...or less abrupt direction change. Even with just adding a 22 degree bend and more pipe I'm a little concerned about restricting the flow.

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Once the pipe is full and not pulling air from the inlet (given enough head on the dam and installation of an anti-vortex device), the increased drop after adding the pipe will probably increase flow considerably rather than decreasing it. That assumes you do not install a T or Y (or any other way for air to get in). That is why my pond pipes all slope down as much as possible leaving the pond. Think about it like starting a siphon and what happens to the flow rate as you drop the outlet side of the tube. The weight of the water dropping actually pulls water into the pipe. Its like having a bigger pipe without the added cost.

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Without an emergency spillway you will have to be extra vigilant about making sure your pipe remains debris-free. If your dam gets over-topped you can lose the whole thing in a hurry. A debris guard can help a lot.

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I wonder if that thin wall plastic pipe can withstand the forces of a full siphon effect if you extend the pipe down the back of the dam. You may not want to install an anti-vortex plate unless the manufacturer can verify it won't collapse.

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Originally Posted By: Redonthehead
I wonder if that thin wall plastic pipe can withstand the forces of a full siphon effect if you extend the pipe down the back of the dam. You may not want to install an anti-vortex plate unless the manufacturer can verify it won't collapse.


That's a good point... I am still leaning toward just using existing large stone stacked wider and then fill all of the gaps with fast setting quickrete to smooth it out into a tialrace. With the sides maybe 6-8" higher than the middle.

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Hello Neo, welcome to the forum. Myself and others wish that we would have found this forum before building our ponds, as I wish that you would have found this forum before constructing your pond.

Hate to be a downer and I hope that I am wrong, but from looking at the pics I suspect its just a matter of time before the runoff from a heavy rain event completely overwhelms that shallow buried 15 drain pipe. The resulting flow of water over the dam could completely unearth the pipe as it carves a deep trench thru the dam, draining the pond

Normally, the top of the primary drain pipe should be about 1ft below the level of the emergency spillway. ( the spillway is a wide flat path going around the end of the dam, over undisturbed soil, often covered with grass, rip rap, or concrete) The elevation of the emergency spillway should be 2 or more feet lower than the top of the dam. The difference in elevation from the top of dam to the spillway is known as the free board. (from the pictures it appears you only have less than 1ft of freeboard above the primary drain)

When determining the size of the primary and secondary drains it is important to calculate the area of watershed which funnels into the pond. From the pictures it appears to me that your pond could receive a massive influx of water during a heavy rain event. (then again, I dont know the total size of the watershed, or how much of the runoff may be diverted around the pond)

Personally, I would either lower the primary drain, or increase the height of the dam, thereby increasing the free board. And I would definitely use a tractor or other equipment to carve an emergency spillway around the dam.

If you do add to the existing drain pipe down the backside of the dam you should probably dig a trench and bury the pipe, as a way of anchoring it. A 15 lightweight pipe pulling a siphon needs to be securely anchored.

Oh yeah, I forgot to mention that your pond is actually pretty cool looking, and I hope you're successful with it.

Last edited by gully washer; 01/30/18 05:31 PM.
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Originally Posted By: gully washer
Hello Neo, welcome to the forum. Myself and others wish that we would have found this forum before building our ponds, as I wish that you would have found this forum before constructing your pond.

Hate to be a downer and I hope that I am wrong, but from looking at the pics I suspect its just a matter of time before the runoff from a heavy rain event completely overwhelms that shallow buried 15 drain pipe. The resulting flow of water over the dam could completely unearth the pipe as it carves a deep trench thru the dam, draining the pond

Normally, the top of the primary drain pipe should be about 1ft below the level of the emergency spillway. ( the spillway is a wide flat path going around the end of the dam, over undisturbed soil, often covered with grass, rip rap, or concrete) The elevation of the emergency spillway should be 2 or more feet lower than the top of the dam. The difference in elevation from the top of dam to the spillway is known as the free board. (from the pictures it appears you only have less than 1ft of freeboard above the primary drain)

When determining the size of the primary and secondary drains it is important to calculate the area of watershed which funnels into the pond. From the pictures it appears to me that your pond could receive a massive influx of water during a heavy rain event. (then again, I dont know the total size of the watershed, or how much of the runoff may be diverted around the pond)

Personally, I would either lower the primary drain, or increase the height of the dam, thereby increasing the free board. And I would definitely use a tractor or other equipment to carve an emergency spillway around the dam.

If you do add to the existing drain pipe down the backside of the dam you should probably dig a trench and bury the pipe, as a way of anchoring it. A 15 lightweight pipe pulling a siphon needs to be securely anchored.

Oh yeah, I forgot to mention that your pond is actually pretty cool looking, and I hope you're successful with it.


Thank you gully washer.... Well the concern about the drain being so high was also a concern of mine when I spoke with the creator of the pond, but he told me it would be fine... I suppose building up the dam would be the easiest and less costly fix for increasing the free board. I have a tractor that I could use and plenty of places on the property to get the dirt.

On the emergency spillway, I am not sure clear on what you are saying, but I assume it is a spot in the dam that is higher than the exit pipe and around 2 ft less than the top of the dam.... I could possibly do that on the far right side of the dam, as it was natural hillside and not man made. So I guess if I built the dam up all the way across except for a few feet on the right side of the exit pipe that could work..?

The inflow of water is from a small stream, actually just ravines that direct are naturally directed down toward the pond from the hills behind it. Normally it is fairly dry to non-existent, but I have seen it flow fairly good if we have had a lot of rain.

I appreciate the input...and the way you presented it. I have attached a few more epics to show it during the building phase...

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Welcome to the PB Forum
Agree with Gully, all it takes to block the water flow is a couple of sticks and if enough water coming in it's over....... not good! Looks nice though. Look into spillway options as well

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Originally Posted By: Pat Williamson
Welcome to the PB Forum
Agree with Gully, all it takes to block the water flow is a couple of sticks and if enough water coming in it's over....... not good! Looks nice though. Look into spillway options as well


Thanks Pat... will do.

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An E-spillway is a must, and you got the idea right....the level of the E-spillway should be about 6"- to a foot above the top of your primary drain, slope AWAY from the dam slowly, and be at least 15 feet wide and well vegetated to prevent erosion/washout if used.

As for your drain pipe.....2 ways to fix it...pull out the rip-rap, re-grade and compact the eroded soil in a curved shape to channel the water, then put down heavy plastic or geotextile fabric and re-lay the rip-rap....this will eventually fail again years later.......best way is to pour a quality, concrete tail race, also in a channeled shape with rip rap on the edges to help keep water on the concrete.

As for the pond side of your pond, check out PondDamPiping.com and get the correct sze trash rack to prevent blockages

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if money isn't an issue, i would install a siphon system. i am a fan of a traditional standpipe, but it is way too late for that now.

did you all calculate your water shed? sure looks like you have a lot of hills that are going to dump water in your pond during a rain.

and like said above, an emergency spillway is a must.


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Getting the dam higher such that water will run around it, not over it, during flooding would be my priority (feet of free-board above gently sloping emergency spillway in virgin soil). Just dumping more dirt on top is unlikely to work well. Any topsoil on the top of the dam really needs to be removed, and the core of the dam raised in elevation in compacted layers. I know this is not what you want to hear, but if you can swing the cost now, it will likely save you money in the long run. It would also be money well spent to lower your water level and install the drain pipe properly now. On my second pond, I went through 2 contractors before hiring folks to fix problems which included replacing a poorly installed drain pipe and raising the dam height. In my case, I did this before the pond filled and I have a worry-free pond. I had not budgeted for this, but things don't always go as planned, and there are a lot of dirt pushers out there. Finding a pond builder is a lot harder. I built my 3rd pond myself but it has yet to be tested, so I cannot claim success yet.

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