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KiwiGuy Offline OP
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Hi team,

I've been reading a bit of the forums to try and figure out the steepest reasonable slope I can get for a small pond, but most the of the replies seem to be basing the slope on ease of maintaining the side and/or being able to easily get out of the water. I'm looking for info about the safety of the dam itself - not wanting to have any instance where it might collapse.

I'm looking to build a small (around ~150 cubic metre / 5000 cubic feet) pond/swimming hole on our semi-rural property. We have a small valley (no stream in it) around 4 metres wide with an average slope of 1:4 that I want to dig out and dam at one end. Local regulations are that the maximum height from dam crest to base of dam is 4 metres.

The soil is mostly rock/clay. The plan is to line the dam to prevent leakage, and I'll have to aerate/treat it to keep it clear. This question is mainly about dam construction as opposed to keeping the water clear.

If I go 1:3 slope on the front and back of the dam, it leaves very little area for the pond, and very little depth (only around 1m, whereas ideally, I'd be getting around 3m depth). I'm not concerned about the steepness of the slope for maintenance as the dam is only a few metres wide, and I'm too concerned about ease of getting in and out as I will be building an overhanging structure to get in and out easily.

So to the question - how steep can I go, and how narrow can the crest of the dam be before it's becoming a risk? I'm also not opposed to driving some piles in and retaining the middle of the dam if that helps. The attached diagram is a side on view of the slope the dam will be on. The valley walls rise about 2-3m on either side of this slope the whole way down. The numbers on the bottom represent the width of the valley at each measurement point.

Thanks heaps for any advice, happy to answer any questions.

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I'm not an engineer , I'll leave that to the much more educated and experienced. Welcome to the Forum , Pray the pond is all you hope for and all that use it , use it safely . God Bless You and Yours.


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I believe I am failing to grasp your main problem. Are you limited by the surface area of land available to make your pond?

Your dam height will be measured from the pre-construction land elevation at the mid-line of the dam to the post-construction top of the dam.

However, you can excavate as deeply as you want in front of the dam. That is not typically considered part of the "dam height". You could dig 10m deep in front of the dam. If your dam breaks catastrophically, you are not sending a wall of water 10m high into your neighbor's property. It would always be no greater than the height of your dam above the ground surface.

IMO, you need to design your full pool water elevation and then add some freeboard to your dam height ABOVE that level. Never let a heavy rain allow the water flow in your little valley to rise over the top of your dam. It will quickly cut out your dam.

You need an outlet pipe and/or a surface spillway to protect your dam and pass through excess water.

Once you have your design elevation for the water level, you just need to excavate the pond area, side slopes, and bottom contours to create your desired pond. I expect you will have excess soil available if you create your water depth to 3 meters. You can spoil this around the pond to make attractive landscaping or just turn your straight dam into a "U" shape.

As to your dimensional questions:

For most embankment dams, the front slope and rear slope should NOT be greater than 3:1. Typically if you have a significant amount of non-cohesive material in the fill (such as sand, gravel, or rocky debris), then the downstream slope should be further reduced to about 2:1.

Further, my guide says 6' of minimum top width for a dam of up to 10' in height. Use a minimum of 8' for the top for a dam of up to 14' in height. (I apologize for switching the units out of meters.)

However, for the small dimensions of your swimming pond you may be able to conceive of the project differently. Imagine you are digging a hole with a volume of 150 cubic meters. You are then landscaping your beach/view/yard to place up to 195 cubic meters of spoils. (There may be a fluff factor as high as 20-30% for your excavated volume after it is spread and compacted into place as your spoils volume.)

A pond that is up to 10' deep can probably be sealed with a clay blanket of 12" minimum thickness in all dimensions. The blanket must be constructed by using sub-soil material with the proper clay content, and moistened and compacted in lifts of 6" or less.

The exception would be your "dam" to the down slope side of the valley. Even being "sealed", it must have the mass to resist the pushing force of your impounded water. However, I think you could do that only 3-4 feet above grade. (Assuming I am understanding your project correctly.)

Finally, another affordable option for a pond that size would be to use a pond liner. You could have nearly vertical sides to a depth of 3m in the places the big kids are jumping and diving. You could then build a very gentle slope on the other side and cover the liner with sand to make a walk-in beach for the smaller kids.

Hopefully you can translate my gibberish into some useful ideas for the guy that actually has eyeballs on the project.

Good luck on your new pond!

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it basically depends on how stable the ground is after it's compacted. Since you are using a liner , you don't have to worry about erosion on the pond side, but on the back side of the dam, you have to worry about it eroding there before you get the ground cover established.

How wide or how narrow the dam can be? It has to be wide enough so you can get equipment on it to compact the core trench and the dam. Heck, if you can properly brace it so it didn't push out, you could make the dam out of 1/2" thick sheet steel and have a dam that is 1/2" wide. ..........


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When you said, "The plan is to line the dam to prevent leakage ...", I interpreted that to mean lined with compacted clay. esshup interpreted that to mean you are using a pond liner.

You probably need to clarify that part to get on point advice.

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Originally Posted by esshup
Heck, if you can properly brace it so it didn't push out, you could make the dam out of 1/2" thick sheet steel and have a dam that is 1/2" wide. ..........

I'd like to see the muskrats try to burrow through THAT dam!



All joking aside, has anyone (esshup and others) seen a dam that used a vertical piece of pond liner to make the seal instead of a clay-packed core trench? I am imagining a situation where the pond site has very little clay, but you can reach a clay layer at the bottom of the core trench excavation.

Could you anchor the bottom of the pond liner in the clay and hang the liner against the trench wall, and then carefully backfill? You might have to make the dam a little wider than usual so your dam fill of sandy and gravel material could better resist the pushing force of the water in the pond.

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KiwiGuy Offline OP
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Hey, thanks for all the responses.

You always think when writing these things that you've articulated the problem but it doesn't take long to realise you haven't! My apologies - but you are correct, I am limited by space above the dam to excavate as it's at the top of a valley. I c ould puch a touch further at the top but there are bunch of native trees that I would hate to cut down.

The primary problem is that, because of the slope of the valley is about 1:4, and the fact that the dam height is measured from the crest of the dam to the the point where the dam meets the natural ground level (further down the slope), that 4-metre height limit mean that the dam is actually only about 1m above the natural ground level at the crest of the dam. This means I've got a lot of excavating to do above the dam to get the necessary water depth. I would prefer to avoid this as it's already 2-3m from the valley walls to the valley floor, ideally, I would use that depth for water rather than dig much deeper.

Oddly enough, dam height is regulated but dam slope isn't. If I built a vertical dam (impossible with compacted dirt I know) the crest would be 4m above the natural ground level, the shallower the slope, the lower the final height of the dam. I've attached the same picture but with a 1:2 slope front (on the right) and a 1:3 slope back (on the left) dam overlaid on the slope to help illustrate what I mean. I've also attached a photo of the valley from the top and bottom.

I think I might be misunderstanding this part a touch:

Quote
For most embankment dams, the front slope and rear slope should NOT be greater than 3:1. Typically if you have a significant amount of non-cohesive material in the fill (such as sand, gravel, or rocky debris), then the downstream slope should be further reduced to about 2:1.

Wouldn't reducing the slope go the other direction (e.g 4:1). I am presuming when people day 3:1 they mean 3m horizontal, 1m vertical?

In regards to lining - the plan is to use a pool liner or similar as opposed to clay.

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Originally Posted by KiwiGuy
Wouldn't reducing the slope go the other direction (e.g 4:1). I am presuming when people day 3:1 they mean 3m horizontal, 1m vertical?

Yes. Typing fast while thinking about all of the ways to accomplish your goals and making a math error is pretty bad. Glad you caught that I was NOT reducing the slope with those numbers!

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Now that I am closer to understanding your situation on the ground, is there any room to move farther downslope to some flatter ground? If you moved to just beyond the end of the slope break, then more of your dam height as allowed by the regulations would be "effective" dam height.

If you are building the pond near the head of a valley, what will be your water source? (However, is your setting in a rain forest? Your flora look awesome.)

If you are using a pond liner, why can't you go nearly vertical for the pond side slope of the dam? That would help your geometry considerably. Just spoil your excavated material downstream of the pond liner location.

Do you have any low-growing vegetation that you like for the area that has a thick network of roots. Most of the dam design rules of thumb we utilize are for much wider dams than your project. You might be able to go as steep as a 2:1 slope on the backside of the dam if you immediately stabilized the soil fill with a solid plant cover so there was no chance of your backside material slumping down the slope. That would further help your geometry problem and gain a little more "effective" height for your dam.

How are you excavating the pond basin itself? The equipment you use in a tight space might also change your design parameters a little.

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KiwiGuy Offline OP
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Yes it's a beautiful area - our own little slice of native New Zealand bush. The slope continues down some way unfortunately, so it's there or nowhere.

The water source will be trucked in (pricey!) / rainwater collected from our property, there's no other option unfortunately.

Quote
If you are using a pond liner, why can't you go nearly vertical for the pond side slope of the dam? That would help your geometry considerably. Just spoil your excavated material downstream of the pond liner location.

This is exactly the info I'm looking for thanks! Everything I had read has suggested a 1:3 slope on the water side - but reasons ranged from ease of exit to decreasing chance of water leak. My concern (seeing as in am lining it) it simply dam strength. Does the strength only come from the non-water side and crest width, or does it come from the water side too? Ideally, seeing as I'm lining it I'll go for a steeper uphill slope and the problem will be solved.

Down hill I cant see a way I can get steeper than 2:1 with an earthen dam. I'll plant it with lomadra which has a super strong root system to minimise the chance of slippage.


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