New pond with bog plan (mkII)

[Pharian]

Hi all,

Thanks to the great advice from you guys regarding my last plan I have ditched it and gone for a more traditional plan. However, I would love some advice again please.

The primary thing I am after is a self contained pond that doesn't need me to drain it and clean it out. I keep reading about people having to drain their pond every 2-3 years and that sounds like a horrendous task. So I'm keen to do whatever I need to do to avoid that. I also don't want to have to use a vacuum cleaner to suck up the gunk off the bottom. I am keen to build it so it keeps itself clean without my intervention (or very minor intervention).

The plan is to have a good percentage of the pond surface area dedicated to a bog. Perhaps 25% surface area and 12 inches deep with 1cm pea gravel (1/2 inch approximate). I'm assuming pea gravel is this stuff? We call it river gravel here in Oz: https://anlscape.com.au/Products/gravels-pebbles/nepean-river-pebble-10mm

The pond is underneath a massive 50 meter (150ft) Ghost Gum tree, with other gum trees around. They are evergreen but they still drop some leaves and lots of bark in summer. So I am going to end up with some leaves and bark in the pond. I'll get most of the big pieces of bark manually (some pieces are massive) but smaller bits will sink too fast, and leaves will also sink sometimes.

I have already bought a 4900Lph pump - Messner Eco x2 4500 (I hope it's enough! I'd like to just need 1 pump) https://lovemypond.com.au/product/messner-eco-x2-4500-filtration-waterfall-pump/
I have also bought a piece of Firestone 7.5m x 6.1m (25ft x 20ft).

The position I have to build it is a weird bit of land on top of a stone retaining wall. The wall is not mortared, but it is stable. However, I cannot dig too deep or the pressure from the water will push out and put pressure on the retaining wall. For that reason I am going down a small distance into the ground, but a lot of my pond depth is coming from building embankments (which I will then build a second retaining wall around to make pretty). That way the pond will be sitting primarily "ON TOP" of the ground with weight pushing down, rather than sitting "IN" the ground and weight pushing outwards. It might make more sense looking at the photos attached.

The pond is relatively small. I've worked out the approximate liters and it is between 4000L and 5000L (which is a bit over 1000 gallons). Of that, around 500L is bog. The deepest spot is currently planned to be about 2 feet deep, essentially a 2 foot channel through the main pond. That may get a bit larger or deeper, if I have to build systems into the design to keep it clean, but unfortunately it's not feasible for me to build a much larger pond at the moment. I could maybe go down another half a foot, or 1 foot in places. And maybe a bit wider. But I'm limited by wife-not-letting-me-use-all-the-grass (fair enough). I'm also limited by the size of embankment that I can reasonably build. They can't be oppressively large. If I need more volume to get better filtration going then I can dig it out some more, within reason (and within the scope of my Firestone sheet).

So really, my questions come down to:

- How do I stop the bottom of the pond filling up with muck and gunk that I have to drain the pond to clean?
- How do I stop the bottom of the bog filling up with muck and gunk that I have to drain to clean?
- Does this plan look like it will work?
- What stupid mistakes am I making this time please?

Options that I have been considering (added to the bog and working with the bog)

- A manifold intake system on the bottom of the pond (just down the deepest end), to suck in gunk and muck from the bottom. I have seen this used a lot in koi pond builds. It's like a manifold you might use in the bog, but with bigger holes. Then you put a layer of 75-150mm (3 inch to 5 inch) river pebbles in the bottom of the pond to cover the manifold. The manifold then sucks all the muck up from the bottom of the pond and dumps it into the bog. However, then you have a mucky bog. Does the bog clean muck and gunk? Or does it just take the nutrients from the water?

- A home made skimmer to suck some of the leaves off the top before they sink. But I'll still be left with a ton of muck on the bottom of the pond won't I? And if I put my pump inside the skimmer, then can I suck the entire volume of the pond through the skimmer every hour? I could conceivably build a splitter so the pump sucks from the skimmer and also from the bottom of the pond? But even then, would that just dump all the muck into the bog and turn that into a muck pit?

- A barrel of bio balls (open top or airtight and pressurized?) that the pump sends the water through first, the thinking being that the gunk will primarily pool in the barrel and then I can flush the barrel out periodically, avoiding having to clean out the pond. Ie, the pump at the lowest point in the pond where it will suck up the most gunk, then that goes into the barrel, then that into the bog. However, there is then issues of the nutrients being filtered in the barrel, because the bog needs those nutrients for the plants.

- Or just an empty box that the water passes through first, that has a grate or netting to catch the gunk?

- Or do I just make a bog, put the pump in the bottom of the pond so it sucks up all the gunk, and then the bog will deal with that gunk and break it down? That would be fabulous. I'd love to just have a bog and nothing else. But several people I have spoken with have said that you need some additional system to get the gunk out, otherwise I'll need to clean it out all the time. A bigger bog?

- I'm not keen on the idea of using a pond vacuum. I'd rather keep it filtered naturally if possible.

Anyway, I hope that makes sense. I'm not averse to there being the usual layer of algae and sediment in the bottom of the pond, but I know from ponds my mother had when I was a kid that the sediment can quickly build up into something foul, and I want to avoid that.

Attached are some photos of the site and a couple of sketches of the layout plan.

The top down view shows the current state of the pond. You can see the main retaining wall at the lower toe of the pond with the black cloth holding it in place for the time being. I will be building that retaining wall all around the pond, and up to an equal level. The bog will be in a separate pool higher than the main pool, but all using the same piece of liner.

The other photo shows the wall from ground level. You can also see the slope of the land (which is another reason for the retaining wall, to level the pond area). That was taken a few weeks ago though, and I haven't started the retaining wall yet.

I can take some more photos tomorrow if that would help at all.

Anyway, sorry for the essay! Ha. Any help or advice would be very much appreciated! Cheers

 

[bagsmom]

My brain isn't entirely switched to the "on" setting yet (need more coffee!) so I just skimmed your post. I think your pond has a lot in common with mine. It isn't huge and I don't want to do a lot of maintenance. I don't have a bog but I do have a waterfall filter. The first thing that strikes me is that you need something to help minimize the amount of debris that can fall to the bottom. (My pond is under a tree and I use a net in the Fall - but there is stuff that blows in year round.) The answer for me is a skimmer. I have a fish-safe Helix skimmer, which was expensive, but worth the time it took to save up for it. Fish can swim in and out, since there is no door to get stuck behind. It seems to me that the current "fashion" in pond skimming is to make an intake bay. Both a skimmer and an intake bay provide a place to put the pump (rather than just going into the bottom of the pond.) They catch leaves, sticks, and whatever blows in. The dirty fish poo water (minus the debris) flows into your bog or waterfall filter, where beneficial bacteria will do their thing. I would suggest hopping on you tube and looking at videos featuring intake bays. Rather than having to get out a skimmer basket, you just use a little net to scoop leaves out of the intake bay. The Pond Advisor (Mark Wilson) has some very good how to videos. Most are within other pond build videos but he may have some specifically about intake bays. Ed the Pond Professor and the Aquascape team do too. I don't know if that is something you are able to consider adding to your plan at this point or not. I need to make more coffee and read your post in better detail. Anyway, that is what came to my mind: if you don't want to have to fool with a lot of cleaning and maintenance, having a place to collect the debris before it sinks down would help a lot. I know others will chime in. Good luck - and I can't wait to see your progress, whatever you decide to do!

 

[bagsmom]

Wait - I see you mentioned a homemade skimmer but worry there will be a ton of muck at the bottom anyway. I'm sure every pond is different. I have a reasonable fish load that Mother Nature seems to take care of. I don't feed the fish except occasionally, so they eat eggs and the population is somehow pretty stable. I have a little bit of what Addy1 calls "mulm" down near the bottom. It's like a little airy cloud layer of tiny bits and pieces. When I use my fine mesh net to try to scoop it out, it's like there is nothing really there. The fish seem to enjoy goofing around and foraging down in it, but it is very insubstantial and not at all gross or ugly. I've only had the pond for three years now but with the skimmer functioning well, I have never had a bunch of crud at the bottom. (I HAVE had to get in and "scuba dive" for gol' danged acorns by the hundreds, though. That was a pain!)

 

[brokensword]

Hi all,

Thanks to the great advice from you guys regarding my last plan I have ditched it and gone for a more traditional plan. However, I would love some advice again please.

The primary thing I am after is a self contained pond that doesn't need me to drain it and clean it out. I keep reading about people having to drain their pond every 2-3 years and that sounds like a horrendous task. So I'm keen to do whatever I need to do to avoid that. I also don't want to have to use a vacuum cleaner to suck up the gunk off the bottom. I am keen to build it so it keeps itself clean without my intervention (or very minor intervention).

The plan is to have a good percentage of the pond surface area dedicated to a bog. Perhaps 25% surface area and 12 inches deep with 1cm pea gravel (1/2 inch approximate). I'm assuming pea gravel is this stuff? We call it river gravel here in Oz: https://anlscape.com.au/Products/gravels-pebbles/nepean-river-pebble-10mm

The pond is underneath a massive 50 meter (150ft) Ghost Gum tree, with other gum trees around. They are evergreen but they still drop some leaves and lots of bark in summer. So I am going to end up with some leaves and bark in the pond. I'll get most of the big pieces of bark manually (some pieces are massive) but smaller bits will sink too fast, and leaves will also sink sometimes.

I have already bought a 4900Lph pump - Messner Eco x2 4500 (I hope it's enough! I'd like to just need 1 pump) https://lovemypond.com.au/product/messner-eco-x2-4500-filtration-waterfall-pump/
I have also bought a piece of Firestone 7.5m x 6.1m (25ft x 20ft).

The position I have to build it is a weird bit of land on top of a stone retaining wall. The wall is not mortared, but it is stable. However, I cannot dig too deep or the pressure from the water will push out and put pressure on the retaining wall. For that reason I am going down a small distance into the ground, but a lot of my pond depth is coming from building embankments (which I will then build a second retaining wall around to make pretty). That way the pond will be sitting primarily "ON TOP" of the ground with weight pushing down, rather than sitting "IN" the ground and weight pushing outwards. It might make more sense looking at the photos attached.

The pond is relatively small. I've worked out the approximate liters and it is between 4000L and 5000L (which is a bit over 1000 gallons). Of that, around 500L is bog. The deepest spot is currently planned to be about 2 feet deep, essentially a 2 foot channel through the main pond. That may get a bit larger or deeper, if I have to build systems into the design to keep it clean, but unfortunately it's not feasible for me to build a much larger pond at the moment. I could maybe go down another half a foot, or 1 foot in places. And maybe a bit wider. But I'm limited by wife-not-letting-me-use-all-the-grass (fair enough). I'm also limited by the size of embankment that I can reasonably build. They can't be oppressively large. If I need more volume to get better filtration going then I can dig it out some more, within reason (and within the scope of my Firestone sheet).

So really, my questions come down to:

- How do I stop the bottom of the pond filling up with muck and gunk that I have to drain the pond to clean?
- How do I stop the bottom of the bog filling up with muck and gunk that I have to drain to clean?
- Does this plan look like it will work?
- What stupid mistakes am I making this time please?

Options that I have been considering (added to the bog and working with the bog)

- A manifold intake system on the bottom of the pond (just down the deepest end), to suck in gunk and muck from the bottom. I have seen this used a lot in koi pond builds. It's like a manifold you might use in the bog, but with bigger holes. Then you put a layer of 75-150mm (3 inch to 5 inch) river pebbles in the bottom of the pond to cover the manifold. The manifold then sucks all the muck up from the bottom of the pond and dumps it into the bog. However, then you have a mucky bog. Does the bog clean muck and gunk? Or does it just take the nutrients from the water?

- A home made skimmer to suck some of the leaves off the top before they sink. But I'll still be left with a ton of muck on the bottom of the pond won't I? And if I put my pump inside the skimmer, then can I suck the entire volume of the pond through the skimmer every hour? I could conceivably build a splitter so the pump sucks from the skimmer and also from the bottom of the pond? But even then, would that just dump all the muck into the bog and turn that into a muck pit?

- A barrel of bio balls (open top or airtight and pressurized?) that the pump sends the water through first, the thinking being that the gunk will primarily pool in the barrel and then I can flush the barrel out periodically, avoiding having to clean out the pond. Ie, the pump at the lowest point in the pond where it will suck up the most gunk, then that goes into the barrel, then that into the bog. However, there is then issues of the nutrients being filtered in the barrel, because the bog needs those nutrients for the plants.

- Or just an empty box that the water passes through first, that has a grate or netting to catch the gunk?

- Or do I just make a bog, put the pump in the bottom of the pond so it sucks up all the gunk, and then the bog will deal with that gunk and break it down? That would be fabulous. I'd love to just have a bog and nothing else. But several people I have spoken with have said that you need some additional system to get the gunk out, otherwise I'll need to clean it out all the time. A bigger bog?

- I'm not keen on the idea of using a pond vacuum. I'd rather keep it filtered naturally if possible.

Anyway, I hope that makes sense. I'm not averse to there being the usual layer of algae and sediment in the bottom of the pond, but I know from ponds my mother had when I was a kid that the sediment can quickly build up into something foul, and I want to avoid that.

Attached are some photos of the site and a couple of sketches of the layout plan.

The top down view shows the current state of the pond. You can see the main retaining wall at the lower toe of the pond with the black cloth holding it in place for the time being. I will be building that retaining wall all around the pond, and up to an equal level. The bog will be in a separate pool higher than the main pool, but all using the same piece of liner.

The other photo shows the wall from ground level. You can also see the slope of the land (which is another reason for the retaining wall, to level the pond area). That was taken a few weeks ago though, and I haven't started the retaining wall yet.

I can take some more photos tomorrow if that would help at all.

Anyway, sorry for the essay! Ha. Any help or advice would be very much appreciated! Cheers

okay, where to begin...

I'm a landscaper and I routinely see some folks doing the labor to plant grass seed under their large trees and diligently taking care of it, being really happy upon new growth and full of satisfaction––until about 3 months later when the summer hits it's peak, the trees have shaded the area completely and sucked up all the water. By the 4th month, the grass has withered away, almost back to what it used to look like. Moral of the story here? You can't have grass growing under trees that block the light.

The story above is like yours; you're trying to have a pond that won't need upkeep and yet you're setting up an 'upkeep' situation by being under the trees. The only way would be to shield the pond, be it net or structure, so the trees don't contribute to your mess. And a net might not handle any 'large pieces of bark falling' on it. Leaves, sure. The idea here is you're going to HAVE To accept either some maintenance as necessary or, do something about the trees. This is MY message to those people trying to grow grass under large trees; either cut the branches up or give up on grass and plant some ivy. This is your situation.

Basic concepts here; DON"T push any debris toward your bog--you'll regret it later. Your pump should be OFF the bottom of the pond. Dig your pond deeper if possible, including reinforcing your stone wall if you have to--again, you'll be glad later. IMO, ponds don't need bottom drains. Bogs are good--it would be better to keep bog separate and higher than pond so you can use gravity to let it feed back.

I don't/haven't drained my pond ever, over 10 years; it isn't necessary if you have good filtration. A bog will do this for you; aim for 30% of your pond surface area. 12" deep for your pea gravel (your link shows proper size, shape--don't use anything with sharp edges as it'll just pack).

I'd seriously talk with your wife and let her know that larger is better/easier re pond. The smaller, the quicker you get problems and the more you have to keep the parameters under control. The larger the pond, the harder to upset the balance.

Keep your pond edge above ground level to avoid water runoff getting into your pond.

If you can, get two pumps instead of one, for reduncancy; if one fails, you don't then have to panic while you source out and acquire another one. Means a bit more plumbing but you get more options. Re pumps, put an adaptor right at the pump that is larger than the outupt, then a wye that downsizes back to output size. The reason is you get much better flow and efficiency when you do this plus now you have 2 leads to use; as in one for a water fall and one to the bog. Use valves and unions for ease and control of the flow.

Use a float valve on your pump so it can't empty your pond. Use a timer on your hose when you fill up so you won't overfill/flood your pond.

Skimmers are for floating debris; if you're more worried about sinking, skimming won't help. A long handled net and moving slowly will rid you of any buildup, or like mentioned above, use a net to keep the debris from getting to your pond in the first place. Some make netting such that when you want to enjoy your pond, you just lift/roll back and when not in use, netted over the pond again. Others, like myself, have permanent solutions that keep debris and whatever (predators, for example) out all the time.

In your bog, use shallow rooted but aggressive growth plants; thin as they take the pea gravel over as you WANT this aggressiveness because it means the roots are pulling out nitrates.

Wider bog is better than deeper, but both help.

Use floating plants with aggressive root systems (water lettuce, water hyacinth) as they're great for cleaning the water column.

Hope some of this helps.

 

[Mmathis]

Having a mechanical skimmer (either a skimmer or using a net) and/or periodically netting stuff off the bottom are about the only ways to keep gunk off the bottom (or an intake bay as @bagsmom brought up). Although it still seems like work, skimming isn’t that much of a chore — think of it more like routine maintenance rather than “something that has to be done.” And you don’t need to get every little piece of stuff off the bottom. I will have to look up the Ghost Gum tree. I guess how much of a problem it will cause will have a lot to do with the consistency of the leaves and whether they might have harmful or toxic properties.

 

[poconojoe]

That's a mouthful! I'll admit, I too had to start skimming through the post.

Maybe you're overthinking it.

All I can do is give you a description of what I have...

My pond/bog is pretty much maintenance free. It's about 1800 gallons and it's gotten overpopulated due to the fish multiplying. The pond is around 3 feet deep.

My bog's surface area is a slight bit over 30% of the pond's surface area. It keeps my water absolutely crystal clear. No exaggeration, crystal clear. Before the bog, I had two pressure filters and a UV light and my water was always solid green. You could only see the fish when they came up to eat.

Over 30 fish. Four large koi, two huge koi, Shubunkin, various goldfish and some hybrids.

I have a lot of trees.
I do not use a skimmer. I tried that years ago and to me, it was just a hassle. Fish on rare occasions would get caught in it and the pump would get jammed up by debris that got past the sponge.

Do I get some build-up of debris on the bottom? Sure, a bit, sometimes. I scoop it out once in the Spring and once in the Fall. I use a bag style pool net and it's not that difficult or time consuming.
Before the Fall, I erect a frame made out of 3/4 inch PVC. That frame is covered with standard nylon pond netting. It stays up until the Spring.

The point is...if your bog is properly sized and built, it will be all you will need to keep your water crystal clear.

I suggest you read addy's bog showcase and my post on my add-on bog for pointers.

My add-on bog...
https://www.gardenpondforum.com/threads/my-add-on-bog-build.26848/

Addy's bog showcase...

Bog building, also called upflow filter, eco filter, wetland filter

I was asked to start a thread on how to build a bog. This is my rendition on bog building 101............. Building a bog: Before you decide to build a bog, you need to know what you want its primary purpose to be. Total pond filtration bog filtration combined with another filter. If you...

www.gardenpondforum.com

 

[Pharian]

Fabbo, thanks guys. Great advice as always.

Poconjoe's bog is essentially what I am planning, although a smaller version for a smaller pond and using the same piece of liner as the main pond. Basically that design in a raised bed above the main pond, that then runs off down a very tiny waterfall into the main pond.

So from those posts:

- I'll forget the idea of a manifold in the bottom of the pond (phew, that was going to be a logistical nightmare)
- I'll also forget the idea of a barrel of bio balls
- Sounds like I am going to have to suck it up and use a net if the bottom gets too full of gunk. But perhaps the bog will manage to deal with that for me (fingers crossed).
- I'll try and add another foot of depth I think, try and get to 3 foot deep. And another 1-2 foot of width. That would be maxing out though.... unless I get creative with positioning the bog elsewhere... hmm.

The skimmer I was planning to build is basically a home made job out of a big plastic 80 litre (20 gallon) tub (see images). The pump fits happily in the bottom. However, I have been concerned about the whole 1000+ gallons per hour going through the skimmer (that's a lot of flow) so I think I'll put the pump in the pond (off the bottom) and build the wye system with valve taps to regulate flow. One in the skimmer and one from the pond itself.

I guess if I install the skimmer then I can always stop using it, while installing it later will be a serious pain.

I'll look into a float valve, but I have heard conflicting reports of them being made illegal here now due to the endless droughts and water shortages. I'm trying to find more concrete information but not having much luck (which suggests they aren't illegal after all).

I may have overstated the leaves issue. The hole for the pond has been there now for a few weeks and there is only a light smattering of leaves in there. A skimmer would grab a lot of them off the top before they became an issue. I definitely want to get most of them out because gum trees aren't great for fish in large doses. Australian native fish aren't affected much, and the hardy goldfish laughs in the face of gum trees (so go guppies), but still better to have most of the leaves out.

Thanks once again for everyone's time and input. It's enormously valuable and hugely appreciated. Cheers!

 

[GBBUDD]

A skimmer can be anything from your tub design to a lower edge of the pond where water overflows into a second chamber of the liner T The more the ponds bottom is rounded toward a central depression the easier it is for cleaning.
I am a absolute firm believer in over filtration. if the EXPERTS say a 5x8 filter will do the job i make it almost twice the size. yes twice the price but money in the bank down the road.
A pond that is never vacuumed.
a pond that is rarely netted
Is a mature pond , for the first couple YEARS not months you need to be willing to do WHAT EVER the pond requires. You may find flaws in your design that are detrimental like dead spots where stuff gathers all the time . will a small power head do the job? will netting take care of the problem will age resolve the issue ? No two ponds are alike we have seen time and again my neighbor and i have the same pond shape but he has no algae and i have tons of string algae. My guess is more the human interanion that maybe the root cause but that's only speculation. So many people believe i have to feed my fish 3 times a day and what they can consume in 5 minmutes............. Way wrong thinking in any aquarium or pond i have had. Watch tones of you tube videos from the professionals .Be cautious of dyi ideas though at some point every successful design starts as such. i have zero pads on my pond i rely only on baskets to collect heavy solids and the over sized bog to do it's thing so far so good. it did take a good year and a half for the water to truly get crystal clear for algae's to stabilize and not require intervention.
right wrong or indifferent some use sludge detoxifiers to break down sludge build up i do not the only addition i add is a biotope additive. Brian with aquascapes adds sludge remover to his pond so he says and has not cleaned on or vacuumed the pond in ten years.

pictures, drawings and before construction starts is where this community can help the most.

 

[Pharian]

Cheers GBBUDD, much help again. I realise it will be a process (and likely a long one) to get the pond right. I'm rather looking forward to that. But I'd like to at least attempt to head off problems in the construction phase rather than having to start over.

On advice from others above, I watched a ton of videos on negative edges and intake bays rather than skimmers. Unfortunately a negative edge and large underground watershed system will not be possible for me, but I think I'll do an intake bay rather than a skimmer. So my current plan is now to use the big tub as a pump vault and get a small aquablox, then cut a hole in the tub and poke the end of the aquablox into it. Then partition that off with a wall of rocks. I was concerned about the volume of water going into a skimmer, but the intake bay nicely spreads that volume out while still skimming the surface.

So my plan is now just an intake bay pumping up to a big bog filter (30% of the pond), and that's it. Fingers crossed! Next couple of weeks should see it dug out and in progress

 

[bagsmom]

I will look forward to seeing pictures and reading about your project! I am planning an intake bay for my second pond - same set up, with it pumping into a bog. It will just be a smallish intake bay, more for the purpose of collecting debris without a skimmer than for water collection. Mine will be more like the ones The Pond Advisor does, rather than the large scale ones I've seen the Aquascape Team do. Have fun!

 

[Pharian]

Yep those ones The Pond Advisor does are the ones I'm modelling my intake bay off. I don't have the space or depth for those big ones (or the cash now I mention it, ha!)

I'll be sure to post pictures as it progresses. Cheers!

 

[GBBUDD]

Unfortunately a negative edge and large underground watershed system will not be possible

It does not need to have the large storage area and many aqua blocks as the water falls over the negative edge in my set up i have my main pump where the pipe is cut into an aqua block so it is in the center it is burred to the bottom and i have the entire bottom filled with 2 to 4" river rock. This way no matter how clogged things get there should be enough displacement where the water being pulled from an area of 36 x 18 x24 or what ever the actual size was the water can be drawn from all around/

 

[GBBUDD]

And instead of a soft walled tub those blue barrels are a heck of a lot stronger when altered

 

[Dave_Aus]

The position I have to build it is a weird bit of land on top of a stone retaining wall. The wall is not mortared, but it is stable. However, I cannot dig too deep or the pressure from the water will push out and put pressure on the retaining wall. For that reason I am going down a small distance into the ground, but a lot of my pond depth is coming from building embankments (which I will then build a second retaining wall around to make pretty). That way the pond will be sitting primarily "ON TOP" of the ground with weight pushing down, rather than sitting "IN" the ground and weight pushing outwards. It might make more sense looking at the photos attached.

Hi Pharian, I'm new to the forum and also in Aus, but can't add much with respect to bog and pond design.

However, i read your paragraph about building on top of the land rather than digging in, to reduce pressure on the wall. Unfortunately, your thinking is exactly opposite to what is probably going to happen. I'm not a geotechnical engineer (so this is not professional engineering advice) but the weight from a load spreads down and "out" - how much "out" depends on the soil's properties.

What that means, is that for the same load, I believe the higher you place it, the more there will be load on the wall. If fact you'll have the weight of the ground, the pond and the berm (embankments) holding up the sides of the pond all putting additional pressure on the wall.

I suggest you consult a local consulting engineer.

For Australian fish species and pond design, Ozponds (website and Youtube) are pretty good IMO.

Good luck and I look forward to seeing pictures of what I'm sure will be a magnificent pond. Your site looks beautiful.

 

[Pharian]

Hi Dave,

I get what you're saying, and for a static solid weight like a rock you are entirely correct.

However, if I remember my old physics classes right, with a liquid you have both the weight you are speaking of (which goes down and out at an angle dependent on the density of the ground it is sitting on), and you also have a horizontal force equal to the height of the column of water X surface area / volume (or something like that... it's been a while). It is that horizontal outward pressure that was worrying.

Think of a dam. A dam must hold back all the weight of the water behind the dam. The downward force of the weight of the water is pushing on the ground beneath the dam (like a static weight), but the entire weight of the column of water (since it is liquid) is also pushing on the edges of the container, pushing on the weakest point, which is usually the dam itself.

Similarly, if you bury a pond then that horizontal liquid pressure is put on the surrounding ground. That normally wouldn't matter, since it's just ground and can take whatever pressure or weight no problem. But with a retaining wall made of unconcreted stacked up bush rock about 25cm away from the toe of the pond I was concerned. It would be pushing outward through the narrow bit of ground, pushing against the retaining wall like a dam. The lining would mitigate the force a little, but not much.

With embankments then the liquid pressure is on those embankments. If the embankments fail then my yard gets wet, no big deal. But if the retaining wall is pushed outwards and collapses then that is a major problem.

Realistically it was unlikely for anything to happen, it's only a relatively small pond. But better not to risk it.

Plus, the ground in that spot is clay mixed with a gazillion tons of ancient building rubble. It's best to leave it be if possible.

Regardless, the pond is built now and so far no problems (touchwood). Build thread is here.
https://www.gardenpondforum.com/threads/new-pond-construction.28493/

Cheers!