Using graduated gravel sizes (large to small) in your bog. I think it doesn't matter.


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I'm just about finished building my Aquascape style bog filter—snorkel/centipede design with water matrix settling chamber and 3 different sizes of gravel.

I do think that the snorkel/centipede and settling chamber aspects of the design make it superior to the basic slotted pvc + pea gravel design because it allows backflushing that would be impossible otherwise.

But after finding this in some Aquascape literature from way back when (excerpt from The Pond Builder's Bible), I think that using graduated gravel sizes may be unnecessary:

Screen Shot 2021-10-21 at 5.27.20 AM.png


The whole idea behind the graduated gravel sizes and putting large cobble at the bottom and small gravel at the top is to allow the water start out moving slow at the bottom of the filter and speed up as it rises due to the reduction in void space as gravel size gets smaller. Theoretically, it should also make backflushing more effective because the increasingly smaller gavel should trap increasingly smaller bits of sediment that can then be backflushed into a larger void space below it, allowing it to fall down into the centipede and be pumped out.

But looking at the chart above and the explanation that goes with it, the water storage volume of each type of gravel is very consistent across 3 of the 4 types measured. That would indicate that water velocity (and space between rocks) would remain basically the same all the way up the water column.

This defies common sense—bigger rocks should have bigger spaces between them. That's obviously still true, but perhaps it doesn't matter in this case because the sizes are not different enough and the total volume of gravel used is not great enough for a real difference in water storage volume to play out. Maybe if you built and enormous filter and used bigger rocks on the bottom and pea gravel on the top you'd get closer to the expected results.

All that to say: If you're building an Aquascape style filter, you may be able to simplify the design by using a single size of gravel (maybe 1 - 2") without losing any functionality.
 
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I believe there's a great value when it comes to backflushing; imagine trying to 'unclog' the bottom-most surface of 36" of pea gravel FROM THE TOP of the bog vs only 12". I doubt you'll get enough pressure and flow from the backflushing pump to break it free. And of course, the larger rocks below give the 'backflushed' crud someplace to go where it ends up at the bottom of a sloped floor that ends at the cleanout stack.

The multiple size rock in layers bog is what I have. Too, quite different from most bogs here (except maybe GB's) is I use 4" manifolds, not 2" pvc, so my water is slowing down once it hits the bottom of the bog. And, need it be said, the price of the aquablox system is, imo, obscenely high. I may not have a full 12" of unadulterated open space at my bog bottom, but I also don't think it necessary. Nice, yes, necessary, no.
 

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I may not have a full 12" of unadulterated open space at my bog bottom, but I also don't think it necessary. Nice, yes, necessary, no.
happy with the necessary ..............no...............since I have only pea gravel down to the bottom of the bog.
 
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I doubt you'll get enough pressure and flow from the backflushing pump to break it free. And of course, the larger rocks below give the 'backflushed' crud someplace to go

The argument I am making is that this may not be true. If water storage capacity in the different layers of rock are essentially the same, then the void space for crud to get flushed through in each layer is essentially the same as well. So, it may be no different trying to flush crud through 3' of 1/2–3/4" as it is to flush it through 3' of 4–6" rock.

Again, this seems to defy common sense, but that is what the numbers tell me. Perhaps I misinterpret the numbers.
 
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The argument I am making is that this may not be true. If water storage capacity in the different layers of rock are essentially the same, then the void space for crud to get flushed through in each layer is essentially the same as well. So, it may be no different trying to flush crud through 3' of 1/2–3/4" as it is to flush it through 3' of 4–6" rock.

Again, this seems to defy common sense, but that is what the numbers tell me. Perhaps I misinterpret the numbers.
I don't believe that, CW; just logic makes it wrong. Can you see taking the outlet of your pond pump and thrusting it into the pea gravel AT THE TOP, believing that such pressure and flow there can dislodge crud 36" down? The clogging CAN be inside the pea gravel column but the surface/plane you're trying to break is at the BOTTOM of the pea gravel layer. Once that gets filled/clogged, the bog water rising will channel to the sides, against the liner, where there IS no crud because it's not going to grab the liner as it would interstitial voids between stone. I've had this channeling--that was my clue as to why I was losing water (my bog v1 sides were too short and the water was going over, albeit barely).

So pushing water through 36" vs 12" would logically dictate you need MORE pressure and flow from the top. I doubt any of us have such a pump that could do that. Just trying to push through 12" takes some time to completely get the particles from within the pea gravel layer down into the more open spaces of the large stone, and then to the cleanout. The first time I tried, I mistakenly thought I could use house hose pressure; ah, no, not even close. Was frustratingly bad. I ended up digging up the whole thing and washing it by hand. I then contacted the pond professional upon whose advice I was acting and was told I needed the pressure from the pond pump, that I needed more pressure AND flow.

Not to disparage Aquascape, but I don't believe their info in this regard; I look toward data from people/places that have no profit in them.
 
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So pushing water through 36" vs 12" would logically dictate you need MORE pressure and flow from the top.
My argument doesn't have anything to do with that. Totally agree that the deeper your gravel, the more pressure and flow you need.


Just trying to push through 12" takes some time to completely get the particles from within the pea gravel layer down into the more open spaces of the large stone
Look at the chart. Pea gravel compacts much more densely than the other 3 rock sizes. So yes, no disagreement. But of the other three rock sizes, the "more open spaces of the larger stone" is essentially the same amount of open space!

Not to disparage Aquascape, but I don't believe their info in this regard
In what regard?

I look toward data from people/places that have no profit in them.
That is aquascape's own data. Aquascape published The Pond Builder's Bible. And it seems to conflict with what they recommend—graduated gravel size.

In any case, I don't think we're focused on the same thing here? You seem to be making one argument and I am making another because what I posted doesn't disagree with anything you said!

Now off to wash some more f***ing rocks. Day 6.
 
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My argument doesn't have anything to do with that. Totally agree that the deeper your gravel, the more pressure and flow you need.



Look at the chart. Pea gravel compacts much more densely than the other 3 rock sizes. So yes, no disagreement. But of the other three rock sizes, the "more open spaces of the larger stone" is essentially the same amount of open space!


In what regard?


That is aquascape's own data. Aquascape published The Pond Builder's Bible. And it seems to conflict with what they recommend—graduated gravel size.

In any case, I don't think we're focused on the same thing here? You seem to be making one argument and I am making another because what I posted doesn't disagree with anything you said!

Now off to wash some more f***ing rocks. Day 6.
Let me see if I'm following you; there's the SAME amount of open space between larger rocks in a layer (of X size depth) and pea gravel of the same size depth? Ah, I don't think so; just the image of 8" rocks surrounding another 8" rock would give you a decent amount of open space for a bunch load of pea gravel.

Now, if you're saying water flows similarly up through same size depths of different size stone, then I can agree, until the smaller size gets clogged and inhibits this flow, which it would take a longer amount of time (even if possible as now you'd need LARGER size debris/bioload to even clog the space) to make happen.

Btw, pea gravel doesn't compact at all, not by definition--it's why it's recommended (round stone of any size) because it won't compact. Now, if you mean it fills a volume better, sure, and the interstices are smaller than those provided by larger round stone.

Re Aquascape; if they're contending that pea gravel of X depth is similar to cobblestone (2" for instance) in regards to water flow, that's where I disagree. I'd wager water flows easier and faster (with constant pressure behind it) faster than the pea gravel layer.

Okay; just looked at the chart again and if your point is 'there's no difference using 8" or 2", then I can see it their way. I thought this was more about pea gravel (only, in totality in the bog) and using graduated stone of various sizes.

I think using the larger stone below (8") if more for settling, ala the aquablox, which you'll have to admit is far cheaper and easier to set up and stronger to boot re holding up the layers above. And, given a choice of which size layer I want to backflush through, you'll be hard pressed to convince me debris/bioload won't flush down easier through 8" stone than 2". Be an interesting experiment, I guess.

Didn't mean to cause any trouble, CW.
 
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But after finding this in some Aquascape literature from way back when (excerpt from The Pond Builder's Bible), I think that using graduated gravel sizes may be unnecessary:
I have not read the book but i would strongly disagree. Larger stone allows for larger stronger flow to evenly disburse across the bog so you use every square inch of pea stone and other layers for colonization of bacteria and helps to evenly spread out any sediment to the tighter layers above
 
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I do think @brokensword is missing the part where you said "no including the pea gravel" and is focusing on exactly that - the pea gravel. Correct me if I'm wrong @brokensword . It's the remaining three sizes that seem to be identical in the volume of water they hold.

Having said that, I will also add that we have yet to have a need to backflush our bog, almost ten years in. So this may all be a moot point.
 
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If I may, the results regarding _storage volume_ don't seem that counterintuitive once you think more on them, if you consider that a container of large rocks has relatively fewer spaces for water than a container of small rocks, so even if the individual spaces between the rocks are bigger, there's fewer of them.

However, there's a substantive difference between storage volume and flow through rate, no? Those are like apples and oranges. That graphic says nothing about the flow rate through the different mediums.
 
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Ah, I don't think so
Fully agree
if you're saying water flows similarly up through same size depths of different size stone
Have to disagree on that one point being go to the gravel yard after a rain and pick up some 2 -4" rock 12 hours later and watch how it is almost dry. now go pick up some 3/8 pea stone and it will be dripping with water That's why we use it it slows down the flow allowing bacteria and plants a chance to suck on and out the nutrients in the water column thus the benefit of the bog

Btw, pea gravel doesn't compact at all, not by definition--it's why it's recommended (round stone of any size) because it won't compact. Now, if you mean it fills a volume better, sure, and the interstices are smaller than those provided by larger round stone.
It's my understanding that yes it has a lot to do with not compacting but even more so it will not allow sediment to stick and get locked in uneven jogs and notches in shards of chip rock. river rock/ pea stone is smooth and allows sediment to get pushed up and out as an area does start to clog and pressure builds. another reason for the layers of different sized rocks it spreads out these pressures evenly

I think using the larger stone below (8") if more for settling, ala the aquablox, which you'll have to admit is far cheaper and easier to set up and stronger to boot re holding up the layers above. And, given a choice of which size layer I want to backflush through, you'll be hard pressed to convince me debris/bioload won't flush down easier through 8" stone than 2". Be an interesting experiment, I guess.
of course it will flush much easier through larger rock as the gaps are far larger and there is far less as many layers

I hope there's no ill will taken or meant it's a discussion this field in particular is hardly this is the way you MUST do it there are so many variables to consider so many different water supplies and dust in the air here is loaded with pollens / organics while someone in the south west has sand and someone in the rust belt probably has top soils that contain fertilizers. so while the practices are the same variations are beneficial in one area that are probably not needed else where. kinda like a skimmer in the desert. there may be no use for one while someone in the forest who has tons of leaves may need two skimmers.
 
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owever, there's a substantive difference between storage volume and flow through rate, no? Those are like apples and oranges. That graphic says nothing about the flow rate through the different mediums.
Its all about the flow and having to large a space for sediment to clog the space until your at the higher levels
 
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Let me see if I'm following you; there's the SAME amount of open space between larger rocks in a layer (of X size depth) and pea gravel of the same size depth?
NO! Look at the damn chart! lol :)

Didn't mean to cause any trouble, CW.
Don't worry about it. Just a small fee is in order. We can add it to your November dues.

so even if the individual spaces between the rocks are bigger, there's fewer of them.
This makes sense.

However, there's a substantive difference between storage volume and flow through rate, no?
I don't know, actually. Is there? You said flow through rate, but maybe it's more about velocity. I'm trying to compare it my mind to using 1 big pipe for your plumbing vs several small pipes. Like using a single 3" pipe vs nine 1" pipes. It would take about 9 1" pipes to equal the open area of a 3" pipe.

In this case, flow is the same. Whatever the pump is pushing, that's the flow rate. But what about velocity? Is velocity faster in the nine 1" pipes? It sure would be if there were only one of them. But since we have the same amount of area, water is distributed across all of them, reducing the flow of each pipe, and velocity evens out.

So no, I guess it's not really velocity, either. But nine 1" pipes has 3x the circumference of a single 3" pipe, so there should be more friction in the system of 1" pipes, which would reduce flow.

Hell, I don't know. This is what happens when it starts raining and I go inside to think about what I'm doing rather than just doing it. Hopefully I'm not totally off base and leading folks astray. I was mostly interested in what seemed like an inconsistency between Aquascape's own research and what they specify for their wetland filters.

@GBBUDD's point about catching different sized particles in the different sized voids of larger vs. smaller rocks makes sense. A barrel of cobble stones will not polish water the same way a barrel of sand will, for instance. And a barrel of sand will clog much faster than the cobble since it's trapping more. Aligns with the idea of efficient filtration by using progressively finer media. I guess even if total void space is the same, the size of junk being trapped is different because the size of the voids is different.

Maybe that's it. I've had too much wine tonight.
 
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The argument I am making is that this may not be true. If water storage capacity in the different layers of rock are essentially the same, then the void space for crud to get flushed through in each layer is essentially the same as well. So, it may be no different trying to flush crud through 3' of 1/2–3/4" as it is to flush it through 3' of 4–6" rock.

Again, this seems to defy common sense, but that is what the numbers tell me. Perhaps I misinterpret the numbers.
Hi. The chart makes sense if you are just concerned about the water volume in a container of a certain size rock.… but Just wondering if you thought about water flow and distributIon. Does the larger rock help distribute the water more evenly across your bog than smaller rock would? I would think the more compacted the rock the more resistance there would be and the water current would stay localized and not be evenly distributed across the bog.
 
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I don't know, actually. Is there? You said flow through rate, but maybe it's more about velocity. I'm trying to compare it my mind to using 1 big pipe for your plumbing vs several small pipes. Like using a single 3" pipe vs nine 1" pipes. It would take about 9 1" pipes to equal the open area of a 3" pipe.

In this case, flow is the same. Whatever the pump is pushing, that's the flow rate. But what about velocity? Is velocity faster in the nine 1" pipes? It sure would be if there were only one of them. But since we have the same amount of area, water is distributed across all of them, reducing the flow of each pipe, and velocity evens out.



@GBBUDD's point about catching different sized particles in the different sized voids of larger vs. smaller rocks makes sense. A barrel of cobble stones will not polish water the same way a barrel of sand will, for instance. And a barrel of sand will clog much faster than the cobble since it's trapping more. Aligns with the idea of efficient filtration by using progressively finer media. I guess even if total void space is the same, the size of junk being trapped is different because the size of the voids is different.

Maybe that's it. I've had too much wine tonight.
Ever see a pond pump's output flow decrease as the input grid/filter casing gets clogged? This is what happens at the bog end too, but the clogging is on the other side. That is, as your bog clogs, no matter the amount, there's back pressure put on the pump, right? So, you won't even get full flow/efficiency from it. Too, the pump doesn't slow down--that water/pressure has to go somewhere, and it's backwards.

So, consider our modern bog filters; there's large spaces between the large rocks but no way you're pushing anything large into the bog so that's a moot point. I'd say you won't get much large-ness of debris that would clog the spaces between 2" cobble, either. It's only at the pea gravel layer where you'll begin to see problems, imo.

As I said before; large rocks on the bottom are equivalent to aquablox in that they provide settling area and slowing of the flow upward. The middle layer evens out the flow toward the pea gravel. And it's the pea gravel with much more SSA that is doing the yeoman's share of the job. It isn't necessarily the pea gravel column that you worry about, it's the plane at the bottom; if that clogs, then you have a less efficient/non-functioning filter as the water is going to force it's way up the sides and avoid most if not all of the pea gravel.

I do think Criscar has it right; you're looking at a chart that shows static water storage by volume. That is, the chart shows you how much water you have IN your bog, probably more for dosing issues than anything else as you'd need to calculate the total gallonage if you start adding chems. I DO believe when talking re filtration, that flow through is definitely the important point and I think logic still tells you that forcing water up through more open areas (even if less than in number than the layer above) is easier than through smaller openings (the pea gravel area).

So for my money, the chart is only to let me know that I have more water storage at the bottom and middle of my bog than at the top portion and that my type bog holds more water than someone with a pea gravel only bog. There's nothing that convinces me, from the chart, that if I'd put all 2" stone in, that I should have eliminated the large ones at the bottom. Water storage is about the same between them but I know flow upwards is aided by larger open spaces below the decreasing spaces above. Sort of like how they size ductwork in that you start out large area/volume and work it to smaller ones at run's endpoint(s). There's a pull associated there and I think is mimicked with the water filtration system. I may be off in that as I can't cite any sources, but I'll lean toward that theory as on the surface, it's logically the same.
 

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