Not that it matters but I think its 817 ft per minute. Using the 2" pipe would drop flow down a lot so it'd probably be more like 600 ft per minute.

Not sure if you're planning to send the entire flow to the bottom or divert some from falls. Being underwater blunts flow pretty fast so you could drill larger holes if you wanted.

There's no real way to predict flow thru the holes because many factors would have to be known first. Way more than a rational person would want to deal with. I think your approach works best, just go by total area. But I would use a 3" or 4" diameter for total area. Unless there is some reason to restrict the amount of water being pumped.

You didn't ask but...

Putting the 8200 on the skimmer is the max the skimmer can take...when completely clean. As leaves pile up the amount of water the pump is moving will drop while you'd still be paying a lot for electric. Putting a large pump on a skimmer doesn't really improve skimmer very much skimmer wise. It just causes the weir to be pulled down more which pulls in a lot of subsurface water which doesn't do much. You really want as thin a sheet of water falling over the weir as possible so surface tension pulls the surface water, and stuff floating on the surface. Skimmers work based on surface tension, not raw flow power.

Putting a smaller low head pump on the skimmer would save like $500 on the pump and maybe around $250 per year on electric and I think you'd get pretty much the same performance. With the saved money you could add another skimmer if needed.

I thank you for your magnificent reply and for catching my mathematical error... it is 817 feet/minute. Let me provide a few more details. The total pond volume is on the order of 4,000 cubic feet. The skimmer can handle 8,000 gph (max) and the 8,200 GPH pump is right at the limit. All filtration will be from plants and freshwater clams in a biofiltration basin, which is 25% of the total pond volume. The high pump flow rate is not for water turnover (i.e. filtration), it is about getting decent flow over my numerous waterfalls. The Upper Waterfall Basin has an 18" one (dropping 3 feet to the Main Pool) and a 36" one (dropping 12" to a Mid-Level Waterfall Basin). Water exits the Mid-Level Basin via two 18" waterfalls -- one dumping 2 feet to the Main Pool and another dropping 12" to the Biofilter Waterfall Basin. The water exits the Biofilter Basin via a single 18" waterfall, dropping 12" to the Main Pool. This series of waterfalls will create a circular motion in the Main Pool and direct water into the skimmer. So as you can see, the flow over each waterfall is about 150 GPH per inch of width (i.e. 8,000 GPH / 54" total width). The intake and discharge openings on the pump are 2" ID, so that's why I said 2" pipe, but I can split that flow into multiple 3" or 4" pipes once I get the water to the Upper Basin and if I quadruple the cross-sectional area, the exit velocity should be cut by a factor of 4x, right? OR, I could run the 2" pipe from the pump into the bottom of a plastic 55 gal drum (redneck waterfall) and let that cascade into the Upper Basin. In the latter scenario, the pipe would expand from 2" to about 24", a huge increase in area and a consequent decrease in exit velocity. Were you suggesting maybe I should pull half my pump's intake water directly from the pond?

The intake and discharge openings on the pump are 2" ID, so that's why I said 2" pipe
It's not intuitive but the size of a pump's discharge isn't related to the size of the pipe that should be connected. It may sound strange but the compartment holding the pump's prop has to have kind of high pressure otherwise the turbulence would pull water back into the pump from the exit pipe. So this pump having a 2" output is almost like a check valve, with 800 ft/min exiting it would be hard for water to move in the wrong direction.

However, once the water is thru that 2" opening you're free to do what you want. For example, say you didn't have any pipe connected. The water would come shooting out of the pump like a cannon and the pump would run just fine. So generally we screw an adapter into the pump output so a 3" or 4" pipe can then be used.

You've already found the velocity calculators, kudos. Determining what size pipe you use a "pipe friction calculator". Using that you'll see the pressure in a 2" pipe is around 2 psi (I think) and head loss is like 5' for a 20' length of pipe (not counting fittings which can really make things worst). With the head loss you can check your pump to see how much flow is reduced by that amount of head loss. Depending on the pump, for 5' that could drop from 8000 GPH to 7000 or 6000.

You don't really have to go that far. I just look at the pipe size where friction drops a lot. For example moving from 2" to 3" pipe the head loss drops from 5' to like 8". Checking 4" pipe the head loss drops to like 6", not very different from 3" so I figure 3" is pretty good. If the pipe run is really long or there will be a lot of fittings maybe 4" is worthwhile.

but I can split that flow into multiple 3" or 4" pipes once I get the water to the Upper Basin and if I quadruple the cross-sectional area, the exit velocity should be cut by a factor of 4x, right?

Yup. For 8000 GPH:
2" pipe = 3.1 sq in opening = 817 ft/min
3" pipe = 7.1 sq in opening = 363 ft/min
4" pipe = 12.6 sq in opening = 204 ft/min

However, the length of the 3" or 4" pipe and its direction would matter. A 10" length for example, even if pointing upward, would probably not be enough to drop the velocity very much. It would still be a geyser. This normally isn't a problem because we bump up the pipe diameter at the pump.

OR, I could run the 2" pipe from the pump into the bottom of a plastic 55 gal drum (redneck waterfall) and let that cascade into the Upper Basin. In the latter scenario, the pipe would expand from 2" to about 24", a huge increase in area and a consequent decrease in exit velocity.
Yup.

Were you suggesting maybe I should pull half my pump's intake water directly from the pond?
Something like that. This is separate from the pipe sizing just to be clear.

Your skimmer only needs 1500 GPH to work. Even then, if the basket gets jammed with leaves water flow thru the pump can decrease while you still pay just as much for electric. The bigger the pump the fewer the leaves needed to reduce flow.

So yes, 2 pipes going to the pump. One from the skimmer, one from some place in the pond, generally what's called a mid pickup, meaning somewhere on the side. Too low it pulls in crap. Too high and a vortex can be created which sucks air into the pump which can be very loud and isn't good for the pump. You should put a ball valve on the mid pickup in case you need to tweak flow.

The high pump flow rate is not for water turnover (i.e. filtration), it is about getting decent flow over my numerous waterfalls
I see. Couple of things you might consider:

You could put a 1500-2500 GPH pump on the skimmer and a separate pump to boost the falls flow to what you want. This can actually cost less electric for the same amount of flow (but generally not huge). It makes plumbing easier. And for example the skimmer pump has to push water kind of a long way probably where the booster pump could be placed right next to the falls. Less plumbing cost, less friction loss.

This also gives you more control over your falls. There may be times when you'd prefer 2000 GPH over the falls rather than 8000. People (men type people mostly) often want Niagara Falls and when first fired up they have a huge grin on their face. Later, after having a cook out they realize people can't hear each other talk that maybe Niagara Falls isn't so hot.

Plus there is no reason to run a falls 24/7. Falls are for looks, they don't benefit the pond very much compared to other options. Only turning on the falls when you want to hear/see them means you're not likely to have a bunch of string algae covering the falls. The electric savings could be like $5,000 over 10 years.

The other thing to consider is a low head pump. The HEX8200 is a high head pump. This means the prop is designed to push water high (high pressure / high head). But you don't (or shouldn't if pipe sizes are right) have any need for high pressure / high head. So you can choose a low head pump which will move more water for less electric.

Last thing...biofalls aren't very good at bio filtering compared to other options like Trickle Towers, Moving Bed and Bakki Showers. Plus biofalls are expensive and a pain to clean imo. I'm not sure why you would even need a biofalls at all from what I've read in your posts. If this is just some web site or clerk telling you all ponds need a bio filter that is completely false.

I cannot thank you enough for your time an invaluable advice on my project. I need to get this watergarden up and running so I can get my two goldfish out of their 30-gallon trashcan into their palatial home before Fall arrives. I have uploaded a 2D diagram of my pond layout, which I have framed out with 6"x6" PT timbers and hardened, 10" steel screws from FastenMaster. It seems like every time I provide more details, you give me a clearer understanding of how I need to proceed. I might point out that my experience level is ZERO, but that's exactly how much experience I had building before I started construction of my house in 2004 (yeah, it's still in progress; working full time and building a house mostly by yourself is a slow process).

Let's start at the beginning. The pond has a surface area of 12'x16', but is divided into 5 discrete "boxes", each lined with 45 mil Firestone PondGard and underlaid with scrap carpet and padding. The objectives are: (1) Provide natural habitat for goldfish and orfe (not koi), small frogs, native crayfish; (2) Provide an area for the adults and an English Springer Spaniel to cool off in Summer; (3) Provide entertainment for our 5 cats; (4) Provide visual appeal for the adjacent 16'x16' patio area. I live in Southcentral PA where ponds freeze over every Winter. Consequently, each "pool" section is exactly 38.5" deep. Because of the frogs/crayfish, I was advised not to include a bottom drain, as I had originally intended, because they will hide in it and get sucked into the pump. The pond is not near any vegetation and high leaf loading is not anticipated. The Helix skimmer is a interesting concept and it is mainly there to give me a minimally intrusive way to get water to the external pump and provide some filtration for the pump. Waterfalls are designed to provide aesthetic appeal, some oxygen, and to create a circular motion in the Main Pool to direct floating debris into the skimmer. The Helix skimmer can house a submersible pump beneath a single blue matala pad, but it is my understanding that external pumps in flooded suction applications are more energy efficient and last longer being they are not constantly submersed in water. I intend to suck water out of a 2" bulkhead fitting at the base of the skimmer through an elbow, a 2" check valve, another elbow, and into the pump. Based on your input, I will use a 2" to 3" adapter on the discharge side of the pump. The 3" pipe will proceed into two elbows and through about 15' of pipe underneath the Upper Waterfal Basin and into one or more plastic drums filled with possibly lava rock and/or a series of matala pads to enhance water quality. The water will then cascade into the Upper Basin like a waterfall (not shown in diagram). By running the 3" pipe under the Upper Basin, I can keep the water in it protected from freezing. The vertical rise from pump intake to water level in Upper Basin is prolly about 4'. At most, I will use 6, 90°elbows and 15' of pipe. The "plant biofilter" will be "all natural" and consist of floating islands and natural rock shelves holding aquatic plants, mainly water hyacinth, yellow flag iris, and submerged oxygenating plants. In addition, there will be snails and freshwater clams from William Tricker, Inc. The drums I mentioned are only to be used to cut the discharge velocity to

Thanks for the diagram, helps a lot and makes things way more clear. Earlier you posted the pond would be 4000 cubic feet which is about 30,000 gal and the 8000 GPH pump seem to fit that so I didn't think any more about it, and why I suggested considering more than one skimmer. The diagram says 4500 gal so I'm going with that from here on.

I might point out that my experience level is ZERO,...
Still you have more going for you than many. Aren't afraid of math, open minded and not defensive. Being able to consider options is a big asset imo.

..but that's exactly how much experience I had building before I started construction of my house in 2004 (yeah, it's still in progress; working full time and building a house mostly by yourself is a slow process).
Sounds like me too. Learning is fun. "For the things we have to learn before we can do them, we learn by doing them" -Aristotle Smart dude imo.

Posting your objectives really helps too. Step 1 imo is figuring out what kind of pond you want. Without that a person can't really decide what pond info applies to them. For example if you posted in a Koi forum they'd tell you the plants are really bad. If you posted in a Wildlife Pond forum they'd say add more plants. Both are correct, but for completely different ponds. Confuses and frustrates many people.

I think you're wanting more of a Wildlife Pond than a Water Garden. But I'll double check...you really want frogs, crayfish, snails and calms? If you're wanting the crayfish, snails and calms solely to clean the water I think you're making a mistake. Yes, these critters do eat waste but they also produce waste. The net benefit really isn't very good. If you want these critters just because you think they're cool and fun then super, I'm with you. The calms especially are not easy to grow and really need a lot of food suspended in the water column or they starve. So you kind of have a cross purposes working against you. I think you'd have to decide whether you want the crayfish and clams to survive or you wanted clear water. Both is tough in a small body of water. Lots of info on this from aquarists.

So if you want clams just for their cleaning ability I'd pass on them. Not having the substrate they require also makes cleaning easier.

(2) Provide an area for the adults and an English Springer Spaniel to cool off in Summer;
I've had problems with dogs in the past digging thru EPDM liner. I'm not seeing why this would be a problem for your pond but just FYI I mortar over the liner mainly because I like the look but it also protects the liner and makes cleaning a tiny bit easier.

I live in Southcentral PA where ponds freeze over every Winter. Consequently, each "pool" section is exactly 38.5" deep.
The first pond I ever had was a Wildlife Pond in upstate NY. A few years it froze ground to better than 4' (I was in construction and had to dig footings so am sure the ground was frozen to 4').

38" is probably enough below grade...but if you go 48" or 60" below grade the risk to fish and critters goes way down.

Aside from freezing solid there are 2 basic things we should worry about. Gas exchange and being too cold. Both will kill fish. Gas exchange just means O2 moving from air into water and toxic gases produced in the pond like CO2 and others moving from the water to the air. Completely ice covered greatly reduce gas exchange. So O2 decreases, CO2 and other harmful gases increase.

Lots of people in forums do things to keep an opening in ice. Problem solved, maybe, maybe not. Lots of people don't seem to know, or care about, that an ice covering also protects fish by keeping water warmer. Warm water rises right? Not exactly. Water is its heaviest at 39F. When ice covers the water becomes very still because wind is blocked. That allows the water to stratify. 39F at the bottom and temp decreases to 32F as it gets closer to the ice. Running a blubber or pump to keep ice open mixes the water so the entire pond becomes colder.

There's no way to predict which would kill fish faster, lack of gas exchange or cold water. Fish can certain live in sub 39F water, but the longer they have to the more they're stressed. Just like you can survive outside in winter for some period of time, but eventually you're probably going to croak. Same thing with gas exchange, fish can live for some period of time under the ice.

You can reduce risk by going deeper. That gives water a better chance to stratify and more thermal gain from the ground. It also leaves more unfrozen water so gas problems are reduced. If you clean the pond in fall, remove the plants, it reduces gas problems (reduces load) greatly too.

So to me a little bit more digging, a little bit more $$$ for liner, reduces risk and can eliminate the need to even consider blubbers and heaters to try and keep ice open which is a pain imo. I like simple.

Because of the frogs/crayfish, I was advised not to include a bottom drain, as I had originally intended, because they will hide in it and get sucked into the pump.
A properly designed bottom drain system imo is what's called gravity feed, meaning the pond overflows thru the drain pipe into a sieve filter. So the critters wouldn't be sucked into the pump. But a ride into a sieve is no picnic either. There are ways to create skimmer type basins to catch frogs and crayfish and keep them safe. When I make multi-pool ponds I place such a basket between pools, where a pool overflows into the next pool. That way I can keep fish in separate pools and none are harmed.

However, I see no reason to add a bottom drain system to your design. Especially if you want all those critters. Removing the waste via a BD would just starve the critters. Or even if you want a clean pond I prefer a vacuum system for your type of pond. BD systems are more for when needed to keep fish healthy when there's a high fish load. If just for reducing maintenance I'm not so sure it's a big win.

I'll write more later.

Where are you in this project? Drawing or digging?

I have constructed all of the "boxes", so the depth is fixed at about 36 inches, not feet (oops). I couldn't go any deeper, because the site actually sits atop my incoming underground electrical main and outgoing sewer main, the depth of which is not far below. I was tired of mowing the steep bank down to my North patio and so I dug it out for this pond, but had to interlock the wooden "boxes" to keep it from moving anywhere once it is full of 4,500 gallons of water. The two goldfish I mentioned were won at a county fair about 5 years ago and have been living in a 30 gallon trash can sunk into the backyard ever since... no filtration, no feeding, no water circulation, so they are quite hardy, despite having 4 inches of ice capping their abode every Winter. I am at the "late Summer, Fall is coming fast" phase in which need to buy the liner, pump, skimmer; get that all installed, haul a truckload of rock for subsurface shelfs and auatic structure, and get the pond filled with water... so i can get those twin, 6" goldfish moved before Winter hits again. I call this project my watergarden. It's definitely not going to be laden with fish, but I want to have as balanced of an ecosystem as possible where the water is somewhat clearer than a native pond, but not crystal clear. In other words, I'd like to be able to see the critters in the water down to at least 2 feet, if not the bottom, as opposed to some murky pool, hosting the Loch Ness Monster. I'm going to modify my mental vision of the goal as things progress, ease into this slowly with regards to aquatic life until I get a handle on the water quality/clarity. Just the two fish and the bareroot Irises (salvaged from the banks of the Juniata River) this year. The clams, crayfish, and snails are all optional for biodiversity. The frogs will come from a local farm pond. I used to lay in bed and listen to my Pap's frogs croak all Summer long and that's why I want them. No bullforgs, though, they're appetites are too voracious. Incidentally, I've got a M.S. in Chemistry from NM Tech (Socorro) and I'm very open-minded, particulary when it comes to listening to advice. That 4 foot square box near the skimmer is where my skimmer and pump are going. Also happens to be where my duplex electrical outlet and electrical meter are. The top of the Upper Basin is exactly 38.5 inches about the top of the Main Pool, just to put things in perspective. It's kind of like a square spiral staircase, descending in a clockwise fashion from the Upper Basin to the Main Pool, and then getting withdrawn via the skimmer and pumped back to the top again. I'm very interested in any input you have on the plumbing/pump(s). I was thinking of running the water line from the pump under the Upper Basin to a 55-gallon drum on the far right side of the Upper Basin to protect it from frost (that'll mean >12 feet of horizontal pipe) or I could Run it straight up the sidewall between the Skimmer "Box" and the Upper Basin and maybe build an insulated box around it? I guess I should mention that the left side in the diagram is completely exposed to the air (i.e. no soil to insulate it, although I will install 2 inches of rigid insulation under my liner to cut heat losss) and the right side is cut 6 feet into the soil bank. I was going to just let the pond surface freeze each Winter, figuring I will be OK if I keep the fish load and decomposition to a minimium. This picture I uploaded shows you where I started a year ago. Note the patio pavers on the left, the location of the phone junction/electric meter, and the soil line on the wall. I'll try to upload a few pictures of the current state. Thank you so much for helping me!!! Life can become overbearing at times trying to figure out everything by yourself -- very satisfying when finished, but quite a burden in the interim.

Waterfalls are designed to provide aesthetic appeal, some oxygen, and to create a circular motion in the Main Pool to direct floating debris into the skimmer.
Falls don't add much O2. Directing pump output directly into the pond creating a current moves more water to the surface where gas exchange happen. Falls only improve gas exchange in the sense that you need moving water to create a fall, it isn't the falls directly.

That's just kind of nitpicking in your case because you sure shouldn't have any O2 issues at all. It's only in high fish load ponds that a keeper would rather use a tangential pond return to create a stronger current than have a falls. But a person can improve total gas exchange and still have falls for the same costs. I call this a waterfall catch basin.
{{gwi:203722}}
Installed:
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The idea is to direct all the water from the falls to the pond bottom which pushes low O2 water at the bottom to the surface where gas exchange happens.

It also calms the surface of the water making fish view better. It also catches foam that can be produced by falls that float around the surface. You still get the look and sound.

Falls don't really direct stuff to the skimmer (although many web sites say different). Skimmers work because of surface tension.

The Helix skimmer can house a submersible pump beneath a single blue matala pad, but it is my understanding that external pumps in flooded suction applications are more energy efficient and last longer being they are not constantly submersed in water.
I've never noticed any difference in life span. Some pumps last a month, some seem to last forever. My best lasting pump was a 900 GPH mag drive. Raccoon pulled it out of the skimmer once and I found it laying on the ground the next day still running. Put it back in the pond and it ran fine.

Submerged pumps are made to be submerged. A mag drive for example doesn't have any seals to wear out.

Energy efficiency I don't think has anything to do with external vs submerged. Pumps vary a lot in that regard. So you can find cases where one or the other wins.

Where you really win energy wise is picking the right pump. Low head when appropriate for example. But also the size.

...into one or more plastic drums filled with possibly lava rock and/or a series of matala pads to enhance water quality.
This is a really old type filter. There are much better. I would not bother. I don't like cleaning mats, especially when they don't improve the water.

I think you'll falling into a trap that I and pretty much every other pond keeper falls into. Laying awake at night dreaming of magnificent filters and clear water? Most filters do nothing, yet many owners will say the reason their water is clear is because of their filter(s). This is especially true in the DIY world. They see what they want to see, that their invention is the best thing ever created.

Get yourself a UV filter, installed, sized and maintained correctly and you'll never have green water, 100% of the time.

Longer term water can get a little crappy. Water changes takes care of all these causes. In fact, there is a newer type system a few people are using that's based on the oldest fish keeping practice, 24/7 drip water change. A drip emitter runs 24/7 into the pond and an overflow takes away the extra water. IMO there is no better system based on what keepers have long said about normal water changes. If the overflow is directed to landscaping that would normally need water there's no extra water cost. People using this system have also not seen green water but too early to tell if this is always true.

My next pond will be 24/7 drip for sure.

By running the 3" pipe under the Upper Basin, I can keep the water in it protected from freezing.
I don't think pipes can be protected from freezing since ground freezes to some depth and the pipe are above grade at some point I assume. Freezing pipes in ponds aren't generally an issue because they're open to the air. Closing them will valves can create a problem.

...submerged oxygenating plants.
They produce O2 only when the sun is out. At night they consume O2. The net result is they are a negative. They're also ever difficult to keep alive.

The drums I mentioned are only to be used to cut the discharge velocity to
That can be addressed by changing the pump size or simpler ways. If you really want drums then cool. I used to love drums, had them all over the place. I slowly got over the thrill of lots of complex plumbing and started removing stuff. Turned out I didn't need any of it. I'm not saying you shouldn't have complex...it's fun for awhile...you learn a lot...but there are other options.

You mentioned two pumps: one for the skimmer at 2000 gph (more or less) and a 2nd one pulling a slip stream directly from the pond, mid-depth. I have a single 20A circuit for power. I agree that I want the weir floating and not being sucked full down and so two pumps might work better, but there would be no filtration for the 6000 gph pump to supply the waterfalls. That concerns me.
Why the concern? You mean no pre-filter? External pumps can have a strainer pot before the pump to protect it. But a mid level pickup normally will be pretty clean water.

Submerged pumps can be placed so clogging is reduce. Placing them on the bottom is bad imo, get empty the pond.

The helix skimmer is rated at 8000 gph and I might be able to use two of them side-by-side with a 4000 gph pump for each. What about that?
I personally wouldn't. It would work of course, but then you have 2 skimmers to clean out. Extra $500, extra install time, another thing to go south. I like simple.

If you want 8000 GPH for aesthetic you should absolutely have 8000 GPH. Ponds should always be built to want the owner wants...it's the whole point for building a pond.

But if this is mainly to create water flow for filtering or driving stuff to the skimmer or because someone said a falls should have 150 GPH per inch then you have options.

For example, I had a pond once that was 7600 gals, 6 connected ponds, 2 skimmers, one waterfall, all driven with a single 900 GPH 42 watt pump. The waterfall was 2' wide, but the water sheet was less. Much higher fish load than you, no bio filter. Aesthetics totally aside, this functioned fine technically.

Water was clear to 4.5'. It took me awhile (several years) to figure out how it get the water clear (this was before UV filter were cheap), but it is possible with a lot less flow.

One thing you might consider for example is making the 3 outer pools into a single pool and maybe going for 1 waterfall. Again, I really don't want to talk you out of your vision...just not sure if aesthetics or technical issues are driving the design. I know it's partly both, but is there room to down size the pump?

8000 GPH for this type of pond is a lot. Which is fine if that's what you want.

The plants, the bio filter stuff...if you want all that because you want all that then cool. I love stuff like that. But don't be fooled into thinking this will all give you clear water or make water cleaner or improve fish health. Plants add more waste than they remove in total. There are certain things some plants do remove, like nitrate, which is cool. But with a couple of goldfish you are never, ever going to come remotely close to having a nitrate issue. Plants like the Iris will need to be fertilized, which will add nitrate to the water. Not a problem, but don't believe people that say these kinds of plants remove waste.

Waste water treatment plants do use vegetation to reduce some chemicals. But these are huge and nutrient levels very high which they're only trying to knock down. Backyard ponds aren't in that ballpark.

Also, you mention that Helix 8200 is a high head pump, I wasn't aware of that. How can you tell?
The max head is over 20'. A low head pump the max head will be 10-15'. There are even very low head pump schemes where the max head is an inch or two. You also see the difference when you compare GPH per watt, although pumps vary because of other things. Lower head pumps should be higher GPH per watt, otherwise they're really bad pumps.

Lastly, I was not planning on any compressor for subsurface air. I'd prefer to rely on natural oxygenation and purification of the water, if practical.
Your pond doesn't come close to ever needing more O2.

For comments on the plumbing I'd need a diagram. Text is too hard to follow for that. But in general, I'd just pump directly into the upper pool. Like make 2-3' of total pipe, maybe one 90. Instead of a drum you can just build a rock/block.brick wall inside the pool, on top of the liner...I should draw this.

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Basically just build a wall (in green) in the upper pool. The distance from the skimmer compartment can be very little, even 2" if you liked. I'd go maybe 6-12" out.

That would calm 8000 GPH down.

The wall could be submerged or a short falls. You could leave gaps in the block/bricks toward the top to reduce velocity more.

The diagram shows a submerged pump (in red), but an external could be used. Pipe could go over the top if you liked.

I build my own skimmers. So if this were my build and I wanted a 8000 GPH pump I'd build a big skimmer box and have 2 or 3 weirs and 2 or 3 baskets. You could have one huge basket but kind of heavy and awkward. I think building skimmer is actually easier than buying and installing...plus the time needed to earn $1000 for 2 skimmers.

Also the way I build I would have a single pond, one liner. Then inside I would build the walls for the pools on top of the liner, but this requires a concrete pad too on top of the liner. This does a few things for me...much less liner to buy and lay. I can shape the walls any way I like, so I can get creative. Easier to tweak later. Want a smaller pool, no problem. Want to remove one pool, no problem.

I would also add an auto fill and an overflow. Allowing a pond to overflow where can be an issue, like right next to that building. Easy and cheap to add an overflow so good insurance imo.

This is a West-facing view of my water garden taken circa 11 AM today. The location of the two pumps and skimmer will be on the left front by the electric meter and phone junction box. The Main Pool is front and center. The Plant Biofilter is on the right front. Left rear, the tall section, is the Upper Basin and of course the Mid-Tier Basin is directly behind the Main Pool. The cutouts represent the rough openings for the waterfalls. I will need to cover those sharp edges with scrap carpet and there will be a seperate liner in each of the 4 sections that contain water. native rock will be added on top of the line to creat shelves and subsrface structure. Per your suggestions, I will have a 2000 gph submersible pump in the skimmer and a 6500 gph external pump, drawing directly from mid-depth in the Main Pool. I will use 3" Flex Hose from the dischard end of each pump up over the Upper Basin Wall and down into the water. I think i will need inline check valves on the discharge end of both pumps so when I turn them off, the water from the Upper Basin will not siphon to a lower level and cause flooding. I can connect an overflow pipe from the skimmer to my foundation/gutter drains that run underneath that front section. Because of the combined 8500 gph suction from my 1500 gallon Main Pool, are you saying I should have an automatic fill valve to prevent excessive drawdown? It's hot and humid here in PA, but nothing like the extensive dry heat of the AZ Sun, so I wasn't anticipating a lot of evaporative losses in the Summer.

"Basically just build a wall (in green) in the upper pool. The distance from the skimmer compartment can be very little, even 2' if you liked. I'd go maybe 6-12' out."

This is a Northeast-facing view, with the Upper Basin on the right. I'm not sure I fully understand your quote above. The Upper Basin is 4 feet wide and 8 feet long; depth as with all the sections is 38.5 inches. Are you suggesting that I build the internal wall in this basin at a horizontal distance of 2 feet from the skimmer or 6-12 feet? A horizontal distance of 8 feet puts the pump discharge end in the right foreground... what would be the purpose of that... frictional pipe losses to slow the velocity? I was thinking of maybe building a rock wall inside the basin with an opening in the bottom of the wall (like your catch basin idea), with the flex pipe ends facing the opposite wall (the one in the foreground in this pic). Incidentally, I could have gotten more depth in the Upper and Mid-Tier Basins, but I backfilled them with 3 feet of dirt to act as a "dead-man" to keep the whole structure from moving anywhere. As an added countermeasure, I have 16-foot 4x6s braced against a 3-tiered planter on the other side of the patio. Those two braces serve to frame in my patio pavers. I'm going to steer clear of UV for now. Any advice on high quality, high effciency, low head pumps? I'll prolly run the skimmer pump 24-7, but shutoff the higher cost waterfall pump at night.

I see, you've already built the walls. Forget my drawing.

The cutouts represent the rough openings for the waterfalls. I will need to cover those sharp edges with scrap carpet.
I assume you're going to use EPDM liner? I wouldn't bother with carpet on any of the wood. EPDM is tough enough to handle being against wood. And the sharp edges at the top are no problem either for EPDM. There's not a lot of pressure on the liner up there. Or hit the edges with a belt sander if you like. You can add carpet on the wood but kind of a pain for little gain. I have used landscape fabric (felt type) against wood but don't think it was needed and haven't since. I don't pad vertical concrete block either..

Carpet on the ground is a good idea.

I will use 3" Flex Hose from the dischard end of each pump up over the Upper Basin Wall and down into the water.
You could hide it a bit by going thru the wood above the liner in the main pool and then up the inside of the upper pool between the wood and liner. Not saying you should, just thinking.

Not sure what kind of flex hose you're thinking of using...make sure it can be connected to the pump some way. Some flex hose isn't made to be connected to smooth pipe, or require special fittings provided by the hose manufacturer.

I think i will need inline check valves on the discharge end of both pumps so when I turn them off, the water from the Upper Basin will not siphon to a lower level and cause flooding.
Good thinking. Or if the pipe out flow is above the water line in the upper pool, with the pipe pointing down shooting water into the upper pool. Whatever way.

I can connect an overflow pipe from the skimmer to my foundation/gutter drains that run underneath that front section.
Putting the overflow inside the main pool would be better. The water level in skimmers varies a lot.

Because of the combined 8500 gph suction from my 1500 gallon Main Pool, are you saying I should have an automatic fill valve to prevent excessive drawdown?
Not really. I don't think draw down will be much if all the upper pools are full. Having to top off once during pump start up is easy enough.

It's hot and humid here in PA, but nothing like the extensive dry heat of the AZ Sun, so I wasn't anticipating a lot of evaporative losses in the Summer.
Yeah, but you'll still be having to add water every so often.

Penn State says the average net water loss (including rain gain) in the Susquehanna river watershed (ballpark close?) is 5.8" for July. In you pond all evaporation will appear in the main pool, which is 1/3 of the total pond. So you should expect the main pool to drop more than 17.5" in July. I say more than because waterfalls can greatly increase evaporation and plant transpiration can add a lot too. So probably in the 4" to 6" range per week in summer. To keep the skimmer running you'll probably have to be adding water at least once a week in summer.

The main issue really is forgetting to turn off water when topping off. Lots and lots of posts of people doing this and killing fish. For years I topped off by hand and I don't think I ever remembered to turn off the water when I should have.

To me, having to top off every whatever is a pain. Vacation? But my main issue is I just don't like the look of a pond with a lower level. See more liner or a dirty tub ring around the pond isn't something I like. And an auto fill is like $20-40 and easy enough to add if a water source is near by. I'd even run a water line.

This is going to be a unique pond. Tons of falls. The main pool is going to be like a mountain stream with 8000 GPH pouring into it. Can't wait to see it running.

"Basically just build a wall (in green) in the upper pool. The distance from the skimmer compartment can be very little, even 2' if you liked. I'd go maybe 6-12' out."

This is a Northeast-facing view, with the Upper Basin on the right. I'm not sure I fully understand your quote above.
Doesn't matter anymore. You already have the walls up.

Any advice on high quality, high effciency, low head pumps?
Not really. I've only bought maybe 100 pumps in my life, so not really enough to know if one is better than another. Put another way I don't look for a specific manufacturer. I just look what's a available and compare cost, flow at the head I need and watts.

I'll prolly run the skimmer pump 24-7, but shutoff the higher cost waterfall pump at night.
Originally I assumed 8000 GPH meant you were building a big stone waterfall. String algae on those is a problem. Now that I see what you're doing string algae on the falls shouldn't be an issue. But turning it off to save electric is of course a good idea.

Kind of side note...code requires electric panels to be at least 5' away from the wall of swimming pools and water features. Each locale do differ on what is considered a swimming and/or water feature. Normally this is only an issue when selling a property, most people don't really care if a property owner wants take the chance. But, it is possible, probably not likely, that the electric company comes out to do something and pulls the meter. It's a tricky area for them because the meter is really their responsibility. Someone gets killed and maybe some smart lawyer goes after them.

In most cases you can get around this by building a wall or fence to separate the panel from the water. For example in your case building a deck over the skimmer pool and a wall/fence should be enough to keep it to code. But you'd have to check.

I cannot thank you enough for your input, waterbug! I have uploaded my vison of the pond plumbing schematic. Water will be drawn from the pump from two depths: (1) from the surface via the Helix Skimmer and (2) via a mid-depth via a seperate line with an intake screen (to keep out the aquatic life). Each intake line for the pump will be 2" PVC, with a check valve on each. The lines will join at a 3-way valve to control flow from each branch and then pass into a leaf basket / pump primer. The pump will be flooded suction placement within 18 inches of the skimmer and will ahve threaded unions so I can remove the pump prior to Winter. Discharge from the pump will be via a 3" PVC line with a check valve above the surface of the water in the Upper Basin (to allow me to drain the vertical 3" pipe when i remove the pump, but prevent the Upper Basin from emptying). Water will discharge from the pump into the top bung hole of a 55 gallon poly drum at 6-7,000 gph (due to head losses). The poly drum will serve as a "splash chamber" to help tame the flow and water will enter the bottom of Upper Basin via the bottom of the poly drum. The exit from the drum will face the opposing wall at a horizontal distance of roughly 6 feet. Hopefully, I will not have a geyser! I would most hearitly appreciate any comments you might have. The Helix pump has a 3-yr warranty and is, imo, relatively efficient (only 450 watts), but quite expensive ($800 list less 20% discount). I did some research into other pumps, but my total dynamic head (TDH) is going to be 5-10 feet and lower head or cheaper pumps were going to take too much of a flow hit. I also looked into twin pumps, per your suggestion, and decided against that. You are far more experienced, for sure, but I'm just going to have to learn for myself that you are right about using two pumps. I will either run the single pump 24-7 or shut it off during the night... yeah, I know, that's when the aeration from the falls is most needed.