Unfortunately because waterfalls and streams are so unique an accurate calculation isn't possible. Pools for example could consume a fair amount of water even in a falls. But because you'd have almost no stream, just 4' (falls height), the time it takes water to get back to the basin is hardly anything.

So just ball parking it I would guess the falls would hold 50 gals, maybe 100 gals depending on design. You need some water in the basin to cover the pump intake, or cover the pump depends on water for cooling. The shape of the basin kind of defines that amount of water. If the pump needs say 10" of water there is a big difference between a 5'x5'x10" and 1'x1'x10".

The intake on a 10,800 GPH pump should be 18-24" below water imo. The intake can create a whirlpool type deal and suck air down thru the water and into the pump. That can cause problems with the pump and be noisy. Breaking up the water flowing to the intake can reduce that risk. But the closer to the surface the intake the greater the risk.

The other issue is how the basin is constructed. You can buy box type deals that are strong enough for people to walk on and be covered by rock but are hollow inside so can hold a lot of water. I don't personally care for these, cost, not so easy to install, but they're fine. In tight spaces they can be the only choice.

I like to dig a hole, lay EPDM and fill with 2" river rock. I generally use a submersible pump and put that in a 5 gal bucket, with holes, in the river rock. But I'm normally in the 1000-5000 GPH range.

In your case a 10,800 GPH pump might need those water feature containers. For example a Savio WW1000 well can handle a flow up to 15,000 GPH. Aquascape's Vault with an AquaBox can handle 12,500 GPH. There are others. If you choice one of these that would kind of define the basin size, depth. Each manufacturer provides detailed instructions.

I would still make my own with concrete, concrete blocks, whatever, but that's me. You can view the instructions each manufacturer provides to get an idea of what kind of dimensions are needed.

For that size falls you need a pretty large area to catch water, so that kind of defines part of the size for you too.

I suggest you add an auto fill. I use a float valve but bring the water supply from a lawn sprinkler valve. I can set the sprinkler valve to only go on for a couple of minutes a day, or every few days. That way if the float valve fails, and they do, I don't get a flood and a huge water bill. It also can tell you if you have a leak where a float valve by itself will keep up with most leaks so you never see a problem.

You can't use a wheelbarrow to move material? That's how I move most material. But I've done the bucket procedure too. It's certainly work, but doable. Good work out.

Like I said, I wouldn't buy the manufactured systems either. Just cheaper and easier to built it myself. I like doing this type of work.

The main issue I think you would have is the speed at which water could flow between rocks for such a high GPH. 2" drain rock and a 5 gal bucket is OK for say 2000 or 3000 GPH. Of course you could just use 3 or 4 pumps.

What I would do for a single 10,800 GPH pump is dig the basin and lay EPDM liner. Then a 2" thick floor of concrete, no steel, over the liner.

You can get a little fancy and form a trench for the pumps if you like. Dig a V type bottom before laying the liner. Then final form with concrete.

Then I would dry stack concrete block to form open areas.
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I'd cover with 1/2" Hardie board. JamesHardie sells in the Netherlands but there are no doubt other sellers of cement board in your area. The blocks are stacked so the board on top doesn't have to span more than say 6-8". The board is not rated to span any distance, but I find it OK for 6-8" spans for occasional walking. Certainly OK for holding up even 6" deep layer of stone.

When I back fill I place rocks large enough to block gaps between blocks, and then smaller and small stone until I get to the top of the blocks. Then the cement board and then cover with the final layer of stone.

You don't have to cut holes in the cement board, should be plenty of gaps around the edge.

I do cut a small piece of cement board to cover the pump or pump intake so that's easier to get at.

That should get you 500 gals of water easy which should be way more than needed.

You can install 2 or 3 submersible pumps instead of one external to get the amount of water you want. Getting the sound of a waterfall the way you want it can be tricky. Many people do want Niagara falls. And they love it when it's turned on. But about 2 minutes later they realize they can't hear anyone talking unless they shout in their ear. Then they realize they can't turn down the flow and are not happy. Not saying this will be true for you, but it happens a lot.

And there are different moods you may want. Sometimes a tiny bit of noise, other times a lot. Multiple pumps can provide that. And three 4000 GPH submersibles can be cheaper than a single external to buy and run.

The biggest factor to me is pump noise. I've read many times in forums where people said their external pump was quiet, and they can be. But a water feature in a small space the last thing I want to hear when I turn it on is the customer say "what's that hum?" I don't want them to hear anything other than water.

Also a super cool piece of equipment is a remote control switch. These are only $10-20, plug into an outlet and then the pump plugs into that and you turn it on and off with a little remote. Way better than having to plug cords in an out and even better than wall switches imo for the cool factor. You can also buy more complex controllers so the falls can switch on automatically just before you wake up in the morning or come home from work. They're kind of expensive, like $100-200.

No I couldnt use a wheel barrow to move anything because access to the backyard is only possible by going through the house (townhouses) and my kitchen is only 6 ft wide and only 2 ft wide of a path out of the 6 down the middle of the kitchen which is how you get to the back yard. I have no idea what concrete board is I googled it and I see what it looks like but I have no idea what people actually use it for. The thing that I was planning on putting over the basin are these things

http://www.vanrooy.nl/producten/100132/485/vloerroosters-gegalvaniseerd

Its in Dutch sorry. Basically they are the perforated galvanized steel floor grating. They come in different sizes and reasonably priced. I am confident they will be able to support the weight of stones if they are meant to support people walking on them.

I will use 2 different pumps I think. One stronger one for the taller waterfall and one smaller one for the one that is less high up. I think your right about the rushing water sound being overwhelming so perhaps installing some sort of valve to regulate flow would be in order.

The pump for the tallest waterfall would need to be 7200gph at 6-10ft of height if I take into the 4ft height and slightly over 10ft of hose into consideration with fittings. The other pump would need to be 3600gph at 5ft of height. In theory though I could use one pump and split the line I think (could be wrong)

I totally agree with you about lining the walls of the basin and a concrete bottom. I seen a nice youtube video where a guy did just that and it seemed simple enough.

Cement board is mainly used when tiling a wall or floor. It's nailed or screwed to the wall or floor and tile is mortared to it Lasts better than plaster type backing in wet areas. Cement board can be fully submerged and be OK. Hardie has a little display stores sometime put out that has a small piece of their board in a clear plastic sealed case of water to show it holds up.

Any grating rated for floor would work and you could use fewer blocks maybe depending on the grate's rated spanning ability.

Galvanized is not going to last a really long time. Depending on many factors could last a few months, could last a few years. Getting say 5 years out I wouldn't have much hope. Unless the manufacturer rates it for below water use. But depending on price that may be OK for you. In theory cement board would last many decades.

I don't normally line the walls with concrete. You're right, not very difficult because it doesn't have to look perfect. I do normally cover the top edge with concrete to protect that. The loose stone protects the walls. But all concrete makes me feel safe.

For the walls instead of just concrete you can mortar rocks, like a wall. This is even easier than just concrete. Put down a little concrete and push a rock or brick or chunk of broken concrete into it. It sets up fast and little risk of sagging.
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With just concrete you have to be careful of how much water you use. And because the liner is waterproof the concrete slips and the more you work it the more it wants to slide down. If you get into that problem you can just build the wall up a little, like 6-12" and come back the next day and built up some more. Don't use any steel in the concrete. It rusts and cracks the concrete.

Pump flow can be split. A valve is only needed on the pipe going to the lower discharge. As you close that more water if forced up the higher discharge.

My preference would be two 3600 GPH pumps instead of a single 7200 GPH for the reasons I already wrote. But that's just me.

What is taht a picture of? It looks nice. I mispoke before when I said concrete wall I meant cinderblocks with a concrete floor sorry for the confusion. I found this website regarding the effects of water on galvanized steel. From what I read its not as bad as you thought it would be http://www.galvanizeit.org/hot-dip-galvanizing/how-long-does-hdg-last/in-water

I know you advocated for concrete board but i dont see how water is going to pass through it into the reservoir to refill the basin. The floor grates are like waffles so the water will just pass right through the rocks and into the basin.

The water doesn't pass thru the cement board, it passes thru the stones around the edges of the "vault" and thru the large gaps in the vault wall.

The link about galvanized steel is completely true...in a perfect world. The issue is not how well zinc holds up in water. The issue is how well does the zinc protect the steel. If the material is cut there will be exposed steel, which rusts, rust expands popping off zinc, and so on. Any flexing of the material can cause cracks in the zinc allowing O2 to get at the steel, rust expands, etc. Rocks grinding against zinc as the rocks are placed or stepped on can damage the zinc coating. Less than perfect manufacturing can mean voids between the zinc and steel which rust and pop the zinc more. For example hot dipping a weld is really hard to get a perfect seal.

Galvanizing steel can greatly increase the life of the product, but it still has a limited life.

The next issue is the thickness of the steel. Thicker the steel the longer it takes for it to weaken to the point of failure. Thinner the steel can also mean more flex, which can be more damage to the zinc coating.

All kinds of plating stuffer the same problem. Chromed steel will rust in most cases. Nickle, brass, copper. The weak link is the bond. Stainless steel on the other hand mixes the chrome into the steel mixing the bond throughout the material.

I'm not trying to talk you out of using the grate material. It's perfectly fine to use. Just didn't want you to be surprised if you saw some rusting.