What are some interactive design ideas for a negative edge skimmer?

[combatwombat]

I was sitting on the deck last night staring at my big hole in the ground and started dreaming about a negative edge skimmer—the type where the pond just cascades over a waterfall, through some gravel, and into a storage vault.

I started wondering if you could set one up so that it's more than just decorative and actually interactive. Could you build a fire pit on top of one? Some sort of patio? Something else fun and interesting?

I went looking for examples and didn't find much on Youtube or Google Photos. Found lots of beautiful negative edge designs, but nothing where the top of the basin is reclaimed for useful space.

Can anyone share pics or links to designs like this?

@Lisak1: I know you have a negative edge. Two questions that just popped into my head:

1. Do I remember correctly that you tried placing filter fabric/pea gravel on top of your basin and it caused you some problems?
2. Is it correct that, with a negative edge, you need a pump that can overcome more head pressure because the "water level" that your pump is pulling from will be lower (hidden underground)?

 

[MTguy]

I am attempting to make mine somewhat interactive in that it will look essentially like the surrounding flower/rock bed. I'm hoping to have it done soon, so I can post a picture then. I've got to do some edging around it yet and finish the actual negative edge, but the vault area is covered with the rock and is very stable to walk on. I'm considering adding a couple of plants to soften the area as well.

For the pump question, the way I understood it is that the head pressure is based on the height it has to push the water (and some factors such as elbows/check valves/etc) but "more" pressure would be relative to the alternative pump placement locations. If you were going to put it in the bottom of a pond that is three feet deep rather than a basin that is 2 feet deep, the basin location would have less pressure.... I think!

 

[combatwombat]

I'm hoping to have it done soon, so I can post a picture then

Can't wait to see some photos! What are you using for storage matrix? And how many gallons of storage?

If you were going to put it in the bottom of a pond that is three feet deep rather than a basin that is 2 feet deep, the basin location would have less pressure.... I think!

Head pressure is calculated starting at the height of water level. If you pump is below water level, then the mass of water above it is, essentially, pushing down on the water that the pump is drawing, allowing it to lift the water back to water level for "free." The "dynamic" part of dynamic head pressure comes into play once you choose a pipe size, length, and any fittings you add.

Since, in a negative edge design, the basin is outside of the pond and the water level in the basin will be below the water level in the pond, the difference in that height would be added to your total head. As long as the basin is full, probably no biggie. Where it could get dicey is if you have a 6 ft. deep basin that is almost empty. Now you have 6' of added head pressure. So I think you just need a plan to keep the basin full to continue to get the flow rate you're expecting from your pump.

 

[MTguy]

We’re on the same page. If you are comparing an in pond submersible at the bottom of a 3’ pond to a 6’ deep negative edge chamber the difference would be 3’ more of head pressure.

In my case, I used septic chambers for my basin so my negative edge area is actually only a little over 2’ deep compared to my pond at 3’ deep. If we calculated from the bottom I’d have less head pressure in my overflow area than my pond.

 

[Bluerooster]

Just for spits and giggles, I usually figure head as the vertical height that the pump must move water. then I figure in about 30% on top of that.
So, if I want to move 86gpm, with 6' head, I'll get a pump capable of 86gpm at 9' head. (what I have in my swimmin' pond) The pump that I have at my spring is 2000 gph at 180' head. It's working into an 80' head, so I have plenty of reserve. (not working the pump too hard).
What I've seen is folks going "just big enough" based upon calculations, with no room for unforseen circumstances. Thus working their pump to death in a short time.

Thing with the "vanishing edge" is that you must have a balance between pump, pond, and catch basin. The basin needs to be big enough to catch, and hold 100% of the water flowing from the pond, until the pond quits flowing (full pool, pump off) Plus enough to not run dry when re started. Then the pump needs to move enough water to keep the flow going without pumping the basin dry. Of course the flow over the edge is based upon the GPM of the pump. For a true vanishing edge you want about an inch of water over the edge. Depending on the pond size, and edge length, That could be a lot of water. Think about this: 10'x20' pond 3'deep (full to very top). Now, fill it to 3'1" and keep it that way. That's 28800 cubic inches of water, over full pool. 231 cubic inches per gallon. close to 125 gallons over full pool. I would hazard to guess that it takes about 2 minutes to drain down to full pool with the pump turned off. So, you will need roughly 250gpm to hold full pool plus 1" at zero head. And a bare minimum of 250 gallon basin.

 

[Lisak1]

Yes - we were advised to use layers of landscape fabric over the blocks and between the different layers of stone. Don't do that.

As for the pump - probably, but I'm not an expert on that one. We put in a variable pump so we have the ability to increase and decrease the pump speed - highly recommend. We also have several ball valves that we can use to direct flow where we want it - to the bog, to a secondary waterfall/stream, or to a small water feature that we sometimes set up in the bog.

As for reclaiming space - most of our underground storage is, well, underground! Only a small portion of it is actually "visible". It's about three times as wide as the gravel portion would lead you to believe. The rest we covered over and filled with dirt and reclaimed as yard space. Not the best photo, but here you can see how wide the underground storage ended up being where the grass is cut out.

Here's the finished size - you can see we "reclaimed" lots of space on either side:

I've seen these done where they actually plant grass over the top and just have a small bubbling boulder or other water feature on the top to keep the water circulating.

 

[combatwombat]

Don't think I've ever seen your pond from that angle, @Lisak1. Love it. The build photos help a lot, too. Cool to see how you made use of the space on top of the cistern. Do you feel any flex in the ground when you walk over that space or is it totally solid?

 

[Lisak1]

100% solid. You'd never know it was under there. If we had built the pond first and patio second, I would have extended the underground storage under the patio - I'd love to have twice the size.

 

[combatwombat]

@Lisak1: Remind me of the size. 1000 gal?

 

[GBBUDD]

Negative edge does not need to have a drop. But who wants to waist the opportunity for a water fall.

You definately want to have a good amount of water in resivour. And you also need to have a ability to collect the water from the pond when the pump shuts down. The hard part is making the height of the pond at the stone for the negativevedge water proof so tge water doesn't continously leak and tge pond level drops thus needing more reservoir space.

Lisa's misfortune with the fabric gave me an idea I made the falls drop into a section of fabric that's made similar to a net but it is set so that it's top is below the top of tge cistern.. so when the fabric bag fills it overflows and just drops into the cistern but will still catch large debris.
Yes you can easily convert your catch area into a patio or a deck. I recommend boxing in the cistern with rubber so there's little to no evaporation loss.

If you look at the latest video you'll see tge negative edge to shts creek that drops to hells gate / the filter net before dropping into the cistern

 

[combatwombat]

Of course the flow over the edge is based upon the GPM of the pump. For a true vanishing edge you want about an inch of water over the edge. Depending on the pond size, and edge length, That could be a lot of water. Think about this: 10'x20' pond 3'deep (full to very top). Now, fill it to 3'1" and keep it that way. That's 28800 cubic inches of water, over full pool. 231 cubic inches per gallon. close to 125 gallons over full pool. I would hazard to guess that it takes about 2 minutes to drain down to full pool with the pump turned off. So, you will need roughly 250gpm to hold full pool plus 1" at zero head.

I hadn't started thinking about pump flow rates in a negative edge set up yet. Thanks for the heads up.

In your example where there's 125 gallons above the spillway that would take 2 min to drain down, wouldn't you only need 62.5 GPM to maintain? A 2 min drain time on 125 gal = 62.5 GPM of drainage over the edge, not 250. Or am I misunderstanding?

The hard part is making the height of the pond at the stone for the negativevedge water proof so tge water doesn't continously leak and tge pond level drops thus needing more reservoir space.

That's a good point. Have seen John Adams talk about this on his channel. I was thinking I would build up the height of my spillway over the liner using a few large, flat boulders and foaming them in, but that might cause problems during an extended power outage.

I made the falls drop into a section of fabric that's made similar to a net but it is set so that it's top is below the top of tge cistern

I had this same idea. I was thinking it would be cool if you could have some pooling over top of the cistern rather than the water just disappearing at the bottom of the falls. You could build up a "mini pond" with it's own 1" negative edge that falls over into the void space. A negative edge within a negative edge.

Yes you can easily convert your catch area into a patio or a deck.

Any thoughts on how to install a fire pit over this area? Would need a really bullet proof way to keep the bottom of the fire pit from melting a hole in the top of the aquablox.

If you look at the latest video...

I did see that!

 

[MTguy]

On my negative edge transition I am planning on using a bib liner to get the exact height I want. Basically, just a scrap of liner taped to the liner with pond tape at the right height. I’ll probably sandwich this between 2 flat stones to hide it. I saw a video where they used foam as well. It works, but won’t hold the level indefinitely apparently.

 

[Lisak1]

Aquascape is selling some cool new "fire and water" features. I'd check those out.

And yes - 1000 gallons underground. Our whole pond was born out of the desire to find a way to store water for emergency use. Lots of people use the big blue barrels, but those have to be dumped and refilled on a regular basis, and you need a place to store and fill them. It was doable, but not desirable. While pondering this idea we came upon Aquascape's RainXchange "system" which is really just Aquablox and a pump vault - bingo. So then we talked about a small water feature to keep the water circulating. Then we said "well, it would be a shame to have all that water underground and NOT ENJOY IT. And here we are!

 

[combatwombat]

Our whole pond was born out of the desire to find a way to store water for emergency use.

Were you mostly concerned with having water for the garden during drought? I'm kind of wondering what my insurance company is going to do once I tell them we built a pond. On one hand—it's an added drowning danger like a pool would be. On the other, I live in an area at risk of wildfire, and the pond is a hell of a fire break and source of water to extinguish flames...

 

[combatwombat]

I got an idea today about how I could construct a negative edge skimmer on my pond that could be both a pooling area and a fire pit.

Here's a quick sketch:

The basin would be built with a fire pit in the center and there would be two overflows. The first overflow would be a "fire pit overflow" and set just below the top of the water matrix blocks and would have a valve on it.

The second overflow would be the main overflow and however high I want it (probably just below the top of the fire pit—just below wherever I set the top of the liner. This could also just be a low spot in the liner; no need for a pipe.

Most of the time, the valve on the fire pit overflow would be closed, and water would rise until it reaches the main overflow. This would give the basin the appearance of a "lower pond" that the main pond is spilling into.

But when you want to sit around the fire pit, you would open the valve on the fire pit overflow which would drain your basin below the gravel covering the water matrix blocks, allowing you to set some chairs down there to enjoy a fire and watch the waterfall over the infinity edge.

I got this idea after remembering how I constructed the condensate drains on the indoor unit for my heat pump. I have two drains, a main drain and a second one just 1/4" higher. The higher one is plumbed with clear tubing. When everything is working correctly, the second drain is totally dry. All the condensate drains out of the main drain. But if the main drain ever gets plugged, the water level in the unit will rise until it starts running out of the second drain. Since it's clear tubing, I can see water running in that tube, and it alerts me that the main drain is plugged and needs to be serviced.