By Demi Fortuna, Atlantic Water Gardens
Like most of us out there, I had been filling my pond-free reservoirs with gravel, usually 3/4" - 1" screened, sizing the reservoir so the water around the stones would suffice to run the stream and falls. As you already know, most of the reservoir space gets taken up by that gravel in there, so I learned how much volume to excavate the hard way - I undersized my first reservoir badly enough that I had to re-dig it.
Let me give you an example of an extreme. Imagine a beautiful backyard under tall oaks, sloping up away from the deck behind the house to create an almost perfect amphitheater carpeted with hay-scented ferns. Envision a 100-foot long stream winding its way down to a koi pond, which then overflowed into a reservoir where the pumps were located. The idea (not mine) was brilliant. No skimmer would be necessary since debris would simply overflow onto the top of a buried reservoir; the level of the water in the koi pond would always stay constant; the reservoir level would rise and fall instead of the pond.
The idea was great, but the math was off. The stream needed about a foot of water by an average of 3' wide to completely fill the 100-foot run and start recirculating, a total of 1 ' x 3' x 100 '= 300 cubic feet of water. The reservoir held only about two thirds that much, but it still might have worked if the stream had been able to retain most of the water, easily achieved with liner and proper design. See the Detail.
If the stream pockets had retained 2/3 of their water, both the stream and reservoir could have been filled with a hose; and if only a third of a foot were needed to get things started - 1 /3' x 3' x 100 = 100 cubic feet, about half the reservoir - it would have worked. See the Stream Diagram, the bottom stream with stream pockets waterproofed with liner.
Unfortunately, the materials actually used to retain the water in each stream pocket - stones glued together with foam and cement - were not waterproof, so they didn't retain water for very long. The pump would empty the reservoir before water could recycle, then have to be switched off until the reservoir was about to overflow, then on again until empty, and so forth. Not fun. It took a day to get it started, and finally all was well - until it was shut off. 300 cubic feet of water from the stream tried to fit into 200 cubic feet of reservoir. The lost 100 cubic feet would need replacing and the tedious job of filling while cycling the pump on and off would start all over again. There are over seven and a half gallons of water in a cubic foot. I'll bet you can imagine how much damage 750 gallons of water can cause every time the power went out, which happened there about once a month! Talk about a learning experience!
The formula presented itself: I wanted to have at least 4 times as much water in the reservoir than I needed to get the feature recirculating. That way, when the pump went on, the reservoir would only drain down a quarter of the way before an equilibrium was reached, with the water from the stream replenishing what was being pumped out. To keep things straight, let's call the amount of water needed to completely fill the stream top to bottom and start refilling the reservoir, the Dynamic Volume. The Reservoir Volume had to be at least four times the Dynamic Volume, so the water in the reservoir would never drop by more than one-quarter on startup:
(Note that the amount of water that the stream holds in the pockets and pools when the pump is off doesn't change the formula. It's only the water you need to add to get the feature recirculating that matters. The less water you need to get things started, the smaller the reservoir can be. It takes a 25% larger reservoir to supply a stream that needs 4" instead of 3" of additional water to get started. You save a lot of work, time and materials by decreasing the Dynamic Volume.)
Well, it seemed easy enough - but what about the gravel? I needed to know just how much larger the reservoir would have to be if I backfilled it with gravel, so I turned to a man who never takes anything for granted. I'm pretty sure Bill Hoffman, Pond Supplies of Ohio, has actually checked every measurement and formula that I've just taken for granted off a chart. Sure enough, he had checked volume of water in graveled reservoirs. Bill told me he had filled 5 gallon pails with clean gravel in a variety of sizes, and, regardless of the size, each displaced about two-thirds, leaving onethird water. That meant I would need to dig a hole three times larger than the volume of water I needed, to accommodate the gravel. Tripling the original formula of four times the Dynamic Volume to accommodate the gravel left me with the startling realization that the Graveled Reservoir Volume had to be at least TWELVE times the Dynamic Volume!
Volume of Graveled Reservoir = Dynamic Volume x 12
Let's see how that would work for my extreme example above. Even if I only needed to supply 100 cubic feet of water to the stream, that means 400 cubic feet of water, which would take a 1200 cubic foot reservoir if I was backfilling with gravel. Ouch! That's a hole 4' deep by 15' by 20', and I would need 40 yards of gravel to fill it! That's not just a lot of gravel to buy, it's a tractor-trailer load that has to be delivered, dumped and installed!
Obviously larger water features like this need a different strategy.
Enter the Water Matrix. These sturdy blocks take the place of the gravel in the reservoir, and they're engineered to take the load. Typically internally reinforced with inner walls, these closed rectangles are no milk crates; water matrices were first developed for stormwater retention under parking lots. Some, like the Atlantic Eco-Blox for example (see Eco-Blox illustration), can handle over 7 tons (!) of distributed weight, so a properly constructed reservoir can be safely buried anywhere in the yard, under water features, patios or walks, without fear of collapse and are completely invisible. These matrices are 95% open space, so there's no need to dig three times the reservoir, and forget all that additional gravel! We're right back to the simple original formula of a Matrix Reservoir of only four times the Dynamic Volume:
Volume of Matrix Reservoir = Dynamic Volume x 4
Let's look at the reservoir for our extreme example now, using matrices instead of gravel backfill. Instead of that monster 1200 cubic foot reservoir, if we don't need the space for the gravel, we're back to around 400 cubic feet. Instead of a 4' x 15' x 20' pit, we can now dig a hole one third the size at 4½' x 8' x 12', filled with two layers of 24 matrices each, with a Pump Vault and Vault Extension to house a pump, the whole reservoir capped with a modest layer of 10" of gravel, totaling about 3 cubic yards. The cost of the 48 matrices might be slightly more than a tractor trailer of gravel (not where I'm from on Long Island!), but consider the savings: digging one-third the hole; moving one-thirteenth the gravel; transporting all the matrices in a shortbed pickup; assembly and installation in half a day, no strength or exertion required; and what about avoiding the cleanup costs of moving 50 tons of gravel! OK, but 100-foot long streams are certainly not everyday projects; what about smaller features? The savings translate down in scale just as well.
Let's say for the sake of argument that you've contracted to build a 5' high waterfall, running down about 9' total of stream and falls before it gets back to the gravel where the vault is located. Let's guesstimate that, in our hypothetical illustration, the 9' long stream varies in width from 1' to 4' wide, but the average width of the stream and falls is about 2' wide; we'll guess that the average depth we have to fill including every pocket and pool is about 4" before the water flows back into the gravel. If we calculate the volume of water we'll need in the stream and falls to get the feature running at 9' long by 2' wide by 4" (1/3 foot), we'll need about 1/3 of 18, or 6 cubic feet of Dynamic Volume. To have enough room for the water and gravel besides, we'll need 12 times the Dynamic Volume for our graveled reservoir, or 72 cubic feet to hold the 24 cubic feet of water, that's a hole 3' x 4' x 6 ', plus almost 3 yards of gravel. That's a hard day's work for two men by hand, not much less with a machine when you consider the cleanup.
Using Water Matrices, we can store the same volume of water with one-third the digging, about 2' x 3' x 4' and 4 matrices, only half a yard of gravel and less than half the time required. It's no wonder Water Matrices are what serious contractors are using these days, for pond-free waterfalls, bubbling rocks and urns, rainwater harvesting cisterns, even greenhouse thermal storage mass! If you aren't already using them, give them a try; you won't go back to backfilling with gravel.
Remember, it's always better to learn from the mistakes of others! Make sure you calculate the reservoir volume you will need to make that stream or pondless feature work the way you intend it to.