Vanishing Edge Pools

Paolo Benedetti discusses the intricacies and complexities of vanishing, infinity, negative, knife edge, perimeter overflow, slot overflow, disappearing edge or flooded edge pools...
What ever you want to call them, a vanishing edge pool is a pool wherein one (or more) sides of the pool are slightly lower than the operating water level. The most generic term for these edge effect pools is "a water in-transit pool."
While strikingly beautiful, they are actually quite hydraulically & structurally complex. Because they are oftentimes built on a mountaintop or the edge of a cliff, they require a geo-technical investigation and appropriate structural engineering.
Hydraulics
The hydraulic design of these types of pools is very complex. In fact, these pools require 2 separate filtration systems. The primary system is utilized to filter, heat, and sanitize the main pool - just like a regular swimming pool. The second system, draws water from the surge tank and returns it to the upper pool. A significant quantity of water may be required to raise the level of the pool, to create the "spill over" effect. The catch basin may be attached to the main pool or it may be remote holding tank. These 2 systems are in addition to the pumps & piping required for the spa jets, water feature(s), pool cleaner, and solar panels.
The volume of the plumbing needs to be calculated on some styles of these pools, as some rely on gravity drain lines to return water to the holding tank - lines that must be filled each time the system starts. The surge tanks need to be adequately sized to accommodate bathers' displacement, wind displacement, and storage of the "water in-transit" necessary to raise the main pool to the appropriate level to create the edge effects.
The with proper pipe sizing, the primary filtration pump can be as small as 3/4 HP. Depending on the project requirements, the secondary edge pump may need to be as large as a 3 to 10 HP. This is usually due to the quantity of water required to both raise the water level in the main pool and overcome the surface tension of the weir materials.
Small pumps or large pumps - Confused yet?
Don't worry, most swimming pool contractors and designers are confused as well.  The vanishing edge system (effects pump) requirements are determined by the linear feet of edge to be flooded, the edge tolerances (materials highs to lows & installation perfection), pipes to fill and flow rates required to ensure that the gravity gutters & pipes are scoured (water moving fast enough to carry away any debris).
Those that don't understand these complexities oftentimes under design or go for the overkill.  Either way the property owner loses... a system that does not operate correctly or one that consumes way more energy than is remotely required.
Taking the easy way out - edge filtration
Many pool builders & designers in a effort to "save the client money" (and make more money for themselves), eliminate the secondary filtration system. They may either delete the entire secondary system or just it's filter tanks.
By eliminating the filter tanks, they cause this system to blow the debris that collects in the catch basin back into the main pool. Since the edge is a giant weir, a majority of the debris in the main pool ends up in the catch basin.
Imagine what will happen every time the edge pump comes on.... a cloud of dirt and fine debris will be blown into the main pool, clouding the water. Just what a client desires when showing off their pool to guests or hosting a party! I know of a pool builder who routinely deletes the edge filtration system from projects that we design. I know, because I get the call once the project has been completed - complaining that I created a "faulty design." After viewing the finished project, it becomes readily apparent what the cause is... Yet this builder continues to build in this manner. I am sure he will only change his ways after a client's lawsuit.
If the secondary system is eliminated entirely, then the main pool filtration system will be forced to draw it's water from the catch basin. This forces the pool builder to install a pump that is significantly larger than what would normally be required... remember that the primary filter pump can be a mere 3/4 HP vs 3-10 HP for the edge system.
Though there was an initial savings of about $5,000 by eliminating the edge system, the electricity costs over the life of the project will grossly exceed any savings. This is stepping over dollars to collect pennies!
Auto-fill
An electronic automatic fill device is another requirement. Many pool builders & designers rely on inexpensive mechanical toilet float style auto-fills. While they are functional, they are the wrong choice for a water in-transit pool. But, hey they can save $200! If there was no wind and no one ever swam in the pool (and therefore were no waves), these mechanical auto-fills would work okay. The problem is, these pools are dynamic. Wind causes waves, swimmers displace water & create surges, and some plumbing lines for the edge system may need to fill when the system activates. These variables cause the mechanical auto-fill to surge, turning on & off with the wave action. The repetitive hissing sounds from the auto-fill is annoying enough. Add the rythmic water hammer on the plumbing system within the residence, and you have enough to cause most homeowners to immediately voice their complaints.
An electronic auto-fill has a built-in delay, thereby ignoring minor bather surges, waves, or fluctuations in the water level. This eliminates the annoying hissing & resultant water hammering in the plumbing. It also has a fail-safe timer, that prevents it from becoming stuck in the "on" mode for ever.
Back flow prevention
Another requirement, is the need for failsafe back flow prevention (in addition to check valves) on EVERY plumbing line that returns to the upper pool from the surge tank. When a check valve fails (and they WILL fail - not IF they will fail, but when!), the back flow prevention device will prevent the back siphoning of the upper vessel into the catch basin. On a hillside, dumping 30,000-50,000 gallons of water down the slope will be catastrophic for the foundation of the pool, the hillside or the living room of the downhill neighbors! Imagine all of that water and mud sliding into the neighbors backyard, followed shortly by the swimming pool?!
Complexities
Because these style of swimming pools are frequently built on the top of mountains or on the edge of cliffs, their structural engineering oftentimes requires drilled caissons or other complex foundation design. This complex structural engineering requires specialized training, construction practices, and independent testing lab inspections & material samplings/testings. The complexity of these concrete structures is similar to that of building a bridge overpass - beyond the licensing, training & expertise of most swimming pool contractors.
While these projects can be quite lucrative for a swimming pool contractor, the failure of such a vessel will result in major lawsuits. Many swimming pool contractors have attempted to build such complex pools, thinking that they are easy, only to make every possible mistake. If they are not an expert in such vessels and their intricacies & nuances, do you really want to be their learning curve??
Even if they have built "a few" of these type of vessels, due to the significant quantity of possible mistakes, do you want them making "just one" on your project? While the rewards of a properly executed project are astounding, the opposite results are disastrous!
I saw a pretty picture...
I recently heard of a California Horticulture contractor who attempted to copy a detail from one of our projects (Yes, another landscape contractor who ventured into pool design & construction - a realm they really do not understand). They attempted to place the spa along the vanishing edge, and have it appear that the spa was spilling over the horizon just like the pool. The problem, is that they did not understand the hydraulics & mechanical systems of creating such an effect.
Remember, that when the main pool filtration system is on, it draws water from the main pool skimmers & returns filtered/treated water to both the pool & spa? Because the spa also spills over the backside & into the catch basin, the water level in the main pool drops & the level in the catch basin rises.... eventually to the point where water spills out of the catch basin & down the hillside. Though they were able to "patch" a fix through creative programming of the automatic valving - it is a matter of time before an automatic valve fails to turn or a switch is accidentally left in the wrong position ...and the downhill neighbors get 50,000 gallons of free water! Greed won, brains lost!
This is just another example of how good intentions go bad.... biting off more that one can chew. Because someone has a mastery of an art, the execution of their craft appears easy to an outsider... because they know what they are doing! But by trying to execute a project for which they do not have an understanding or the expertise, is a time bomb waiting to explode. There is absolutely no acceptable excuse for these types of mistakes, especially when there are design consultants (like myself), classes (like the Genesis 3 courses) and reference literature available.
A vanishing edge on every hillside - not!
Just because a site is sloping lot, overlooks a body of water, canyon, or precipice, does not automatically qualify a project for a vanishing edge pool. I have actually talked clients out of a vanishing edge pool, when the lines of sight were not correct or when soils reports indicated that a majority of their budget would be required for foundations instead. Sometimes, a vanishing edge pool just "isn't right" for the site. A pool does not have to be a vanishing edge to be perfect for a view or hillside lot.
Exponential Cost
Lastly, property owners should not expect the cost of a vanishing edge pool to be comparable to a similar sized pool on a flat lot. There is a lot of additional finish work that needs to be completed. There is the secondary edge filtration system and complex structural engineering & foundations. The entire weir wall is a "free standing" wall, that requires additional steel, special form work, and unique talents of the shotcrete crew. There are the additional finish materials & labor for the catch basin & the backside of the weir wall (remember it is usually the length of the pool & 4-6 feet high!). Often times the outside of the catch basin also needs to finished. There is the added cost of constructing an equipment pad on a hillside (it should be placed at or below the level of the catch basin, which oftentimes requires footings, retaining walls, and access steps.). There is the matter of where to drain the overflow/rain water, without eroding the hillside.
Get shoehorned into a Yugo
In an effort to bring these expensive pools into the realm of mainstream affordability, some fiberglass & vinyl liner pool builders have attempted to build such vessels. I proffer that they are almost always lacking the proper geo-technical reports, structural engineering, and lack the proper details to prevent eventual catastrophic failure. These are usually package pools that are meant for flat parcels, being modified (by Tim Allen types) to wedge into a particular budget. The ones that I have reviewed always had a seam that was "bootleg" sealed or an under water joint that was critical in remaining waterproof. Again - if you cannot afford to do it right, DON'T DO IT! We'd all love to drive a Bentley or Maybach, but are resolved to drive a Ford or Chevy, because that's what we can afford. Just because a client wants a vanishing edge pool and cannot afford one done properly, does not mean that a builder should whore himself to create a modified vanishing edge pool from a kit!
How in the hell do they fasten a vinyl liner pool to the hillside? It is impossible to structurally tie the vinyl to a foundation or to contain a leak. They are just too fragile and susceptible to rips/tears to be placed in such a location.
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Paolo Benedetti - Aquatic Artist
"Creating water as art."™
Aquatic Technology Pool & Spa
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