Perimeter Overflow Pool, Flooded Deck Pool, Slot Overflow Pool... or who forgot to turn off the fill line?

Whether you call them waveless pools, overflow pools, perimeter overflow pools, slot overflow pools, knife edge pools, flooded deck pools, slot edge or wet edge pools, zero edge, wetted edge, no edge, or flooded coping pools, or wet deck pools, they all provide essentially the same appearance... that someone forgot to turn off the fill line. They are not to be confused with vanishing edge, negative edge, infinity edge, or disappearing edge pools, though the basic hydraulic principals are the same.

While these pools appear simple, they are actually quite complex. It is what you can't see that makes them complex. To make their design even more difficult, their hydraulic systems are a hybrid of pressurized returns and gravity drain lines.

Adding further to their complexity, they must have remote holding tanks, called surge tanks. The size of these tanks must be carefully calculated to provide enough water to raise the pool (to flood level) and continue the flooding process until that water returns to the surge tank. This quantity of water must be held in storage, plus a quantity necessary to prevent the tank from being pumped dry (a common mistake).

The capacity of the gravity drain lines must be carefully calculated, so that the amount of water held in reserve can be calculated. The time it takes for the water to begin to return to the holding tank is another critical calculation. The pitch (slope) of the gravity drain lines also determines the velocity that the water travels. Too slow, and the water cannot scour the debris in the lines. Too steep, and the entry point of these lines within the tank, drops significantly below grade. The further away from the pool the tank is located, the larger (or deeper underground) the tank must be placed - as the descending slope of the drain line gets lower & lower the further from the pool it travels. These gravity drain lines also cannot be calculated at their maximum capacity, as they must maintain some "freeboard." That is air in the top of the pipe. This prevents the formation of suction & a really annoying sucking sound!

The tank must have additional storage capacity to allow for displacement from the pool. Displacement comes primarily from wind and from bathers. Now there's a novel idea - people actually swimming in the pool! I cannot tell you how many of these tanks I have seen that were undersized. Every time someone went swimming, a few hundred to a thousand gallons of water was forced out of the tank's overflow line, only to have to be replaced when the fill line turned on.

On the opposite side, the lack of understanding of the hydraulic complexities of these pools, oftentimes results in the surge tank being WAY over sized. While there is nothing wrong with an over sized tank, it is a waste of money, labor, materials, and energy to build them over sized. Having to filter & chemically treat a few thousand gallons of extra water adds up over time.

The lack of proper planning, oftentimes leads to the neglect of maintenance of these tanks. This is where all of the large debris from the pool settles. If the builder designed the tank improperly, this debris is never drawn into the suction line, and therefore the pump strainer basket or filter.
These tanks should be outfitted with main drains, inlets to direct the flow towards these drains, auto-fill lines to also assist in the debris management, ladder rungs for personnel access, vacuum ports for cleaning, personnel access hatch over the ladder rungs, lighting (when fiber optic or low voltage is used on the project), and most importantly, a strainer. The strainer sifts out the large debris before it is deposited onto the floor. I have my own design for a SS basket placed in the path of the incoming line. A smaller lid is placed over this strainer basket to facilitate inspection and servicing of the basket.

By failing to make servicing these tanks easy, the service personnel will simply fail or neglect to service these tanks. At least 2X each year, they need to be drained, rinsed out, and vacuumed with a shop vacuum. An alternate approach, is to drain them to within 12" of the floor, and if one was installed, utilize the internal vacuum port in the floor with a pool vacuum to collect the debris into the pool filter system. This is best performed immediately before the filters are serviced 2X annually (cartridge or DE).

This brings us to filter selection... DE filters are not the wisest choice. They are simply too difficult to recharge after they are back washed or serviced, without the installation of a slurry line or tank.

The detail that is used on the edge of the pool where it meets the water, is one of personal preference. The current rage and most difficult to execute, is the knife edge. I will discuss this technique in detail in another blog.


Paolo Benedetti - Aquatic Artist
"Creating water as art."™
Aquatic Technology Pool & Spa
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