The Hydraulics of Zero-Edge Construction

A vanishing edge is the most photographed feature in luxury pool design and the most misunderstood. Homeowners see a sheet of water dissolving into a view. What they are actually looking at is a precise hydraulic loop. Water leaves the visible pool over a dead-level wall, falls into a hidden basin, and is pumped back over the edge on a continuous cycle. The illusion is simple. The engineering behind it is not.

This brief covers the three forms people lump together: the vanishing edge (one wall open to a view), the slot or zero edge (water disappears into a narrow deck-level slot), and the perimeter overflow (the pool brims over every side). They look different. They share one core problem. All three move water off the vessel and have to catch it, hold it, and return it without flooding or starving.

What a zero-edge actually is

A standard pool holds a fixed water level inside its walls. A zero-edge pool does the opposite. Its working water level sits exactly at the crest of the overflow wall, so water is always spilling. That spill has to land somewhere. It lands in a catch basin, a second concrete vessel built below or beside the visible pool, hidden from view. A dedicated pump pulls from the basin and pushes the water back up and over the edge.

So a zero-edge pool is two vessels, not one. The upper vessel is what guests see. The lower vessel, the basin, is where the real work happens. Most edge failures are basin failures.

The catch basin is the whole problem

The basin has to hold every gallon that can be in motion or displaced at once. Get this volume wrong and the system fails in one of two visible ways. Undersized, it overflows onto the deck the moment the pumps stop or a group of swimmers steps in. Oversized, it wastes structure, water, and heat, and the auto-fill runs constantly.

Four things compete for basin volume:

1. Water in transit. When the edge pump is running, a real volume of water is in the air and on the wall on its way down. The pool surface drops by that amount, and the basin has to be carrying it.
2. Drain-back on shutoff. When the pump stops, the water in transit plus everything sheeting on the wall drains back into the basin. The basin needs freeboard to absorb that surge without spilling.
3. Bather displacement. People entering the upper pool raise its level, which pushes more water over the edge and into the basin. Design for a full bather load, not an empty pool.
4. Evaporation, wind, and splash-out. A sheeting wall loses far more water than a still surface. The basin and the make-up supply have to keep up.

The honest answer is that basin sizing is a calculation, not a rule of thumb. It depends on edge length, sheet flow rate, wall height, and expected bather load. Any builder who quotes you a basin size before those numbers exist is guessing.

The edge pump and the sheet

The edge needs its own pump, separate from the filtration pump. Sharing one pump is the most common shortcut and it removes your ability to tune the sheet without disturbing filtration. A dedicated, variable-speed edge pump lets you set the look: a glass-smooth full sheet, a textured cascade, or a quiet trickle for evenings.

The flow you need scales with the length of the overflow wall. More wall means more water to keep the sheet continuous. Too little flow and the sheet breaks into dry patches and mineral streaks. Too much and it roars and throws water past the basin. The target is a continuous, even sheet at the lowest flow that achieves it, because every extra gallon per minute is energy you pay for forever.

Two details decide whether the sheet is even:

• The weir must be dead level along its entire length. Tolerance here is measured in fractions of an inch over the whole run, not inches. A wall that is high on one end will run dry there and heavy on the other.
• The plumbing should be oversized and smooth. Friction in undersized pipe forces the pump to work harder for the same flow, which raises total dynamic head, raises the power bill, and shortens equipment life. Bigger pipe is cheaper than bigger pumps.

Water level and make-up

In a standard pool you control the water line. In a zero-edge pool the upper level is fixed by the height of the wall, so you cannot raise or lower it. The level you actually manage is the basin. Auto-fill and the water-level sensor belong on the basin, not the pool.

Because a sheeting wall loses water fast, the make-up line has to be sized for real loss on a hot, windy day, and the basin needs enough working depth that a few hours of evaporation will not expose the pump intake. Starve the pump and it pulls air, the sheet stutters, and the seals cook.

The Techne position

A vanishing edge is a hydraulic system that happens to be beautiful. We design the basin first and the view second, because the basin is what makes the view repeatable. That means a real volume calculation for water in transit, surge, and bather load, a dedicated variable-speed edge pump sized to the wall, oversized plumbing to keep head and energy down, and the level control on the basin where it belongs. We set the weir to a survey, not an eye.

Done this way, the edge runs as a glass sheet at the lowest sustainable flow, holds its level through a full party, and does not flood the deck when the timer clicks off at midnight. Done the other way, it photographs well on day one and disappoints by the first warm weekend.

Failure modes we design against

• One shared pump for filtration and the edge, so the sheet cannot be tuned independently.
• A basin sized by rule of thumb instead of by transit, surge, and bather load. Floods on shutoff or overflows when guests step in.
• A weir set by eye. Dry patches on the high end, heavy flow on the low end.
• Undersized plumbing that drives up head, energy, and noise.
• Auto-fill and level sensing placed on the upper pool, where they cannot work, instead of the basin.
• Make-up water sized for a calm day, so the basin drops and the pump pulls air in real summer conditions.

A vanishing edge is not where you cut the budget. It is where the engineering either shows up or shows.