The Knife Edge Part 1 by Benjamin Lasseter

In recent years the Vanishing Edge pool design has evolved into overflow details only limited by the designers imagination. A detail that started as the simple single wall Vanishing Edge, now has countless iterations in form, execution and detail. We at Design Ecology have designed and built more than a dozen of perimeter overflow and Lautner Knife Edge pools over the last 10 years and are constantly pushing the boundaries of design and construction. This is the first in a series of articles that will guide you through the challenges of designing and building head turning overflow edges.

With every new knife edge project, we seem to modify our mechanical and plumbing design to improve over the previous. One of my major points of concern with the knife edge is minimizing the sound of water echoing through the drainage plumbing and creating unwanted noise near the pool. Its an easy detail to overlook, because all of the pretty photos your client sees of a knife edge pool are just that, photos, they dont make noise! Therefore, they are probably expecting a quiet serene environment associated with this type of pool.

The first few knife edge pools I built, we used a 3” vertical pipe spaced roughly every 5” within the overflow channel. Each of these vertical drop-ins had a 1.5” vent line that that teed off and returned to the channel. This vent line may be the single most important part of the plumbing because it allows the air to escape the pipe on not create a vacuum. If the draining water creates a vacuum, you will have an immense amount of noise and gurgling escaping the pipe!

Eventually I modified the way we were plumbing the vertical dropin to a 45 degree drop in, still using the 1.5” vent line. This was an improvement in noise abatement, but left room for improvement.

My most recent Lautner Knife edges have been installed using 4” pipe installed horizontally, so that the pipe invert is level with the bottom of the channel. The pipe then drops at a 45 degree angle into a trunk line will vary in size depending on the cumulative amount of water the drain is collecting. With this detail, we eliminate the need for a vent line, because the 4” pipe laid horizontally will have enough air space in the pipe to self-vent. However, you must take your flow rate in the channel into account when you calculate the spacing of your inlets. Your horizontal inlet pipe should NEVER be more than half full of water at your maximum flow rate. I have found that he horizontal drop-in with properly sized pipe and edge flow almost eliminates noise created by the gutter plumbing.

We have done several pools that have a Lautner Knife edge in combination with a vanishing edge. This style can eliminate the need for a separate surge tank because you can dump the water from the channel into the basin. The trunk line will be carrying the entire volume of water that overflows the channel when it reaches the basin. If you try to put a 90 on your trunk line (which might be 6”-10” depending on flow rates) to get trunk line pipe invert down to the basin level, you will create a significant amount of noise within the pipe. To avoid this, we like to put a 45degree slope on the pipe from the pool level to the point where it enters the basin. For added level of noise reduction, I put a vent on this transition. If you have to use a vertical pipe in the transition, we recommend you increase the pipe diameter significantly to avoid a diaphragm or piston effect inside of the vertical drop. If you are using a shared basin or a separate surge tank, you will encounter a problem with debris. The entire edge will act like a skimmer and move debris from the pool surface into the tank or basin. If you have done your flow calculations and pipe sizing properly, you will not need any clean outs in the collection plumbing. Bather surge should be enough flow to scour any debris from the pipe. I recommend using a variable speed pump so that you can periodically increase the flow over the edge to help scour the drain lines. We will typically put a debris handling drain like a Paramount MDX R-3 close to the inlet, and will usually have a Paramount In-Floor cleaning system used in conjunction. By doing this, you dramatically reduce the amount of time your client or service tech spends vacuuming the debris that dumps into the basin floor. If you are using a separate surge tank, there are numerous ways to set up a skimmer basket to help catch the debris before it dumps into the floor of the tank.

In the coming issues we will construction techniques related to forming, reinforcing and concrete placement, masonry, decking and huge cannonballs!

Contact me anytime for any questions.

BENJAMIN LASSETER SWD Registered DESIGN ECOLOGY AUSTIN, TEXAS

ben@designecologyaustin.com