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200804BVB0By Brian Van Bower

As we discussed last month, perimeter-overflow details are among the most distinctive and challenging of all features in today’s custom pool market.  

In March’s column, we defined the different types of these edge designs, then described the complex, exacting process of building a knife-edge overflow system.  This time, we’ll get into the hydraulic finesse needed to make these systems work.  This includes everything from calculating bather surges to sizing the plumbing and surge tanks needed to make these effects function reliably for the long haul.

Before we get into the numbers, however, let me restate the caution I offered last month:  Designing and building these systems is not for beginners:  If you’re thinking about tackling a perimeter-overflow system for the first time, it’s important either to bring in an expert consultant or take all the time or classes you need to gather information and nail down every detail as tightly as possible.  Otherwise, you might run into big trouble once the system is supposedly complete.


Hydraulically speaking, the first thing to keep in mind is that these systems work by gravity flow on the suction side of the pump.  Water is added to the pool to the point at which it flows over the edge, moves into the trough, proceeds down through plumbing drops installed (in our case) at five-foot intervals and then, finally, enters a trunk line that slopes down to feed a surge or collector tank.  

In other words, it’s all about gravity until the water reaches the surge tank, at which point a pump applies suction to draw the water through a filter before it is reintroduced to the pool.  Keeping this gravity factor in mind is important because it influences the sizing of the trunk-line pipes.  

That all seems clear now, but several years back, when my Genesis 3 partners and I started teaching about water-in-transit designs, we advocated using a chart that based plumbing selection on how much water a pipe of a particular size would handle based solely on the slope of the line.  In other words, sizing was calculated relative to a slope of one-eighth-inch, one-quarter or one-half inch per foot – and that was that.

In the field, however, we observed that slope wasn’t the only factor we had to consider, basically because the charted calculations were based on flow through a full pipe – a condition that prevails in pressurized systems but occurs only rarely in gravity-flow systems.  It didn’t take us long to recognize that basing plumbing size solely on flow down the slope wasn’t enough and that we had to base our choices both on slope and the flow over the totality of the edge.

To make a long story short, this led us to specify significantly larger trunk-line pipes than slope-alone calculations would indicate.  The result is that, today, we teach our course participants that the calculation for plumbing size begins not with slopes, but instead with the amount of water flowing over the edge.

Let’s illustrate all of this with a simple 40-by-20 foot rectangular pool with a knife-edge overflow system and, therefore, an edge 120 feet long.

Assuming you can achieve a tolerance along that entire edge of plus-or-minus 1/32 of an inch, we start by calculating the flow over the edge as one-half to one gallon per linear foot per minute.  For purposes of illustration, let’s assume a flow rate here of one gallon per linear foot per minute.  (Yes, it’s possible to achieve the edge effect with a lesser flow, but experience shows us that, to make up for bather surge and displacement fast enough to maintain the edge effect, you can’t work reliably with a flow rate of less than one-half gallon per foot per minute.  A variable-speed pump can be a help here.)


Working with that one-gallon figure, we’re looking for a total of 120 gallons to flow per minute over the 120-foot edge of the pool and into the gutter.  If we base our plumbing capacity on this flow over the edge rather than just on the slope of the plumbing, it’ll soon be obvious that we need to upsize the plumbing beyond the basic guidelines of strict gravity flow/slope calculations.  

We need to think in terms of larger pipes because we know that a four-inch pipe at a quarter-inch slope will handle only 53.4 gallons per minute when half full (according to a standard chart on approximate discharge rates in drains flowing at half full).  Increase that to a half-inch slope, and the same four-inch pipe only handles 75.5 gallons per minute – still not enough.

Keeping in mind our desired flow rate is 120 gallons per minute, we know that the four-inch pipe just isn’t going to work.  If we go up to a six-inch pipe at a quarter inch slope, however, we know it will handle 157 gallons per minute half full, moving the water at a rate of about 3.75 feet per second – well within our desired maximum line velocity of six feet per second.  

Now we have a plumbing size that more than handles the 120-gpm flow rate.  And the fact that this flow rate is based on the line being half full – the common status of gravity-fed systems – means we’re well within an acceptable performance range.

(As an aside, if you get in a situation where you’re building a much larger system with a much longer edge, you can accommodate the larger flow rate without increasing the size of the plumbing – and blowing out the size of your bond beam, as we discussed last month – by splitting the trunk-line system in two so that water flows away from a central high point in opposite directions.  That way, the plumbing handles only half of the total flow at any given point, meaning it can be smaller than if it’s set up as a single loop handling the entirety of the flow over the edge.  In another option, we just completed design work on a very large pool that calls for four separate eight-inch trunk lines spaced around the pool to handle the tremendous flow.)

The water flowing through the trunk line eventually flows to the surge tank – another area in which sizing calculations are critical.  In fact, I probably get more questions about setting up these tanks than I do about any other system detail because tank sizing is a genuinely complicated issue.  

The big questions about surge capacity have to do with the impossibility of knowing just how many bodies might jump into the pool at the same time, the size of those bodies and the overall level of displacement.  There’s also weather-related displacement.  In fact, I’ve seen a high wind blow two inches of water out of a pool in rapid order.   

There are a variety of ways to look at this issue, but the Genesis 3 perspective – based on years of collective practical experience and consultation with numerous experts – leads me to calculate surge capacity based on two inches of displacement for the entire surface of the pool. Yes, more than that might be displaced were an entire flock of sumo wrestlers to descend upon a given pool all at once; under most circumstances, however, we’ve found that the two-inch figure for surge capacity keeps everything well within an acceptable working range.

This is, however, strictly a guideline, and you need to draw your own conclusions based upon what you know about the project, the clients and the setting.  Size is also a factor, and you may see the need to adjust your thinking if you’re working with a small pool or spa – or when you’re facing a very large pool.


Getting back to our 40-by-20-foot pool, we have 800 square feet of surface area to consider.  A square foot of water contains approximately 7.5 gallons.  Multiply the square footage by 7.5 and then divide it by six, and you get to the desired total volume of two inches of water for the entire pool. In this scenario, that calls for approximately 1,000 gallons of surge capacity – and now we can begin sizing the surge tank.

That word “begin” is important, because there’s a lot more to consider here beyond bather surge.  That number, for example, does not include the minimum operating level within the tank – that is, the water volume needed to keep the pump from cavitating and to maintain proper flow through the system.  In a typical project, that minimum operating level will require the presence of one foot of water in the surge tank at all times.  

Let’s assume that we’re building a surge tank out of concrete in an area that allows for interior dimensions of five by eight feet.  This means that this surge tank has a flat surface area of 40 square feet.  

To accommodate the minimum operating level, you multiply 40 square feet by the 7.5 gallons in a cubic foot of water, which gives us 300 gallons for every foot of depth.  So now we know that, in addition to the 1,000 gallons of surge capacity we need to handle flow through the trunk line, we also need to add in the 300 gallons to accommodate the minimum operating level – in other words, we’re after at least 1,300 gallons of surge-tank capacity.

Doubling back, we know that we have 300 gallons per foot of depth, so, dividing 1,300 by 300, we find the tank needs to have a depth or four feet, four inches.

But collector tanks should always have freeboard above the calculated capacity to allow for installation and proper functioning of an auto-leveling system as well as overflow to waste.  So if we think about an additional eight inches of depth (adequate for functioning of the auto-leveler), we’re looking at a tank that will need to be five feet deep.  

But now there’s an X factor:  In sizing a surge tank, if you’re going to be off in any way, you always want to err in favor of making the tank too big.  While one that’s too small will create a range of problems, all that having one that’s too big means is that it can accept an excessive surge should it occur.  In other words, in this scenario, if you made the tank five and a half or even six feet deep, that only means the system is able to handle more water – and that’s a good thing.


There’s another key issue with surge tanks that bears consideration – that is, the level at which they should be placed relative to the pool.

In many commercial applications, these tanks are set at the same level as the pool, meaning the maximum level in the tank (in this case four feet, four inches) and the point at which we’d install the overflow line.  This all makes sense, because it eliminates concerns about equalizing the levels of the two bodies of water.

The drawback in this arrangement, however, is that when the system is shut off, water seeks its level and there won’t be any water at the level needed to wet the top of the edge.  This compromises the pool’s appearance, and we know that clients opt for this detail mainly because they like the way it looks.  To address this situation, you either need to run the system all the time or accept the fact that it just won’t look as good when it’s shut off.

Moreover, when there’s no movement of water through the gutter, you lose what’s known as “gutter scrub,” which is the flow that carries debris to the drop pipes (which are never grated because you want debris to flow through the system) and ultimately into the surge tank.  When the maximum level of the tank and the pool are the same, water won’t move through the system when it’s off and debris will stay in the gutter.   

Keeping Things Clean

With perimeter-overflow systems, there are always questions about maintenance of the gutters and the surge tank.

When we first started building these systems, we placed removable pieces of deck material at the corners of rectilinear pools so that they could be removed and someone could use a hose to blow out the gutters.  This was aimed specifically at situations in which excessive amounts of debris would be forced into the system – as by a hurricane, for example.

Through experience, however, we’ve found that this detail isn’t really necessary.  With a one-inch opening in the gutter slot, you can easily take a garden hose with a pressure nozzle and rinse the gutter to displace debris of any sort (such as beach sand) that might not readily flow to the surge tank.  It’s really that simple.

Surge tanks are a bit more involved when it comes to upkeep.  The debris that collects in these tanks can affect water quality if left in place for more than a limited time, which is why it’s important to stress maintenance with homeowners and set things up in such a way that someone can gain access to the tank to net it out or vacuum it.

To ease this process, we’ve been experimenting with in-line nets or baskets placed just below the main trunk line’s entry point to the tank.  This has some potential, and we’re continuing to play with an array of available options.   

Finally, in all of our designs, the perimeter-overflow system is on a completely separate loop from the primary filtration system, and water flowing out of the surge tank is always filtered separately. That doesn’t take care of large debris on the bottom, but it does help maintain overall water quality.  


Given these factors, my preference is to install the surge tank so that its maximum operating level is lower than the bottom of the gutters. That means that when the system is turned off, gravity will continue to move water through the system until the gutters are evacuated and debris has flowed into the surge tank.  (For more on debris and system maintenance, see the sidebar at right.)

Finally, it’s a hard and fast rule that in all water-in-transit systems, without exception, the vessel that collects the water must have an automatic leveling system.  The reason is simple:  If the water in the tank drops below its minimum operating level, the system will run dry and the pump will cavitate.

In my practice, we use an auto-leveling systems manufactured by Levolor, a longtime supplier of these systems that was recently acquired by Jandy Pool Products of Petaluma, Calif.  The model we use has two sensors, one at the minimum operating level and the other at the maximum level (that is, at the overflow).  When water drops below the lower sensor, it activates a solenoid that opens a valve connected to the water supply and adds water until the sensor is wetted once again.

The upper sensor is used to activate the circulation system (if it’s off) when the water reaches the overflow level.  This is useful in case of excessive bather surge or weather-related challenges and ensures that the system doesn’t simply lose water but rather circulates and filters it when surge is excessive.  

The only downside:  If it rains for 40 days and 40 nights, the system might stay on for the duration – a possibility we discuss with clients.  All in all, however, that minimal possibility is far outweighed by the upside of having these dual-function auto-levelers in place.


As with most complicated design challenges, when you break down all of the components and considerations into individual bits, no single element of installing a perimeter-overflow, knife-edge pool is all that daunting.  Still, the issue with these systems is that they are extraordinarily unforgiving:  Every single detail discussed last month and this month must be done right the first time, because if anything goes wrong, it’s never an easy or inexpensive fix.

And while we’re talking details, let me make a last few hydraulically relevant points:  First, the water drawn from the collector tank must always be filtered before it returns to the pool.  Second, the return piping must have a loop that reaches above the maximum water level and must be rigged with in-line check and vacuum-relief valves to prevent equalization.  Third, the water should then be returned to the pool at a low level or through a floor return to minimize turbulence – a topic we’ll address more fully in another column.   

If you have questions or doubts, it always makes sense to bring in an expert who knows these systems front to back.  Just reading through these two columns in no way qualifies anyone to build one of these without further (and extensive) support:  There are simply too many variables, and as anyone deeply involved in custom watershaping already knows, every situation is going to be different and present its own, specific set of challenges.

For all that, the upside is that these water-in-transit designs are indisputably a wonderful way to deliver something very special to your clients.  When you master these features, you can be very proud of the fact that you’ve just moved to the head of the class.


Brian Van Bower runs Aquatic Consultants, a design firm based in Miami, Fla., and is a co-founder of the Genesis 3 Design Group; dedicated to top-of-the-line performance in aquatic design and construction, this organization conducts schools for like-minded pool designers and builders.  He can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it..

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