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Reasonable Choices

Suction-entrapment incidents and their causes have been of passionate interest to mechanical engineer and commercial pool designer William N. Rowley through more than 30 years of tests, investigations, debates and high-profile lawsuits. Here, he uses his standing as an expert to define what’s involved – and to promote safe engineering and construction practices while offering a comprehensive look at ways of preventing these incidents.

Suction-entrapment incidents and their causes have been of passionate interest to mechanical engineer and commercial pool designer William N. Rowley through more than 30 years of tests, investigations, debates and high-profile lawsuits.  Here, he uses his standing as an expert to define what's involved - and to promote safe engineering and construction practices while offering a comprehensive look at ways of preventing these incidents.

It’s one of the most horrific things that can happen to anyone who enters a pool or a spa: One moment you’re having fun or relaxing, and in a terrible instant you’re caught in a devastatingly painful and potentially fatal situation.

Most people who become entrapped by pool, spa or wading-pool plumbing do survive, but all too often they suffer life-altering injuries. As with any aquatic safety issue, we all agree these incidents should be prevented, and a great many talented people from government, trade associations, research institutions, equipment manufacturers and consumer-safety groups have invested a tremendous amount of time in examining suction entrapment. For all of that effort, however, seeing our way to adequate solutions has proved a difficult and persistent challenge.

The fact that there are different types of suction-entrapment incidents that happen in varying combinations of conditions and in various settings is what makes determining exact causes and suggesting remedies so frustratingly complex. Furthermore, the fact that children are often the victims of these gruesome accidents makes emotions run so high that reason is often a second casualty.

But fortunately, suction entrapment is a matter of applied hydraulics. As a result, we can turn in the clear light of day to hard facts of science and engineering in searching for answers. By studying how and why these incidents occur and applying what we learn with scientific testing in controlled conditions, a much clearer picture of the problem – and its solutions – unfolds before us.


Let’s begin with a basic definition: A suction-entrapment incident is one that involves a bather becoming trapped on a drain. The bather is either injured by the force of the entrapment itself or drowns or nearly drowns.

There are four basic types of suction-entrapment incidents:

[ ] Body entrapment; in which (usually) the stomach, abdomen, hip or posterior becomes trapped on a “single-suction” main drain with a broken or missing cover.

[ ] Limb entrapment, in which the person has a hand or leg sucked into the suction pipe in the sump of an uncovered main drain.

[ ] Evisceration, in which someone sits on an open or broken main drain and is disemboweled.

[ ] Hair entrapment (entanglement), in which a person is trapped by entanglement of their hair in a grate. This is not a result of suction alone, but rather has to do with turbulence caused by water velocity.

There is a fifth type of incident included by some who study these issues: This category is referred to as mechanical entrapment, in which someone gets a digit stuck in a hole of some kind and can’t get it out. This doesn’t have anything to do with suction, and the steps you’d take to prevent the four main types of suction entrapment have very little to do with preventing mechanical-entrapment incidents.

None of these incidents occur with any great frequency. From 1985 to 2002, the Consumer Product Safety Commission (CPSC) documented 147 entrapment incidents, 36 of which resulted in death. Additional incidents may have gone unreported during that time frame, but it’s safe to say that all known incidents – especially those involving serious injury or death – have been investigated in painstaking detail.

Many of these incidents have been the subjects of lawsuits, some resulting in huge monetary awards to victims. A small number of these cases have been covered extensively in the media and have generated lots of rhetoric from sources both informed and uninformed.

What stands out is that each incident, whether notorious or obscure, fatal or survived, involves unique sets of circumstances and a highly individualized profiles of exactly what went wrong.


The most dispassionate way to examine this collection of incidents is to break everything down into statistical terms as matters of hazard/risk analysis. In simplest terms, hazard equals risk multiplied by exposure.

In applying such measures, it must be considered that entrapment incidents occur in three different types of bodies of water – swimming pools, spas and wading pools – with each presenting a different level of hazard.

Technically speaking, the risk in all three bodies of water is the same in that suction-entrapment risk can be defined as occurring anytime someone comes in contact with an open main drain – more specifically with a single-suction main drain in which only one line is plumbed to a pump.

Bear in mind, however, that the exposure represented by these three categories of bodies of water is completely different, which is why the hazard is different between them. For example, you would expect a child playing in an 18-inch deep wading pool to sit on a drain. Likewise, it’s not unexpected for someone to come in contact with an open drain in a three-foot-deep spa.

In a swimming pool, however, main drains are typically (but not always) found in deeper water, so the exposure is not the same. You won’t typically see people sitting on main drains in deep ends of pools; what you will see, with far greater likelihood, is someone who sticks his or her arm into an open drain.

Each type of incident in each type of vessel carries its own set of statistics for different sorts of circumstances. For example, I personally know of only one hair-entanglement incident that has occurred in a swimming pool. We also know that most hair entanglements occur in spas, while limb and body entrapments (but not eviscerations) occur in pools. Eviscerations typically only happen to small children, and almost exclusively in wading pools.

How you approach the study of these problems and how you weigh the statistics must depend on the conditions that influence the hazard.

As you look closely at how these incidents occur, key observations emerge that can be used in drawing important conclusions. I’ve looked quite closely myself and have participated directly in several studies on entrapment – including various tests in which I have used my own body as a test subject, deliberately “trapping” myself on exposed drains under carefully controlled conditions.


One of the important things we’ve learned through such testing is that when you get near an open drain, you can’t feel any suction at all, even when you’re just a few inches away. To become trapped, you have to get right down on the drain, at which point lightning strikes and you’re in trouble.

We’ve also learned that with a drain cover in place – be it an anti-vortex drain cover or a standard drain grate – it is virtually impossible to suffer a body or limb entrapment or an evisceration. Moreover, we’ve observed that in systems with split main drains, you cannot become trapped even if a grate or cover is missing.

The facts observed from incidents in the field are consistent with these findings: In every case of suction entrapment (with the exception hair entanglement), you’ll find a broken or missing main-drain cover combined with single-suction plumbing.


To determine what happens in suction-entrapment incidents under observable, recordable conditions, we developed a test stand that features drain lines set up with a variety of covers and grates. (There are also open sumps to replicate situations in which a cover or grate is missing.) The stand was placed in the shallow end of a pool for actual testing. During these tests, valves and flow rates were precisely monitored and controlled.

That’s a huge problem, given how many pools built through the years have been plumbed with single-suction drain configurations, and the hazard is compounded when the drain covers are missing in these vessels. (It’s important to note that testing as well as anecdotal evidence show that entrapment incidents simply do not occur in pools, spas or wading pools equipped with dual or multiple main drains.)

Controlled studies and field data also indicate that proper flow rates established in conjunction with appropriate drain covers and grates will prevent suction entrapment. I don’t know of a single incident in which a cover approved by the American Society of Mechanical Engineers (under ASME/ANSI A112.19.AM-1987, “Suction Fittings for Use in Swimming Pools, Wading Pools, Spas, Hot Tubs and Whirlpool Bathtub Appliances”) and operating within the specified flow rates has been involved in any kind of entrapment incident.

A study conducted by the National Swimming Pool Foundation (NSPF) in 1997 concluded that maintaining a pump suction velocity of less than six feet per second with covered drains (either single or dual) will relieve entrapment concerns. To be sure, excessive flow rates may result in hair-entanglement incidents even if the cover is in place – but only in situations in which the flow rate exceeds the cover’s or grate’s specified operating conditions.

Obviously, flow rates are a different issue in systems with extremely large plumbing, such as those you might find in a waterpark, lake or reservoir. In these systems, where plumbing might measure 18 or 24 inches in diameter, you run into a condition where, even at slow flow rates, the kinetic energy of the water’s flow can suck a person into an uncovered pipe.


Moving back to residential and commercial pools, the maximum-flow standard of six feet per second is endorsed by CPSC, NSPF and the National Spa & Pool Institute and has been accepted by a majority of health departments throughout the country.

There has been less consensus among the experts, however, when it comes to whether or not anti-vortex drain covers provide a greater level of safety than do grates – and no statistical evidence to support the superiority of either type.

In my view, both grates and anti-vortex covers are effective safety measures, the key being whether or not they are properly attached. I’ve always been somewhat concerned that, in shallow-water applications, anti-vortex drain covers present a tripping hazard because of their slightly raised profile, but the bottom line is clear: Both systems work in preventing accidents – and no expert I’ve ever encountered has ever questioned that assumption.


For the testing, I entered the water and positioned myself over the operating test stand, attempting to entrap myself on individual grates, covers or open sumps under a range of test conditions. Those conditions were quite realistic, as the welts on my midsection testify. What we determined is that entrapment occurred only with single-drain configurations; no such entrapment occurred with split-drain systems.

Keeping any drain cover attached is the key to long-term safety and is essentially a service/maintenance issue of simple but profound importance. Indeed, it is possible that the number of entrapment incidents could be reduced by an ongoing campaign to encourage service technicians, health-department inspectors and certified pool operators to make the checking of grates and drains a top priority.

(Along more mechanical lines, it has also been suggested that manufacturers could help by making bolts or other fasteners for drain covers in a different color from the drain itself, making for easier visual inspection from the surface of the water.)

What is increasingly clear in most conversations about entrapment issues is that drain configuration is the other key component here, and the one that has proved most controversial. For my part, I am a strong advocate of the concept of split main drains and see them as a logical remedy for entrapment problems. At the same time, I see opposition to split drains as ill founded and a denial of statistical data and information gathered from the field.

Categorically, I do not know of one suction-entrapment incident that has ever occurred in a system with a functioning split main drain, regardless of the presence of a grate and even in a system with an excessive flow rate. In my view, this simple fact means that every pool, spa and wading pool built should be equipped with split drains at least three feet apart (as defined by a number of studies).

I’ve conducted tests and investigations in the field during which I’ve tried to trap myself onto drains with missing grates in the presence of flow rates well in excess of six feet per second. So long as the drain is split, absolutely nothing happens.


All of this is why, in 1997, the California legislature passed Senate Bill 873, a law requiring that all wading pools be plumbed with split main drains. So compelling was the reasoning behind it that the law was actually made retroactive and closed all existing, non-compliant wading pools until they could be remodeled.

A great deal of controversy and debate surrounded passage of this bill, largely because of the key provisions that made it retroactive. But some aggressive lobbying on the part of the pool and spa industry, a good bit of late-minute politicking and the fact that there really aren’t residential wading pools ensured the bill’s passage.

It is worth noting that there has not been a single entrapment incident of any kind in any California wading pool since the bill became law. Also in California, the legislature passed Senate Bill 1726 in 2002. This law requires split main drains on all new spas and pools.

The challenge of passing legislation of such clear merit points up the difficulty of mandating the split-drain solution as a virtual cure-all for the entrapment problem: There are literally millions of pools and spas out there that were built with single-drain configurations, and there’s no practical way to mandate retrofitting for every single one of them. As a consequence, with single-drain systems we must rely on proper flow rates and especially on covers to prevent entrapment incidents.

In some circumstances, a drain can become plugged – a point often made by those who go on to claim that split drains plug more easily than do single drains. Nothing in the data suggests that this is the case, although I know of one case in which a single pressure-test plug was left in a split-drain system, causing it to function as a single drain. To that point, it is a matter of common sense to note that for split drains to be effective, two or more drain lines must be open.

Also, it’s important to note that testing has revealed that the skimmer does not function as a second drain when it comes to preventing suction entrapment. This means that a single, uncovered drain sharing a suction line with a skimmer can still invite entrapment. We’ve found that because skimmers are set at different elevations from the main drain, they function differently than do drains at the same elevation and therefore do not provide the safety of split drains.

The design standard we use, as called out in the California legislation, is based on the simple idea that if one of the drains is plugged, the other will continue to operate within design parameters with respect to flow rate through the grate or cover.

As mentioned above, these drains must be three feet apart. This is based on anthropometrics of the human body: With the drains set three feet apart, human beings are simply not big enough to lie across and block both drains. In addition, the T for the split between drains needs to be far enough away from both drain apertures so that a person can’t reach down into the drain and get his or her hand caught in a single-flow situation if one of the drains is open.

In any new installation, it is in my opinion simply dangerous and obsolete to install single drains. Furthermore, retrofitting existing bodies of water with split drains should be suggested, promoted and encouraged at every turn and in any way possible.


All of this anecdotal and statistical data boils down to some straightforward assumptions about situations that are dangerous: Specifically, swimming pools, spas and wading pools with broken or missing drains covers or grates and single drains combined with excessive flow rates are all conditions common to the vast majority of entrapment incidents.

At the close of its 1997 study of suction entrapment, NSPF stated flatly, “Only if all three hazardous conditions are prevented does a safe condition exist.” I agree wholeheartedly with the inescapable logic of that conclusion and the attendant notion that safety must therefore be defined as redundancy within any given system.

Breaking the Circuit

Suction vacuum-release systems, or SVRSs, encompass a relatively new category of sensitive mechanical or electro-mechanical devices designed to sense a change in suction pressure and, in response, either open a vent valve or turn off the pump to prevent suction-entrapment incidents.

My own exploration and evaluation of these technologies leads me to believe that they are yet to be perfected, but it is nonetheless sensible to suggest that such devices provide yet another layer of safety. And certainly, such systems may prove useful as an aftermarket item for use on pools with single-suction drains.

I do not, however, view SVRS technology as the magic bullet some system promoters have suggested. To my mind, there is no substitute for sound hydraulic design and proper construction practices in preventing suction entrapment.

More specifically, because SVRS systems operate by sensing a change in suction pressure, they do nothing to prevent hair-entrapment incidents, which do not always result in complete blockage of a drain grate or cover. Furthermore, medical data indicates that in evisceration incidents, disembowelment occurs almost instantaneously, and SVRS systems will never be able to deactivate a pump in time to prevent injury: Shutting off the pump does nothing immediate to stop the inertia of water flowing through the system.

Proponents of these technologies are working hard in legislatures and courts to make their point that equipment manufacturers should be required to rig their pumps with SVRS devices. It is my view that this is the wrong solution for suction entrapment: Pumps do not cause suction entrapment; rather, improper hydraulic design, installation and maintenance do.

To my mind, prevention is best achieved using the principles of hydraulics and what science teaches us about the physical characteristics of the human body.

— W.N.R.

According to CPSC, NSPF and ASME, the first safety measure required to prevent suction entrapment is a grate or cover in good operating condition, attached so that it can’t be removed except with the use of tools, meaning a screw driver or a wrench of some kind. Second, the cover should be approved and should operate as designed. (ASME established these operating standards in 1987 – a development that stands as one the key advances in the fight against suction entrapment.)

By most definitions, split drains are a second layer of protection, and it’s important to point out that they needn’t be limited to two in number. I’ve heard some people argue that using two drains simply means that two people can drown instead of one in the unlikely event that two people could get stuck on two uncovered drains at exactly the same instant.

This has not happened to my knowledge, but there’s nothing that says three or four drains can’t be used instead of two. I would argue that the likelihood of three or four people sticking on three or four open drains at the same time is so incredibly unlikely that it represents no measurable hazard.

As stated above, suction flow rates at or below the six-feet-per-second threshold are crucial. With commercial pools, the situation changes somewhat because you can end up with designs with enormous pipes and massive pumps turning over hundreds of thousands or even millions of gallons of water in a matter of hours. At that level, attention to line velocity and plumbing configurations is even more critical, which is why, in typical 50-meter competition pools, we’ll install up to four main drains, each covered by a pair of 18-inch-square grates, giving each a total of 36 by 18 inches of grated coverage. These grates are so big and the flow through them is so slow that there is no chance that anyone could become trapped on them.

A further safety measure that can be employed with any sort of drain system, commercial or residential, involves the use of atmospheric vent tubes – essentially standpipes that are plumbed to the main drain line and that, in the event all drains are blocked, allows a pump to pull air into the suction line. This causes the pump to lose its prime rather than dead heading and is a proven solution to entrapment problems; obviously, however, it can only be used with new installations or in the event of major renovation.

Shut-off switches are another measure of safety, and they’ve been successfully employed in numerous spas and wading pools. With large commercial pools, however, you run into the problem of people shutting the pool off for other than emergency reasons, be it a prank or just a mistake. When you deal with the cost and time involved with restarting some of these complicated systems, many of which run on three-phase power, these infrequent shutdowns can become a maintenance and operational nightmare.

Finally, there is a new class of devices known as suction vacuum-release systems (SVRSs) that can also be used as an added measure of safety in single-suction systems. For more on this technology, see the sidebar just above.


The encouraging and abiding truth about suction entrapment is that it is entirely preventable. For all of the complexities and case-specific intricacies of these incidents, we know what solves the problem: proper covers or grates, split drains and specified suction-side flow rates. Based on what we know today, to argue against these measures is to resist the preponderance of evidence and the weight of reason.

The problem, of course, is that the real world is rife with bodies of water that are lacking one or more of the necessary safeguards and can and will regrettably become sites for future entrapment incidents. For that reason, it is incumbent on everyone in this industry to work in any way we can to promote and insist on these fundamental solutions and to continue to study the problem with the further objective of refining our understanding of this vexing issue.

William N. Rowley, PhD, is founder of Rowley International, an aquatic consulting, design and engineering firm based in Palos Verdes Estates, Calif. One of the world’s leading designers of large commercial and competition pools, his most notable projects include partial designs for the competition pools used in the Olympic Games in Munich (1968) and Montreal (1972), and he acted as aquatic consultant for the design of the Olympic Pool Complex in Los Angeles (1984). His projects also have included a wide range of non-competition pools, including the White House pool in Washington, the Navy Basic Underwater Demolition Training Tank in Coronado, Calif., and the resort pool at the Hyatt Regency at Kaanapali Beach on Maui. Dr. Rowley is involved in a range of local, state and federal entities, consulting on construction and safety-code requirements and was recently named a fellow of the American Society of Mechanical Engineers.

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