Reflecting Absence: The Engineering Behind the 9/11 Memorial Pools

0:00The cat tells the chemistry is good by looking at it in closing clear yesterday from the last time. The saltwater pool is a chlorine pool! This is the Talking Pool's podcast with pool pros from every region in the country. If it happens in a pool, you'll hear about it here. Everything from tips and hacks to the latest tricks and trends. Breaking news? We lay it on the line, we tell it like it is because we think you deserve to know. Before we get into today's episode, I need to acknowledge that today marks 25 years since the attacks of September 11th.

1:02That morning, 19 Alcata terrorists hijacked four commercial airliners. They took planes filled with innocent people and turned them into weapons against civilians. It was calculated mass murder carried out by cowards. They murdered 2,977 people, firefighters, police officers, paramedics, military personnel, office workers, airline crews, travelers, and ordinary Americans who started an ordinary Tuesday believing that they'd make it home. But those murderers do not get the final word. We remember the passengers and crews who fought back. We remember the first responders who ran toward collapsing buildings while everyone else was trying to escape. We remember the strangers who carried one another, the families who endured the unimaginable and a country that refused to stay on its knees. As a veteran and the Long Islander, September 11th will never be just another gate on the calendar.

2:06We remember those who were murdered. We honor those who faced the danger. And we remember the first responders, the recovery workers, the survivors who made it home but carried the injuries and illnesses of that day with them. No joke, no clever transition. We remember. Rising to the cold United we stand. We are Americans. We'll fight. We'll fight for Constitution. God shed His grace on thee. For the fallen heroes who gave their lives to freedom. September 11th, 2001 will always be a fixed point in the life of America. The loss of so many lives left us to examine our own. Each of us was reminded that we're here only for a time.

3:07And these counted days should be filled with things that last in matter. Love for our families. Love for our neighbors. And for our country. Land of the free. Thanks to the brave America. To understand water. These are not swimming pools. Nobody is confusing them with swimming pools. And I would never reduce their meaning to just pumps, filters, hydraulics and chemical treatment. But you know what happens when you spend your life in this industry. You can stand in front of the most powerful water feature in the country.

4:12Feel everything. It was designed to make you feel and party your brain still starts tracing the water. Where's it going? How is it being collected? What kind of surge capacity are they dealing with? How are they keeping the edge that even? The public sees two enormous voids and hears water falling into absence. We see that too. We also see catch basins, pumps, filtration, treatment, sensors, controls, stainless steel, stone, waterproofing, debris removal, and maintenance crew that cannot hide a bad day under a solar cover. This is sacred ground and beneath it some serious engineering. The design is called reflecting absence architect Michael A rad and landscape architect Peter Walker created the winning concept.

5:19It was selected in January 2004 from over 50 to 100 submissions representing 63 countries. The memorial opened on September 11, 2011, 10 years after the attacks and open to the public the very next day. The two pools sit in the footprints of the North and South Towers each one covering nearly an acre. The bronze parapids surrounding them carry 2,983 names, 2,977 people murdered on September 11, 2001, and the six people killed in the World Trade Center bombing on February 26, 1993. The names are not alphabetical. They are arranged through what the memorial calls meaningful adjacencies. Water drops about 30 feet into each square basin from there. It moves toward a smaller opening in the center and it falls another 20 feet disappearing completely from view. Michael A rad described the idea as making absence visible. Water keeps entering the void, but the void can never be filled.

6:26Let's talk about what it takes to make that experience work. Start with the edge. If you've ever fought a vanishing edge, you already know water is the most honest level on the job site. The concrete can look perfect. The tile can look dead on. Then you fire up the pump and the water points out every high spot and every low spot. Like it's been waiting all week just to embarrass somebody. Now stretch that edge around a vessel nearly an acre in size. Each memorial waterfall weir is just under 700 feet long. Across both pools, you're looking at roughly 1400 linear feet of weir. Delta fountains, out of Jacksonville, helped engineer and build the stainless steel fabrications and redesigned the weir structure according to Delta. The early mockups called for about an inch of water over the weir once they tested it, however, they brought it down to roughly half an inch.

7:27It just performed better that way. I think about that from our side of business, half an inch of water flowing evenly across almost 700 feet per pool. That's insane. That is not close enough. They installed stainless steel sill plates on the concrete walls. The rails held bracket supporting small one ton industrial jacks placed every five feet. Those jacks allowed vertical adjustment of as much as two inches while the rails allowed lateral movement for expansion and contraction. Delta says the finished run was adjusted to within one sixteenth of an inch. We've all seen six foot tile lines that could not hold one sixteenth without wandering off these people had to hold it across nearly 700 feet while stainless steel expanded contracted and took everything knew your weather could throw at it. And the water will tell on you. Dry section breaks the effect too much water and another section changes both the appearance and the sound on this project. The weir is not just a piece of the hydraulic system. It is part of the entire message that we at pump numbers.

8:42And this is where you and I need to be careful. The published figures do not agree during construction the memorial reported ten large pumps operating at roughly 40 pounds of pressure and circulating about 52,000 gallons per minute across the two pools. Seamonds engineering account later described each pool is holding about 600,000 gallons moving as much as 30,000 gallons per minute and filtering roughly 6,000 gallons per minute pool magazine. Our friends Joe trusty and Marion trusty both fantastic people they reported 16 pumps eight per pool moving approximately 26,000 gallons per minute delta fountains kind project page says the two recirculating systems together move as much as 18,000 gallons per minute at peak performance. Those numbers conflict I'm not going to pretend they don't they may come from different stages of the project they may describe different pump groups different operating modes waterfall for flow versus treatment flow or design capacity versus actual operation without the current mechanical schedules and piping diagrams.

9:57There is no honest way to match them together and declare one magic number what we can say responsibly each pool has been reported at about 600,000 gallons depending on the source and which part of the system is being described published water effect flow ranges from 18,000 gallons per minute combined to 30,000 gallons per minute per pool. Seamonds separately reported filtration at approximately 6,000 gallons per minute per pool you already know why that distinction matters water moving over waterfall is not automatically water moving through filtration and treatment we cannot grab the biggest flow number divided into the vessel volume and announce a turnover rate without knowing the plumbing configuration. Some of that water may exist strictly to create that visual effect some may be moving through the treatment loop different parts of the system may run at different rates under different conditions a giant gallons per minute number looks great in an article give me the hydraulics schematic and then we can talk turnover what has been confirmed is that the water is collected in catch basins and recirculate.

11:16Delta says the system includes chemical feed that can respond to weather conditions along with a network control system monitoring more than 1,000 operating points more than 1,000 points for two water features that are supposed to look effortless and that is the point of those controls they let the operators see what the public should never have to see. Change in conditions equipment status flow problems and any small warning before it becomes visible across an acre dark granny. Recording from the time described the water as being chemically treated in a way that's comparable to swimming pool water and heated for winter operation. In a piece I wrote in 2019 I identified bromine as the sanitizer UV as the secondary oxidation system and eight pumps per pool but I have not seen a more current public operating specification from the memorial or from Delta confirming today sanitizer residual targets UV equipment filter type controller or water balance parameters.

12:28So I'm not going to hand you seven year old information and call it a current specification. You know better and so do I equipment changes treatment strategies change operating requirements change I read it somewhere is not a spec sheet. Now look at the surface they have to maintain the visible basins are lined with dark granite during construction the memorial reported the installation of nearly 4,000 smooth granite towels each weighing about 420 pounds. That dark surface gives the water its reflection in depth but from a maintenance standpoint it is absolutely unforgiving scale, standing debris even flow none of it has anywhere to hide. Then look at the environment the plaza contains roughly 400 swamp white oak trees they belong there they are part of the design but you and I both know what 400 trees beside open water means. Leaves do not pause out of respect in 2021 filmmaker Joshua Charo documented James Morone in the crew cleaning the polls at night time reported that most of what they remove his leaves although plenty of personal belongings end up down there too.

13:44Chief engineer Anthony LeCosto has also been identified in reporting about the operation pool magazine reported that a three person crew clean the pools five nights a week and that vacuuming and brushing each pool took about eight hours. The waterfalls have to be shut down the crew enters the basin and works beneath the walls about 30 feet high they vacuum they brush the dark granite vessel at night knowing that families will be there in the morning looking for the name of somebody they lost. There's no such thing as good enough at that site maintenance is not cosmetic it is part of protecting the dignity of the memorial. The memorial staff places a white rose at a victim's name on that person's birthday that is huge because two thousand nine hundred eighty three can become a statistic if you let it. The roses remind us that every inscription belongs to one person with a family with a story with a life that should have continued every pump sensor stone panel control point brush stroke and overnight maintenance shift serves the act of remembering those people that is what separates these pools from every resort water feature or record setting found we've ever seen.

15:05The engineering is incredible but the engineering is supposed to disappear nobody's standing at that parapet should be thinking about the one ton jacks under the we're they should not hear a bad pump burning see a dry edge smell treatment chemicals or watch control system struggle with the weather the system succeeds when nobody notices it machinery disappears the meaning remains that may be the highest standard any aquatic system can meet. There are a few things here that should hit home for anyone in our industry first mockups matter your regional we're concept changed after they put water on it and saw how it actually behaved a drawing could not prove what 1400 feet of fallen water would do testing could second water does not care what the plan say tolerance is matter most wherever water can expose them you can argue with a tape measure you cannot argue with a dry section of a water. The dry section of a wet edge third effects flow and treatment flow are not necessarily the same thing before we talk turn over we need to know which water is actually going through filtration and treatment.

16:16Fourth automation does not eliminate maintenance more than a thousand monitor points will not remove a single leaf. The vacuum one square foot of granite it will replace the judgment of the person responsible for the system controls give trained people information they do not make trained people optional also the design has to survive the real world it is not enough for the water effect to look good during a call mockup it has to deal with heat when freezing temperatures rain contamination and expansion and contraction finally maintenance access as part of the design. If the people responsible for the feature cannot safely isolate it enter it inspect it clean it and adjust it then the job was not completely designed it was drawn. Here at this memorial this museum we come together we stand in the footprints of two mighty towers graced by the rush of eternal waters.

17:26We look into the faces in nearly 3000 innocent souls men and women and children of every race every creed from every corner of the world. We can touch their names and hear their voices and glimpse the small items that speak to the beauty of their lives. The wedding ring a dusty helmet a shining badge here we tell their story so that generations yet unborn will never forget. I visited the memorial in 2019 I went there as a new yorker I went there as a veteran and because I spent most of my life working around water I could not help but see it as a pool professional too.

18:27The scale of the system is staggering but when you stand at that edge the specifications stop being the point you read the names you hear the water lower man hat and keeps moving all around you. Traffic construction voices all of it but the sound of the fallen water changes the space. It separates that ground from everything happening beyond it that did not happen by accident it took architects landscape architects structural engineers mechanical on electrical engineers found the specialists fabricators control specialist construction crews operators and maintenance professionals. The pools make absence visible the people who care for them make remembrance possible every day. You and I can appreciate the engineering without ever losing sight of why it exists to pools nearly two acres of surface area more than a million gallons of water.

19:27A 30 foot waterfall followed by another 20 foot drop nearly 1400 feet of precision weir thousands of granite panels more than a thousand monitored operating points and two thousand nine hundred eighty three names. 19 years ago on this day at this very hour on this field 40 brave men and women triumph over terror and gave their lives in defense of our nation to the family members of flight 93 today every heartbeat in America is wedded to yours your pain and anguish is the shared grief of our whole nation. Heroes of flight 93 are never lasting reminder that no matter the danger no matter the threat no matter the odds America will always rise up stand tall and fight back today we pay tribute to their sacrifice and we more deeply for the nearly three thousand precious and beautiful souls who were taken from us on September 11th 2001.

20:47I'm really stankwits this is the talking pools podcast today we remember. I just wanted to give a minute to say thank you for listening today I'm hoping you enjoyed the episode as much as we enjoyed putting it together for you listen it's been a couple of wacky crazy screwed up years from pandemic to pool again. I just want you to know that we are all in this together if there's anything that we can do for you send me an email at talking pools at gmail dot com again that's talking pools at gmail dot com we're here this is your podcast we are the pool people's podcast of the pool people for the pool people by the pool people's podcast this one is about you so thank you for tuning in listening to me a favor click subscribe before you go that way you don't miss an episode.

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