[samldev]

I was a college swimmer, qualified for Olympic Trials in 2012 and 2016. There are absolutely slow and fast pools. It basically comes down to two things:

1. The depth - which is only 7ft in Paris, unusually shallow for a competition pool.

2. The sides. Does the water spill over the sides into the gutters, or smash into a wall and bounce back, creating more chop.

A trained eye can see all the swimmers in Paris struggling in their last 10-20 meters (heck, an untrained eye can spot some of these). Bummer that it makes the meet feel slow but at least it generally affects all the swimmers equally

[madaxe_again]

I actually did a fairly lengthy research project on pretty much exactly this as a physics undergraduate - I wasn’t looking at swimming specifically, but rather boundary separation and Reynolds number in an open channel of varying depth.

The setup was simple - a constant head vessel to provide a constant but adjustable flow of water in from one end, and a little plastic boat sat in the middle of the channel, attached to a force gauge at one end of the channel. The outflow of the channel had a gate with an adjustable height in order to vary the depth. Also, a couple of dye injectors at different heights in the channel in order to see turbulent vs laminar flow.

The key finding was that at shallower depths, turbulent flow began much more rapidly and resulted in erratic but overall higher resistive forces on the boat. Deep water remained laminar for much longer, and could flow much faster before turning turbulent near the surface. This was the expected result, but it was nice to experimentally prove it.

So in short, the pool depth almost certainly impacts the point at which turbulence kicks in, and therefore athletic performance. It’s probably the dive/entry that is being most impeded, as that’s when the swimmer will largely be experiencing laminar flow.

[bonestamp2]

Is there a formula to calculate the ideal depth at which going any deeper results in diminishing returns? In other words, how deep should future competitive swimming pools be built to provide the most world record opportunities (and perhaps more excitement for spectators due to increased world records)?

[bjoli]

Does it manifest directly? The water has had some time to settle when they start.

[askvictor]

At some (shallow) point, does a 'ground effect' kick in? Or any sort of constructive interference?

[conductr]

My untrained eye has noticed. But I also think it's not really a big deal. There are so few events where the conditions are exactly the same every 4 years. Just kinda the luck of the draw if you happen to be competing in the most ideal conditions for WR setting in any event

[kergonath]

Also, the important bit is fairness for all competitors. As the OP said, the same conditions affect everybody. I have little sympathy for the (few) swimmers complaining. They are not owed a world record and if they’re that good, they’re going to get one anyway.

[trhway]

> But I also think it's not really a big deal.

the difference of the resulting turbulence from the wave bounced back from the bottom surface at 2m here and from the more traditional 3m is a big deal. The water is pushed by the swimmer's hands with the speed of something on the scale of 2 meters per second, so, as the swimmer moves forward, that turbulent movement of the water reflected by the pool bottom may as well come behind the legs in the 3m depth case while in the 2m depth case it would catch the legs decreasing the efficiency of their movement.

[LargeWu]

Additionally, anybody good enough to be prevented from setting a world record because of this pool will undoubtedly have multiple chances in other competitions with faster pools.

[thaumasiotes]

> There are so few events where the conditions are exactly the same every 4 years.

The Olympics used to be held at Olympia. It's not difficult to make the conditions the same every four years.

[tweetle_beetle]

I'm no expert, buy there also seems to be loads more stuff lying on the bottom of the shallow floor than I remember from previous Olympics. Not even sure exactly what it is - large white panels and other equipment that the swimmers don't look that far off touching when underwater (not the robot cameras which are relatively unobtrusive).

[ranie93]

digital lap counters to help the swimmers keep track

[jonhohle]

Wouldn’t the underwater camera on a track cause currents?

[mysterydip]

Would the swimmers in the center lanes have any advantage by being furthest away from the wall-induced chop?

[etempleton]

Yes, those on the outside will have choppier waters as the water bounces off the sides of the pools. The modern competition pools do a pretty good job of reducing this effect, but it is always there.

[hgomersall]

Supposedly, that's why the fastest qualifiers get the middle lanes.

[jschulenklopper]

And having a better view on the competition in the neighboring lanes (just like in track running). There's even some applied psychology here, that competing swimmers 'push' each other to higher speeds because they can see each other more clearly (and 'feel' the push from someone just lagging).

Putting the faster qualifiers in the middle lanes is also a better view for the spectators on both sides of the pool.

[johnp314]

It would be interesting to see a comparison of lane effect, say for instance, re-running a race after let's say a weeks rest with the top finishers now nearest the side walls and the lowest finishers in the center lanes. Oh and for incentive, let's say the average of their two times determines the winners.

[ldng]

Supposedly, all the Paris 2024 new accommodation were designed with re-usability for the general public after the Olympics. Is it possible that had an impact the swimming pool on design choices ? Put differently, are performance design at odd with more general/accessible design in the case of a swimming pool ?

[tw04]

Given the pool was put on top of a rugby pitch, which resulted in the shallower depth because they didn't want to destroy the pitch (a full depth pool would have weighed too much) - unlikely. The pool isn't a permanent structure, it's not going to remain there after the olympics are done.

I would be very surprised if they re-use it at all - an "above ground" pool of that size seems like it would be more trouble than it's worth to maintain over the long run.

A timelapse of the pool being put together: https://www.youtube.com/watch?v=jTuFidqy0yI

[physicsguy]

Given the abandonment of olympic venues all the way round the world, I don't think this is a bad thing.

There's a lot in the book Soccernomics about how the economic value promised by politicians lobbying for hosting things like this rarely pan out.

[femtozer]

In this case, the pool is temporary. La Défense Arena is usually used for rugby (Racing 92) and concerts.

[Scoundreller]

Dumb question I never thought about: do the circulation/filtration pumps get turned off during races? And for what minimum time before hand to let things settle?

If so, I guess this would be a serious competition only thing because you wouldn’t want them off for hours.

[dredmorbius]

FINA competition pool standards have recirculation minimum standards, and pumps run continuously. Further, "water distribution has to be such that no appreciable current or turbulence is created. 'Appreciable current' is defined as water movement that can move a floating basketball (filled with 6 litres of water to obtain the right buoyancy) in one direction for more than 1,25m in 60 seconds."

<https://resources.fina.org/fina/document/2022/02/08/77c3058d...> (PDF)

Most competitive swimming pools have a large number of inlets with diffusers on them, laid out every 2m or so across the pool floor.

Those are circular disks about 10cm in diameter, looking vaguely like this:

  __________
  \________/
   |      |

    ^^^^^^

The carets indicate inlet water flow beneath the diffusers. The effect is that water entering the pool largely moves perpendicular to the pool floor, and slowly diffuses upwards. Water return is through the (large, wide, deep) gutters.

Because the gutters are continuously removing water from the pool, circulation needs to be on to maintain a consistent fill level.

[sandworm101]

They do, but circulation cannot be totally stopped. This is a greater problem in outdoor pools but any pool will have some sort of temperature gradient that will inevitably result in circulation. Any water movement means slower times, at least in those events longer than 50m (1 length).

[croes]

>A lot of this is perception vs. reality,” he said. “If you were to talk to many very accomplished coaches, they would say the pool has to be a minimum 3 meters deep. Most of our research shows that anything over 2 meters is frivolous. … Obviously, some depth is very important. But after a certain point, it's diminishing return.

Maybe it's just the swimmers and not the pool as such

[lou1306]

But a lot of the swimmers themselves are not able to go anywhere near their personal bests: this is a sharp reversal from past Olympics, where many athletes reached new personal records.

Plus, they are the worldwide foremost experts on competitive swimming. Definitely I would be more interested in their evaluation of a swimming pool rather than trust "research results" from the company that built the pool in question.

[derbOac]

At the end of the article it notes thqt quote is coming from the company who built the pool.

On some swimming forums competitors were complaining about the bidding process for the pool construction and giving a different opinion, noting that the depth is less than what was recommended by international standards bodies. There's also something about video equipment at the bottom of the pool?

I'm not sure what to think, as there are things to consider both ways, but there's a bit more out there than swimmers versus pool officials.

[qzw]

Notice the “most of…” and “diminishing returns”. The vagueness is doing a lot of heavy lifting in that statement. Since world records are often broken by mere tenths or hundredths of a second, I would think that an Olympic games would err on the side of extra spending for exactly those diminishing returns. The excitement and extra viewership from having many new WRs would more than offset the added cost anyway.

[Xcelerate]

I doubt that. What is P(set of slower times across swimmers | pool is not slower)? Seems like it wouldn’t be too hard to calculate if we make a few reasonable assumptions about observational data.

And if you don’t like inferring causation, one could just directly perform an experiment to test this pool vs another pool using swimmers who didn’t quite make the Olympics.

[etempleton]

There are more things that influence if a pool is fast or slow than just the depth.

How the pool gutters neutralizes or doesn’t neutralize waves; water temperature; the design of the lane lines; design of the starting block; the electronic touch sensors(how hard are they - do you get a good solid feel for push off?); etc

Depth is probably only part of the reason the pool is slow. It would be very unlikely everyone happens to be slow at the Olympics this year.

[rscho]

Perception is everything. If the pool 'feels fast', you'll feel like you're on your way to your best, which is a huge boost to your motivation. The converse is also true.

[roughly]

I mean, "diminishing effects" - these folks are the absolute apex of swimming struggling for the absolute maximum speed in the water that any person has ever achieved (under the watchful eye of someone with a stopwatch, at least). Small effects don't matter unless you're going for a world record, and then they might.

[dredmorbius]

The one aspect which isn't equal at least amongst Olympic and other competitions is that a slower (or faster) pool will generate fewer (or more) records overall. One measure of the current games' bias would be the number of new Olympic and world records set as compared with other events --- prior or subsequent Olympics, and other world-level competitions within the current year, say. I expect the quants at sites such as 538 would be looking at this.

[alistairSH]

Are records generally evenly distributed over time? Or do we see massive spikes due to single generational talents?

[rightbyte]

> Bummer that it makes the meet feel slow but at least it generally affects all the swimmers equally

Shouldn't the middle swimmer be worse off with "fast" pools? There would be less waves on the side lanes, compared to a "slow" pool where there are reflections.

[pdpi]

> at least it generally affects all the swimmers equally

> 2. The sides. Does the water spill over the sides into the gutters, or smash into a wall and bounce back, creating more chop.

My only experience with competitive swimming is playing some water polo some 25 years ago, but wouldn't that effect disproportionately affect the swimmers on the outermost lanes?

[davidmurdoch]

To a lesser effect, there's also the surface tension of the water, which can be adjusted with salt and borates.

[HPsquared]

Density and viscosity must also play a role, or are those managed via temperature control?

[ClassyJacket]

This is yet another reason on the huge list of reasons to keep the Olympics in Athens and just use the same facilities each time.

[adolph]

Why aren't swimming lanes stadium shaped like running tracks or speed skating? Floor mounted LED indicators could advise competitors of the relative distance ahead and behind of nearest adversaries.

[sschueller]

Shouldn't that be part of the standard "Olympic Pool" definition? Depth, water height on side and overflow etc. seem to be as important as the nittting of a football goal net.

[manojlds]

Why doesn't it affect more the people closer to the sides?

[mb7733]

It does. This is why the fastest qualifiers get to swim in the middle lanes.

[notatoad]

this is the reason that the leftmost and rightmost lanes of the pool aren't used for competition.

[jmalicki]

For point 2, wouldn't the reflections off the sidewalls create more issue with chop on the outer lanes than the middle?

[palijer]

The outer lanes are where the slower swimmers are placed already anyways, so if they were having more chop, it wouldn't likely impact a medal contender and cause a big upset... Not saying that outer lanes haven't come out on top in the last, but less likely.

[myworkinisgood]

All swimmers are affected equally so I don't see a problem.

[Yawrehto]

Off-topic but congratulations for qualifying for trials!

[croes]

So now that two swimming worlds records have fallen, we can't blame the pools anymore, can we?

[umanwizard]

Why does it matter how deep the pool is?

Edit: the article addresses this, so if anyone else is curious like I was, I suggest clicking.

[creshal]

Because it means waves bounce off the bottom faster (less distance travelled) and much more importantly, with far more energy (square cube law works against you). So the waters far more choppy far faster, since you have 50% less water volume to absorb all the energy.

[defrost]

If it's too shallow the swimmers arms hit the bottom ...

Slightly deeper and there's drag from the floor as their arms barely miss it. That effect persists until it doesn't .. now it's deep enough.

It needs to be deep enough that vortex's created by swimmers have disapated by the time they reach bottom and reflect back to the surface so as to not interfere with following swimmers or swimmers returning.

[HPsquared]

Edge effects affecting the flow field around the swimmer. I suppose the floor might trap turbulence near the surface rather than dissipating into the depths.