Consider a swimming pool as being a hole in the ground. Sometimes it's not, of course, but in this case we definitely need a hole in the ground.
If your planned pool is already going to be a hole in the ground, then it needs to be at least twice as deep as you planned. If your plans are for an above-ground pool, you merely need a hole in the ground a little larger than that.
After building the hole, and constructing appropriate sides and bottom for it, add a roof. If you are planning a hole-in-the-ground pool, the roof should be located at about the half-way mark, with respect to the depth of the hole. For an above-ground pool the roof would be at ground level.
Your actual swimming pool is now to be located directly above the underground hole; the roof just mentioned is the bottom of the pool. Anyone who has ever heard about the existence of penthouse swimming pools knows that a roof can certainly be built strong enough for this to work.
Once most of the construction is completed, but before actually filling the pool with water, a slight modification to the preceding needs to be mentioned. DURING construction of the roof, a LOT of screened valves need to be installed in it.
OK, now fill the pool with water. Invite swimmers.
Eventually, probably, especially if "eventually" is long-enough, someone will get into trouble and need help.
Push a button.
All the valves open up, and all the water in the pool quickly drains through all those valves into the lower underground chamber. The screens covering the drain openings prevent anyone from doing what the water does.
After the needy person has been rescued, press another button to close all the valves, and then press a third button to pump water from the underground tank back into the swimming pool.-- Vernon, Aug 24 2011 I see someone at the bottom of the pool, trying feebly to reach the surface, and maybe succeeding, until the valves open and suck them back to the bottom, and hold them there until the pool empties. There's probably a math formula for when they would have reached the surface versus how long until the surface reaches them.
You are going to need a lot of valves and a lot of digging. [ ]-- baconbrain, Aug 24 2011 [baconbrain], yes, I did specify a "LOT" of valves. I didn't specify that the valves need to be fast-opening, but they do. I also didn't mention the air vent that is needed to allow air to leave the underground tank as fast as water rushes in to fill it, but it should be obvious that that is needed, also.
As for how fast the pool will drain, this is actually a fairly simple calculation. First, compare the surface area of the bottom of the pool by the surface area of all the valve openings. You can imagine, for each valve, it being surrounded by PART of the pool bottom, and the column of water above that area-of-bottom is what that one valve would drain.
Now conduct a simple thought-experiment, involving a vertical length of hose, full of water. Gravity can empty a 10-meter length in something like 2 seconds; think of the water as being a simple falling object. A mere 2-meter length of hose (equivalent to a 2-meter-deep pool) will empty rather faster.
The thought-experiment needs to be adjusted for the fact that, with respect to part-of-the-pool-bottom and one valve, the valve is the smaller area. Simply multiply the hose-emptying time by the larger-to-smaller ratio of the two areas.
I think it is quite possible to drain the entire pool in less than 10 seconds, if there are indeed a LOT of valves on the bottom!-- Vernon, Aug 24 2011 It would be cheaper to have a buoyant grid (e.g. made of wood) secured to the bottom of the pool. When someone gets into trouble, release the grid which will float to the surface with everyone on top of it.-- hippo, Aug 24 2011 //the valves open and suck them back to the bottom//
Wait a minute, [bb], how could the valves suck the person away from the surface of the water? Towards the bottom, yes, but not away from the surface I think; surely the maximum suck-speed afflicting the drowner is limited by the rate at which the water surface falls, and can't exceed it (unless there are some fiendish fluid dynamics going on that I haven't thought of)?-- pertinax, Aug 24 2011 You forgot to mention the whirlpool effect of draining a body of water through a hole, the air takes some of the valves surface area, slowing down the transfer of water.
Unless the area of the valve is bigger then the surface area of a human, you will get people clogging the vents.
Basically, more trouble then you started with.-- nickthird, Aug 24 2011 What [hippo] said.-- mitxela, Aug 24 2011 //people clogging the vents// [marked-for-tagline]-- FlyingToaster, Aug 24 2011 [hippo], this being the HalfBakery, I'm free to post something at least partly ludicrous (an expensive way to do something simple; I had to delete my somewhat less-ludicrous "pool bottom net" Idea because someone else had already posted it years ago). And you are free to post your own notions independently.
[nickthird], the whirlpool effect is at least partly related to a need for air in the place where the water is going, to get out of that place. However, if a separate air vent exists (as mentioned in my last anno), then no whirlpool effect need happen, at all.
And by the time people clog the vents, the water level will be low enough that those same people can simply stand up on the vent-screens at the pool bottom, putting their heads well above the water.-- Vernon, Aug 24 2011 It'd work faster if the lower tank had no air in it.-- Ling, Aug 24 2011 //It'd work faster if the lower tank had no air in it.//
Interesting. Zero drowning fatalities, but multiple deaths from water-hammer.-- MaxwellBuchanan, Aug 24 2011 random, halfbakery