Rainwater collection

[ScienceNostalgia101]

I previously discussed, as part of a broader thread about flood mitigation/adaptation, the notion of concave-up ellipsoids over cities to collect rainwater and prevent flooding. It was noted that they would be weak at the bottom, because of the immense pressure of all the surrounding water. On further consideration, cities are probably not the best place to put them in light of that.

 

However, thinking about this further, I've since come up with another idea; a concave-up rain-collector (whether as ellipsoid or half-cylinder, depending on whichever is more practical) at sea, directly underneath the intertropical convergence zone.

 

This way, rainwater, distillated by nature, could be siphoned off to artificial reservoirs on land. This would kill two birds with one stone; the need for desalinized water, and the rising ocean levels. (Imagine how much by which they could be decreased if a disproportionate chunk of the 100 inches of rainwater that would otherwise annually fall into the ocean were moved to reservoirs on land!)

 

As well, another advantage to making it at sea is that both buoyancy and pressure at a point in space relate to the mass above that point in space. Therefore, even though fresh water is less dense than salt water, this would not matter, as any rate of freshwater-siphoning adequate to keep the rain-collector afloat would be adequate to keep the pressure on the inside of its walls comparable to the pressure on the outside of its walls, hence the forces pointing outward and forces pointing inward being nearly in balance.

 

So why hasn't such a project been pursued?

Edited October 21, 2018 by ScienceNostalgia101

[OldChemE]

Essentially, what you are proposing is a huge boat that you would allow to fill with rain water.  Once it fills beyond a certain point, (determined by buoyancy) it will sink-- unless it is supported from below.  The feasibility of that support will depend on the depth and/or technology used to support it (pontoons or equivalent).  I don't really know-- but I suspect the cost would far outweigh the cost of simpler solutions such as desalinization.

[ScienceNostalgia101]

The idea is more so that the water would be siphoned off to said reservoirs before it gets beyond that point.

 

I'm not sure why it would need to be an outright boat, though. If it were a simple concave-up structure, wouldn't buoyancy itself act as a restoring force toward equilibrium if it even so much as began to tilt to one side?

 

Also, don't desalinization plants continuously use electricity while desalinating  water? Wouldn't constructing a rain collector be an investment in the short run that pays off in the long run through savings on electricity? Or would water siphoning consume even more electricity?

[Frank]

3 hours ago, ScienceNostalgia101 said:

The idea is more so that the water would be siphoned off to said reservoirs before it gets beyond that point.

I'm not sure why it would need to be an outright boat, though. If it were a simple concave-up structure, wouldn't buoyancy itself act as a restoring force toward equilibrium if it even so much as began to tilt to one side?

Also, don't desalinization plants continuously use electricity while desalinating  water? Wouldn't constructing a rain collector be an investment in the short run that pays off in the long run through savings on electricity? Or would water siphoning consume even more electricity?

Fine if it rains.  Need to keep the sides of the reservoir above sea waves, like by flotation.  I was told Australian pools by the ocean sometimes get sharks in them.  "Because in Australia, sometimes you have to pick a shark out of a pool" https://www.goodthingsguy.com/environment/australia-shark-pool/.  So the sides might need to be high enough to keep out marine life.

 

For the ocean rise part, it seems unlikely the mass of antarctic glaciers plus volume expansion caused by temperature increase would equal what is siphoned off and never returned to the ocean (somehow).  That would be a lot of very large reservoirs.

 

[John Cuthber]

5 hours ago, ScienceNostalgia101 said:

The idea is more so that the water would be siphoned off to said reservoirs before it gets beyond that point.

 

If you can get a syphon that runs uphill then we have solved the problem (And most of the world's  other problems too)

[Frank]

On 10/22/2018 at 2:26 AM, John Cuthber said:

If you can get a syphon that runs uphill then we have solved the problem (And most of the world's  other problems too)

So "pumped"?  Good point but a bit pedantic.

 

[DrP]

On ‎22‎/‎10‎/‎2018 at 12:03 AM, ScienceNostalgia101 said:

I've since come up with another idea; a concave-up rain-collector (whether as ellipsoid or half-cylinder, depending on whichever is more practical) at sea, directly underneath the intertropical convergence zone

Being honest - the surface area of the combined rooftops across town is quite a large area already...  all that water gets collected by the guttering on the run off of the roofs. Putting a big bucket over the city won't collect that much more...   It maybe needs to be directed somewhere other than the drainage system/sewer once it runs off the roof?...

 

2 minutes ago, Frank said:

So "pumped"?  Good point but a bit pedantic.

 

....  maybe by collecting it somewhere and pumping it off to a reservoir or something.

[ScienceNostalgia101]

On 10/23/2018 at 10:56 AM, DrP said:

Being honest - the surface area of the combined rooftops across town is quite a large area already...  all that water gets collected by the guttering on the run off of the roofs. Putting a big bucket over the city won't collect that much more...   It maybe needs to be directed somewhere other than the drainage system/sewer once it runs off the roof?...

 

....  maybe by collecting it somewhere and pumping it off to a reservoir or something.

Indeed, that's why I've since replaced the "over a city" part with "at sea." The "under the ITCZ" qualifier is simply because you get more rain there than at the polar front. However, the key point is the "at sea" one. Here's a quick visual of what I'm talking about, which I probably should've used sooner...

Since seawater is 1.02 to 1.03 grams per mL, while water is just 1; and since volume is proportional to r^3; my idea was that the column height for freshwater could be 1.0066 to 1.01 times that of the surrounding seawater, if hull mass is considered negligible. (Depending on necessary thickness, resilience, and lightweight nature of the material.) For a, let's say, 100m seawater column height, this could correspond to a 1 metre water level difference, allowing it to be pumped to a 1 metre tall reservoir with a net zero vertical displacement. (Misinterpreted the definition of siphoned, but the idea is to avoid using more pump power than necessary.)

 

Of course, on further consideration, it's doubtful one could use a large enough (by surface area) region of Earth for 1m to work for the "offset ocean level rise" purpose anyway. Earth's surface is 155 million square km; if it all rose by 2m, (ie. 0.002km) that'd still be 310 thousand cubic km. You'd need a 68km by 68km by 68km reservoir to store that. Highest mountains are less than 9km, so it'd be more like 9km by 204km by 204km. Any geology majors here know of any large valleys surrounded on all sides by material impermeable to water? Or at least of large enough regions of the Earth's surface from which water can't seep into the surface from above to save on floor construction?

 

However, for the "alternative to desalinization" purpose, that still leaves behind the question of whether or not a pump could use electrical power more efficiently than desalinization.

 

. . .

 

For the record, I'm well aware that most uses of desalinized water result in it evaporating and eventually forming rain again.

Edited October 25, 2018 by ScienceNostalgia101

[tinkerer]

I did not see mention of the fact that many localities now have made rainwater collection illegal. Believe it or not. I believe the first case cited was in California, perhaps a year or two ago, when drought was a leading newsprint seller. Just sayin'......

[Frank]

Musk wants to use his boring company TBM for sewers.  Maybe rainwater/storm sewers could also work?  Might be a way to retrofit where no separate storm sewers exist.