Sea levels are rising

[henryzz]

The global sea level is currently rising by around 3.7 mm per year according to https://en.wikipedia.org/wiki/Sea_level#Change

I recently had the crazy idea that we could potentially store enough water on unused land in order to reduce sea level by a noticeable amount. I realised that this was unlikely but that it was still an interesting idea to think through.

Although estimates vary(partly because it changes as sea level rises), estimates for the surface area of water on earth are around 361,740,000 km².

If we wanted to reduce the sea level by 1cm(about ~3 years worth), then we would need to store 3617 km³ or 3.617×10^15 litres of water. This is a lot of water. However, there are man made artificial lakes already of significant capacity such as Lake Kariba(180 km³) and Lake Volta(148 km³) which suggest that this may be theoretically possible although extremely difficult. Both of these also had the advantage of providing significant amounts of hydroelectricity. It is worth noting that it was probably easier to contain large quantities of water in these areas due to existing geography.

There are significant issues that would need to be overcome to make this possible. We have plenty of inhospitable land(deserts for example) that could be used for storage if water could be contained. Filling and keeping reservoirs filled would also be a significant problem.

I have not a clue how much this would cost and suspect that it would be nearly impossible to make a significant dent in sea level rise while we attempt to reduce it other ways.

Have I missed anything above(probably)? Does anyone have any suggestions that could solve some of the issues with this idea?

[kruoli]

A related idea (which would have held a lot of water): northern river reversal.

[kriesel]

About 71% of the earth's surface is ocean, 29% land.
So to uniformly cover all land with ocean water, to effect a 1cm change in level of existing oceans and seas, the average land coverage would be 71/29 cm = 2.45 cm.
Since people will object to keeping nearly an inch of salt water on their driveways, roads, roofs, lawns, gardens, farm fields, parking lots, and on forests, lakes, rivers, wetlands, glaciers, etc, and there are severe practical engineering problems with doing so despite slopes on roofs, fields, roadway interchanges, hills, mountains, etc., only a small fraction of total land area could be considered. Assume feasible fraction of land area is bounded by 1%. That would require a barrier of at least 245. cm ~ 8 feet, disregarding waves, tides, etc. If this was a single circular area, for efficiency, placed in a nearly flat portion of US or African desert, for dual use as salt production & perhaps water desalination, and perhaps wind/solar array, the area would be 3617 km³ / .00245 km ~ 1.476E6 sq km,
diameter 1371 km,
and the circumference would be 4307 km,
so the lower bound of wall area would be ~10.6E6 m².
The horizontal area would require full impermeable membrane with bentonite layer place for self healing of small tears or punctures.
Site selection would include low earthquake incidence, both for preventing massive shear tears in wall and membrane, and to avoid slopping the contents over the wall.

Now let's compare that land area to total area of some US states;
Arizona 2.95E5 sq km
California 4.24E5 sq km
New Mexico 3.15E5 sq km
Texas 6.96E5 sq km
total 1.73E6 sq km, 1.17 times the area estimated needed. Not all would qualify as low earthquake incidence.
(This area is far from circular, but it appears wall cost though considerable is not the primary cost)

This could also serve as a considerable marine obstacle to unauthorized immigration, along the ~2000 mile US-Mexico border, and would create a need for numerous shallow-draft Coast Guard boats for control of smuggling contraband. Allowing jet skis for recreational use on the newly created ocean area would complicate enforcement considerably.

If we use the low value at https://homeguide.com/costs/precast-...rete-wall-cost for cost of a wall, $90/foot, and assume a sprayed protective layer on the salty side is negligible cost by comparison,
$90/foot x 4307 km * 3280.8 ft / km = $1.27E9.

If we use a couple figures for sheet membrane and for bulk sodium bentonite purchases, to prevent salination of underlying aquifers, about $5/sq foot for impermeable barrier installation appears plausible.
$5/sqft * 1.48E6 sq km * 3280.8^2 ft2/km2 = $79.6E12. That's ~$80 trillion, which is a lot. We should check on the known world resource for sodium bentonite clay.

For comparison, the total value of all US residential real estate in 2017 was estimated at ~32 trillion, and California's total real estate $4.4 trillion in 2010.

The cost of relocating tens of millions of people and any economically relocatable assets and land acquisition would far dwarf the cost of perimeter wall construction.
The likely political opposition would be quite considerable, and numerous environmental impact studies required.

There's a stark disparity between wall and floor membrane cost, ~1/63000.

It looks like a taller wall and smaller area would be less disastrously uneconomic.
Wall cost is worse than linear with height, probably quadratic or greater.

Probably should multiply wall extent by a factor of 5 to 10, to make compartments and containment structures, so a single failure point doesn't dump the whole thing.

And that is for 3 years of rise. Then it's someone else's turn.

[Dr Sardonicus]

Just pull the big plug to drain the excess water into the hydroplate

Instead of trying to corral the increased volume of sea water, it might be more effective to relocate some real estate upwards. There is ample precedent for this - in the Netherlands, they've been building "terp mounds" for ages. Aboriginal Americans built mounds to mitigate the effects of flooding along inland waterways.

In the second half of the Nineteenth Century, Chicago raised a goodly part of itself out of the swamp, then reversed the flow of the Chicago River to send its sewage to the Mississippi River instead of into its primary source of drinking water, Lake Michigan. See this post for more details.

[henryzz]

Originally Posted by kriesel

If we use a couple figures for sheet membrane and for bulk sodium bentonite purchases, to prevent salination of underlying aquifers, about $5/sq foot for impermeable barrier installation appears plausible.
$5/sqft * 1.48E6 sq km * 3280.8^2 ft2/km2 = $79.6E12. That's ~$80 trillion, which is a lot. We should check on the known world resource for sodium bentonite clay.

I was anticipating filling the reservoirs with fresh water rather than salt water. This means that a membrane for the entire base should be unnecessary hopefully. This will mean increased difficulty in filling it as there will be some drainage but hopefully that should be possible if we consider river redirection for example.


I was also anticipating a much higher wall than the 8 ft you described. Based on your calculations substituting some area(lowering the costs for a base if needed) for a higher wall is a viable cost saving method.
A 10x higher wall may raise the cost/foot by 100x but should also reduce the needed circumference by a factor of sqrt(10) = ~3.16 resulting in a ~30x cost for the wall in exchange for a 10x reduction in the cost for a membrane.
Reducing the area also should make finding a location easier. Hopefully natural geological features could be used to help contain the water.

[kriesel]

The best topography for fresh water storage has already mostly been exploited.
Hoover Dam / Lake Mead is at historic low levels, as are some other reservoirs. Parts of the US are in serious contention over fresh water shortages, now. (Some rivers are oversubscribed by law. Search water rights. Some only flow seasonally, the Gila for example. In spring it's white water rafting, but in August it's dusty stones in an empty bed.)

Some degree of sealing near a dam is sometimes necessary, to prevent seepage undermining the structure resulting in later catastrophic collapse.
I may have used a much too low cost for the 8' wall per foot of length. The page said $90-320/lateral foot, for 6-8' high. And it was actually a concrete wall, not a dam with lateral pressure. An alternative is considerable earthworks with membrane applied on the pool slope, and facing of concrete or sod on the other. Bentonite when wetted as in when it is to perform its sealing function is limited on how high a slope it will cling on, before sliding down and failing. So it takes far more soil volume than concrete does.
Given fresh water scarcity worldwide, I thought desalination and conservation were worth considering.

And yes, the 8' was just an easy first guess to run some initial numbers on, not an optimization of any sort.
But I would not want to live downstream of a 250meter tall dam holding thousands of cubic kilometers of water. And quite a distance would be downstream once that gets itself and a growing amount of swept up debris moving, including whole vehicles, trees, boulders, and parts of various structures.
See also https://en.wikipedia.org/wiki/Missoula_floods

[kriesel]

Originally Posted by henryzz

there are man made artificial lakes already of significant capacity such as Lake Kariba(180 km³)

... at which hydroelectric generation will cease or from which electricity will be rationed, due to low water levels, depending on the country drawing it down. https://www.aljazeera.com/news/2022/...s-dulow-levels

[kriesel]

Originally Posted by kriesel

I may have used a much too low cost for the 8' wall per foot of length.

Typically hydropower dams are arch dams with the convex side toward the water. An encircling wall dam on a plain is necessarily concave toward the water overall. The initial deflection of an arch dam creates a compression stress in the concrete, while an encircling wall develops tensile stress in response to the internal pressure from the water. Concrete is prone to cracking under tension, and is much stronger in compression than in tension. Reinforced concrete contains inserts that withstand tension well, usually steel re-bar. Steel reinforced concrete beams such as those that support some highway bridges have steel in the bottom portion to carry most of the tensile stress developed from bending, while the upper portion carries compressive stresses.

[chris2be8]

A more practical way would be to pick places such as the Qatarra Depression in Egypt that are below sea level, so all you need to do is dig a canal or tunnel from the sea to fill them with water. This assumes it's empty desert so no one will be living there now. And it saves the cost of a surrounding wall.

The area round the Salton sea in the south west USA is another possible candidate, but I think it's inhabited now which would raise the cost (you would have to buy out the current population).

[Dr Sardonicus]

Originally Posted by chris2be8

<snip>
The area round the Salton sea in the south west USA is another possible candidate, but I think it's inhabited now which would raise the cost (you would have to buy out the current population).

Regarding the Salton Sea, this story may be of interest.

The surface of the Dead Sea is around 430 m (over 1400 feet) below sea level. And it keeps dropping, because the water of the Jordan River is being diverted. Not too far away is the city of Jericho, well over 800 feet below sea level. I suppose it's only a matter of time until sea water starts entering the Great Rift Valley in substantial amounts.

Here in the good ol' USA, Death Valley is about 86 m (280 feet) below sea level.