The Covenants book has a concept for a group of magi given a narrow strip of land along a beach, who then create magical dykes to create more land.
Obviously, this is just with big CrTe rituals to create the stone. Question is, how wide should the walls be? How high above sea level? I'm not a civil engineer or an oceanographer.
Another question I have, is how to the wizards then manage such land? Obviously, more water will get in, which will then have be periodically drained out, right? And won't the land sink or rise without the water pressure on it? My guess is that it would be a function of how deep the soil used to be under water and its composition. (Clay vs sand vs rock)
The more I think on this, the more it could be a neat basis for a Saga.
If you are summoning a building you are only worried about how many factors of 10 you need to hit your goal. Weather you need 1 ft above sea level or 10 is probably not going to come into play compared to the depth of the sea. If you are really worried you could look for the heights of real life dykes (probably want to use different wording on the google search...). Either way Magi don't need as large of a safety margin since you could easily respond to a storm buy temporarily raising to higher or warding the covenant against water (also because you have the equivalent of modern drainage tech, see below). I suppose you would also be using the dyke as a fortification so you would want it considerably higher than a period sailing ship sits above the water (pretty high to say the least).
I doubt the sea floor is compressible enough for anything to matter. Tides routinely cover and uncover fairly large stretch of land and I haven't heard of anything like swelling. If anything you would lose height as things dried out.
Sea floors aren't known for their good natural drainage properties. You are right there. Just to manage rain you would want a ditch/storm drain system that leads to a PeAq ring spell.
First of all, you can instead transport the stone from elsewhere. If you have a source of rock nearby, it's really easy to use low level (spontaneous!) ReTe to quarry big blocks and move them to where you need them. Depending on how shallow the sea-floor is, if it's sand or gravel you can use that as building material, again with simple ReTe. Or summon a daimon or faerie god and ask for help!
Second, you need not build in stone. You could, for example, magically alter seaweed to grow into thick, tightly weaved "walls". Or temporarily create giant sea vermin that will grow and leave behind perfectly mundane, giant sea shells to form your dykes. Or just freeze the water and make giant walls of ice -- or, in a similar fashion, Rego-up walls of salt! These will melt, but not that quickly ... and not at all if kept from doing so with a Ward. In fact, you could create an enchanted device that simply keeps all water away, up to a certain distance, without the need of a physical barrier.
The answer to the first question (how thick?) is: it depends on how deep they go. There are areas where the sea slopes gently down, and even at 1Km from the shore its depth is no more than 3-4m. There are areas where it drops sharply. In general, pressure is proportional to depth -- this was understood by the ancients, and in particular to Archimedes; about 1 ton per square meter per meter of depth (oh, the beauty of the metric system). If you are building in rock, having the walls about as thick as they are deep at any given point is quite safe. Sand of gravel? Multiply that by 5. You can do slimmer stuff, potentially significantly slimmer, but it gets complicated, and it's not really worth it.
The answer to the second question (how high above sea level?) is: it depends on where you are building. You have to account for a) tidal excursion (the difference between high and low tide) and b) waves. As a rough guideline: in the mediterranean, build 2/3m above low-tide level. In the atlantic, double that.
Probably, though not necessarily. A lot depends on the nature of the sea floor. Wards against sea water would help a lot. Keep in mind that the problem with sea water seeping slowly in is not the water (which, in general, will evaporate soon enough), it's the salt it leaves behind, which will make the land lifeless. Of course, again depending on where you are, rainwater could be an issue too.
The land will indeed rise without the water pressure on it. But so little it can be safely disregarded.
The Frisian dykes weren't very high - but in 1287 the St Lucia's day flood causes colossal flooding and loss of life and changes the geography of the area.
The seawalls of Constantinople are 12-15 metres high by the shore - I don't think the waves get anywhere near that high, but that's as tall as they need to be to stop any naval threat the Byzantine Empire thought might occur.
I would avoid a lot of this. Anything that takes persistent magic to operate is super cool but sort of impractical. Personally I wouldn't want to put myself in a situation where a single PeVi could destroy the entire covenant. I would totally keep a ward against sea water on tap as a precaution but not as the only way of keeping the ocean out.
One of Transforming Mythic Europe or Germetic projects has a lot of sample spells for many of these discussions (I forget which is which).
A thought - if you are going to radically alter the coast by setting up a dyke then why not just create more land? Dump monstrous amounts of dirt and rock with a Creo ritual onto the beach and water and settle it. The dyke is a great idea but it still allows a very obvious attack to wreck all you’ve built.
A few notes. Creating a permanent, mundane wall through magic that temporarily affects the creating agents (so: seaweed and giant seashells) obviously does not fall under the criticism.
Creating a mundane wall that must be renewed by magic or otherwise fails very slowly, over several weeks at least (wall of ice, or of salt) is partly immune: it's not as if the magi won't be able to respond to problems. Furthermore, there's a great to such a magic-sustained solution: it dispels any ideas that a mundane lord might get about grabbing the newly acquired land for himself.
The same "don't-even-think-about-it" advantage can be had from a magic item that keeps water away as a Constant effect. It's not entirely clear what type of PeVi effect would be required to bring it down. A disenchant Ritual on the item? A tiny PeVi spell cast at its very edge? Hard to say, but it hardly seems more fragile than a thick, mundane stone dyke. If you are bringing the supernatural in, destroying stone is no harder than dispelling magic.
Finally, keep in mind that in such a scenario the sensible thing is to keep the abode of the magi and trusted servants on the (former) beach The land is for farming. Should the water reclaim it, the covenant loses its source of income, a few replaceable mundane labourers, and little else!
I do agree that having multiple solutions up simultaneously is better Utlimately, though, it's a cost-benefit analysis. Unless you have a lab text, creating a big, dyke-creation CrTe ritual might be significantly more expensive in terms of time than some of the other solutions. And mind the botch!