I think the cornerstone would be hydraulic works
We have lots of fun places to live, but many of them are busy growing crops...or should be.
We also let gigatons of water flow uselessly out to the sea, e.g. Out the Columbia (200,000 cubic feet per second), Mississippi (600,000 CFS) and St. Lawrence (300,000 CFS). Meanwhile, some places have way too much water at times, like Houston.
You build mega-canals to take that wasted water to arid areas. You also include massive reservoirs in suitable places. Large reservoirs are not too hard if you berm (or use natural terrain) to impound very large areas.
If you have canal systems capable of moving 600,000 CFS from the lower Mississippi River to the arid southwest, whilst simultaneously supporting navigation and recreation. These are some big canals. You build them so (at cost of stopping navigation) they can super-flow to triple capacity, e.g. 1.8 million CFS for all of them, and you do that for flood control reasons.
Normally, the southwest irrigates off the daily flow. However there are huge reservoirs, which you keep largely empty for flood-control reasons. When you have a storm, you do several things at once. First, you stop drawing from the Mississippi normally, and let it flow to the sea. You then drain down all the Mississippi valley and southeast Texas reservoirs, so they have room to absorb storm surges. Then, as the storm hits, the canal and now-empty regional reservoirs work together to draw water away.
Consider Hurricane Harvey. It dropped 9 trillion gallons (1.2 trillion cubic feet; 28 million acre-feet). Well, that's the capacity of Lake Mead alone. Since the mega-canals can supply daily needs, there's no longer the need to store years' worth of water, so that existing capacity can be used for flood control.
Oh yeah. Thermal management.
The water enables massive growth of flora. The flora has the cooling effect in the normal way it does that. Not least, all 600,000 CFS of Mississippi water ends up being evaporated (partly via the flora) in the Southwest.
Consider the evaporative cooling effect of 600,000 CFS. Boiling one cubic foot of water takes 70 million joules. So that flow rate removes 42 terajoules/second of solar heat. Assuming 600W of solarization per square metre half the time, that nullifies all solar heat for 138,000 square kilometers. Or about half the size of Arizona.