Droughts are pretty easy, you can do it right now if you stop trying to maximize evaporation and start doing something reasonable like drip irrigation.

Flooding is a bit different in that I assume we're speaking about the Greenland glaciers and some of Antractica melting. You can either prevent them from melting or just build high walls around particularly valuable pieces of real estate. Shorelines are never stable, anyway -- continental plates rise and sink, estuaries silt up, hurricanes rearrange barrier islands, etc.