From StrategyWiki, the free strategy guide and walkthrough wiki

• Disaster: Flood

The easiest way to handle the flooding borders on cheating: at the start of the scenario, pause the game and bulldoze all the coastline. Be sure you get every tile. With no coastline, flooding cannot occur. If you're not careful, though, it'll usually turn out that you have one or two tiles of coastline left, and a flood will occur there.