Start off with your overall central topic: in this example, we're starting with flooding. What comes to mind? Major storms that have created flooding here (Allison, Harvey, Imelda), the recovery efforts (FEMA, flood insurance, mold remediation), the possible causes (climate change, land development)...
You're trying to accumulate lots of ideas at this point! Big picture. Make connections, and write whatever comes to mind. When you start getting stuck, turn to Google, Credo Reference, and Wikipedia to get more ideas.