Usage patterns
洪水が起こるfor a flood to occur
Heavy rain caused the river to swell, and a flood occurred.
洪水に見舞われるto be struck by a flood
That village is hit by floods almost every year.
洪水で〈場所〉が浸水するfor a place to be submerged due to a flood
Many houses were flooded due to the flood.
〈物〉の洪水a flood (overwhelming amount) of something
The internet is overflowing with a flood of information.
洪水を防ぐto prevent a flood
The dam plays a role in preventing floods.