The phrase "unprecedented flooding" means that a flood event is happening in a way that has never been seen or experienced before. It suggests that the flooding is extremely severe and unusual. Full definition
Plus, with unprecedented flooding in Houston, we take a moment to see how prepared Kansas City is for heavy rainfall. (kcur.org)
Over the past two years, half the populated islands of the Torres Strait have experienced unprecedented flooding from surging king tides. (humanrights.gov.au)
Hurricanes Maria and Irma follow Hurricane Harvey, which brought unprecedented flooding to Houston and the Texas coast. (christianitytoday.com)