I don't think it would be considered redundant. The flooding could have been the worst experienced in the last 10 years, the worst experienced in the last 200 years, or the worst ever experienced. ever just emphasizes that no matter how far back you go in time, still no such flooding was experienced.
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cbstehmy grammar checker flagged that sentenceAh, yes. You have to be careful with some of that software. The software doesn't really "understand" what you are trying to say, and just points out mindlessly whatever patterns it is programmed to find. But you knew that already.