How was Noah’s miracle the Flood, seeing that it happened after his people disbelieved him, and miracles come so that people may believe the prophet? And what are the miracles of Shu‘ayb and Hud, peace be upon them?

Every prophet was given a miracle suitable for his people, and the Quran and Sunnah do not explicitly state the miracle of every prophet. The miracle of Noah (peace be upon him) was the building of the ark and his salvation along with those who were with him in the flood, as well as his clear argumentation with his people. As for the miracles of Hud and Shu'ayb (peace be upon them), their miracles may have been the clear argumentation and eloquent exposition in their دعوتهم (call/preaching), or signs that the Quran did not mention. For every prophet came with one or more signs confirming his truthfulness.