The Beach City Flooding is a disaster that occurred within Beach City within the Season 5 episode "Post Haste".
Effect
The disaster itself was, at first, brought on to be the result of an oncoming Hurricane, even accompanied by a emergency broadcast to Beach City, Bayburgh, and Empire City, mainly as a fake ruse possibly made by Acid and Cyanide in order to evacuate the town and have the Crystal Gems locked up in the Temple for Hurricane Israel. Acid and Cyanide then weakened the Beach City Water Tower's legs, making it collapse, and land onto the U Store. The flood itself, released by the Water Tower, effected the entirety of the city.
The residential area was completely washed away, being right in the path of water. Sadie Miller, Sour Cream and Onion, Lars Barriga, the Frymans and Pizzas all were confirmed homeless, whereas only people weren't left homeless was the Crystal Gems who were far enough away from the flood, and Bill Dewey's family who's home stood uphill from the Water Tower. It's unclear if Jamie had lost his home in the disaster. Many store establishments still stood, but got flooded out, many of the inner workings washed out to sea. The U Store got completely destroyed. Much of the freshwater had been removed entirely from Beach City, as all of it came from the Water Tower in question. Power lines were lost in Beach City, leaving the city without power and/or internet connection.
Outcome
From the flood, it took months for the town to fully recover from the flood, taking all of the following winter in order to have most of the residential area properly built, as revealed in Season 6.