Water. You probably already knew that. But where does all that water come from?
I'm not going to discuss Tsunamis here, except to mention that it's an undersea earthquake, and if you're ever on the beach and the tide suddenly goes out, and out, and out, then the thing to do is run as fast as you can for higher ground, because that's the only way to be saved. In the2004 tsunami, 280,000 people were killed.
No, I'm talking about a more frequent event; floods caused by storms and hurricanes (a hurricane is a storm, only much stronger).
The storm is carrying lots of moisture, which it picked up from the evaporating sea (see my article on hurricanes). When the storm is over land, it is no longer getting the warm air from the ocean, so it cools down, and cooler air can't carry as much water vapour as warm air, so the water vapour condenses to water, and falls as rain. Lots of rain. Harvey dumped 52 inches of rain on Houston. Lots of people are shorter than that.
But that isn't all. There's also a storm surge; the storm winds push offshore ware onshore ... and then you get high tide.
Add all this together, and you get a flood, because all that water is trying to get away from where it is, down to the sea, but if you're on level ground, the water doesn't know where to go and just hangs about.
But that isn't all. Rain is clean, but when it piles up four feet above the ground, it gets all kinds of muck swirling around in it, including sewage. Don't drink it! And if there's a toxic chemical dump nearby, it'll pick that up too. What kind of city has a toxic chemical dump near a residential area? Cities that don't have restrictions on what you can build and where. Like Houston.
Eventually, the flood subsides, and people can get down off their roofs (don't go up into an attic, unless you can break through into the roof, because otherwise, you'll be stuck) and start the clean-up. Which will be massive, because water damages pretty much everything.