Tsunami

I watched one video, of the water overwhelming the farms beside the canal, taken from a helicopter.

From so high the crawling chaos seemed almost slow as it slid from side to side swallowing neat farms and gardens and stacked hay into a black stew littered with shattered buildings. But it wasn't slow, as you can see from the cars speeding away on roads that (God help them) did not lead straight away from death.

You see all human order lost in nightmare, with nothing you can do to turn it aside.

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And to add to the mess of shattered buildings and drowned land, a little blast at a power plant. They had previously detected cesium and iodine, so the core had been breached somewhere. Hot uranium plus water = uranium oxide plus hydrogen, leading to a little boom? That'll be messy to deal with. And one report said they were only now getting the last reactors back online after Kobe.

Update They use uranium oxide, so it was likely steam interacting with other structural elements.