洪水

1. Origins, structure, and basic meaning

• Structure: 洪水（こうずい） means a flood in which water rises and overflows.
• Basic usage: It can refer to a past disaster, as in 昔の洪水, or to an assumed flood scenario, as in 洪水を想定した避難訓練.

2. Common pitfalls and similar expressions

• Flooding versus inundation
  • 洪水 focuses on water rising and overflowing; 浸水 focuses on water entering or submerging an area. In 洪水で浸水した範囲, the first example links the flood to the area that was inundated.
• An actual event versus an assumed situation
  • In the second example, 洪水を想定した identifies the assumed conditions of a drill. Reading this phrase distinguishes a description of training from a report that a flood actually occurred.