▲若指一次洪水，为可数名词。如：

We had several bad floods this year. 今年我们这里发了几次洪水。

After the heavy rain there was a big flood and water came into our house. 那场大雨后闹了洪水，我们的家都淹了。

Chicago was blacked out yesterday after a freak flood.  芝加哥昨天因为一场罕见的洪水而全城断电。

▲若泛指一般意义的洪水，则为不可数名词。如：

The area near the river is liable to flood.  河流附近的地区容易遭水灾。

Tornadoes, earthquakes, and flood are natural disasters. 龙卷风、地震和洪水都是自然灾害。

▲有时用用复数形式，并不表示多次洪水，而是强调“洪水”之大或严重。如：

The town was declared a disaster area after the floods. 洪水过后，这个城镇被宣布为灾区。

The floods were a cataclysm from which the local people never recovered. 洪水造成灾祸使当地人民再也没能恢复过来。