What is the definition of a flash flood?

A flash flood is characterized by a rapid rise of water in a short period, typically caused by intense rainfall over a small area. This phenomenon can occur within minutes to hours of the excessive precipitation, leading to potentially dangerous and fast-moving flood conditions. Flash floods can be exacerbated by various factors, such as urban development, which increases runoff, or the topography of an area, which may funnel water into specific locations.

The other choices reflect different flooding scenarios or conditions that do not align with the specific characteristics of a flash flood. The slow increase in water levels would describe a gradual flooding event rather than the immediate and intense nature of a flash flood. Seasonal changes in river flow and regular occurrences in drought areas pertain to distinct hydrological phenomena that do not fit the definition related to the rapid onset of flooding.