ANSWER: Both
Explanation:
A flood, which is caused by heavy or excessive rainfall in a short period of time over a relatively small area, is referred to as flash flood. In flash flood, water level rises and falls quite rapidly with little or no advance warning.
Typically, flash floods occur in areas where the upstream basin topography is relatively steep and the concentration time of the basin is relatively short.
Flash floods occur primarily at night and when there is an abundance of atmospheric moisture; in addition, there is usually little, if any, vertical wind shear present.
Flash flooding can be produced by large, slow-moving storms or as a result of "train effect" storms (i.e., sequential mature storms that release precipitation over the same area). Train effect storms can be part of multi cell cluster or squall line storm systems.
Soil moisture, soil permeability, soil surface alterations, and vertical soil profile are important soil characteristics that affect runoff production and hence help define flash flood prone areas. Basin characteristics, (e.g., size, shape, slope, land cover) influence runoff and hence flash flood occurrence potential.
