Flash floods may trigger hazardous events such as mud and landslides, structural bridge failures, and other threatening conditions. Rainfall intensity and duration are the primary source of flash floods.
Intensity is the rate of rainfall, and duration is how long the rain lasts.
Other factors include topography, soil conditions, and ground cover. Floods are a long-term event and may last several days, or even weeks. There are also floods that have characteristics associated with the geographic areas they are in, such as river, coastal and urban flooding. River Floods in Hawaii are usually triggered by hurricane or tropical storm rains. Coastal Floods are caused by winds generated from tropical storms and hurricanes or intense offshore low pressure systems that can drive ocean water inland and cause significant flooding.