Flash floods are short-term events, occurring within 6 hours of the causative event (heavy rainfall, dam break, levee failure and rapid snowmelt) and often within 2 hours of the start of high intensity rainfall. A flash flood is characterized by a rapid stream rise with depxhs of water that can reach well above the banks of the river, carrying with it large amounts of debris and causing high damage due to its suddenness. Flash flood damage and most fatalities tend to occur in areas immediately adjacent to the river where they weaken the soil and cause mud slides, damaging homes, roads and property. 

Heavy rainfall and landslides upstream can cause flash floods downstream. Since Sikkim receives a high amount of annual rainfall, especially during the monsoons and landslides occur frequently, flash floods are a common phenomenon and pose a constant threat to life and property. Also the presence of large dams in Sikkim, poses a great danger of flash floods in case of dam breakage.