Floods that happen within six hours of significant rainfall, a dam or levee failure, or a sudden release of water held by an ice jam are referred to as flash floods. Their unpredictable nature makes them more destructive than regular floods in terms of potential damage.

Causes of flash flood 
Natural reasons:

• Rainfall: Heavy seasonal rains causes rivers to overflow.
• Silt: Heavy sediment load from the catchments, especially in the Himalayan ranges, coupled with inadequate river carrying capacity.
• Cyclones: Cyclones with strong winds and high tidal bores inundate coastal regions and cause floods.
• Cloud bursts: Sudden heavy rain result in flash floods, as seen in Uttarakhand floods.

Anthropogenic causes:

• Drainage: Human impact on natural drainage reduces its capacity to carry away the rainwater quickly to streams/ rivers.
• Flood plain encroachment: Population pressure leading to encroachments into flood plains aggravates the problem.
• Urban planning: Improper town planning, inadequate drainage arrangement can lead to urban floods as seen in Chennai.

The impact of flash floods are 

1. Death or Serious Injury – Flash floods can seriously injure or kill people, especially when occurring at night.
2. Immediate Property Damage – The force of water and large debris like boulders can cause heavy structural damage to homes, making them uninhabitable and carrying away large pieces of property.
3. Loss of Critical Infrastructure – Large debris and floodwaters can damage bridges, roadways, telephone, and cable lines.
4. Deposited Sediment & Silt – Floods can contaminate ground water and leave behind large amounts of silt and other debris, harming local flora.