Flash floods occur when heavy rainfall exceeds the ability of the ground to absorb it and a large volume of water turns into runoff. In India, flash floods have been commonly witnessed in cities like Chennai and Mumbai. Depression and cyclonic storms in the coastal areas of Orissa, West Bengal, Andhra Pradesh, and others also cause flash floods. In the Himalayan states, flash floods are often associated with cloudbursts, which cause huge devastations downstream.

Reasons for frequent occurrences of flash floods in India:

• Short but intense rainfall: Phenomena like cloudbursts or thunderstorms where heavy to very heavy rainfall occurs over a catchment over a very short period of time overcome the absorbing capacity of the catchment leading to the huge volume of runoff. Himalayan regions generally witness such flash floods during monsoon seasons due to cloudbursts. Coastal areas of Orissa, West Bengal, Andhra Pradesh, Karnataka, and Tamil Nadu also witness flash floods due to intense rainfall.
• Changed characteristics of drainage surface: Wildfire, concretization, and other changes in landscape change the characteristics of the drainage surface, thus, reducing their absorbing capacity, which in turn leads to rapid runoff.
• Himalayan terrain: Most of the rivers in the Himalayan terrain flow through narrow gorges abutting moderate to steep slopes with sharp bends and meet tributaries on steeper slopes. Rolling of debris along the constricted course of the rivers leads to short-term damming of the river flow, resulting in the creation of temporary lakes. When the backwater pressure of the lake exceeds the retention capacity of the barrier, the accumulated water gushes downstream with powerful force inundating otherwise safe settlements.

Impact of flash floods on low-lying regions in India:

• Damage to infrastructure and economy: Flash floods damage critical infrastructures like dams, hydropower plants, roads, rail tracks, electricity telecommunication infrastructure, and bridges. Flash floods also have an immense impact on the economy of the region and the safety of the local population. They damage crops along the way and have adverse effects on activities like tourism. Also, restoration of damaged infrastructure strains the financial capacity of the state and industries
• Loss of life: In 2013, the flash floods triggered by very heavy rainfall and cloudburst in the valleys of Uttarakhand affected 12 out of the 13 districts in the state. About 6000 people lost their lives in this disaster. Also, in the 1993 flood in Naptha, more than 200 people were killed.

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• Loss of agricultural lands: The majority of the settlements in the Himalayan area are located on the middle slopes. People tend to exploit fluvial terraces as those are adjacent to the rivers. These areas are considered to be the most fertile locations but at the same time are most vulnerable to floods caused by the breaching of landside-induced dams. Evidence of past damming in different river basins of the Indian Himalayas suggests that the river was blocked many times, particularly at the confluence with tributaries. Flash floods damage these agricultural lands. The damage due to flash floods has increased in the recent past due to an increase in the frequency of floods, encroachment of flood plains, and other anthropogenic activities like mining, deforestation, etc.