Flash flood

What is a flash flood?

A flash flood is a rapid flooding of geomorphic low-lying areas such as rivers, dry lakes and basins. Flash flooding occurs when it rains rapidly on saturated soil or dry soil that has poor absorption ability. Flash floods are distinguished from regular floods by having a timescale of less than six hours.

What are the causes of flash floods?

Natural Causes

It may be caused by heavy rain associated with a severe thunderstorm, hurricane, tropical storm, or meltwater from ice or snow flowing over ice sheets or snowfields. Flash floods may occur after the collapse of a natural ice or debris dam.

Man-made causes

1 . Population pressure: Because of large amount of people, more materials are needed, like wood, land, food, etc. This aggravates overgrazing, over cultivation and soil erosion which increases the risk of flooding.
2 . Deforestation: Large areas of forests near the rivers/catchment of cities are used to make rooms for settlements, roads and farmlands and is being cleared due to which soil is quickly lost to drains. This raises the drain bed causing overflow and in turn urban flooding.
3 . Trespassing on water storm drains: The areas which were essentially created by the storm water drains to let their flood waters pass freely being tress-passed for developmental purposes result in obstruction of water flow and thus contributed immensely to the fury of floods.
4 . Poor Water and Sewerage Management: Old drainage and sewerage system has not been overhauled nor is it adequate now .

What are the measures through which flash floods in urban areas can be avoided to a certain extent?

1 . Mapping flood zones: By combining field surveys, historical records, satellite imagery and infrastructure assessment, identification of vulnerable areas should be done. Such maps and data should be shared with citizens so that they could decide where to live there or not. More importantly, this data should be used to regulate development. Cities should be divided into zones on the basis of risks.

2 . Compliance: Only preparing data will not be adequate, unless compliance according to data is not enforced. State governments should be willing to commit to zero tolerance against non-compliance.

3 .  Extensive water discharge tunnels should be built to divert and store floodwater.For example Tokyo has one of the largest underground tunnels to hold diverted flood water, which is later pumped into safe watercourses using turbines.

4 . Reclaiming the existing water bodies, which have been reduced to very small areas.

5 . There is need for early warning and dissemination of reliable information about floods and rescue.

6 . Strict action against illegal encroachments should be taken by checking real estate mafia involved in illegal construction and usurping of water bodies. Clearing flood channels and riverbeds where illegal buildings stand.

7 . Regular maintenance of the drainage system by desilting the drainage system across the city.