Flash floods usually result from intense storms dropping large amounts of rain within a brief period. Flash floods occur with little or no warning and can reach full peak in only a few minutes.
It is important to recognise that urban floods begin with an unanticipated high rainfall event which seriously disrupts public transport, electricity and communications and also plays havoc with the urban economy.
Some examples of flash floods
On December 1, 2015, even in 2005 and 2006, heavy rains from the months of October to December had flooded Chennai.
Located in a relatively flat area, Chennai depends on natural water bodies, canals and rivers to drain the heavy water runoff during rains. Its drainage and storm water network, which is absent in many places, is inadequate even to convey water during moderate rains.
The phenomenon of metropolitan cities receiving excessive rainfall, leading to sudden flooding, has been seen in cities such as Mumbai and Kolkata also.
Natural Causes
- Heavy Rainfall: Water of Heavy rainfall concentrates and flows quickly through urban paved area and impounded in to low lying area raising the water level. It creates more havoc when a main drain or a river passing through the area over-flows or breaches
- Lakes: Lakes can store the excess water and regulate the flow of water. When lakes become smaller, their ability to regulate the flow become less and hence causes flooding.
- Silting: The drains carry large amounts of sediments and deposited in the lower courses making beds shallower thus channel capacity is reduced. When there is heavy rain, these silted drains can’t carry full discharge and result in flooding.
Man-made causes
- 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.
- 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.
- 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
- Un-Authorized colonies: Un-Authorized colonies have been developed by the local colonisers without consideration to the city plans ,drainage, sewerage etc. and thus subjected to flooding during heavy rain falls.
- Poor Water and Sewerage Management: Old drainage and sewerage system has not been overhauled nor is it adequate now .All the drainage and sewer system in many parts of Delhi has collapsed resulting in flooding. This can be seen during rainy seasons every year.
Recommendations
Mapping flood zones: By combining field surveys, historical records, satellite imagery and infrastructure assessment, they have identified vulnerable areas.
Such maps and data are shared with citizens so that they could decide where to live there or not. More importantly, this data is used to regulate development. Cities have been divided into zones on the basis of risks.
Indian cities can take lessons and try to come up with the clear risk zones.
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.
Enhance mitigation: Kuala Lumpur and Tokyo have built extensive water discharge tunnels to divert and store floodwater. Tokyo has one of the largest underground tunnels to hold diverted flood water, which is later pumped into safe watercourses using turbines.
While this system can be extremely expensive, what India can do reclaiming the existing water bodies, which have been reduced to very small areas.
Response measures: Rio de Janerio has spent $14 million and created a real-time monitoring centre of infrastructure and traffic flows. There is need for early warning and dissemination of reliable information about floods and rescue.
Check Illegal constructions: 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.
Maintenance of existing infrastructures: Regular maintenance of the drainage system by desilting the drainage system across the city
