• Over 17,000 people died in floods and heavy rains between 2012 and 2021 in the country, Minister of State for Jal Shakti Bishweswar Tudu said in Rajya Sabha on July 31.

• The minister said the increased incidence of high intensity rainfall of short duration is mainly responsible for urban floods, which is further compounded by unplanned growth, encroachment of natural water bodies and poor drainage system. 

• The state-wise data on damages due to heavy rains and floods in the entire state (rural and urban areas) has been compiled by the Central Water Commission (CWC), and as per which the total damages, including crops, houses and public utilities stood at Rs 2,76,004.05 crore.

• Several initiatives have been taken by the government to encourage groundwater recharge and other nature-based solutions to tackle urban flooding in flood-prone areas. 

• Master Plan for Artificial Recharge to Groundwater-2020 has been prepared by the Central Ground Water Board (CGWB) in collaboration with states/UTs envisaging construction of about 1.42 crore rain water harvesting and artificial recharge structures in the country to harness 185 Billion Cubic Metre (BCM) of water.

Floods

• India is one of the most flood-prone countries in the world. 

• Out of the total geographical area of 329 million hectares (mha), more than 40 mha are flood-prone.

• The principal reasons for flood lie in the very nature of ecological systems in this country, namely monsoon, highly silted river systems and the steep and highly erodible mountains, particularly those of the Himalayan ranges.

• Most of the floods occur during the monsoon period and are usually associated with tropical storms or depressions, active monsoon conditions and break monsoon situations.

• Floods occur in almost all river basins in India. The main causes of floods are heavy rainfall, inadequate capacity of rivers to carry the high flood discharge, inadequate drainage to carry away the rainwater quickly to streams/rivers. Ice jams or landslides blocking streams, typhoons and cyclones also cause floods. 

Types of Floods

• Flash Floods: Floods occurring within six hours, mainly due to heavy rainfall associated with towering cumulus clouds, thunderstorms, tropical cyclones or during passage of cold weather fronts, or by dam failure or other river obstruction. This type of flood requires a rapid localised warning system. Flash floods occur due to high rate of water flow as also due to poor permeability of the soil. Areas with hardpan just below the surface of the soil are more prone to floods as water fails to seep down to the deeper layers.

• River Floods: Floods caused by precipitation over a large catchment’s area, melting of snow or both. Built up slowly or on a regular basis, these floods may continue for days or weeks. The major factors of these floods are moisture, vegetation cover, depth of snow, size of the catchment’s basin, etc. 

• Coastal Floods: Floods associated with cyclonic activities like hurricanes, tropical cyclones, etc generating a catastrophic flood from rainwater which often aggravate wind-induced storm and water surges along the coast.

• Urban Flood: As land is converted from agricultural fields or woodlands to roads and parking lots, it loses its ability to absorb rainfall. Urbanisation decreases the ability to absorb water two to six times over what would occur on natural terrain. During periods of urban flooding, streets can become swift moving rivers, while basements can become death traps as they fill with water. 

• Ice Jam: Floating ice can accumulate at a natural or human-made obstruction and stop the flow of water thereby causing floods. 

• Glacial Lake Outbursts Flood (GLOF): Many of the big glaciers which have melted rapidly and gave birth to the origin of a large number of glacier lakes. Due to the faster rate of ice and snow melting, possibly caused by global warming, the accumulation of water in these lakes has been increasing rapidly resulting in sudden discharge of large volumes of water and debris and causing flooding in the downstream.

Structural Measures to Tackle Flooding

Embankments

This has been one of the major structural approaches in which the river is restricted to its existing course and prevented from overflowing the banks. Usually embankments are constructed with earth but at some places masonry and concrete walls are also used. However, divergent views have emerged concerning the effectiveness of embankment. Studies have pointed out some of the problems such as poor congestion in protected areas, silting of rivers which not only means rising river bed level and consequent decrease in carrying capacity but also depriving neighboring agricultural land from fertile soil and groundwater recharge. There have also been concerns on the issue of embankment capacity to withstand erosion.

Dams, Reservoirs and other Water Storage Mechanism

Dams, reservoirs and other water storages, both natural and man-made, are effective means for reducing the flood peaks in the rivers.

Channel Improvements

The aim of channel improvements is to increase the area of flow or velocity of flow or both to increase carrying capacity. Normally this measure involves high cost and there are also problems of topographical constraints to execute such a measure.

Drainage Improvement

This aims at construction of new channels or improving capacity of existing channels to decongest and prevent flooding. However, what is important is to ensure that such an approach do not cause congestion and flooding in downstream areas 

Diversion of Flood Water

This involves diverting all or part of flood water into natural or artificial constructed channels which may be within or outside the floodplain. The diverted water may or may not be returned to the river downstream. Usually effective to prevent flooding around cities.

Catchments Area Treatment

The aim in this approach is to provide protection to catchment areas through measures such as afforestation which minimise siltation of reservoirs and silt load in the rivers. This can be a very useful approach to control flood peaks and suddenness of run offs. 

Seawalls/Coastal Protection Works

The construction of seawalls and other such work, try to prevent flooding from sea water. These are highly cost intensive apart from the fact that complexity of sea behaviour and other environmental aspects should also be considered.

Manorama Yearbook app is now available on Google Play Store and iOS App Store